Can a jet on a treadmil fly?

[WillBrink]

This is a well known mind bender and creates debates even among those who study the topic for a living, and it's a topic that far as i can tell put to bed, but I'm still looking for the conclusive source. Q goes like this and two basic versions:

The first goes like this (as depicted on the image above): “Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?”

Is that Q as written is not actually possible, it can't actually be answered as such; "The object will not stay motionless because we have unbalanced forces. So we can not design the conveyor belt to move at the same speed as wheels."

Hence, that Q is moot. The Q that makes more sense that most think of when they think of that Q, that is possible and does not ignore Newtonian physics is:

“An airplane cannot take off from a runway which is moving backward (like a treadmill) at a speed equal to its normal ground speed during takeoff“ - as shown in Mythbusters – is a different matter, and yes, the plane would fly.

Answers to both:

http://c-aviation.net/plane-conveyor...ined-debunked/

[Artos]

How does it get lift being in the same spot??

[1168]

Airspeed =\= groundspeed. Is anybody this silly?

[flenna]

I watched the video and it looked like the plane's forward motion was faster than the conveyor going backwards, which gave it enough air movement over its wings thus creating lift. Which is what I would have expected with any forward movement. What I thought they were going to do is keep the plane basically stationary as the belt went backwards, at which point no lift is created. Am I missing something?

[Adrenaline_6]

It should be able to take off. The thrust from the engines is pushing it, not the wheels driving it like a car. The thrust will overcome the pull of the treadmill rolling backwards.

Now if you want to play devils advocate and put zero physical constraints on the treadmills capabilities, in theory I guess, the acceleration of the backward pulling treadmill could match the thrust of the engines, but that acceleration would have to be never ending. In reality that is impossible.

[TomMcC]

If the the thrust of the engines equals the speed of the belt, there is no air flow over the wings, thus no lift. The differential air flow over the wings does the lifting not the thrust of the engines.

Didn't think that one through sufficiently!

[Renegade]

This is embarrassing.

Next up - https://www.montyhallproblem.com/

[TomMcC]

Oh I see my mistake now. I shouldn't give it away I suppose.

[boss_hawg]

Air has to move over and under the wings to create lift.

Lift is about airspeed over the wings creating a low pressure region.

If the wing has zero airspeed, plane’s not going anywhere.

[TomMcC]

But the wings will have airspeed.