Episode 97: Airplane on a Conveyor Belt
Air Date: January 30, 2008
An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff.
busted
First some small-scale tests were performed with a model airplane on a treadmill and the plane was able to take off. For the large-scale test, the MythBusters used a 400 pound ultralight aircraft with a 2000 foot tarp under it. The tarp was pulled backwards to simulate a moving runway. The ultralight pilot had no trouble taking off. This is because the thrust of the airplane engines acts on the air, not on the ground.
A cockroach can survive a nuclear blast.
busted
The MythBusters irradiated cockroaches and several other bugs. The cockroaches all eventually died, and they died sooner than some of the other bugs. Nonetheless, cockroaches would survive longer than humans.
If you freeze a can of shaving cream, cut it open, and then put the foam in a car, it will heat and expand to fill the car.
busted
This myth was first tested with one can of shaving cream; there was little effect. Even with 50 cans of opened shaving cream in the car, the foam did not significantly expand. Lastly, the MythBusters tried putting a large quantity of polyurethane foam in the car, and it expanded to completely fill the car.
Newer: Episode 98: James Bond Special 2
Older: Episode 96: Lead Balloon
It’s all about traction. A wheel-propelled machine depends on the connection with the ground to generate forward trust and can therefore be countered by a threadmill.
Airplanes, being propelled by a jet or propeller not so, backwards momentum won’t transfer fully because the airplane will lose traction with the ground progressively as it gains lift.
Your analysis is incorrect. There is no traction to lose.
Hypothetically, if an airplane had no lift, all the momentum generated by the treadmill would transfer to the airplane resulting in a standstill. Perhaps I used “traction” wrong, but I think I made my point.
Point is, people intuitive think the forward moment of an airplane would be neutered by the same amount of backwards momentum, but the momentum won’t transfer by the connection the plane has with the thread because of the lift of the airplane
No it has nothing to do with the lift of the airplane. The plane would move forward at it’s normal take off speed even without wings.
Your analysis is wrong.
I glad you are up for an argument. So many post once and then leave.
Lets have a tug of war. A cart is sitting on a motionless treadmill. A rope comes off the front to me and off the back to you. Lets say we are equally strong and the tug of war is a draw. The cart does not move.
Now you use your secret weapon. You turn on the treadmill. Do you imagine the treadmill will change the result? How? The treadmill simply spins and the wheels spin with it. The backward force it creates is tiny. We would not even feel it.
Now maybe you could have someone in the cart who would apply the brakes. As he applied them harder and harder you would get more and more force from the treadmill until you would pull me over. But the pilot in our myth isn’t going to apply any brakes. The wheels spin freely so you get (almost) no help from the treadmill.
Now I use my secret weapon. I have an wingless airplane attatched to the rope and I turn on the engine and propeller. Are you going to hold back an airplane? Of course not.
Airplanes are not wheel-propelled, if they were then planes that take off from bodies of water could never rise into the air.
The wheels are simply a means by which an airplane can slide on the ground until the engine propels the plane forward with enough speed that the air moving over the wings generates enough force to raise the plane.
No air moving over the wings == no lift. No lift == no flight.
Therefore a plane on a treadmill moving in the reverse direction at an identical speed cannot generate lift and cannot fly.
I find it hilarous how many people talk about all these irrelevant things like the ball bearings in the wheels and the pilot’s breakfast.
Steve wrote:
>No air moving over the wings == no lift. No lift == no flight.Therefore a plane on a treadmill moving in the reverse direction at an identical speed cannot generate lift and cannot fly.<
How do you come to this incorrect conclusion after your correct statements above?
The plane moves THROUGH THE AIR at it's normal speed. The treadmill moves in the other direction but has no effect on the plane moving THROUGHT THE AIR. The wheels spin freely at twice normal speed. the wings create lift because they are moving THROUGH THE AIR at normal take off speed.
Steve wrote:
Airplanes are not wheel-propelled…
No air moving over the wings == no lift. No lift == no flight.
Presto replies: Correct!
Steve then concludes:
Therefore a plane on a treadmill moving in the reverse direction at an identical speed cannot generate lift and cannot fly.
Presto asks:
How do you come to this incorrect conclusion after your correct statements above?
The plane moves THROUGH THE AIR at it’s normal speed. The treadmill moves in the other direction but has no effect on the plane moving THROUGHT THE AIR. The wheels spin freely at twice normal speed. the wings create lift because they are moving THROUGH THE AIR at normal take off speed.
I wonder, does airplanes have a free rolling wheel? can it even be moved backwards if it’s idle and nonactive by a treadmill?
yes, planes wheels are free spinning. Also moving backward is irrelevent. The plane moves forward.
I realize that many won’t read this far down, but I’ll post anyway. Some of the failures in logic I see amongst the “naysayers” is that they are focused on Lift, the belt and the wheels.
Firstly, as any pilot should be able to tell you, flight is a matter of balancing four forces. Lift versus gravity, thrust versus drag. The “no fly” crew are certain that you don’t get lift because of the drag of the belt. But it is thrust that negates drag and the wheels do not affect drag as much as you would think. Thrust comes from the engines spinning the prop fast enough to push back the air.
Second, the tires. All this “spin,” “no spin,” “but breaks on the tires can stop/hold in place a plane” are forgetting little things like planes that use pontoons or skis. Yes there are things on the bottoms of planes other than wheels that logically have even less to do with forward motion than wheels. So imagine skis if you really have to wrap your head around how little the wheels matter.
Finally, you’re forgetting that few if any conveyor belts are 100% free standing. There are drive wheels in the belt every so often. So the plane’s wheels would have something to push off of every so often, just as the ultralight had with the conveyor belt the mythbusters had set up.
So, in the end, the thrust provided by the engine will produce enough speed to make the conveyor belt pointless. The wheels will spin FAST, but they won’t hold back the plane.
I agree with most of you post.
However let’s look at this paragraph:
>…few…conveyor belts are 100% free standing. There are drive wheels in the belt every so often. So the plane’s wheels would have something to push off of every so often,…<
You really lost me here. Earlier you explained that the wheels have nothing to do with forward thrust but here you state the wheels “push off” of something?
Also you don’t have to get into the details of how a conveyor belt is constructed. The point is that it is a moving surface on which the plane sits. The belt can hold a car still because the car is driven by wheels. It can’t hold a plane still because the plane is driven by moving air.
Pontoons and Skis are designed for low friction, thus low drag. The wheels perform the same function on a tarmac.
When free wheeling, the only drag on the plane is the friction of the bearings, which is very small. This is how a man was able to pull a 747 in his teeth.
When the brakes are on, the friction creating drag is the friction between the wheels and the ground, which is very high and thus creating much more drag. Would the friction be enough to stop the plane from taking off? I doubt it, but that’s one I could be wrong about.
To presto, I think he’s referring to the belt would sag between the drive wheels, thus when the plane hits the drive wheel it would “Bump” upwards, likely losing contact with the belt momentarily, thus losing any drag created by the belt and building forward momentum. Honestly I think it’s kinda irrelevant to the argument.
The problem is that this in not the original version of what is essentially a thought problem. This originally appeared on a Russian site. There the conveyor belt matched the speed of the planes wheels. It was meant to get people to realize that it does take energy to accelerate the wheels of the airplane. Now it takes a totally unrealistic treadmill and unrealistic wheels for this to work, but you can calculate how fast the conveyor belt would have to accelerate to keep the plane in place. It has been calculated on some sites that it would take an acceleration of at least 50 g’s for the wheels to produce enough force to keep an airplane in place.
A constant speed conveyor belt will not keep a plane in place, an accelerating one can. Unfortunately this thought problem is on the same order as the question “What would happen if you drilled a hole from pole to pole and dropped an object down it?” You could answer mathematically, but you can’t do it today, and you probably never will be able to do it. So a real life conveyor belt will not keep a real life plane in place. But a purely theoretical one can.
>The problem is that this in not the original version of what is essentially a thought problem. This originally appeared on a Russian site.<
Why is this a problem?
The myth being discussed is clearly printed at the top of this thread.
You’re correct, if you also include a set of theoretically indestructible bearings in the wheels. Since I think the think problem doesn’t include the plane falling apart :)
Presto, the problem is, most of the arguments people make on this page are based off their perception of the original myth, rather than the specifics the mythbusters outlined.
To those people I say
“A real life conveyor belt can not hold the plane still, or even significantly slow the plane’s acceleration enough to prevent takeoff.”
The only way for the bearings to be indestructable would be for them to be frictionless. Thus once again, the aircraft takes off.
For bearings and tires to be creating enough friction (I’ll forget about the brake packs for now) to prevent an aircraft taking off? They wouldn’t last long at that acceleration.
presto you are wrong, an aircraft tire DOES have traction.
from Wikipedia: traction – adhesive friction or force in the context of vehicle.
It is exactly this reason that Subduction Zone is wrong, it IS possible for a constant speed conveyor belt to keep a plane in place.
The friction between the free spinning wheel and the conveyor creates a small force in the direction of the conveyor belt movement, “rearward” in the system. A free spinning wheel only has zero-friction in your high school physics text book, not in the real world.
If the forward thrust created by the spinning propeller matches the rearward force created by the wheel friction, the resultant force is zero and the airplane stays stationary. Obviously this requires MUCH less than takeoff power. Why are you assuming that the engine must be at full power? That was never a condition of the experiment.
Jeff, you seem to think the objective is to keep the plane still.
This indicates that you fell for the trap. You read “Treadmill” and reasoned “vehicle stands still”.
Read the myth at the top of this thread. The treadmill is going at the planes NORMAL TAKE OFF SPEED. If you think it adjusts you are looking at the wrong myth.
Lets say the planes NORMAL TAKE OFF SPEED = 60mph for example. The treadmill goes 60mph in one direction. The pilot handles his airplane EXACTLY AS IN A NORMAL TAKE OFF which causes it to move 60mph (RELATIVE TO THE AIR BECAUSE IT IS POWERED BY A PROPELLOR).
The pilot flys normally and feels no effect from the treadmill because the slight friction on the rolling wheels is inconsequential.
>If the forward thrust created by the spinning propeller matches the rearward force created by the wheel friction, the resultant force is zero and the airplane stays stationary. Obviously this requires MUCH less than takeoff power.<
You are completely correct here. I agree completely.
Now re-read the myth at the top of this thread.
Dude you’re making yourself look bad. I didn’t mention anything about the plane moving forward, taking off, or how the myth is written at the top of the page. All I said was the plane’s tire has traction, and it is POSSIBLE for the plane to remain in place on a moving treadmill. I’m not saying the objective of the myth was for it to stay still. My point is many people in the more recent posts seem to believe that it’s not possible for a plane to match the speed of the moving treadmill because the wheels are free spinning and power is not transferred through the wheels or something like that. All I’m saying is that it is possible, and it has nothing to do with the myth.
fell for the trap?? What trap is this? I didn’t say anything about the myth. What trap anyway? Interpreting the myth exactly the way the myth busters portrayed it on screen with their animation?? The ambiguous definition from the show, and the one at the top of this page are not in agreement.
The pilot handles his airplane EXACTLY AS IN A NORMAL TAKE OFF ?? Where ANYWHERE in the myth does it say this is a condition? Nowhere. You are making more assumptions than the people who think it should remain still! At least the people who think it was supposed to remain still have some reason to think that. Just watch the beginning of the show where they explain the myth. They totally misrepresent what they actually tested. They show an animation of an airplane standing still on a moving treadmill, and then flying into the air with no apparent forward motion. You can’t blame people for interpreting the myth the way they did, because the myth busters didn’t explain what they were testing very clearly. Any logical person would see that animation, and deduce that the airplane is standing still relative to the ground, and is rolling on the treadmill. Your ungrounded assumptions about the conditions of the myth are the only reason that your explanation holds any water.
Yah I know what the myth says. And I agree with the conclusion, given that definition. But in a post above you say and I quote “The belt can hold a car still because the car is driven by wheels. It can’t hold a plane still because the plane is driven by moving air.” which is not so much wrong, just a naive explanation. Whether a vehicle is powered by turning the wheel or moving air does not matter. It makes more sense to talk about free spinning vs. locked wheels and that gets into Subduction Zone’s explanation in which he says it’s impossible.
Naive? Ridiculous. I understand this myth very well and I think I explain it about as clearly as any one.
free spinning vs locked wheels? Read the myth again. Where do locked wheels come into it?
>Whether a vehicle is powered by turning the wheel or moving air does not matter<
What??? That’s everything! That’s the whole crux of the matter. Airplanes are powered by a propellor acting against the air and cars are powered by wheels. Failure to understand the distinction is the entire point of the puzzle.
>All I said was the plane’s tire has traction, …<
Of course we both understand that the tire has traction on the treadmill. The point is that since the wheel is free spinning the traction is not signifigant. Technically any time someone says “I’m stuck, I have no traction” you could correct him and say “Well you have traction you just don’t have enought”. In my opinion that would be a silly thing to say.
>…and it is POSSIBLE for the plane to remain in place on a moving treadmill.<
This is not relevant to this myth. The question is can the plane take off. It is not can the pilot make the plane stand still. The pilot would get in his plane and pilot it as normal and take off.
So yes, The idea that the pilot would get in the plane and take of as normal is part of the myth. It is not my assumption. He’s not going to stand beside the plane and twiddle his thumbs and he’s not going to get in the plane and push gently on the controls in an attemp to keep the plane still. Ridiculous.
It is not POSSIBLE for a plane to remain stationary on a moving treadmill. Unless it’s tethered, but I guess that makes it pointless.
And the treadmill is a simile. Nobody is supposed to be talking about a treadmill at all.
>It is not POSSIBLE for a plane to remain stationary on a moving treadmillAnd the treadmill is a simile. Nobody is supposed to be talking about a treadmill at all.<
Huh?
the question is not if the plane takes of but does it move (relative to the ground and air) on a treadmill. yes it does and thus it flies. if it did not, it would not.
Actually no. The question IS will the plane take off. (Read the myth at the top of this thread).
This myth was not tested properly at all. The plane took off yes, but this is ONLY because the plane accelerate faster than the truck pulling the tarp did. The myth is described as the plane does not move at all in relation to the ground, but in this episode the plane clearly stayed at the same speed to start with, but then took off because it was moving forward faster than the truck was. To PROPERLY test this the plane must not move even 1″ in relation to the ground, so therefore the plane should NOT pass any of the cones they have set up. This will result in the plane’s wheels traveling very fast, but the plane itself will have a speed of 0 mph and without air passing over or under the wings will NOT lift off the ground. The plane must do the same thing their car did on the conveyor belt and not accelerate beyond the speed of the belt. DUH…
>The myth is described as the plane does not move at all in relation to the ground<
No.
This is the incorrect assumption people make when they read the myth. You saw "vehicle on treadmill" and thought "vehicle standing still".
This is only true for vehicles that are driven by their wheels.
Please think about this and don't get defensive. Most people make the same mistake.
This is the crux of this myth. This is the whole point. Everything else is window dressing.
