Comments on: Episode 97: Airplane on a Conveyor Belt http://mythbustersresults.com/episode97 Outcomes from all MythBusters Episodes Sun, 14 Mar 2010 05:37:40 +0000 http://wordpress.org/?v=2.9.1 hourly 1 By: Andrew http://mythbustersresults.com/episode97/comment-page-13#comment-7479 Andrew Sat, 13 Mar 2010 00:36:48 +0000 http://mythbustersresults.com/episode97#comment-7479 It seems to me that the controversy here is largely centered around the understanding of the part the conveyor belt plays in the myth. There are two different views on the conveyor belt. The first is that the conveyor belt is running at a *fixed* velocity equal in magnitude but opposite in direction to the plane's required take off velocity. The second is that the conveyor belt runs at what ever speed is necessary to render the plane immobile. 1) In the case of a *fixed* velocity conveyor belt the answer is simple: the plane takes off. This is exactly what the episode showed, and for reasons explained in the episode and by previous posters the plane takes off. Specifically, the wheels are free spinning (nothing in the plane's mechanisms affect the speed) and the thrust comes from the propeller (or jet in a larger scale) which is still in contact with the medium that provides it thrust (i.e. the air). The result is the mass of the plane is still enacted upon by the force of thrust and moves forward. The wheels spin faster than the plane is moving to compensate for the backwards velocity of the conveyor. 2) If the conveyor is supposed to move at a speed that renders the plane immobile, the myth has entered the realm of theory. In the previous example the wheels are actually spinning faster than the plane is moving, meaning you would have to increase the velocity of the conveyor. Unfortunately the wheels will simply accelerate to compensate, meaning you would again have to increase the conveyor's velocity which would of course be compensated for in the wheels resulting in mutual exponential acceleration. Barring reaching the speed of light and relativity, this indicates that it is actually impossible for the conveyor to move fast enough to keep the plane stationary. If you were to somehow fix the plane to a location on the ground without restricting it's vertical movement potential (it would be an interesting rig), then you would find something else entirely. In this situation (not at all the original myth) the plane would not move forward and no lift would be generated. It bears clarifying that in this specific case you have changed the parameters of the myth from "plane on a conveyor belt" to "plane incapable of forward acceleration". As a final note, friction actually has very little to do with this situation, at least in the sense many seem to think. The wheels are not present to create traction (another word for "good" friction) with the ground, but to actually reduce the effects of friction contact with the ground has on the plane's ability to produce thrust. With this in mind, anything that removes friction would actually assist the plane taking off rather than hinder it. For those that claim the Mythbuster's conveyor was flawed due to contact with the ground providing friction needed to take off, elevating the conveyor would likely have no effect on the results. If anything, it would actually make the plane take off faster (though I doubt this as friction is a result of the properties of the surface, not whether or not that surface has air underneath it). It seems to me that the controversy here is largely centered around the understanding of the part the conveyor belt plays in the myth. There are two different views on the conveyor belt. The first is that the conveyor belt is running at a *fixed* velocity equal in magnitude but opposite in direction to the plane’s required take off velocity. The second is that the conveyor belt runs at what ever speed is necessary to render the plane immobile.
1) In the case of a *fixed* velocity conveyor belt the answer is simple: the plane takes off. This is exactly what the episode showed, and for reasons explained in the episode and by previous posters the plane takes off. Specifically, the wheels are free spinning (nothing in the plane’s mechanisms affect the speed) and the thrust comes from the propeller (or jet in a larger scale) which is still in contact with the medium that provides it thrust (i.e. the air). The result is the mass of the plane is still enacted upon by the force of thrust and moves forward. The wheels spin faster than the plane is moving to compensate for the backwards velocity of the conveyor.
2) If the conveyor is supposed to move at a speed that renders the plane immobile, the myth has entered the realm of theory. In the previous example the wheels are actually spinning faster than the plane is moving, meaning you would have to increase the velocity of the conveyor. Unfortunately the wheels will simply accelerate to compensate, meaning you would again have to increase the conveyor’s velocity which would of course be compensated for in the wheels resulting in mutual exponential acceleration. Barring reaching the speed of light and relativity, this indicates that it is actually impossible for the conveyor to move fast enough to keep the plane stationary. If you were to somehow fix the plane to a location on the ground without restricting it’s vertical movement potential (it would be an interesting rig), then you would find something else entirely. In this situation (not at all the original myth) the plane would not move forward and no lift would be generated. It bears clarifying that in this specific case you have changed the parameters of the myth from “plane on a conveyor belt” to “plane incapable of forward acceleration”.
As a final note, friction actually has very little to do with this situation, at least in the sense many seem to think. The wheels are not present to create traction (another word for “good” friction) with the ground, but to actually reduce the effects of friction contact with the ground has on the plane’s ability to produce thrust. With this in mind, anything that removes friction would actually assist the plane taking off rather than hinder it. For those that claim the Mythbuster’s conveyor was flawed due to contact with the ground providing friction needed to take off, elevating the conveyor would likely have no effect on the results. If anything, it would actually make the plane take off faster (though I doubt this as friction is a result of the properties of the surface, not whether or not that surface has air underneath it).

