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Aircraft on treadmills

12/13/06 | by Adam | Categories: Technology, Silly

Link: http://www.boingboing.net/2006/12/11/airplanetreadmill_pr.html

BoingBoing has a hysterical discussion on the old conundrum of whether an aircraft on a treadmill could take flight. Being neither a physicist, engineer nor a mathematician some of the erudite discussion escapes me, but as far as I can tell there are two answers.

1) It takes off. This is due to the flow of air being sufficient to force it off the ground. In the case a propeller aircraft, it's the flow of air being pushed over the wings. In the case of a jet aircraft, it's the force being expelled from the combustion of the fuel. The wheels on the treadmill are simply there to keep it from sinking into the ground while this all happens. One of the commenters describes this very aptly: chain the aircraft to the ground and try the experiment again; although it'll be unable to go anywhere, the plane should be able to effectively hover as the chains will counter the forward momentum but not the lift.

2) It doesn't take off. This, to be true, is the nitpicker's solution. The argument is that due to friction between landing gear and ground, and the instant increase of the treadmill to match the forward velocity, eventually the tires burst, the bearings seize and the whole aircraft becomes a flaming wreck thrown somewhere behind.



Comment from: Nimble [Member]  

If the plane is unable to get forward momentum, I’d say it won’t take off, unless the treadmill itself somehow conspires to generate a lot of wind.

I think the wheels would be insufficient to prevent it from taking off, though - the forward moving power does not come from the wheels. Chaining the aircraft down, though (unless it’s to a central tether or something), will prevent it from taking off.

Forward momentum is the key to everything, though it’s all related to wind.

Harriers (jump jets) and helicopters are the only examples I can think of that could do otherwise, and that’s only because in the Harrier case, the thrust vector is straight up and powerful, not forward fighting a treadmill, and in the helicopter case, the wing action, such as it is, is rotary.

I remember some wing simulation software way back when on the C64. That lift from wind is all important - the greater length of path of air over the wing generates much lower pressure than the straighter, under the wing path, and the differential force is what produces a great deal of lift.

If you want to turn the question around, take the “opposite” example: an airplane held stationary in a wind tunnel. You can generate significant lift coefficients even though the apparatus cannot move forward. (Simulations like this: http://wright.nasa.gov/airplane/tunnl2int.html can be useful)

12/13/06 @ 22:59
Comment from: Adam [Member]  

I think you’re missing the point. A prop aircraft generates its own lift by grabbing the air and pushing the air past the wing, meeting the lift criteria. A jet aircraft works by expelling exhaust therefore giving it forward momentum; in this case it just needs to be directed in the appropriate direction to take off. It’s not technically any different from a rocket in that regard.

I didn’t mention a helicopter as there’s no perceived forward momentum that the treadmill could counter.

Basically, the treadmill is just a red herring.

12/14/06 @ 11:47
Comment from: Nimble [Member]  

The propeller-generated lift is present, but not large. If it were large enough to cause lift, single-propeller planes would flip sideways from the torsion. It’s the propeller-generated thrust that is the key to the take-off.

By the way, I’m not missing the point. The planes will take off unless there’s actually a chain holding them back (which was the ‘further example’ I was actually denigrating :) All of my commentary past “Chaining the aircraft down, though (unless it’s to a central tether or something), will prevent it from taking off.” was related to the chaining-down example.

Yes, the treadmill is a red herring. The only effect it would have is what the nitpickers would claim: it could lead to mechanical failure since it would be making the wheels go ’round at twice the usual speed when the plane finally lifts off the ground.

12/14/06 @ 18:51
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