Wrong. The myth states that the treadmill moves in the opposite direction at the same rate of speed as the plane. Therefore the plane has no motion relative to the ground (as would a jogger jogging on a treamill). Since a plane’s lift is generated by the movement of air over the wings, the only lift that would be occuring here is whatever the wind’s doing or whatever wind the propeller blows over the wings (assuming the propeller’s exhaust even moves across the wings)
I agree with you steve. I caught this rerun yesterday and have been bothered by it since. I think the pemise is misleading. If a plane stood perfectly still (no FORWARD motion) on a treadmill, how can just the spinning of propeller generate enough lift to create actual flight? If it could, why the need for all the wing surface? Why the need for a runway? Why not floor it and pull back on the stick? haha
I don’t know if you will still read this, but please keep an open mind on this one.
Picture a plane (turned off) whose minimum take-off speed is 60mph (airspeed, not groundspeed) on a treadmill going backwards at 1000 mph (much higher than take-off speed)
Groundspeed = -1000 mph
Airspeed = -1000mph
Now turn the plane on. Any forward thrust given by the props will “slow” down the negative speed, accelerating it forward until the airspeed equals 0 mph
Wheelspeed = 1000 mph
Airspeed = 0 mph
No airspeed = no lift
But here’s where is gets squirrely. Now you have a plane with props putting out enough forward thrust to accelerate a stationary plane to 60mph, and you have a “stationary” plane, so it will accelerate to 60mph!
Wheelspeed = 1060 mph
Airspeed = 60mph
Yes, you are correct here.
People get tricked by the phrase “plane on a treadmill…” and imagine the plane standing still.
But if the pilot accelerates as normal the plane will move forward as normal and take off as normal. The conveyor belt has virtually no effect no matter how fast it is going.
Well the plane did take off. No doubt..
People who said that that the plane will not fly thought the plane will not have relative motion w.r.t ground.
People who thought it would fly say that in the myth there is no mention of the fact that there should be no relative motion of plane to the ground. Hence they say the myth is true.
Well if that is the case then there is also no mention on the size of the conveyor belt. Lets make a conveyor belt whose size is just one feet more than the plane length. Note the belt can move as fast as you want.
In this case the amount of thrust needed for a plane to fly in one feet. (maybe we may have to wait another 1000 years). And when that happens I will reduce conveyor belt extra length to 0.5 feet… and when you match that (I will reduce to 0.25 feet) and so on…
I guess you get the point … so is the myth busted (I think not) this requires some more thinking
You are either confused or unclear. If the conveyor belt was one foot beyond the plane the plane would simply move forward as normal and take off as normal.
are u sure… The plane has just one foot to take off… I do not know of normal planes that can take off in one foot. Plus I am assuming the plane has to stay on the conveyor to take off… If it goes off it it will fall down a …say hill for example
also I assume my conveyor is more like a real treadmill type…. not the type used by mythbusters
I see what you are saying. I’m imagining a conveyor belt level with the runway. You are imagining that it is above the runway like an exercise treadmill in your living room.
In any case the plane would move forward. If the conveyor was level with the runway it would continue as normal and take off.
If the conveyor belt was above the runway the plane would still move forward and fall off. Depending on the height this may or may not be a problem. Completely irrelevent to our myth.
This is all a side show. The point of the question is that most people imagine the treadmill will hold the plane still like a car. This won’t happen. The plane is not driven by it’s wheels. It is pulling itself through the air. It’s like you wearing roller skates on a treadmill and pulling yourself forward with a rope. The treadmill is irrelevant.
that may be true… but the devil is in the detail…its not about what most people think…
It’s funny that 2 years later people are still arguing about this. It appears that many people here just can’t understand that regardless of the conveyor belt’s speed the plane will take off. You CAN’T speed the conveyor up to a point where the plane couldn’t fly – assuming the wheels can handle the speed.
So everyone saying that the myth was running the conveyor so fast that the plane would be stationary – it can’t be done. There is no way to test the myth as you all imagine it.
You stated the situation perfectly
At this point I’m just staying with this thread to see how different people handle a discussion on the internet.
It seems to me that if the belt matches the speed of the plane and, the plane has no motion relative to the ground, There cannot be lift because there is no airflow over the wings. When I watched the show, the plane clearly had motion relative to the ground because it was moving past the cones. My impression was that the plane speed would match that of the belt which means there is no motion relative to the ground.
This puzzle is very clever. It tricks people into winning the wrong argument.
You went right past the question “How does a plane behave on a treadmill” (Because it seemed so obvious that it behaves like a car).
Then you want to discuss the question “Can a plane take off with no forward motion relative to the air”. The answer to this is obviously no. So it is easy to win this argument.
But the first question is the real question. The answer is not obvious. The second question is not even being asked in the myth!
I just saw this show episode for the first time and I guess I was confused by the myth. (Yes I read the myth) I thought the point was the plane had to be stationary relative to the ground. But that is not my question.
Just to be clear…say you where to use a glider with wheels, and attach it to something in front of the conveyor belt. So that it would not be able to move forward or backwards relative to the ground as the conveyor sped up.
No matter how fast the conveyor belt moved the glider would not spontaneously take off like a kite right? If anyone thinks it would take off please explain to me how being on a treadmill would generate enough airflow over the wings for the glider to take off?
>I thought the point was the plane had to be stationary relative to the ground. But that is not my question.No matter how fast the conveyor belt moved the glider would not spontaneously take off like a kite right? <
Of course not. This image of a plane taking off without moving forward is is thing people THINK they are supposed to argue against. But in fact it is not part of the scenario. Because you thought the plane was supposed to stand still you thought you were supposed to discuss a plane taking off with no forward motion.
It would take off like a kite, yes. But, as with a kite, it would need to be a windy day. A very windy day.
The wind is inconsequential. You wouldn’t measure the “normal grounspeed for take off” in 60 mph winds would you?
Discussions of taking off in high wind are a side show. They are not relevent to this myth. The myth IS NOT ABOUT THE PLANE TAKING OFF WHILE STANDING STILL.
The plane was also moving foreward, for the myth to be properly tested, the aircraft needs to remain in one spot!
>The plane was also moving foreward, for the myth to be properly tested, the aircraft needs to remain in one spot!<
You fell for the trap. A plane on a treadmill is not a car on a treadmill. It behaves differently.
-Maybe the myth would be more accurate tested with a heaviest plane (a cessna 172 for example) and in a place without wind.
-A plane fly because can achieve certain speed and “create” relative wind plus airflow in the wings.
-A Piper J-3 can take off in a minimun space with out complication and if exist some headwind (10 knots) easily can do it (i fly a similar plane).
-Think about USAF experiments in the 60′s, if they can use some kind of device similar to the used in the myth they would be using that thing now in the present, instead of develop VTOL a/c. but the history is different.
-Another scenario is had 40 knots headwind and you can make a stationary flight with a light airplane (like the Storch), only because you have enought laminar flow around your wings, not present if the airplane dosen’t move because that is achieved when the thrust is more than drag and generate relative wind in the aerodinamyc surfaces.
-Obviously the myth was false and in the future better make a call to lockheed martin.
Heinz you are way behind. Wind was not involved. The plane took off as normal because no treadmill can impede the motion of ANY PLANE.
If a plane will take off without a treadmill it will take off with the treadmill and will feel no effect. The plane is pulled through the air by it’s propellor (or jets).
The way the myth is stated is obviously false. The plane can accelerate until it is moving at its normal speed relative to the ground(=twice its normal speed relative to the moving treadmill) and then of course it will take off. Which is pretty much what we saw. The only surprise was how easy it was for the plane to get up to that speed which was due to the subtle difference with the wheels that the mechanically clueless(=me) didn’t think about in advance.
It would have been more interesting to me if the MythBusters had started with the plane stationary on the tarp, the pickup had accelerated to full speed and THEN the plane had started up. In that case at least the plane would have had to accelerate twice as much as normal and the pilot would have noticed a difference.
It does get me to whether there would be some way to have a plane take off from a stationary position using a wind tunnel.
Hi Roy,
For some reason you only grudgingly acknowledge that the mythbusters test was correct and showed that the plane will move forward and take off as normal. This result surprised most people and obviously stirs up a lot of conversation.
>It does get me to whether there would be some way to have a plane take off from a stationary position using a wind tunnel.<
If a model plane is held still in a wind tunnel it would behave as if it were moving through the air. This would shed no light on this myth.
If you are wondering about a real plane in a wind tunnel I don’t think they make a wind tunnel that big and again, it would have no relevence to this myth. We already know that planes fly as a result of relative air moving past the wings.
The simplest, cheapest (though not as interesting) way to test the idea is to simply tether the plane in two axis (one reverse and one down)…let the plane rev to the same RPM (or even higher – what the heck) that is normally required for takeoff…and measure the vertical force exerted against the (up/down) tether. If there’s upward force sufficient to lift the plane off the ground, then the point is proven (the air movement over the wings being strictly created by the motion of the prop is sufficient to create liftoff. (I’m thinkin’ any AE will tell you that’s not the case. The amount/velocity of air passing around a wing is quite different than that which is creating thrust with the propeller). That would make a pretty boring test though.
floridafraze
You made the incorrect assumption that most people make when they encounter this myth.
THIS MYTH HAS NOTHING TO DO WITH A PLANE TAKING OFF WITHOUT FORWARD MOVEMENT.
This is from one of my previous posts and applies very well here:
This image of a plane taking off without moving forward is the thing people THINK they are supposed to argue against. But in fact it is not part of the scenario. Because you thought the plane was supposed to stand still you thought you were supposed to discuss a plane taking off with no forward motion
well… Most people would believe that a treadmill would enable a person to run as fast as possible if no restrictions apply… I have already proved that a plane would fall into a ditch and burn to death if it was put on a treadmill no matter how strong the engine is… All I would like is to wipe the smirk of the face of the people who think the myth is true after the mythbuster experiment…. it is not true an I can prove it with the easiest of assumptions
Here is the myth as stated at the start of this thread:
I don’t see where you “proved” anything in your ealier posts. You just seem to have a different idea of what is meant by a treadmill in this case.
I’m not sure what you mean in the second part of your post. Here is the myth as stated at the beginning of this thread.
“An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff.”
So you want to “Wipe the smirk off the face” of people who believe this is true?
Haha wow I can’t believe there has been this much discussion about this. A plane on a SHORT treadmill (as seen in the common images around the web) of course cannot take off because it doesn’t have the available distance to move to gain enough velocity relative to the surrounding air. The wheels will rotate, but the plane itself can’t move enough to take off before it falls off the end of the treadmill.
People here seem to be confusing that with the stated myth (that’s important; scroll up and read it again if you’re not sure about it).
This myth assumes an unlimited length of (moving) runway for the plane to move through. The important point is, the wheels are rotating freely so it doesn’t matter at all what the runway is doing.
The friction of free-rotating wheels on the backwards-moving surface is present, but negligible compared to the forward thrust of the propeller / jet / whatever is pushing the plane. The plane will move forward relative to the surroundings (and thus the surrounding air), and will take off. With enough free distance to move, just about any plane will take off regardless of what the ground is doing.
As for the internet meme (will it take off?), of course not. The plane doesn’t have enough room to move before it falls off the end of the treadmill.
A great summary. Well put. But don’t be surpised if the very next post starts off:
“They did it wrong…The plane was moving past the cones!”
Presto you are a very patient person. you have stated what should be obvious several times and people are still arguing with you.
Never tried to post a comment before…. make me feel anxious. Just want to say thanks. Mainly to Presto but also to the “confused” who kept her/him hardworking with the arguments. I think you’ve all given me enough to write an exam question based on the idea. And I’ll get paid for it!! ( Just a little ) Thanks again ;-)
Thanks Rex!
Here’s yet another attempt to make this whole thing clearer:
Imagine a car, a hover craft, and an airplane driving side by side at 50 mph on an ordinary runway. There are cones on the edges of the runway and all three vehicles are passing them.
Up ahead the runway turns into a very long and wide treadmill moving at 50 mph the other way.
Can you see that when they hit this stretch the car will stop passing the cones and will seem to stand still to observers on the sides.
Can you also see that the hover craft would not be effected by the treadmill? The hover craft has an air cushion below and a big propellor behind pushing it.
Do you think the airplane with it’s free spinning wheels and propellor (or jet) would act more like the car or like the hover craft?
Think about it before you post.
It seems to me that planes often gauge their speed as airspeed and groundspeed. The question as its put forth doesn’t seem to clarify what type of speed is being set for the aircraft. As groundspeed is particularly important during takeoff and landing (especially when taking off from a treadmill) it seems as though they should have made a point of this. It appears to me that this is the source of the disagreement and confusion. The show didn’t clarify this properly. The question is unclear. Provided they reach v1 (true airspeed) the ground can do whatever it wants. See v speeds. It’s easy to get the wrong answer when the question in put poorly :-)
Actually, rereading the post it looks to me as though it’s based on groundspeed. I still think the question is phrased terribly. And the examples in the show make it more confusing. They show the plane staying still inferring that the planes groundspeed is equal to the tarps speed. This is not true.
“the normal ground speed during takeoff” is gauged by true airspeed, and not the actualy groundspeed. There’s too measurements being used interchangably. Make no mistake, the plane is still achieving v1 takeoff speed. You cannot fool physics by moving the ground.
What further complicats this is the fact that the ground is moving. There’s 2 groundspeeds. One in relation to the tarp and one in relation to the static ground.
Ugh.
I bet, if you reworded this question there would be absolutely no disagreement.
I suppose you could claim that any puzzle is just a poorly worded question in that you could rephrase it to remove “confusion”.
But this myth is stated very simply and accurately. It is our own assumption that causes us to analyze it incorrectly at first.
There is no confusion in the question regarding speed. The question concerns “normal ground speed during takeoff” which is easy to understand.
The confusion about what is meant by speed is needlesssly introduced durring the discusion.
Headwind is irrelevent. It is true that some planes can stand still in flight if the headwind is high enough but no pilot is going to take off in that kind of wind.
Whatever wind is present when you measure “normal ground speed during takeoff” would be the same wind you would use for the test so any reasonable person would choose no wind. It just makes the test easier. In any case it would be small.
The episode did include a picture of an airplane on a small treadmill that you might find in a gym. I agree that this confused some people. If you watch the show regularly you know that there are many animated segments and that some are helpful and some are comical.
Strictly the myth as stated is impossible to perform because the myth states that the treadmill must be going backwards at the same speed as the plane is going forwards.
Due to friction, the wheels + treadmill will exert some small backwards force on the plane. I agree not enough to prevent takeoff but still a small amount. Lets say that the treadmill reduces the airplane speed by 1mph.
So if the plane is moving forward at 10mph then the treadmill must be moving backwards at 11mph in order for this to happen. But this breaks the myth because the plane and treadmill speeds aren’t equal.
It’s even easier to see this problem with a car on a treadmill – they even performed this on the show. They had a stationary car sitting on a moving treadmill – except that when you think about this, the myth states that the treadmill should be moving at the same speed as the car.
So the myth would be more accurately worded as “Can a plane take off on a treadmill moving backwards at the same speed that the plane would be moving forward if the plane were on normal ground”. In this case the answer would be yes (and indeed this is what they tested on the show).
Greg, your very first statement is wrong. Please read the myth stated at the top of this thread;
“An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff”
If you can’t see where you made your mistake read it again. Then read a few of my posts.