]]> By: Brayen http://mythbustersresults.com/episode97/comment-page-13#comment-7475 Brayen Thu, 11 Mar 2010 18:27:48 +0000 http://mythbustersresults.com/episode97#comment-7475 Hi, I just wached the episode.......I think they were pulling in the wrong direction. To get a reverse effect they need to pull in the same way the plane is going. I think now ..they just helped the plane accelerate. Hi, I just wached the episode…….I think they were pulling in the wrong direction. To get a reverse effect they need to pull in the same way the plane is going. I think now ..they just helped the plane accelerate.

]]> By: Qichao Zhao http://mythbustersresults.com/episode97/comment-page-13#comment-7370 Qichao Zhao Tue, 16 Feb 2010 19:14:32 +0000 http://mythbustersresults.com/episode97#comment-7370 All of you so called engineering students and pilots who make the claim that the plane cannot take off completely fail at A-Level physics. Draw a free body force diagram for the plane. It is simple to see that the force exerted on the plane by the conveyor belt is very small (rolling resistance and frictional losses in the wheel bearings), compared to the thrust generated by the engine. This force imbalance means the plane will experience an acceleration (using F=ma), and this results in a velocity for the plane (v=u+at), which results in lift and take-off. If the plane needed 50mph to take off and the conveyor surface was moving backwards at 50mph, the wheels would simply have a rotational speed equivalent to 100mph at the point of taking off. All of you so called engineering students and pilots who make the claim that the plane cannot take off completely fail at A-Level physics.

Draw a free body force diagram for the plane. It is simple to see that the force exerted on the plane by the conveyor belt is very small (rolling resistance and frictional losses in the wheel bearings), compared to the thrust generated by the engine.

This force imbalance means the plane will experience an acceleration (using F=ma), and this results in a velocity for the plane (v=u+at), which results in lift and take-off.

If the plane needed 50mph to take off and the conveyor surface was moving backwards at 50mph, the wheels would simply have a rotational speed equivalent to 100mph at the point of taking off.

]]> By: Yves http://mythbustersresults.com/episode97/comment-page-13#comment-7363 Yves Sun, 14 Feb 2010 18:24:26 +0000 http://mythbustersresults.com/episode97#comment-7363 At first I thought they blew it, but then I started thinking about and concluded they only explained it poorly. A car gets its movement from the power transmitted to the ground. For a plane its the air instead of the ground. Also if you hang a car on a rope the wheels will turn but the car itself wont move. If you do the same with a plane the wheels wont move but the plane will. I also refer to the other experiment where they put the cars behind a plane. Behind a car you wont even feel a breeze. For the non-believers There are planes taken off from water. How do you think those planes take off? A second propeller in the water maybe? At first I thought they blew it, but then I started thinking about and concluded they only explained it poorly. A car gets its movement from the power transmitted to the ground. For a plane its the air instead of the ground.

Also if you hang a car on a rope the wheels will turn but the car itself wont move. If you do the same with a plane the wheels wont move but the plane will.

I also refer to the other experiment where they put the cars behind a plane. Behind a car you wont even feel a breeze.

