General Question

waterskier2007's avatar

Can a plane take off on a giant treadmill?

Asked by waterskier2007 (2050points) June 3rd, 2008

this was an argument on a forum that my friend belonged to, but basically can a plane take off if it is on a giant treadmill going in the opposite direction

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15 Answers

Hollister0221's avatar

I doubt. A plane need drag that creates lift. This is only done by speed of air over top of wings

sndfreQ's avatar

No-the purpose of thrust is to generate air current under the wings, producing lift. Lift increases as the air current increases. If the plane is stationary this won’t happen.

The curvature of the wing produces a differential in air pressure-less on top, more underneath, which increases as more air passes the wing. Thrust produces this increase, but only if the plane comes in contact with that great degree of air current. If the plane’s take-off velocity is somewhere around 200mph, then that sustained air current passing the wings would have to be duplicated at a standstill. So far only wind tunnels can simulate this, and mostly on a much smaller scale.

Hollister0221's avatar

are u guys pilots? I learn this in pilot school

waterskier2007's avatar

haha well its kind of tricky, because the treadmill cant keep the plane stationary, because the plane isnt using wheels to move, it used either props or turbine engines, which are not effected by the treadmill, so it would still move forward, and therefore produce lift to take off

jrpowell's avatar

Mythbusters covered this. The plane does take-off. The wheels just spin faster.

A rather lengthy discussion on the topic.

sndfreQ's avatar

I imagine on a very small scale this could work if a plane has excellent lift capability (light weight) and low maximum velocity (e.g. a glider); however in the case of passenger airliners and fighter jets-well, I have yet to see any treadmill designs on aircraft carriers.

sndfreQ's avatar

In theory-no, in practice yes, if the conveyor belt has too much friction in its wheels and pulleys, then the plane’s thrust will eventually overpower the force keeping it on the conveyor belt, and run off the conveyor belt-but not take off from a virtual stand-still. It would still need the proper amount of air current passing its wings to generate lift.

Hollister0221's avatar

I guess too. I’m not sure if I have the complete picture. However, the basic principle is a plane needs lift and in order too acomplish that it needs air speed over wings. To me there is no other way around it. Whatever means to accomplish this, it will fly. And every plane has a different speed to create lift. ie. A smaller plane requiring less

sndfreQ's avatar

@phoenyx: yes, I read the link, and the most plausible explanation I found from there was:

“Basically, the question assumes that moving the runway backward under the plane while it is accelerating forward will cancel out its forward motion. But that’s not necessarily true. The degree to which this is possible is the degree to which there’s friction between the wheels and their axles. In a frictionless environment, it doesn’t matter what the runway is doing.

Looking at it from the opposite extreme, if the runway can push back on the plane exactly as much as the engines are pushing forward, the plane will stand still. Rather than needing any fancy conveyor belt system, you could just put big blocks in front of the wheels. :)

Does this mean that the plane won’t take off? Well, probably. You might want to think that the engines, as they push the air over the wings, will produce some lift, and I guess that this is true at least minimally, but the real lift generated by the wings is the relative motion between them and the air through which they move. Without forward motion, you don’t have that lift. I doubt the engines could run fast enough to even generate enough lift to bring it off the runway at all.

So, the bottom line is that this isn’t a hard problem if you understand that a plane flies because the wings generate lift as they move through the air, not merely because the engines push air over the wings. The engines push air over the wings in some cases just because it’s a convenient design. The purpose of the engines to propel the plane forward. They can be anywhere in relation to the wings (well, excepting worrying about turbulence and stuff).

A more complete answer is that if you attempted this experiment, what would happen is that only at first would the conveyor belt cancel out the engines push forward. But very quickly, that small friction of the wheels wouldn’t matter. The plane will move forward regardless of how fast the conveyor belt is moving backward. So the plane will just be slightly slow to start moving, then once it does, it will quickly act like any other plane taking off.

Alternatively, assuming you really could apply a backward motion equal to the forward propulsion of the engines, then the plane wouldn’t take off. It’d sit there as the engines ran faster and faster because the engines aren’t what generate lift. And the fancy backward-moving conveyor belt is a distraction, because you can get the same effect by just locking the plane against forward motion.”

It is amazing though that once the plane’s wheels overcome the friction of the conveyor belt, they’ll move faster (as johnpowell and the mythbuster team points out), and the plane will move forward. But that’s where the question gets a bit complicated. Perhaps the question could be worded more effectively as: “Can a plane’s thrust overcome a conveyor belt moving under it in the opposite direction?” The answer would be yes, as it would take a minimal thrust force to overcome the force of the belt against the tires.

But in any case, no forward air current, no lift. If instead of some monstrosity of a conveyor belt, as the poster indicated above, the wheels were just blocked up (creating a similar state of “infinite” frictionless force against the tires), then if the tires were not the issue, the plane would just sit still on the tarmac.

sndfreQ's avatar

@phoenyx and johnpowell:

Edit: I didn’t realize this until after the fact, but it was JP that posted the link to the lengthy discussion that I used to quote above.

waterskier2007's avatar

@hollister, i dont think you are catching the point. there is no way the treadmill could stop the plane from moving because the plane moves forward via propellers or a turbine engine, and therefore the treadmill cant stop the plane, if the treadmill speeds up, the wheels of the plane simply spin faster

Hollister0221's avatar

so I guess it could fly

jrpowell's avatar

The engine doesn’t drive the wheels on a airplane. The wheels will just spin at double-time. That is all.

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