Then post your response. The mistake you made is the mistake almost every one makes when encountering this question.
Greg, Although you did misinterpret the stated myth you didn’t quite make the usual mistake. You understand that the plane won’t stand still (most people don’t).
But you wrongly think the plane is supposed to match the treadmill speed. As stated in this myth the treadmill moves at the planes “Normal groundspeed durring takeoff”.
That speed is measured before the test. Let’s say it is 60mph (for instance). Durring the test the treadmill goes 60mph backward, the plane moves 60mph forward and the planes wheels move at 120mph forward.
Why do I choose to to use airspeed for the airplane? I don’t. The pilot is merely trying to take off. He isn’t doing anything different than a normal take off. He applies no noticable extra power. The effects on the wheels are negligable. He doesn’t try to go 60mph that is simply what happens because that is the normal take off speed and this is a normal takeoff.
I havent seen this episode in a long long time, but for some reason I remember an animation of an airplane on a treadmill (which implies no movement relative to the ground). Is this image in my head from something unrelated to MB or was this actually shown? If it was shown that’d be a prime reason for so many people’s misunderstanding of this myth.
Dan,
You are correct. From around three posts up:
“The episode did include a picture of an airplane on a small treadmill that you might find in a gym. I agree that this confused some people. If you watch the show regularly you know that there are many animated segments and that some are helpful and some are comical.”
I’m not sure the tow-behind tarp used in the show could have dragged a stopped airplane around without tearing apart, however even if the test were further scaled upward with a gigantic treadmill that could move the whole plane, the result would be the same.
With the engine’s disengaged, the plane would move backwards with the belt. Once the engine is engaged, the propeller has to produce enough thrust to overcome the normal static friction as if it were to be on a stationary runway. Whether the belt moves faster, slower, at the same speed as the plane, the belt’s presence is irrelevant. The result would be the same no matter what direction the belt moves. I wish they would have marked up the wheels so they could show it with the high-speed, but the answer is simple anyway. With the belt driving South at 25mph and the plane driving North at 25mph, the plane’s wheels would be spinning at the same RPM as if it were driving at 50mph. The only added drag would be that caused by friction within the wheel bearings resisting rotation, but airplane bearings are of very high quality and the pilot shouldn’t feel any difference.
Remember, a plane’s wheels are part of it’s LANDING gear. The only thing they do for takeoff is keep the body of the plane off the ground so that the propellers can create enough thrust to take off.
Well said, Brandon.
I wouldn’t change a word.
thanks Brandon. i hope that ends the confusion
I can’t believe the amount of people that can’t read a simple sentence.
I can’t believe people are so stupid.
I guess if you’re that stupid, the fact that you can’t read and write doesn’t phase you when it comes to arguing with strangers about a topic you have no understanding of.
I guess it helps the human race if you listen to the responding answers and become a little less stupid.
I really want to become a pilot now, since they don’t seem to understand simple physics or know how to read and write, it doesn’t seem such a difficult goal.
I disagree. Many intelligent people get this wrong at first. Congratulations if you didn’t.
Just to answer some of the other questions that came up.
A large or high powered aircraft will not stay in place at full power with the brakes on. It will slide on locked wheels, usually in a curving line dependent on which wheels/piece of tarmac has the most grip. It’s not been tested much, it tends to be the result of an error in proceedures, and tends to destroy tires.
Before you ask, no it is extremely unlikely it will achieve take off speed.
Completely irrelevent. I haven’t seen that question come up. It might be up in there somewhere though so …ok
It is up there. You could’ve just hit Ctrl+F and searched the page for it, but…
You must have misunderstood my post.
a) How a plane with brakes on acts is completely irrelevent the myth being discussed.
b) I haven’t seen the question come up but it might be up there so …ok
Thank you Presto! Your explanation has caused this to finally fully click. No matter how fast the treadmill moves, even it is moving at 1,000,000 mph the plane will still take off. Granted at this speed the bearings would cook, but the plane is still going to move forward just the wheels will be moving SIGNIFICANTLY faster than the plane is in relation to the ground. Also being on this conveyer belt does not either shorten or lengthen the distance the plane requires to take off, it simply causes the wheels to move faster than they would if on stationary ground. It now makes sense thank you!
You’re welcome. And Thank You. I wasn’t sure any one was reading this stuff. A lot of people post once with much confidence and then never post again.
Cmon guys. Man up. If you post an opinion either defend it or let the other guy know he changed your mind!
Well said, that was what I was trying to get at in my post above. What the ground is doing is completely irrelevant, unless of course it’s moving so fast that the bearings fall apart or the friction overcomes the forward thrust of the plane (practically impossible but 1,000,000 may do it, haha).
The only thing that matters is airspeed, which is achieved by a long enough runway.
It is very clear that the propellors drive the plane forward so stationary tarmac or moving conveyor belt is largely irrelevant to that action if the wheels are freewheeling. I say ‘largely’ because with both tarmac and conveyor belt there is still drag on those wheels by contact with the tarmac or conveyor belt surface. Let’s assume the surface is made of the same material. Of course in both cases the propellor more than easily overcomes this small drag to allow the plane to take off. But the difference between stationary tarmac and moving conveyor belt is surely that in the same air distance the plane requires to take off it will freewheel over a greater length of conveyor belt than it does over stationary tarmac. If the plane has to freewheel over a greater length of conveyor belt (and the faster the conveyor belt goes the greater that length is) surely the overall drag on the wheels prior to take off must be greater with the conveyor belt take off. This may in reality be insignificant due the the efficiency of the wheel design but nonetheless the contact of freewheeling wheels with the runaway or conveyor surface represents drag and the greater distance travelled over that surface represents greater drag. So to stay still the plane would have to generate thrust from the propellors only equal to the minimal extra drag cretaed via wheel contact with the conveyor belt surface compare to the stationary tarmac. In reality this most likely would be such a small amount of thrust that it would be very difficult to calibrate exactly. So the plane is bound to move forward with the propellors firing because it easily overcomes the drag on the wheels. In theory the plane should take slightly longer to take off from the conveyor belt because the drag is greater on the wheels compared to the stationary tarmac but the difference is likeley so small as to be not noticeable.
I agree with your post. Thank You.
(I hope you will forgive a small critique; That is a lot to read. In the future you might want to break up your posts into several paragraphs and eliminate repetition.)
Sure thing.
I’m amazed this so called ‘myth’ was given any credence at all since it is so obvious that the propeller pulls the plane and with freewheeling wheels what on earth is there to hold the plane back except the possible measly extra drag I mentioned which probably measures in the millionths as a fraction of the propellor thrust?
It’s just so silly in the end.
(Hope you liked my paragraphs there.. =D)
I think it’s actually a very clever brainteaser.
The phrase “Vehicle on a Conveyor Belt…same speed opposite direction” causes people to form a mental picture which is innacurate. They think of a person or car on a conveyor belt standing still relative to the background and apply that to a plane.
Once they make this mistake they move on to what they think is the tricky part of the question. But now they are doomed to endless arguments about prop wash and wind velocity.
Now some people for various reasons don’t make that initial mistake. I strongly suspect it is because they don’t encounter the question “cleanly”. They come across the argument and read the discussion before they had a chance to think about the question on it’s own.
It’s like if you were walking throught the forest and came upon a bunch of people talking about a trap that was up ahead that had captureed a bunch of people. You then walk ahead and say “Wow this trap is so obvious, those leaves don’t look natural at all. What a bunch of idiots to fall into such a stupid trap”
Most of these people will protest and say “No I didn’t know there was a trap, I just saw it because it was so obvious”. But remember even if you only had a hint that it was a trap you have a huge advantage over someone casually walking by.
(Oops, better follow my own advice and keep this short so I’ll end here.)
I hear what you are saying. Not sure where that question is. What I read was the statement at the top of the screen
‘An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff.’
My friend had told me this ‘myth’ first in exactly those terms so I knew we were dealing with an aeroplane straight off.
This was also after we had a similar discussion about hovercrafts where I had concluded in answer to his question that a hovercraft would go upstream and downstream at the same speed on a flowing river. He wasn’t so sure.
This problem is similar and my response was similar – the conveyor like the river would make no difference (apart from some possible and very minimal extra drag arising out of the moving surfaces). With the hovercraft the possible drag is just from movement of uneven surface causing some air buffeting in a set direction and some splashes of water. You would only see an effect on a small model not a full size hovercraft.
But my friend disagreed with me about possible extra drag from the conveyor contact with the wheels – that’s why I looked the myth up and found this discussion. I was pretty gobsmacked to see some of the discussions to be honest. But I’m happy you agreed with my analysis since you looked like the clearest poster on the issue.
“This was also after we had a similar discussion about hovercrafts where I had concluded in answer to his question that a hovercraft would go upstream and downstream at the same speed on a flowing river. He wasn’t so sure.”
Right. So when you encountered the Airplane on Treadmill myth you had already thought about a hovercraft on water. The hover craft on water is a way of clarifying this myth and getting people to focus on that part of the myth that most people trip up on.
This is a long thread, and I haven’t read all submissions, so this might be previously addressed. Forget the conveyor belt. picture instead a plane sitting on a very long flatbed on a train, travelling at the the take-off speed of the plane. If the plane faces forward, the wind will tend to lift the plane with no motor running. Facing backward, and motor running to get the plane to the same speed as the train, but backwards (that is, at take-off speed when compared to the flatbed) it will be travelling at zero speed compared to the air, and there can be no lift. To take off, the plane will need to be going at take off speed when compared to the air, which will be twice the speed compared to the flat bed, or by extension, a conveyor belt. In the episode the plane’s speed is much more than zero when compared to the surroundings, that is, more than take off speed compared to the belt.
Don wrote:
“and motor running to get the plane to the same speed as the train, but backwards”
This shows that you misunderstand the myth. The plane standing still relative to the background is not part of the myth. Read the myth again at the top of this thread.
Do you see your mistake?
this shows that YOU misunderstand. The background IS irrelevent. Going at a specific speed compared to the belt is all that matters. If the plane is moving at take-off speed, and the belt is moving at the same speed in the opposite direction, then there is no air movement to create lift.
The belt speed is going at the planes normal take off speed, for instance 60mph.
The plane is going at it’s normal take off speed, again 60 mph. But this is a plane not a car. It’s not driven by it’s wheels. The propellor is pulling it throught the air at 60 mph. (Do you see it yet?)
I’m not being condescending. You are where I was at first. Don’t be defensive but take a step back and look at this again.
The airplane exerts tremendous force on the air to push it forward. The conveyour belt exerts negligable force in the other direction.
The airplane takes off as normal and the pilot doesn’t do anything different from a normal take off.
Here’s an example I gave a few posts up: Imagine a hover craft on the conveyour belt. The fan of the hover craft pushes it forward and it is unaffected by the moving belt because it is on a cushion of air, right?
Would a plane act like a car (stand still on conveyor) or like a Hovercraft (move forward as normal)?
Don wrote:
“… If the plane is moving at take-off speed, and the belt is moving at the same speed in the opposite direction, then there is no air movement to create lift.”
Presto replied:
1)If the (plane’s wheels are spinning) at take-off speed (along the belt), and the belt is moving at the same speed in the opposite direction, then there IS NO air movement to create lift.
2) If the (plane’s body driven by it’s propellor) is (being pulled) at take-off speed (through the air), and the belt is moving at the same speed in the opposite direction (touching the free spinning wheels), then there IS air movement to create lift.
Both of the numbered statements above are true. But statement #1 doesn’t happen in this myth. The wheels will not spin at the same speed as take off because this would require the pilot to consciously apply very little forward thrust in an effort to keep the plane still. He isn’t interested in keeping the plane still, he’s interested in taking off.
Statement #2 is what happens in this myth. The pilot applies the amount of thrust that he would in a normal take-off and his plane moves forward thru the air at 60mph. The wheels contacting the conveyor belt spin at 120 mph.
(sorry about two replies to one post but I thought that by looking at your statement specificaly I might be able to make this easier to see)
Happy Thanksgiving All!
Don,
I hope you will continue the discussion. People seem to stop posting too soon for my taste. I want to know if my arguments are convincing or confusing.
The problem (as often) is in the way the conundrum was originally postulated.
One version says that the conveyor moves backwards at a fixed speed (eg the standard take-off speed of the plane.) In this instance, the belt would exert minimal drag via the wheels, and the plane would require only fractionally more throttle to achieve the required speed through the air. (NB it’s speed RELATIVE to the conveyor would be double normal, but that is irrelevant.)
HOWEVER, one version of the conundrum proposes that the convey is speed controlled, such that the plane remains stationary. That is a near impossibility, but if there was some drag from the wheels (ie friction in the bearings) and the belt travelled exceptionally fast, then theoretically the plane could be held stationary and would not lift. The only practical way to test that myth would be to ride the brakes, such that the force exerted by the belt matched the thrust.
HOWEVER, one final piece that hasn’t been considered is WIG effect. If the wings were low, and the conveyor sufficiently rough, it would pull air beneath the wings, allowing even a stationary place to lift.
On a final note, if anyone asks whether a stationery plane can fly, the answer is yes.
Dazz you are correct in regards to this myth and in explaining that there are other versions.
This version (stated at the top of this thread)is really a very clever brainteaser that tricks people into winning the wrong argument.
Let’s try a different layout.
A real 5000 ft long conveyor.
I’m sitting in the middle ( 2500 ft from either end ) and the conveyor starts moving.
The aircraft starts moving backward.
I start the engine and SLOWLY advance the throttle.
As the thrust increases, it gradually overcomes the backward movement caused by the conveyor until the aircraft matches the rearward speed and becomes stationary in relation to everything else except the conveyor.
We’re not flying. I’m actually taxiing at say 20 mph on the conveyor as the conveyor moves at 20 mph ‘backward’.
Power to the wheels is irrelevant. As a matter of fact, the wheels are
irrelevant !! They could be skis and all of this would still be true.
The thrust of the prop is merely overcoming the rearward travel caused by the conveyor and the wheels ALLOW the relative movement between the conveyor and aircraft.
If the speed of BOTH the aircraft AND the conveyor are increased at the same rate, and both speeds are always the same the aircraft will remain stationary REGARDLESS of the speeds.
The aircraft’s indicated airspeed from the pitot tube will be zero because there will be no forward AIRSPEED ( which is what generates lift and results in flight).
If all of this were not true, you could anchor the tail of an aircraft, apply full throttle and the aircraft would rise VERTICALLY (as far as the tether would allow )just as though it were taking off without moving forward through air and it would ‘fly’ in a a hover.
Airplanes will not do this. Tethered they will ‘jump up and down’ violently to dissipate the energy being produced but they will NOT just rise up vertically.
A stationary plane CANNOT fly without sufficient airflow over the wings
( especially the outboard half of the
wings ) and the prop does NOT produce sufficient airflow to do this !
Don’t believe it, try flying an aircraft with the outer half of each wing sawed off.
Sorry folks, the myth is true.
Watch the Mythbusters clip.
The aircraft at takeoff is moving faster than the ‘conveyor’s’ opposing movement proven by the fact that the aircraft is PASSING the traffic cones along the edge of the tarp !!