For the non-believers
There are planes taken off from water. How do you think those planes take off? A second propeller in the water maybe?

]]> By: Presto http://mythbustersresults.com/episode97/comment-page-13#comment-7350 Presto Tue, 09 Feb 2010 21:07:12 +0000 http://mythbustersresults.com/episode97#comment-7350 Andrew L Frazier, You got it. Andrew L Frazier,

You got it.

]]> By: Andrew L Frazier http://mythbustersresults.com/episode97/comment-page-13#comment-7346 Andrew L Frazier Tue, 09 Feb 2010 08:03:39 +0000 http://mythbustersresults.com/episode97#comment-7346 To say it more simply, The speed of the conveyor belt is independent of the speed of the plane. The conveyor belt cannot slow the plane down it is as if the conveyor belt is completely irrelevant. The only difference between a plane that takes off from a conveyor belt and one that takes off from a runway is the speed of the wheels. To say it more simply, The speed of the conveyor belt is independent of the speed of the plane. The conveyor belt cannot slow the plane down it is as if the conveyor belt is completely irrelevant.

The only difference between a plane that takes off from a conveyor belt and one that takes off from a runway is the speed of the wheels.

]]> By: Andrew L Frazier http://mythbustersresults.com/episode97/comment-page-13#comment-7345 Andrew L Frazier Tue, 09 Feb 2010 07:58:41 +0000 http://mythbustersresults.com/episode97#comment-7345 The plane takes off because the wheels really don't matter all that much, if the wheels are spinning at a certain rate of speed, the propeller is always going to pull the aircraft further forward from where it currently is - the only way a conveyor belt can prevent the aircraft from being pulled further forward is if the force of friction of the wheels is equal to the force pulling the plane forward. The result is entirely consistent with mathematics. The whole notion of the conveyor belt keeping the plane at the same place relative to the ground is flawed. Mythbuster WIN The plane takes off because the wheels really don’t matter all that much, if the wheels are spinning at a certain rate of speed, the propeller is always going to pull the aircraft further forward from where it currently is – the only way a conveyor belt can prevent the aircraft from being pulled further forward is if the force of friction of the wheels is equal to the force pulling the plane forward. The result is entirely consistent with mathematics. The whole notion of the conveyor belt keeping the plane at the same place relative to the ground is flawed.

Mythbuster WIN

]]> By: Presto http://mythbustersresults.com/episode97/comment-page-13#comment-7340 Presto Tue, 09 Feb 2010 01:03:11 +0000 http://mythbustersresults.com/episode97#comment-7340 realistc? Cmon realistic. Follow up on your post. I know you didn't post once and then never checked up on it. Tell me you stand by your post or you changed your mind or I misinterpeted you or something. realistc?

Cmon realistic. Follow up on your post. I know you didn’t post once and then never checked up on it.

Tell me you stand by your post or you changed your mind or I misinterpeted you or something.

]]> By: Presto http://mythbustersresults.com/episode97/comment-page-13#comment-7326 Presto Sun, 07 Feb 2010 23:54:02 +0000 http://mythbustersresults.com/episode97#comment-7326 Matt1970, Well said. It seems so simple once you get past the initial misconception. The myth seems to ask "Can a plane take off if it is standing still?" But it is really asking "Can a treadmill stop a plane from moving forward?" Matt1970,

Well said. It seems so simple once you get past the initial misconception.

The myth seems to ask “Can a plane take off if it is standing still?”

But it is really asking “Can a treadmill stop a plane from moving forward?”

]]> By: Matt1970 http://mythbustersresults.com/episode97/comment-page-13#comment-7321 Matt1970 Sun, 07 Feb 2010 18:20:24 +0000 http://mythbustersresults.com/episode97#comment-7321 To really end this, if the plane were on the treadmill, a person on either side could probably hold the plane in place. The treadmill/conveyor belt is really not exerting that much force on the plane and the plane can easily break free of that force with little opposing motion since it is on wheels. To really end this, if the plane were on the treadmill, a person on either side could probably hold the plane in place. The treadmill/conveyor belt is really not exerting that much force on the plane and the plane can easily break free of that force with little opposing motion since it is on wheels.

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