COULD A PLANE TAKE OFF FROM A CONVEYER BELT? - PLEASE READ

[Glockmeister]

Mythbusters says otherwise:

http://www.youtube.com/watch?v=YORCk1BN7QY&feature=related

[enwiabe]

The conveyor belt was not matching the speed of the plane.

The implication that the conveyor belt matches the speed of the plane is that the plane is not moving...

[enwiabe]

Also, the mythbusters experiment used a propeller aeroplane. I specifically talked about a 747, whose engine DOES power the wheels.

[enwiabe]

And 20 hours of a pilot's license? So what? They don't teach you physics in a pilot's license course. Tell me in which hour they explained to you about pressure gradients and the bernoulli equations. They teach you how to flip the right buttons to make sure you fly safely.

If every pilot was required to have a sound knowledge of aerodynamical physics, then it would be a much higher paid job than it is now.

Watch some aircrash investigation episodes, most of them happen because the pilots don't know what to do in highly turbulent situations.

[methodsboy]

i dont think so

[jess3254]

Quick question: Why would you want/need a plane to take off from a conveyer belt anyway? =P

[shinny]

Quick question: Why would you want/need a plane to take off from a conveyer belt anyway? =P

Assuming we could get conveyer belts that move fast enough, it'd make for some pretty awesome helicopter style landing take-off pads made wherever if planes could take off like this =P

[Glockmeister]

And 20 hours of a pilot's license? So what? They don't teach you physics in a pilot's license course. Tell me in which hour they explained to you about pressure gradients and the bernoulli equations. They teach you how to flip the right buttons to make sure you fly safely.

If every pilot was required to have a sound knowledge of aerodynamical physics, then it would be a much higher paid job than it is now.

Watch some aircrash investigation episodes, most of them happen because the pilots don't know what to do in highly turbulent situations.

Actually, they do teach bernoulli equations within aviation theory class (don't ask me about them, I haven't been in a theory class in yonks). Again, don't pretend you know something when you don't.

Most aviation accidents happen because of pilot error, I agree. But I doubt learning bernoulli's equation is going to help you fare any better in an emergency.

Also, a senior pilot could be expected to earn six-figure sums.

Also, the mythbusters experiment used a propeller aeroplane. I specifically talked about a 747, whose engine DOES power the wheels.

The topic says plane, not any specific model of aircraft.

You also never mentioned 747s until I mentioned propeller.

[Glockmeister]

The conveyor belt was not matching the speed of the plane.

The implication that the conveyor belt matches the speed of the plane is that the plane is not moving...

But the belt was matching the speed of the plane.

[kurrymuncher]

And 20 hours of a pilot's license? So what? They don't teach you physics in a pilot's license course. Tell me in which hour they explained to you about pressure gradients and the bernoulli equations. They teach you how to flip the right buttons to make sure you fly safely.

If every pilot was required to have a sound knowledge of aerodynamical physics, then it would be a much higher paid job than it is now.

Watch some aircrash investigation episodes, most of them happen because the pilots don't know what to do in highly turbulent situations.

what are you on about??

[/0]

I had to do it

[Mao]

I cbf going through 3 pages of discussion, so sorry if this has already been posted:

I was under the impression that planes lift off because of the reaction force acting up by air on plane (mainly the wings, it's somehow related to Bernoulli, but I forgot exactly how, but the design of the wings gives a pressure difference hence 'sucks' the plane upwards). In this case, if we want to model this, the force we're looking at is related to the relative velocity with air.

The engine generate a net turbulence towards the back of the plane [takes in air from the front, release the products of combustion as hot gases at the back which expands :. propel plane forward]. Hence, the plane has a driving force forwards [and net acceleration forwards].

The wheels are in contact with the conveyor belt, but if we ignore the friction between the wheel and its axle (and assume the entire driving and breaking forces are all supplied by the engine), the wheels are just rolling freely. They are rotating because of the static friction between the surface (conveyor belt) and the surface of the wheel (try a pen rolling across a table).

That is, the wheels in this case does not transfer any forces from the conveyor belt to the plane. Hence, if the plane is accelerating at +a due to the thrust force, and there is no retarding forces acting on it from the conveyor belt, then its motion relative to air is the same as before (assuming the conveyor belt doesn't generate too huge a turbulence against what the plane is trying to achieve). Therefore, it takes off however it was taking off before, just, with the wheels spinning madly.

Yes, the plane does take off. But I wouldn't want to be the wheels.

[if it was say, a car going up a jump ramp, and the jump ramp was a conveyor belt, then no, the car wouldn't be able to do the jump (wouldn't be able to go up the ramp at all). In this case, the driving force is supplied by the friction between the wheel and the road surface.]

I might have made some critical errors though. I don't have enough time to think this over again in terms of energy, but I have a feeling that the extra rotational energy of the wheels come from the energy of the conveyor belt, which has a driving force, and things should balance. But again taking off can be considered as a 'collision' between Earth and the plane, and there need to be consideration on how this 'collision' takes place in terms of momentum as well... so...

[Cthulhu]

tl;dr but I skimmed over it and that sounds like the solution I've heard.

[Mao]

short version:

The wheels are free-rolling, and cannot transfer force, so the plane's engine's thrust force still propels the plane forward. The situation is as if the conveyor belt is not there, with madly spinning wheels.

[Mao]

For your interests: http://blag.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/

If you would like to take time to read that, I belong to the second group.

In the first interpretation, you will see a non-rotating wheel in the frame of the plane, and the conveyor belt is going at the speed of the plane. The plane takes off as normal.

In the second interpretation, as I described.

In the third interpretation, the equation of motion becomes , where is the velocity of the wheel (i.e. the plane), and is the speed of the wheel surface at the contact point. There are two solutions to this, one is obviously , i.e. the plane isn't moving (like sliding a piece of paper from under a toy car, if you do it at a constant speed, the car will not move). The not-so-obvious one is , where the laws of algebra are violated if you try to subtract from both sides. In this case, we require the conveyor belt and the wheel surface to have a velocity of infinity (the plane moves forward, the wheels have a velocity as well, the conveyor belt moves faster than the wheel can, assuming the wheel won't slip, it must match that velocity, and the conveyor belt must move faster, and so on). It is physically impossible for the conveyor belt to keep the aircraft stationary as the wheel is in contact with it and it is freely rolling. No matter what velocity of the conveyor belt is moving, the wheel is always going to match it, and the velocity of the plane would be unchanged.

/end tl;dr.

Conclusion: if a plane has a net thrust generated by its engines and its wheels are free-rolling, then no matter how fast the conveyor belt moves, it cannot affect the motion of the plane (of course, ignoring a lot of friction forces here).