The speeds of the ‘tarp’ and the aircraft are NOT the same.
This is not the first time I’ve seen them prove or disprove a myth with a set up that wasn’t EXACTLY right.
Robby,
Your grasp of the facts and explanation of how things work is absolutely correct.
The problem is you have not careful read the myth being tested (It’s right at the top of this thread.
The pilot is trying to take off. He is not carefully adjusting the throttle to keep the plane in place.
The surprising fact is he only has to handle the plane in a normal way and it will take off as usual. The effect of the belt is negligable.
If you do not agree please show me in the wording of the myth where it says the pilot must attempt to match the speed of the treadmill.
You won’t find it. It isn’t there. It simply asks if he can take off and the answer is he can as shown by the mythbusters.
Robby wrote:
“Watch the Mythbusters clip.
The aircraft at takeoff is moving faster than the ‘conveyor’s’ opposing movement proven by the fact that the aircraft is PASSING the traffic cones along the edge of the tarp !!”
Presto replied – Correct.
Robby also wrote”
“The speeds of the ‘tarp’ and the aircraft are NOT the same.”
Presto replied –
1)The myth says nothing about the speed of the belt and the aircraft being the same. It simply asks if an airplane can take off from a belt that is moving at it’s normal take off speed.
Ok Robby. I’m ready for a good debate. Please reply to my points as I have replied to yours.
Let’s try a modification of my first scenario.
We are sitting in a aircraft with a takeoff speed of 60 MPH. The aircraft is sitting on a R E A L L Y long ‘treadmill’.
We just sit there and the treadmill starts moving until it reaches 60 MPH,
backwards, relative to a stationary object.
We are now sitting in a plane moving BACKWARDS at 60 MPH relative to a stationary object. If it had an airspeed indicator with a ‘rearward’ pointing pitot tube, it would show an AIRSPEED
of 60 MPH – backwards !
( I don’t care how much money someone may have – DON’T try this = tailwinds, of almost any appreciable speed, on the ground are NOT good things !! )
Now.
Start the engine and advance the throttle as quickly as you want.
The aircraft will start to reduce its backward speed relative to a stationary object as well as reducing the backward airflow over its wings.
Eventually, the thrust from the prop can and will reduce the rearward airspeed to zero and the airflow over the wings to zero.
The flaw in the thinking here is that ALL of the air around the aircraft is moving at the same speed as the air being moved by the prop. It isn’t. ONLY the air being moved by the prop and pushed rearward toward the read of the aircraft is moving at that speed ( and that is NOT sufficient for flight ).
At this point, the aircraft is stationary relative to the heretofore mentioned stationary object.
The thrust of the prop has neutralized the ‘rearward motion’ caused by the treadmill’s movement.
The aircraft’s REAL airspeed indicator reads 0 ( from the pitot tube halfway out on the left wing ) If you had a pitot tube on the side of the fuselage ( some aircraft do ) it would be reading 60 MPH !!!!!
Will the plane fly ?
Nope. You have no airflow over the wings.
The treadmill is moving at the takeoff speed of the aircraft. The aircraft is taxiing at its takeoff speed but has no airflow over the wings in order to fly.
‘An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff.’
That is EXACTLY what you have here and the aircraft will NOT fly.
Can I MAKE the aircraft fly at this point.
Yes, if I can get 60 MPH of airflow over the wings !!
Right now though, I simply holding my own position because the thrust of the prop is equal to overcoming the force of the treadmill trying to move the plane backward.
The wheels ( or ski’s or roller skates for that matter ) ‘think’ we are taxiing at 60 MPH but the wings ‘think’ nope, we’re standing still because the aircraft is actually not moving at all, in either direction ( forward or backward ) in relation to the airflow over the wings.
By the way, for what it’s worth. It is REALLY nice to have an online debate without name-calling or profanity etc.
Thanks
(
Ok Robby you’re a tough one.
Robby,
You understand everything correctly except for one very important thing so please address this point only:
In any of your scenarios why would the pilot stop speeding up when his airspeed tube says zero?
Wow, presto, I thank you a lot; this has been a fun discussion to read for a simple student such as myself.
I was wondering if some conclusions I made stand ground and are correct:
Robby said (in response to you):
“The wheels ( or ski’s or roller skates for that matter ) ‘think’ we are taxiing at 60 MPH but the wings ‘think’ nope, we’re standing still because the aircraft is actually not moving at all, in either direction ( forward or backward ) in relation to the airflow over the wings.”)
(he sketched a scenario on forehand): Does he not mean 120 MPH ? If indeed, like he said, you take a plane + belt with speeds of 60mph and -60mph.. (I understand this isn’t relevant to the myth, though)
Second, am I correct when I say that when on a treadmill/belt/whatever, the aeroplane will need SLIGHTLY more engine power due to the (as good as negligible) fraction the wheels create ? I understand why a plane would not stand still, I’m just wondering if the friction affects a fraction (ha) of the engine power needed to take off ?
Lastly, I read:
“This was also after we had a similar discussion about hovercrafts where I had concluded in answer to his question that a hovercraft would go upstream and downstream at the same speed on a flowing river. He wasn’t so sure.”
I’m not so sure either.. In the scenario formulated, won’t the velocity of a hovercraft going downstream be positively influenced by gravity ? I understand this is all irrelevant to the myth, but I’d just like to form a complete picture in my mind.
Thanks, Roni.
Thanks Roni,
1) In Robby’s scenario the plane is standing still on the treadmill because the pilot speeds up until the point that air flow over the wings is zero. (so at that point the wheels “think” they are going 60mph). Robby is under the mistaken immpression that this is the scenario the myth is about. But it’s not, the myth asks if the pilot can take off so he doesn’t stop speeding up at that point. He simply continues to speed up as normal and takes off as normal. (at that point the wheels “think” they are going 120mph just before the plane takes off)
2) Yes the plane will take sleightly more engine power due to the wheels rolling friction. People get confused because they read other versions of this myth that claim the wheels would turn at crazy speeds but as discussed above they are only going twice normal speed. The increase in friction is tiny. Well within normal take off variations. The pilot would not feel anything.
3) Upstream and downstream refer to the direction of the waters flow. Rivers have a general slope I supposed but for all practical purposes a given stretch of river is “flat”. So I don’t think gravity enters into it. You have to make reasonable assumptions. Hovercraft aren’t out there navigating rapids.
Good questions. Thnaks
Thanks for the quick reply, now I’m a bit embarrassed by my last question hah. You have a very profound and direct way of explaining (this stuff), and I love to see how you politely try to mask your frustration after the millionth question/statement in the same order.
Cheers !
You’re very kind Roni. Thanks. One of the things I do in my job is to challenge people with brainteasers (I entertain in restaurants doing card tricks and such).
My goal is to help people have fun with this sort of thing rather than boost my own ego by making others look stupid.
Magician’s call this “taking the sting out” of a magic trick. This is a puzzle not a magic trick but the idea is the same.
I always let people know that I make many of the same mistakes they make it’s just that I encountered the puzzle before they did.
Thanks again for your very nice comments. You made my day.
I’m glad you appreciate my comment; I really respect the way you treat these ‘riddles’ !
Your job sounds great yet I’m surprised you aren’t a teacher or something, haha (no offence !)
This is a great riddle nonetheless; today I questioned a friend of mine, to know his thoughts about what would happen. He also figured a plane appearing motionless on the conveyor belt, which like you said is the main part of the puzzle which traps people.
Keep up the good stuff !
‘K – I’ll concede.
There is no reason to not add additional power once ‘equilibrium’ is reached.
In reference to a much earlier post ( it may have been yours Presto ), indeed, the question, or myth, is cleverly worded.
It leads one down a thought path that is incorrect.
Of course the aircraft can take off.
It’s just the wheels ( or skis !! ) that ‘think’ the aircraft is at takeoff speed.
Nice discussion.
I enjoyed it and REALLY wish others on other boards could be as civil even if they prove me wrong !!
Robby,
You obviously had a good grasp of the physics involved. (except for that one small detail).
I’m glad you posted again. Many seem to just vanish when the discussion doesn’t go their way.
I wish there was more civility on discussion boards too.
This mith was raplicated badly and misunderstoon. The point was to keep plane still as a resultant of his propulsion versus moving of the traction surface, as seen on episode with shooting ball from moving truck in oposite direction! Therefore plane can not lift because the lift is result of pressure under the wing of plane which is coused by his moving through fluid. The plane in this case stands in one position!
J said:
“The point was to keep plane still as a resultant of his propulsion versus moving of the traction surface”
No J. If you read the myth (posted at the top of this thread) you will see that you are wrong. You have to read very carefully and not make any unwarranted assumptions.
The idea that the plane is supposed to stand still is an incorrect assumption that most people make when they first encounter this myth.
If you don’t understand or if you just plain disagree, please quote the part of the myth that you believe says the plane is supposed to stand still.
J?
So if a plane that’s motionless relative to the ground can somehow generate the lift necessary for flight, then why hasn’t the military or commercial airports built special conveyer-belt style “runways” to launch airplanes? Think about all the real estate tht could be saved.
And what about doing this backwards? If a plane can pop into the air from being motionless, then why can’t airplanes stop in the air and then float down onto a landing pad?
Steve,
You’re way behind. This myth is not about a plane that is motionless in any way.
I hate to sound all smarty-pants, but I didn’t even realize there was such a myth, and I don’t understand why the argument in here. Clearly, the only effect the backward conveyor would have on the plane would be to make its wheels spin faster.
SteveM.
You can’t really judge the situation because you encountered the discussion before you encountered the myth itself.
The way people (even very smart people) get fooled by this is that it seems like the airplane standing still is a given when in fact it is not.
It seems that way on a casual reading because that is what conveyor belts do when we run on them. They hold us still.
Once someone makes this mistake they tend to stick to it and ignore any attempt to straighten them out.
Sometimes I feel people claim to believe this myth or argue about the method used to demonstrate that is’s busted just so others will read their ramblings.
There are only two things needed to understand, which demonstrates this myth is busted.
1) Planes fly due to lift being generated from the relative motion of airflow over the wings. This relative motion is due to the wing moving through the air and has nothing to do with the ground or ground speed. However, we can simplify by removing windspeed and direction from the list of variables and state 0 MPH. In this situation the plane’s airspeed and ground speed will be the same at liftoff and during flight mode. This does not imply that air and ground speeds are always the same, and in fact, are different more often than not due to wind speed and direction.
So, if a plane can move its wing through the air at a speed necessary for lift, it will fly! This is demonstrated millions (pick a number which makes you happy) of times a day.
The myth is not to prove the understanding of basic aerodynamics. If this is your hangup, then come back once you have this understanding. Once you understand and accept that the wing moving through the air at sufficient speed will generate lift, then you can move on to the second understanding needed to bust the myth.
2) How do planes generate air speed? Planes do not generate or rely on tractive forces, such as cars or trains. The motive force is from thrust. Thrust has nothing to do with the wheels, in contact with the ground or otherwise.
So if a plane can generate enough thrust to overcome inertia and resistances (rolling resistance, drag, etc.) it will increase speed. With enough power, it will speed up to the airspeed (even though still in contact with the ground) necessary to generate lift, and will take off.
The myth is not to prove whether the plane can generate enough thrust to attain airspeed. It’s accepted that a plane is capable of doing this.
Busting is now possible. We only need to calculate or demonstrate the thrust can overcome the minimal extra rolling resistance from the conveyor belt spinning the wheels. If there’s an excess of power, which there typically is, the thrust will cause the plane to accelerate to an airspeed which results in lift (aka flight mode).
I read a lot of lame posts which cite the experiment wasn’t done right due to xyz. Again, the procedure busted the myth regardless of the what-if sayers. The plane generated thrust (from the propeller, not the wheels) which overcame resistances and was able to bring the plane to airspeed. The fact that there was a conveyor or that it was moving the plane backward at any speed only added a minimal obstacle for the thrust to overcome.
Myth busted — again!
Before any one else posts that “They did the experiment wrong…”. PLEASE read and understand the above post by Carl.
I hope this issue has not already been answered but much has been added since I was here last.
An airplane with a tailwind equal to the speed of the airplane’s regular take-off speed would not take off at its regular ground speed. It would have to go faster, in this case probably around twice as fast (in relation to the ground – not the air).
If the runway is long enough it would take off though, and since the myth does not specify at what speed the airplane would take off (only at what speed the runway moves backwards)or says anything about which direction the air moves, the myth is busted.
It did not even need an experiment in my opinion.
As a matter of curiosity though, I am wondering how ground speed is measured and what the ground would be in this case. Are we talking about a backwards moving runway the size of the world? Is the runway the ground, or the world the ground, or is ground speed measured by air movement over the plane?
Karsten,
Where does tailwind enter into this?
Groundspeed is groundspeed. There is no special definition needed to discuss this myth. It is the speed relative to the ground (runway).
The myth mentions “Normal Groundspeed at Takeoff”. This is measured before the treadmill is brought into play.
By the way, in the myth being discussed here the runway needed to take off is exactly the same length as the runway length for a normal take off.
This is because IT IS a normal take off. Same engine power, same runway length. The pilot would do nothing different. He would just take off as normal. The moving runway doesn’t change anything. (The slight increase in rolling friction is insignifigant).
If the airplane takes off as normal (and I accept it does) then it will be touching the moving “runway” (conveyor belt) about the same amount of time than an non-moving runway. Since the runway is moving and the plane’s wheels are spinning faster when take-off finally occurs, there will be more meters of runway in contact with the plane for the same time. Hence – longer runway. Unless you just measure the distance from standstill to airborne on the ground “next” to the moving runway. So, what is the ground? And what is the “runway”?
Same with the ground speed definition. If the ground is measured relative to the moving runway the plane is still on the ground at its regular ground speed. And it will be in the air at twice the ground speed it usually needs. Not that this matters, since it will still take off.
Tailwind comes into play if you reverse the conditions but keep the physics the same. In one scenario (mythbusters) the ground is moving backwards while the air stands still. In the other scenario (mine) the ground stands still while the air is moving forward (=tailwind). The airplane still takes off, but it needs a longer runway. And higher ground speed.
Or not?
Karsten,
Lets try to make this simpler not more complicated.
There is no tailwind.
If you want to define LR1 as the lenghth of runway along the solid ground and LR2 as the length of runway along the moving belt you can. But why?
There is no physics puzzle here. The science of a plane taking off is not in question.
It is simply that most people make the mistake of thinking “plane on treadmill = plane standing still”.
Unless I don’t understand the principles of lift at all, ground speed really does not matter. What matters is air speed and direction of flow over the wing and ground speed needs to be adjusted based to get the right conditions.
Ok let me as you this question:
Can the spaceshuttle take off if McDonald Cheeseburgers are on sale two for one?
Now, to answer this question do we have to know how the spaceshuttle works? Do we want to enter into long discussions about how various forces are defined?
No. We just point out that McDonald’s cheeseburger prices have no effect on the spaceshuttle. That’s it. Case closed.
Once you understand that a treadmill has no signifigant effect on the movement of an airplane you will understand that the plane will take off without having to understand how planes work.
Let’s keep this simple. Please
The case is just not as simple. The initial sentence that describes the myth is not phrased well:
“An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff.”
This could be understood in two ways (but should really not be):
1) An airplane cannot take off at its(!) normal ground speed during takeoff on a runway that is moving backwards.
or
2) An airplane cannot take off on a runway that is moving backwards at the speed of the airplane’s (not “its”!) normal ground speed during takeoff.
Since the word “its” is in the original myth, I will conclude that #1 is the sentence one should go with. A runway just does not have a ground speed and therefor the “its” (and the part of the sentence it is in) goes with the airplane. The runway has to move backwards but, as written, does not have to move at a certain speed.
If (and this has not been answered yet) ground speed is relative to the ground the plane stands on, the plane will not take of at the plane’s normal ground speed. The runway will move backwards, the plane will move forward, it will take off, but it will need to go faster in relation to the runway than usually. Just as if it had a tailwind.
And if you were standing on that tarp they pulled backwards you surely would experience that tailwind.
Let’s keep this precise. Please
Consider this. Prior to some moment in time the myth did not exist. It was devised, somewhere, sometime, by someone.
1) Where did the myth come from and why?
2) When you were introduced to the myth, why did you choose to believe?
Answers:
1) This myth probably started as a physics class exercise, for extra credit or to understand Newton’s laws (or just a sadistic instructor), and not as a myth. It’s plausible, but the origin has been lost in antiquity and world may never know.
2) Cognitive dissonance. This is the conflicting feeling you get when two ideas or concepts don’t jibe. Everyone feels this at some point. It’s how you handle the feeling that defines who you are. To overcome this feeling, people must adjust their belief in one of those ideas. Typically, the idea held onto is the first and the new idea is dismissed. If the new idea is in accord with existing ideas, concepts, beliefs, logic, then it doesn’t cause dissonance and is latched onto which reinforces and supports the original idea, concept, or belief.
At first glance, the concept of a conveyor belt is understood and accepted by most people, perhaps reinforced based on some experience. Now the myth comes along… It makes sense to some while contrary thoughts lead to cognitive dissonance. The result is acceptance of the myth to alleviate the conflicting feeling. So they debate their belief, adamantly at times.
To end the debate, arguing and banter, it will help those who believe the myth to accept that debunking the myth does not conflict with or invalidate their idea of how a conveyor belt works. Instead, expand upon this understanding (and adding a new). Conveyors impart a ‘tractive’ force to move an object, such as a plane (when the brakes are applied or when the propeller is not developing thrust). Planes do not use, rely, or otherwise depend on tractive forces (through the wheels) to move about the ground, except for braking. Thrust is the motive force which moves a plane, caused from accelerating a mass of air, by the propeller (F=MA). The effect of the conveyor on the rolling resistance of the wheels and ambient air is not appreciable and is easily countered by the excess of thrust and can be ignored. Thus, a plane will accelerate to takeoff speed and attain flight, regardless of wheels being in contact with the ground or conveyor belt, moving or otherwise. The only consequence is that the wheels will be spinning (free-wheeling) faster takeoff on the moving conveyor.
The myth is busted. Planes can takeoff on a runway moving backward… If you feel otherwise, it’s probably due to a conflicting belief you’ve adopted. You can eliminate the conflict and accept both by examining your initial belief for flaws in logic, assumptions, application or other preconceptions, and align it with the overwhelming facts, logic, reason and proofs. It’s how you handle conflicts that define who you are.
Carl, that is a long post.
A simple muse to reiterate that the myth is busted The plane can take off…
However, I also explain why people believe in myths and offer a way to “take the sting out” and save face.
Take a step back. Examine the logic and assumptions and don’t let ego get in the way. They find, “I’ve been arguing about my misunderstanding all this time. The plane is not limited to the speed of the conveyor (treadmill). In fact, it can go faster and attain flight because the thrust is pushing against air, not the conveyor. The same air as if the conveyor was moving or not. I can finally agree, myth busted.”
It’s really as simple as that.
You are completely correct. It’s probably good to have various ways of explaining it. Your explanation will probably convice some that I couldn’t get through to.
In fact. At this point that is my interest in this thread. Is it possible to have a rational discussion on the internet? Do people really read opposing points of view and react to them? Or do they just stubbornly defend their first thought and resort to personal attacks when they feel cornered?
I think most of us agree that the airplane will take off with its little wheels spinning rather quickly. We seem to even agree that ground speed is movement of the airplane in relation to the runway.
If(!) wheel speed is how one could measure ground speed, the ground speed would show to be rather high when the plane is ready for take-off. Certainly higher than its normal ground speed during takeoff.
And since it says in the (poorly written) original statement that the plane would not take off at its regular ground speed during takeoff, I will say – not busted. Possibly even confirmed.
At this point this is not a physics question any longer. It is a English grammar issue.
Karsten, The myth is clearly written and unambiguous. It says nothing about what speed the plane will take off at from the treadmill.
Here is the myth being discussed here. It is printed right at the top of this thread.
An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff.
Let the “normal ground speed durring take off” = X mph.
The myth now reads:
An airplane cannot take off from a runway which is moving backwards (like a treadmill) at X mph.
It is the treadmill which is constrained to move at X. Not the airplane.
The ambiguity that you see over the definition of groundspeed does not exist in this puzzle.
This was a very stimulating thread. This was brought up at our new years eve party, and I thought I would add my input. I am one of the poor #@$!@# that don’t think this myth was done correctly. I appreciate Presto’s input and agree with all that he says, but I believe he is the one making too many assumptions. If you go to the Wikipedia definition of “Treadmill” it talks about a way to run or walk and stay in one place. Now, when I read this definition, I don’t completely understand why one would think they need a runway length treadmill. The reason you use a treadmill is to keep something from moving and stay in a small space. I am in an industry that uses rollers instead of tread mills, but the way we use them is for the same principle. It is perfectly acceptable to “tether” vehicles to the treadmill so the “stupid” machine won’t fall off of the treadmill during start up and some cases shutdown. I truly believe this is why people get “tricked” as Presto puts it. Everyone knows that if the “lift off” air speed isn’t reached a plane can’t take off. Since treadmills are “ground speed” by definition, the plane can’t take off from one no matter how fast the treadmill goes! Since the treadmill on the show was obviously “self powered”(like Wikipedia says most expensive one’s are), how can you think that a treadmill spinning at any speed will let the plane take off? Presto, you are the one being tricked. The plane’s power source is irrelevant to the myth, and is where you are being tricked. I have seen many tread mills and use them at work. I have never seen one that I couldn’t run off of, and I am ground driven. So here is a question from us poor misunderstood concrete thinkers…Can a person run/step off the front of a real/can buy at the store treadmill when the belt is turning backward at awalking speed? Once off the treadmill, the debate is over. By you all making a super long one so the plane has enough time to get the airspeed up to “liftoff” speed and not staying in place is when the problems start. Thanks.
db,
1) Please let me know what assumptions you think I am making that are incorrect.
2) The myth describes a runway that is moving “like a treadmill”. I used shorthand to call it a treadmill. It doesn’t matter what you call it, an airplane won’t be held in place like a car or a person.
3) The myth says the entire runway is moving like a treadmill. The airplane does not move off the treadmill once it starts. It just moves forward as normal and takes off as normal
Since you named me in your post and I addressed your points I hope you will address mine directly too.
Presto,
Thanks for your answer, please don’t take offense to being named personally, since you are the only consistent person on here I new you were the only one who might reply.
My point which I guess was poorly made is that the myth spacifically calls out a name “tread mill”. the point of a self powered mill or a non powered mill only changes symantics. by definition, a “tread mill” is a special type of conveyor. I believe your and everyone else who believe they answer the myth correctly thinks that a tread mill is the same thing as a converyor. By calling out the name tread mill, it has to mean the object on it would have no forward (or backward)velocity.
I’ll try to word differently than last time… If you don’t teather to a treadmill, it is no longer a treadmill and the reason for calling by name a “treadmill” is no longer correct. The only reason I can even feel ok debating this issue is that they are the ones that specified in the myth it is “like a tread mill”. by not teathering, it is a conveyor, not a treadmill. Any ground drive vehicle/person can escape a conveyor. The restraints and good balance on a “treadmill” are what enables things to stay on it in one place, not what drives them.
Treading water…Tread mill…I’ll go further and say that Treadmills, dyno’s, and yes even wind tunnels used for testing airplanes hold/teather the vehicle in place when they are run. Wind tunnells don’t rely on the motor of the plane, or the cars wheels when they are operating, they use somthing to make wind and watch the effects on the vehicle while is not in motion.
When myth busters first came out, I loved watching it, as time went on, and they did this type of missrepresentation of myths….I don’t watch near as often as I used to….
I hope I was able to be more clear this time. am looking forward to see if I got my point accross this time.
Presto,
And to further the symantic argument. Technically a plane can lift off without moving forward. even if it is teathered to a non-moving treadmill (as every one should agree the wheels don’t matter) and has its motor off. Every one knows that all a plane needs to lift off is a good strong wind that exceeds it lift ratio.
Myth is true, a tread mill can’t cause a plane to take off. A conveyor can! Being as they specified “tread mill” it infers by definition “no physical movemonet of the object”
Interesting that the title says conveyor and the myth says like a treadmill…somting to confuse us as it definatly has.
I was never offended by being mentioned. Quite the opposite.
I understand your point about the word conveyor. Here are two reasons you are wrong in applying it to this myth:
1) The myth says a runway moving backward LIKE a treadmill. It does not call it a treadmill. I used treadmill as shorthand which is in fact perfectly acceptable because:
2) Objects are not always used for their intended or usual purpose. If a treadmill is used as a coat rack it is still a treadmill.
Ask yourself this: What will happen if you put a fast moving model car on a slow moving treadmill? Would you claim it would stand still because that is what treadmills do? Or would you say it only a treadmill when it is used as intended so the question is invalid?
ok….
a tread mill is a type of conveyor. If you chose to not tie the car down, it comes off, which completely ruins the reason for using the treadmill. (see wikipedia definition of treadmill and how to use one). If you are using a treadmill for a coat rack, I agree it is still a treadmill, but it doesn’t change the fact that you are no longer using it like a treadmill. If you are ok with calling something one thing and then not useing the rules that make it that, and then being ok with a flawed outcome, that is ok with me.
If I went to the store and bought a treadmill and it had no way to hold on to it while I walked in place, I would be very upset, because the treadmill would no longer function for it’s intended purpose (for me to walk/run in place).
As I have watched myself question this and type in my opinion, I am actually thinking we all missed the real point of this myth.
We you/me are justifing the plane’s take off potental based on flawed thought. Actually, if a plane can take off is not based on if it is ground driven or prop driven. it is strictly based on lift at the wings. Tread mills don’t create lift on the wing (as I define a treadmill), A conveyor can by moving the plane forward,thus wind creates lift, the engine only will create lift by moving the plane forward based on airspeed, not groundspeed.
I don’t think that at this point anyone will disagree with this. It comes back to if you think its ok to call somthing by a specific name, and then use the rules of the general term. If you aren’t going to use the treadmill for the purpose of the treadmill, is it just as appropriat to say the “The myth is busted, a plane can take off of a coatrack”
I look forward to your thoughts
db wrote “We you/me are justifing the plane’s take off potental based on flawed thought. Actually, if a plane can take off is not based on if it is ground driven or prop driven. it is strictly based on lift at the wings. ”
I’ve made the point repeatedly that once you understand that the conveyor belt does not effect the movement of the airplane the rest is moot.
The airplane takes off just as a normal take off because it is a normal take-off.
The reason the moving runway can’t effect the movement of the airplane is because the airplane is pulling itself through the air and not using wheels like a car.
No flawed thought here.
A further simplification:
An airplane cannot take off from a treadmill if that treadmill is moving backwards at X mph.
X= the planes normal groundspeed at takeoff
Presto,
Go up a bit further. I copy/pasted the original myth (from the top of the this page) and address the language issue in more detail.
Here is the post that I am talking about:
————————
The case is just not as simple. The initial sentence that describes the myth is not phrased well:
“An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff.”
This could be understood in two ways (but should really not be):
1) An airplane cannot take off at its(!) normal ground speed during takeoff on a runway that is moving backwards.
or
2) An airplane cannot take off on a runway that is moving backwards at the speed of the airplane’s (not “its”!) normal ground speed during takeoff.
Since the word “its” is in the original myth, I will conclude that #1 is the sentence one should go with. A runway just does not have a ground speed and therefor the “its” (and the part of the sentence it is in) goes with the airplane. The runway has to move backwards but, as written, does not have to move at a certain speed.
If (and this has not been answered yet) ground speed is relative to the ground the plane stands on, the plane will not take of at the plane’s normal ground speed. The runway will move backwards, the plane will move forward, it will take off, but it will need to go faster in relation to the runway than usually. Just as if it had a tailwind.
And if you were standing on that tarp they pulled backwards you surely would experience that tailwind.
Let’s keep this precise. Please
———————–
Since English is not my native language I discussed this with other people. This is not a physics issue any longer. The physics are clear. The airplane will take off. It just won’t take of at the speed that is clearly required by the original post.
It is just language at this point. You cannot just rewrite the original myth so it fits what you like it to be. Or leave out some words.
Before you go any further, please explain to me how the myth should be read in your opinion and how the word “its” fits into that version. Let’s not simplify. Let’s be precise about everything that is involved here, including (or even beginning with) language.
I can imagine what they wanted to say (and it is what you imagine as well), it is just not written there.
So, confirmed.
If I understand your interpretation of the sentence you read it as:
A plane cannot take off at it’s normal groundspeed if it is on a runway which is moving backward.
It looks to me like that is a valid reading of the sentense but it is absolutely not the one mythbusters tested. Just watch the show.
If this was the correct reading it would be ambiguous as you have stated.
You did open my eyes to a possible reading of the myth I hadn’t noticed.
Original Wording From Top Of This Thread:
“An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff.”
As you said yourself IT refers to the airplane. So the we have:
“An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to THE AIRPLANE’S normal ground speed during takeoff.”
We agree so far.
Here is where we differ: There are two objects in this sentance
1) An Airplane
2) runway which is moving backwards (like a treadmill) at a speed equal to THE PLANE’S normal ground speed during takeoff.”
The myth specifies the speed of the runway. It is the same as the planes normal groundspeed at take off.
That is my interpretation of the wording. I’ll address your interpretation in a separate post to keep things clear.
Oh, I have seen the show. No doubt they tested the runway going backwards at the same speed than the airplane’s normal takeoff speed. Also no doubt that the airplane took off. I have no problem with the physics.
It is just not what they wrote in the myth and the myth (as written) is confirmed, not busted.
The myth the way you (and the Mythbusters) see it is busted.
I meanwhile went out of my way to talk to an English teacher. She agreed (without prompting) that the way I read it is the only correct way if one only looks at the language used.
I am at peace.
Thanks for the discussion.
Please look carefully at my interpretation as explained above. It is a perfectly valid reading of the sentence. (And I’ll take on your English teacher)
As I see it there are at least two readings that re linuisticaly valid. One is the intended meaning.
By the way, you cannot just change the sentence as you have done it.
“Its” makes the end of the sentence belong to the beginning of the sentence (the airplane). If you replace it with “the airplane’s” you make it belong to the runway. That is changing the sentence and its meaning.
That was part of my explanation of how I read the sentence. It is one correct interpretation of the sentence. (The intended one)
I will replace one word in the myth with one I like better as well.
“An airplane cannot take off from a runway which is moving forward (like a treadmill) at a speed equal to its normal ground speed during takeoff.”
I hope this is permissible as an interpretation of the original myth.
Hey, listen. I am done. There is a reason we have courts of law making decisions about this sort of stuff. We will need to agree to disagree. As I said, I am at peace.
In explaing my interpretation (and the mythbusters interpretation) of the myth I changed a word for clarity. I did this openly while explaining it. You don’t have to eccept it. It is just my attempt to clarify.
I’m not sure why this bothers you.
Your change of “forward” for “backward” deliberately changes the meaning of the sentence. What is your point?
It’s fine if your done but don’t expect to get the last word after posting something like that.
I see the point of our disagreement. Af few posts up Karsten wrore:
A runway just does not have a ground speed and therefor the “its” (and the part of the sentence it is in) goes with the airplane.
Karsten agrees with me that “It’s” refers to the airplane but he also thinks erroneously that using “It’s” somehow means that entire part of the sentence must go with the word “Aiplane”.
In English there is no difference between using the word “It’s” and using the actual word, in this case “Airplane’s”.
OK…. I’ll try for the last time to help you understand….
I will just Re-write the myths to suit me and I don’t know if it will help or not since, I guess you don’t agree with me about what a treadmill is.
Myth 1: A plane can not take off of a normal runway if the plane doesn’t have any wheels or engine….Busted because all we need is a large wind or a big fan.
Myth 2: A plane can not take off of a conveyor belt….Busted… because if the plane is being pulled by the conveyor (brakes on) up to lift off speed, it will lift off(even if it has no motor or wheels. Also, if you have the right wind or fan, or if you do the test like the “Busters” did it.
Myth 3: A Plane can not take off of a Treadmill. Busted…You just need a big fan, motor is optional as it will only apply forward force that will be restrained by the properly construct treadmill.
I guess at least I will now agree the myth is busted, I just think they “busted” it wrong
Thanks
It’s not that I don’t understand your point. I just don’t agree with it
You have a problem with the phrase “A runway which is moving backward like a treadmill”.
I have no problem with that phrase. It is perfectly clear to me.
I know exactly what you mean. I suppose this is why people in the world have such a hard time getting along. Most in truth probably agree at the basic level, but no body can communicate because most people (even in the same state, or town, or family) speak different languages. This is the same reason that I suppose at a certain time in peoples lives they seem to turn colder to the younger generation….they have already debated enough things and don’t even care to contribute, they know they are right or wrong and no longer get satisfaction from what the young poeple think is a stimulating conversation.
Thanks for the follow ups!
Nice chatting with you. I’m glad you took my comments in the spirit they were intended.
it was a fun chat,
after thinking about it today, I think the reason This myth disappointed me was that the “Busters” used to try to completely take any question or doubt out of the myth being true/false. Often now they seem to just be in a hurry to get through them, and often leave me saying….what about that part, or this part. Still a great show, just not quit as satisfying as having every aspect covered.
Thanks again
I think they could have explained this better. I hope they re-visit it but they probably don’t want to open this can of worms again.
I agree that the myth as stated is busted. It is clearly shown in both the model plane and ultralight (400lbs+pilot=light?) tests that this is so. However, why have a treadmill or conveyor at all if it doesn’t stop the forward motion of the plane? If you do that, there’s no way it can take off. The SPIRIT of the question makes it a moot point. Usually it’s the semantics that does that.
Jim wrote “However, why have a treadmill or conveyor at all if it doesn’t stop the forward motion of the plane?”
presto replied “No one is suggesting there is any practical value to putting a plane on a moving runway.
It’s a thinking exercise, a cleverly worded question that should teach people to be wary of their own assumptions.”
No one, including me. The “myth” is that the treadmill has a chance to stop the plane from moving. To get a passing grade on the exercise is to figure out that the the wheels are not the motive force and the plane will take off.
It DID make for a good show.
I just didn’t understand what you were saying after the word “However” in your previous post. It’s ok, It sounds like we both agree about the myth.
Silly Question…
From my understanding the following is true:
1. The plane has zero ground speed at takeoff. (from the treadmill)
2. The plane still generates lift from the engine and acquires enough air speed to take off.
Question:
Say we remove the treadmill and the wheels. Then securely clamp the plane to the runway. Turn on the engine and build airspeed (we are still at 0 ground speed). Once the pilot gains sufficient air speed the clamps are removed and the pilot takes off.
Since the wheels are free moving and the treadmill negates any forward physical movement, the plane is always in the same spot. The plane never moves (relative to ground) until takeoff. So clamping it to the ground would have the same effect as putting it on a treadmill.
Ryan wrote:
1. The plane has zero ground speed at takeoff. (from the treadmill)
Presto replies:
No. This is the incorrect assumption most people make when first reading this question. A treadmill does not act on an airplane in the same way it acts on a car.
Ryan also wrote:
2. The plane still generates lift from the engine and acquires enough air speed to take off.
Planes generate lift from airspeed over the wings only. The engines don’t generate any lift themselves. The plane flies because the conveyor belt does not stop it and it gets normal lift from normal airspeed. If the plane did not move forward it would not “lift” but this is irrelevent because it DOES MOVE FORWARD.
There is nothing in this myth about a plane not moving forward. That is you jumping to a conclusion.
The conveyor belt does nothing to the plane. The pilot gives the normal amount of gas, the plane moves forward as normal, viewers from the side see a normal take off because it is a normal take-off.
i joined to say one thing.
presto stfu and stop arguing with ever single post
Presto you are awesome for hanging with this for so long. I am amazed at how many people cannot understand what you have stated very clearly at least a dozen times. I have been laughing for half an hour reading the two years worth of comments.
Andrew,
Thank You. I find this thread very interesting. I’m trying to learn how to be concise and answer peoples points directly. It’s frustrating that so many people post once and then don’t follow up.
I mean if you post and someone answers, you should either address their points or let them know that they convinced you. Surely people don’t post and then never read the thread again, right?
Thanks again Andrew. It’s good to know people are reading this stuff and understanding my points.
presto
no problem. good luck trying to convince the masses how basic laws of flight work.
Correct! The treadmill has ABSOLUTELY NO EFFECT on the airplane. The only thing that might happen is the increased speed MIGHT cause a failure in the wheel bearings which would stop the plane before it could take off. The plane used in the test was so light and needed so little airspeed to take off that this was extremely unlikely.
I admit that I was totally against any possibility that the plane could take off when I first encountered this problem. Only after I realized that there was nothing to keep the plane from moving forward it all became perfectly clear.
Thank you presto for your persistance in this thread.
Thanks Jim,
I was worried that the conversation had gone astray a bit but obviously you followed it quite well.
I personally think the wheel bearings are not an issue at all. The wheels will only end up going twice normal speed.
I’ve only just seen this episode and I’m appalled that people don’t understand this – in fact I’m surprised they took this one on. Maybe some people’s brains are just wired differently, but to me it is so obvious that the plane would take off that I have concern for those that think it won’t.
It’s hard to evaluate this kind of comment without more information. Thinking the plane will fly doesn’t necessarily mean you understand what is going on.
Care to give us your analysis?
MikeWarner,
Ray in the next post is one of those people who see the myth differently than you. I don’t mind taking a break if you want to explain it to him.
I’ve read everything here. The myth is tosh. The wings need airflow for lift, if that comes from the Prop so beit, but it has nothing to do with the belt theory. To bust this myth they allowed actual forward movement on the plane. Didn’t anyone else see the plane move forward in relation to the ground, not the stupid belt? What’s the matter with you geeks? Get back to common sense, and what actually happened on the day.
Ray,
Like most people who first encounter this myth you are so sure that you understand the myth and can clearly see where the mythbusters went wrong.
In fact you made a mistake. Now, don’t get defensive, I make the same mistake at first.
Read the myth carefully at the top of this thread. Quote me the part of the myth that says the plane is supposed to stand still.
This is all about debate so give it your best shot. Be sure to address my point.
Thanks
Planes don’t care about groundspeed. They care about airspeed.
If the treadmill is moving at any speed, and the plane starts on the treadmill, then the plane has a tailwind. Planes can take off in any tailwind, provided the runway is long enough. (They accelerate to zero, then accelerate to takeoff speed. Remember, they only care about airspeed.)
But the easiest way to take off from a treadmill moving at takeoff speed is to turn around.
Decius,
Your first two sentences are correct. The third is incorrect.
If you are up for a discussion then please address this point:
I] If a plane does not have a tailwind on a regular runway why would it have a tailwind (or seem to have one) on a treadmill?
Hi
I haven’t read all the way down this enormous thread, but I get the drift of people’s confusion. It seems Presto has spent quite a while trying to explain this myth.
The Myth at the top if the thread states;
“An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff.
And an initial interpretation of this sentence could be ‘a runway which is moving backward at 60mph would counteract the aircraft which is moving forward at 60mph resulting in a speed of 0’. Of course this wouldn’t be the case with an aircraft because of the fact there is no power to the wheels and this vital part of the myth ‘normal ground speed during takeoff’ .
I haven’t seen this episode only the You Tube clip at the top of the thread and it’s quite obvious that no matter what speed the conveyor runs at the plane will always take off at normal operating speeds. Even if the conveyor is running at 10,000mph the aircraft will take-off each and every time (as long as the bearings in the wheels can take it). The outside air will still be flowing at normal speeds, the aircraft will still be moving forward at normal operating speeds, only the wheels on the aircraft will be going much faster than if the aircraft used a standard stationary runway.
Think of it like this (not exacly like a conveyor but similar), you are sat in an airplane, any airplane, jet or prop on a runway with an incline, not much of an incline but enough if you release the brakes you will roll backward. We’ll imagine that releasing the brakes is the same as starting the conveyor with the aircraft sat at idle. So we release the brakes and we start to roll backwards, (the conveyor has started) to counter this roll back we can apply some throttle and hold the plane stationary on the incline (this is what many posters have misinterpreted as the Myth, holding the plane at the same speed as the conveyor resulting in 0 speed, but this isn’t normal operating speeds), we can now push the throttle to normal operating speeds which will propel us up the incline and into the air.
Simple…….
I think your second paragraph in particular is concise and accurate and will help a lot of people understand things.
Stevo has it. Airplane wings are created to provide lift. This is done by having a flat underside and curved upside. Air MOVES past the wing in the same volume both under and over the wing, but NOT at the same speed. Air moving over a curved surface has further to go that the air moving over the flat surface. As air speed increases, pressure over the wind decreases. This causes lift. You need the air flow over the wings to obtain lift. Same with the propeller generating air flow. The propeller does not supply sufficient air flow to create lift. If it did, then all that would be necessary to have a plane lift off would be to apply the brake, push the throttles to full, and then release the brake and the plane would immediately lift into the air without having to taxi. You would not need long runways, only unrestriced space as the plane climbs.
This is very easy to prove, just google airplane wing structure and physics. You will see countless examples and diagrams.
Only way to get lift by the propeller alone is with either a helicopter (propeller are very long, and displace an enormous amount of air vs. flying weight) or with a model plane where propeller is outrageously oversized as compared to plane frame and weight, and is acutally acting more like a helicopter. This would not be a true to scale representation of real life airplanes. Face it, wings do the lifting. This is why planes drop from the sky when the wings freeze over. NO, it is not because of the weight of the ice (although that is a hinderence to movement) but becausae it changes the wing geometry, balancing out the curves on top and under the wing. Feel free to google that as well. Simple air dynamics. Easy enough to find references.
Absolutely correct. But how does this relate to the myth? It is not necessary to know anything about how a plane flies in order to understand this myth.
Oh, sorry, forgot to state that as fluids move (and air is a form of fluid) at a faster speed, pressure decreases. (Which is the way a venturi works) Air flows over the wing faster than under the wing. So pressue is decreased over the wing and the plane rises. It is not by the slope of the wing or by the rear flaps as most people believe, Although they can and do assist to some effect. The flaps can direct the plane’s forward movement in all directions, but it is that pressure diffirential that provides the lift.
Are you trying to say the plane won’t take off?
If so explain why.
(Everything you have said is correct. You have not supplied ANY reason the plane won’t take off. What element required for take off do you think is missing?
I’m glad I read this thread.
The way most people understand the myth is that if there is any forward speed in the plane’s wheels, the treadmill will be able to counteract this and keep the plane in place. As others have said, something will go wrong mechanically as the treadmill and the plane’s wheels spin to ‘ludicrous speed’. It would also take extremely fine control of the speed of the treadmill to match the speed of the plane’s wheels. Basically, the treadmill and the plane’s wheels would end up going at infinite speed, which is obviously unrealistic. Physicists don’t like things going at infinity, either. I’m not good with the math, but since we’re spinning the wheels at infinity we could have one of two equivalents: either i) it’s a frictionless interaction and we might as well suspend the plane using magnets and the plane will take off as the propellors act against the air and the ground has nothing to do with anything, or ii) regard the point of the treadmill as being to hold it in place and so it might as well be tethered, and it won’t take off.
However, in reality, the plane gets forward motion from the propellors dragging against the air, so as long as the friction of the wheels/skids/whatever-is-in-contact-with-the-ground is surmountable by the engine, the plane will take off. A glider with an engine powering the wheels for taxiing could not take off on a speed-matching treadmill because its forward motion would be provided by the wheels.
Note: the point of reference for the ground speed is important. The normal ground speed for a plane on a fixed runway is equal to the speed of the wheels. But as noted by others the speed of the wheels is irrelevant to taking off. The propellors will be biting against the air and so ground speed at take off will be the same as normal (though it might take slightly longer to accelerate to that speed).
None of what I’ve written takes the rotation of the planet into account :)
The idea that the belt “matches” the speed of the plane is not part of the myth being discussed here. (The myth is posted at the top of this thread).
There are no extreme speeds or frictions involved.
The planes normal speed at take off is measured before the test. Let’s say it is 60 mph.
The belt goes 60 mph in the opposite direction.
The pilot taxis over the belt without feeling any effect because his plane is being pulled by the propellor at 60 mph.
The wheels touch the belt so they spin at
2(60)=120 mph. 120 mph is no big deal for the wheels. Extreme speeds never come into it let alone infinite speeds.
The plane takes off as normal, virtually unaffected by the belt.
That’s my point.
If the purpose of the treadmill is to keep the plane in place, then it might as well be tethered (not that speed-matching the wheels is practical).
If the purpose of the treadmill is to match the speed of the wheels, then since the wheels don’t affect anything, the treadmill might as well not be there. In that case you could actually run the treadmill faster than take-off speed, or any speed, and the plane would still take off.
Hmm, maybe we are saying the same thing but I’m not sure.
As long as we both agree that there is NO SPEED MATCHING BELT in the myth being discussed. The idea that the belt is programmed to match the speed of the plane is an entirely different scenario.
Thanks for posting. I know I am giving you a hard time but for my tastes you are needlessly complicating things.
I’d rather stick to the scenario tested by mythbusters and try to keep people on track rather than going into detail about other scenarios.
I have taken into account everything you have all said & have made the astounding calculation that none of you have girlfriends!
I was trying to think of a snappy come-back but then I realized it’s too true (In my case, anyway) :(
nice one, but I can’t grasp the math behind the conclusion.
only because my wife wouldn’t approve, but believe me, I’ve barked up that tree :)
Just saw this episode, read most of this thread (although I got a little exhausted by all the repetition) and wanted to give an analogy that came to mind (at the risk of adding more confusion).
Seems to me a similar effect would occur if: let’s say you have a car on the top of a mountain. The slope of the mountain is almost vertical, just slightly inclined enough to allow the car to roll down without simply tumbling through the air. You release the parking brake (:-D) and the car starts to roll down the side due to gravity. Suddenly the mountain turns into a giant almost-vertical treadmill and the slope starts running upwards. The car will not stop because gravity is pulling it down and as long as the wheels can spin freely it will not matter how fast the mountain-treadmill speeds up – gravity is gravity and down it comes.
In the myth, think of the whole pulling-through-the air idea the same way as gravity.
I fear I may have complicated things, but thought I would post something that popped into my head about all this. A fun myth.
mrj,
Your analogy sounds quite correct to me. I’m not sure if it makes things any easier to understand but it may click for some people.
Just saw a clip of the episode and as a pilot I was really annoyed by it at first, enough so that I came here and read through a good bit of this thread. After reading what they were actually trying to accomplish I can back their conclusion.
I originally thought they were trying to prove that a plane can take off with no relative motion to the ground, which I’m sure by now we all agree it cannot. The only exception(s) to this would be a headwind equal to the minimum liftoff speed (rotation speed, Vr), or vertical takeoff thrust, like a helicopter, harrier, F-35, etc.
So I will throw some actual numbers in here to hopefully help explain this myth. A Cessna 172R Skyhawk will liftoff at 51 knots (indicated airspeed) at a pressure altitude of 1,000 ft, outside temp of 10 deg Celsius, 10 deg flaps, and calm wind in exactly 1,000 ft of runway at full throttle and 2,450 lbs. The plane will always lift off at 51 knots indicated (the velocity of the wind flow over the wings). The external factors such as wind, temp, altitude, are how we arrive at the distance required to reach 51 knots. On a hotter day, it takes more runway to reach 51 knots. At higher altitudes, more runway, you get the idea.
So let’s take the same Cessna on the same day and put it on a treadmill several thousand feet long. We turn the treadmill on and begin to accelerate it to 51 knots. Without power, the plane (using no brakes) will move backwards, probably with some slip, and some power would have to be applied to maintain zero motion relative to the ground. The plane is now still relative to the ground and the wheels are spinning at 51 knots, indicated airspeed is 0. We then set the throttle to max and begin to accelerate relative to the ground. Some drag will be present based on the friction between the rearward moving treadmill and the tires, which will increase the ground roll distance (1000 ft) by a marginal amount as the plane will accelerate slightly slower. After the plane has physically traveled 1,000 ft (several thousand feet on the treadmill) it should reach an indicated airspeed of 51 knots and liftoff. Just before liftoff the wheels would be spinning at 102 knots forward, the plane moving at 51 knots relative to the ground.
An aircraft ALWAYS lifts off at a given indicated airspeed (in this case 51 KIAS) regardless of altitude, temp, winds, pressure, etc. Whatever way you get there doesnt matter. 51 knot headwind, being towed to 51 knots, parked with full brakes applied on a treadmill moving 51 knots forward, propeller thrust, it doesn’t matter.
Thanks lazypilot. Great breakdown of the myth. I’m glad you were able to plug some real numbers into our hypotheticals.
Not true
If it’s being carried backward at 51 knots, while it’s rolling forward at 51 knots
It’s actual GROUND RELATIVE speed is 0 (ZERO) knots
There is NO WIND under the wings to give it LIFT…
That plane is doing exactly the same thing that peple do running in place in a gym
They are rolling forward on a treadmill going backward.
If you tie winfs to their backs will they take off ?
NO, since there would be no wind under their wings to give them lift
Kuske,
It’s not unusual at 12 to have strong opinions and think people who differ from you are idiots.
As you grow older I think you will see that things are not that simple.
I can’t reply to your post directly because I don’t understand your point. What do you mean “If you want this to work”?
It might be easiest if you just answer this question:
Under the conditions given do you think the plane will fly? Explain why or why not in as few words as possible.
preste,
I want to ask a simple question to you. Theoretically, can’t the friction force between the wheels and treadmill be equal to thrust force which is caused by the engines? I’m talking about large acceleration caused by treadmill.
Yes, I know this is irrelevant from the myth but I just wanted to learn the answer.
Thank you
In the myth being discussed here (printed at the top of this thread), there is no “large accelleration caused by the treadmill”.
The treadmill’s speed is set before the test. It is the same as the planes normal take off speed (for example 60mph). It does NOT adjust to the speed of the plane.
The whole idea that the treadmill adjusts to the speed of the plane resulting in incredible accelerations is a completely DIFFERENT scenario which in my opinion has none of the clever subtelty of the myth being tested by mythbusters. It only causes discussion because it is ill-defined.
Cunli,
Re-reading your question I see that you probably already understood what I just posted.
As for the rolling force caused at incredible accelerationsit is really imposssible to say. You have to define exactly what bearings are used on the wheels and do precise tests on their friction and their ability to withstand incredible speeds. You can’t give a theoretical answer with no friction etc because these details are what answers the question. I find these discussions to be tedious and meaningless.
Think of it this way: Supposed someone heard you enjoyed puzzles and brainteasers so they took 20 feet of rope and knotted it up and told you to unknot it. That is not a clever puzzle it is just tedious work.
just something i’m gonna say about the airplane one.
Just think of the treadmill as the actual runway. Its just the same except the plane doesn’t move along the runway, the runway runs instead (obviously backwards). The plane just doesn’t move off the treadmill and stays same spot on the runway.
It sounds like you have fallen into the trap. This myth is designed to trick you into imagining the plane standing still on the moving runway.
A CAR going 60moh on a treadmill going 60mph (the other way) will appear to stand still to observers.
This is not true of a PLANE. A PLANE in the same sitution will simply move forward and take off as normal.
Do you see why? The car is connected to the runway by it’s wheels. The runway counters the wheels movements.
A plane is connected to the air by it’s propelloer. It pulls itself through the air. The wheels just go along for the ride.
If you don’t agree please post again and explain your thoughts. I’m so tired of people posting one confident point and then never coming back. If you think you are right explain it to me. If you see my point, then let me know.
Presto, you seem like you know your stuff really well, as well as able to articulate and explain well also (see my post on this thread for some back-up to what you’ve been saying)
You also said you work with the public giving them brainteasers and thought problems to get them thinking. I was wondering if you have time and are feeling generous if you could send me some of your favorites, or a great website. I’d love some (rare nowadays) emotionally detached logic discussion
Ok I’m no expert on this by any means – but it seems to me that one they moved from a conveyor belt to paper on the ground the experiment changed dramatically. Isn’t there some force of friction now since the wheels make contact with the ground below the flimsy paper and can get a forward momentum? I saw what the car did on a treadmill – it stayed still right? Does it still stay still on the paper being pulled by the Segway? Maybe so but I’d like to see that as a control.
Think about it a little more. How would making contact with the ground below the tarp propel the plane forward?
Are you under the impression that planes move forward by turning it’s wheels?
The treadmill has no effect on the plane at all. It is pulled through the air by it’s propeller regardless of what the treadmill is doing.
If you saw the plane standing still on a treadmill it was only in a cartoon animation that was meant to illustrate what people think is going to happen. NOT what actually happens.
The mistake you make in picturing the plane not moving is the same mistake most of us make when first encountiering this scenario.
presto – and all the others who have kindly answered this thread with how this works.
I completely understand how the plane would take off (given the 60mph constraints of the treadmill), and I have read all the posts, but can you answer me one thing, which is frying my brains.
If the treadmill were able to continually accelerate so that its speed was constantly matching the speed of the plane’s wheels as it took off, would it essentially be going infinitely fast when the plane took off, as the plane would be able to pass through the air unaffected by the increase in speed of the treadmill?
That is a different question. In my opinion it has none of elegence of the myth being discussed here.
1) In real life there are no infinite speeds.
2) In discussing infinite speeds you have to make some assumptions about the wheel bearings. The results are almost entirely dependent on these assumptions.
If you assume the wheel bearings are realistic they will burn up and thw wheels will fall off.
If you assume they have no friction the wheels might not fall off but what is the poing since there IS friction?
IMHO this question is not interesting.
The original problem was
1) Put a plane on a treadmill.
2) The treadmill is designed to exactly match the forward speed of the plane, but in the opposite direction.
3 Will the plane take off?
———————-
The video and the so-called proof is bogus because the Mythbusters have NO WAY of “exactly matching the forward
speed of the plane”, as per the problem definition.
———————
History
This problem is NOT a “myth”
This problem is an actal problem that is posed in most engineering schools, as a 2-step logic problem.
Step 1 is to restrict you to thinking inside a very small box where you are not allowed to bring in outside factors. (Bbecause bringing in outside factors will usually make most problems far more complicated than they need to be)
NOTE: Because this a logic problem used for teaching purposes, if you do bring in any extraenous factors that are not part of the original problem definition, YOU FAIL.
After step 1 has been solved and analyzed, Step 2 involves solving the problem creatively.
The emphasis in step 2 is to think outside of the box, and to start teach creative/ lateral thinking for problem solving.
————————
So let’s look at the step 1 solution to this problem.
All logic problems have a nice feature.
YOU can redefine how you look at the problem as long as you respect the original definition
So let’s make sure we are clear on the original defintion
We have a magic treadmill that can match EXACTLY the rolling speed of an airplane, but in the opposite direction.
If the plane rolls forwards a x mph, that treadmill carries it backward at x mph.
We don’t care about friction or how the plane actually is propelled
All we know if that if the plane rolls forward at x mph, the treadmill carries it back at x mph. PERIOD.
So then the question is what is the speed of the plane relative to the ground ?
The answer is ZERO mph.
And that is tru NO MATTER what the “x” value is for the plane or the treadmill since
ground speed =plane speed – treadmill speed
ground speed = 0 (zero) at all times
And with a ground speed of zero, there is NO air movement under the wings to create lift
And with no lift there is no flight.
You’re guilty of violating your own step #1.
I would love to see you give an intelligent analysis of the question we are discussing rathter than claiming you’ve got the “real question”.
Read the question that is being discussed here. It is the one mythbusters tested. It is printed at the top of this thread.
Well let’s eee
If we’re going to test whether a plane can take off from a treadmill moving backwards at exactly the takeoof speed of the aircraft, one would probably need to put the plane on the treadmill as a start
Right ??
So do explain how exactly I violated my own step 1..
———————
Transfer the problem to a float-plane trying to move upstream on a river flowing downstream at the “takeoff speed of the airplane.
Rivers carries plane downstream at x knots
Plane chugs up river at x knots
Total spped of plane relative to shore (and air mass surrounding it) 0 (zero) knots
At zero knots plane is going nowhere. It therefore has zero air movement under it’s wing
It therefore has ZERO lift from the wings.
—————————-
Finally we come to all the nonsense from Mythbusters and others about a car being driven by wheels, and an plane being driven by propellers
TO a physics guy, THRUST is THRUST, whether it’s applied to pavement by a rubber wheel, or a propeller to air, It’s STILL thrust…
It’s a force applied to the airplane, that if large enough will start the airplane moving in the direction the force is applied.
THis is all related to the Newton’s 3 laws of motion that you learn in High School Physics, that apparently the Mythbuster guys forgot, if ever they even learned it.
I’ll stop there
Once you’ve answered my question, I’ll be happy to continue with the exetcise
Scanning through the various posts by Presto, I came across a real hmdinger of a booboo, that actually was also made by the mythbusters
From Mythbusters:
“This is because the thrust of the airplane engines acts on the air, not on the ground.”
From Presto:
“A CAR going 60moh on a treadmill
going 60mph (the other way) will appear to stand still to observers.
This is not true of a PLANE. A PLANE in the same sitution will simply move forward and take off as normal.
Do you see why? The car is connected to the runway by it’s wheels. The runway counters the wheels movements.
A plane is connected to the air by it’s propelloer. It pulls itself through the air. The wheels just go along for the ride.
I quote
This is not true of a PLANE. A PLANE in the same sitution will simply move forward and take off as normal.
Do you see why? The car is connected to the runway by it’s wheels. The runway counters the wheels movements.
Sorry technical glitch above
The notion that “A plane is connected to the air by it’s propelloer.” somehow means that the airplane is NOT connected to the ground by it’s wheels is TOTAL NONSENSE
Why ?
How do you think an ariplane taxis around an airport, and comes to a stop ?
It’s NOT because of the propeller..
It’s because of the WHEELS..
IN the same way the “runway counters the wheel movement” of the airplane.
As to a plane sitting on a “moving treadmil”, without even the brakes on, but the engine OFF..
That airplane will stay on the same spot of that moving treadmill because of something called INERTIA, just lke the car would.
Something for you to consider before we go into actualy solving this problem and demonstrating that the mythbusters “test” is COMPLETELY FLAWED and is in opposition of Newtons’ Laws of Motion.
Newsnik wrote:
The original problem was
1) Put a plane on a treadmill.
2) The treadmill is designed to exactly match the forward speed of the plane, but in the opposite direction.
3 Will the plane take off?
And then he wrote:
So do explain how exactly I violated my own step 1..So do explain how exactly I violated my own step 1..
Presto replied:
You violated your own step 1 by mis-stating the question being discussed here. The question printed at the top of this trhead says nothing about a treadmill “matching” any speed”. You brought that in from outside. If you disagree please quote the question from the beginning of this thread and show me anything about “matching”.
And now a question for you (this pertains to all the ‘nonsense about a car being driven by its wheels…)
Q – How will the treadmill impede the forward motion of the plane if the airplanes thrust is acting on the air and NOT the runway/treadmill?
Newsnik asked the telling question:
How do you think an ariplane taxis around an airport, and comes to a stop ?
It’s NOT because of the propeller..
It’s because of the WHEELS..
Presto gently replied:
Sorry Newsnik, airplanes taxi by their propellors (or jets). The wheels are free spinning and their only job is reduce friction.
Presto, the ariplane wheels, not the propeller. are used to steer and stop.
I realize that this problem must be addressed in a structured fashion that will preclude all the wild jumping around and presumptions that you and others have posted.
I’m going to write up a proper experiment procedure that will demonstrate the lgical and physical flaws of the flawed Mythbusters experiment.
It will also put to rest all the ther nonsense that has been posted
I’ll be back…
Newsnik,
I kind of think you’ve painted yourself into a corner. On the other hand it could just be that you’ve retired for the evening.
I hope you will either continue the discusion or let us know if you have a new perspective on this whole thing.
I would suggest that those who wish to argue my presentation do a cut and paste of eeverything that follows into your favorite ttext editor
It will be easier for you to reand and re-read.
And please do read it completely before you resppond.
Otherwise things will get bogged down
————————————-
To properly argue this supposed myth and the mythical proof provided by Mythbusters, we need ot fix down a few concepts to avoid all the nonsense that so many have posted
By also providing a structure, it will make it easier to argue the validity of individual points in a directed and simple fashion.
For this experiment, we will need 3 things
1) A table of terminology
2) A definition of the moving runway-treadmill and it’s operation.
3) A definition of the test airplane with well defined operating characteristics
—
Section 1) Terminology
1.1 Drag – The force of air against a body moving through it
– Also any resistance created by two bodies moving agains each other ( e.g axle against bearings, tires against ground)
1.2 Lift – The upward force create by wind under and over a wing.
1.3 Resistance, Rolling – the force against a body rolling along on the ground, usually created by the wheels against the ground
It can also be created by internal parts such as axle, brakes against axle, or
1.4 Resistance, Internal – any resistance between two moving parts (e.g axle against bearings)
1.5 Resistance, taxu – the combination of drag and rolling resistance
This is what the thrust needs to overcome to get to takeoof speed.
1.6 Speed – speed is how fast something moves with NO DIRECTIONAL REFERENCE
1.7 Thrust – the force created by various means to propel a body forward through space
Thrust is created by a propeller in air or water
Thrust is created by tires agaisnt the ground and the ground against the tire
1.8 Velcity = speed + direction
For our purpose – anything moving right to left will have NEGATIVE velocity
– anything moving left to right will have POSITIVE velocity
1.30 V_air – actual speed of plane through the air
1.31 V_ground – actual speed of plane relative to ground
1.32 V_runway – either ZERO or -40 knots (see section 2)
1.33 V_stall- velocity below which the plane WILL NOT fly (<=40 knots)
1.34 V_takeoff – velocity of a body needed for the wing to start creating lift (40.1 knots)
1.35 V_takeoff_air – speed of air around wing to create lift for V_takeoff (40.1 knots
1.36 V_takeoff_ground – speed relative to ground to creates V_takeoff (40.1 knots)
1.40 S_laser – the plane speed displayed by the laser gun on the right end of the runway-treadmill
Notes:
1.50 Drag is the opposite of Thrust
Drag is NEGATIVE acceleration
Thrust is POSITIVE acceleration
—
Section 2) Runway – Treadmill (from now on just called the runway)
We are going to build our runway inside a dome so that we can keep constant
- air temperature
- air pressure
- air stillness – NO WIND
We wiil build a runway with the following characteristic
2.1 It can either be at rest ( 0 knots) or moving at EXACTLY 40 knots
2.2 to make things easy for discussion the runway is configured to move from right to left and we will describe it's velocity to be either ZERO or -40 knots (for arithmetic purposes)
2.3 we will plant a pair of poles on either side of the runway.
2.4 we will put a very accurate laser speed detector at the right end of the runway aimed at the plane
2.5 we will connect a very accruate speedometer to the runway.
2.6 we will install a very narrow field of view telescope that ONLY shows
the body, but not the engine/propeller
the wheels to see if they are moving or not
a portion of the runway where you can see if the runway is moving or not
(more on this later – may not be needed)
—
Section 3) "The Plane, the plane" (RIP – Hervé Villechaize)
For the plane I propose using a slightly modified Piper Cub.
The original Piper Cub had a stall speed of 33 knots. That is, if the airspeed fell under 33 knots the Cub turned into a brick and stopped flying.
We will modfy the Cub in the following way
3.1 We will modify the plane so that it's V_Stall speed is EXACTLY 40 knots.
In other words, the wind has to move around the wing at MORE THAN 40 knots for the wing to creat lift and the plane to fly.
3.2 We will install a VERY PRECISE Pitot Tube that can measure wind speed (V_air) in 1/100 of a knot or better
3.3 We wil install a very precise speedometer that can measure ground speed (V_ground) in 1/100 of a knot or better
While the treadmill/runway is at full stop, will will use it to set "cruise control".
We will use the laser gun to:
3.4 calibrate and notch the throttle of the plane so that it can be set at EXACTLY 40 knots of ground speed.
3.41 Since the plane is moving at exactly 40 knots we can also calibrate our Pitot Tube to 40 knots as well.
3.5 We now tie the plane to a cable strung between the 2 poles to immobilize it and turn on the runway
When the plane speedometer is at 40 knots we calibrate the runawy speedmeter at 40 kntos
(At this time both the Laser gun and Pitot Tube should show ZERO knots)
3.5.1 Once done remove cables.
——
STEP 1 PRE-EXPERIMENT
At this point we have set up the experiment correctly using proper scientific technique
We also have calibrated ALL our equipment and measuring devices so that we can trust the numbers they will provide.
We have ALSO eliminated any argument about friction or lack of with regards to the plane, the runway and the surrounding air. BECAUSE we have set our throttle speed by rolling the plane down the STOPPED runway at 40 knots
In effect the THRUST of the propeller at this throttle speed is set to overcome BOTH rolling resistance and DRAG. BUt NOT enough ground speed to create lift under the wing
If this is NOT ABSOLUTELY CLEAR, DO NOT GO on to the next step..
If you feel that this in ANY WAY does NOT meet the problem definition now is the time to comment
Once we are agreed on this, then we can proceed to the actual properly executed experiment
This looks very good to me. I’m all for laying things out clearly.
So we have a plane that is giving just enough throttle to remain stationary relative realative to the two poles planted next to the moving runway.
The plane of course does not lift off. It has air moving over it’s wings and thus no lift.
I understand the set up you have described and am eager to see what you propose next.
(In fairness to you I will tip my hand and mention that your set is not the one written at the top of this thread and tested by mythbusters but it is a good starting point for discussion)
in my third paragraph it should read “It has NO air moving over it’s wings and thus no lift”
Presto,
HOLD your horses.. :-))
There is a reason I set it up this way.
And actually I disagree with you..
Notice 2 things with the my set up
1) Although we have specifically marked the throttle setting to have the plane roll along the runway (moving or not) at EXACTLY 40 knots. THer is NO exclusion for increasing the throttle past that point to move (relative to the runway) at a higher speed.
2)
I also suggest that you will be surprised at what will be the result..
Boy do I hate these browser interfaces.
Here is my COMPLETE response
Presto,
HOLD your horses.. :-))
There is a reason I set it up this way.
And actually I disagree with you..
Notice 2 things with the my set up
1) Although we have specifically marked the throttle setting to have the plane roll along the runway (moving or not) at EXACTLY 40 knots. THere is NO exclusion for increasing the throttle past that point to move (relative to the runway) at a higher speed.
2) WE have the Pitot Tube to tell us the EXACT wind speed the plane is experiencing. And by my set up (3.1), the Pitot tube has to show > 40 knots to generate lift.
I also suggest that you will be surprised at what will be the result..
Presto,
I want to you explain to me where my set up does not match the parameters in the problem definition
We need to resolve this BEFORE the next step.
Presto,
I am guessing, but I believe the phrase:
“This is because the thrust of the airplane engines acts on the air, not on the ground.”
may be one possible source of your reservation about my set up.
If I am correct, then consider when a plane is taxiing, there are 2 forces acting on it
THRUST -(created by the propeller) to move it forward.
DRAG – a combination of air resitance to any body moving through it, and rolling resistance generated by both internal parts of the wheel assembly and deformation of the tire against the ground by the weight of the plane. Note also that DRAG increases with velocity. Air Drag at very high speeds is exponentional until you get past the sound barrier. BUt at the low speeds we are using it’s so close to linear, that it’s not worth the argument. :-)
Remembr also that you first need to crank up the throttle to overcome inertia, and once the plane is rolling you can throttle back a bit to keep it going since rolling resistance is usualy less than inertia.
Finally our set up in (3.4, is such that THRUST is strong enough to overcome all DRAG and produce a 40 knot speed.
THis is EXTREMELY important and we need to be in cclear understanding and agreement on this before we proceed.
Ok, I like your set up and explanation. It is not the situation tested by mythbusters but it is a great strarting point.
Since we are all about being clear lets also be clear what we are discussing. Here is the question printed at the top of this thread. It is the one the mythbusters tested
____________________________________
“An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff.”
______________________________________
Well let’s see
1) the takeoof speed of the airplane is ANY SPEED above 40 knots
2) I have set up the runway-treadmill to operate at 40 knots.
3) I am more than wiling to ignore that smal fraction differential, because as I will demonstrate I will prove to be meaningless
So pease explain to me where my set up fails to meet those criteria ?
I have noticed on your numerous other posts that this is where you have repeatedly failed to back up your claim.
So I really need to you to be VERY CLEAR about why my set up fails to meet the test defiition
As to the mythbusters test, my little exercise will demonstrate that they FAILED to meet their own criteria.
Newsnik wrote:
1) Although we have specifically marked the throttle setting to have the plane roll along the runway (moving or not) at EXACTLY 40 knots. THere is NO exclusion for increasing the throttle past that point to move (relative to the runway) at a higher speed.
Presto replied:
With this clarification I am in complete agreement with your set up and we are ready to run the test, Proceed.
I’m off to work now. Back in a few hours. Can’t wait. Many have postec once and then slinked away so I really respect you for bing precise and cordial and engaging in a good debate.
If you prove me wrong I will admit it and congratualte you on a job well done. I expect no less from you.
Newsnik, I was hoping you would post again tonight. I’m curious where you are going to go with this.
——
STEP 2: EXPERIMENT (part 1)
—
Section 1) Terminology (REDEFINED)
Replace 1.31 with 1.37 and 1.38
for more precise terminology
1.31 V_ground – actual speed of plane relative to ground (measure by speedometer)
—-
1.37 V_plane_2_ground – actual speed of plane relative to ground (calculated)
1.38 V_plane_2_runway – actual speed of plane relative to runway (calculated)
—-
4) Pre-amble and review
In part one we have established a couple of points
4.1) We have created a plane with able to takeoff at any speed over 40 knots
4.2) This speed is most accurately measured with the Pitot Tube which measures the wind over the wings.
4.3) All the measuring tools have been calibrated to each other
4.4We have also incorporated all resistances to forward motion by air and ground into the 40 knot takeoff limit.
At this level of speed the both resistances can be considered linear.
4.5 Another interesting note is that we have establish the dragging (as opposed to air drag) resistance of the plane by tying it out and running the runway under it to benchmark the runway.
So all of thse arguments about wheel bearing friction, tire friction, etc have been rendered moot.
The plane will not roll freely and stay in the same place if the runway is moving
It will be carried away by the runway in motion
4.6 this also means that IF the runway is moving, and the plane motor is turned on, to start moving it “upstream” there will be additional rolling resistance
rolling resitance against the belt carrying it backward PLUS the rolling resitance of moving upstream
So if the belt is moving backward at 40 knots and the plane is moving forward at 40 knots, the rolling resistance is the sum of the 2, ie is 80 knots and the speedometer will be showing that speed
—
5) Experiment # 1
The plane is just sits on runway, the motor is off.
The runway is turned off .
V_air = 0 (Zero)
V_ground = 0 (Zero)
V_runway = 0 (Zero)
S_laser = 0 (Zero)
V_plane_2_ground = 0 (Zero)
V_plane_2_runway = 0 (Zero)
Plane stays aligned with the reference poles
—-
6) Experiment #2
This has already been done in 3.5
But let’s make sure we note the numbers
The plane is tied out, the motor is off.
The runway is turned on to 40 knots
V_air = 0 (Zero)
V_ground = 0 (Zero)
V_runway = -40
S_laser = 0 (Zero)
V_plane_2_ground = 0 (Zero)
V_plane_2_runway = 40
Plane moves to the left of the reference poles
—————————————–
We need to make sure that there is agreement with the above results before going further
Newnik wrote:
So if the belt is moving backward at 40 knots and the plane is moving forward at 40 knots, the rolling resistance is the sum of the 2, ie is 80 knots and the speedometer will be showing that speed
Presto inquired:
Could you flesh this out a little more for me? I know the info is probably all there above but there is a lot to scan through. What is the plane moving 40 knots realtiv to? What is the speedometer measuring in this case?
Also is rolling resistance really measured in knots?
I would first like to make the difference between speed and velocity VERY CLEAR
SPEED is an indicator of how fast you are moving, I does NOT say what your direction is
Velicity is SPeed + a direction component
Speed is effect is the absolute value of Velocity.
Think on this
YOu are on the deck of the Carrier Enterprise
The enterprise is going West at 20 knots
YOu start walking backwards on the flight deck at 2 knots
Enterprise speed is 20 knots
Enterprise VELICITS is 20 knots West
Your speed is 2 knots
Your velicity is 2 knots East.
THe question is what is you velicity going West ?
There are 2 answers
1) with respect to the Enterprise, your velicity going west is -2
The Minus sign shows that you are moving in the opposite direction from the West
2) With respect to the Ocean/planet surface, your Velocity is 18 knots West.
(20 Knots West of Enterprise – 2 knots east you are walking)
Also your speed depdends in the refence point
1) Relative to the Enterprise, your speed is 2 knots
2) relaitve to the planet, your speed is 18 knots.
Notice that in speed, I do not specify your direction.
It is VERY important to make sure that you understand and use correctly these terms
Failure to do so will most definitely screw up your conclusion.
To any one reading this so far and feeling a little intimidated just hang in a bit longer. Newsnik and I are just trying to be precise in our language. Nothing is being said here that hasn’t been said before in far fewer words.
presto said:
To any one reading this so far and feeling a little intimidated just hang in a bit longer. Newsnik and I are just trying to be precise in our language. Nothing is being said here that hasn’t been said before in far fewer words.
Peter says
Agreed
But the problem was that people were using the same words with very different meanings
The only way to eliminate the confusion created by that is to make sure we are all clear on the meanings of the words used.
Ok, I’ve got speed and velocity. Could you still address my last question? I just want you to make that paragraph more self contained if possible. Also I think you are using the term rolling resistance incorrectly if you assign it a speed. Isn’t it a force?
presto said:
Ok, I’ve got speed and velocity. Could you still address my last question? I just want you to make that paragraph more self contained if possible. Also I think you are using the term rolling resistance incorrectly if you assign it a speed. Isn’t it a force?
NewsNik says:
You are correct, I misspoke (thinking about one thing and writing about another…
My bad
To answer the question:
Yes your combined “SPEED will be 80″
But your combined VELOCITY will be 0 (Zero)
(See experiement #4)
Ok. Go on.
FINAL PART (3 of 3)
————————
7) Experiment # 3
This is a step up on experiment # 2
But let’s make sure we note the numbers
The plane is tied out, the motor is off.
The runway is turned UP to >>60<>60<>80<>81<< knots
The runway is turned on to 40 knots
V_air = 41
V_ground = 41
V_runway = -40
S_laser =41
V_plane_2_ground =41
V_plane_2_runway = +81
Plane FLIES and moves forward at 1 knot
————————————————————-
The plane must be moving at a COMBINATION of speeds
The speed of the runway + it's takeoff speed to be able to takeoff
Just rolling along on a runway at equal and opposite speeds does NOT create ANY wind speed to be able to fly..
Mythbusters proof is BUSTED.