CRISIS! Plane on a Conveyor Belt!

[CaptainChewbacca]

I've seen this question tear apart boards, so we need an answer and some reasoning:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?"

Now, I have my own answer, but I'll hold off for now to not bias anyone reading it.

[Joviwan]

My limited faculties with science being known by a few people..


I'm going to guess no. I'm under the (potentially misguided) impression that plane engines rely on sucking/pushing air as the means of propulsion, which wouldn't make much of a difference on a conveyer belt. The problem I see is lift. You can be running at six thousand miles per hour, but on a conveyer belt, you sit stationary, which means that the only wind actually pushing up under the wings is whatever the local breeze happens to be.

I've seen videos on youtubes of 747's taking off at full throttle while not acutally going anywhere, thanks to extreme wind.

[Stark]

Dude this is a highschool physics problem. Jesus.

[Dooey Jo]

This is at least the third time.

Yes, the plane will take off, unless it has very high rolling friction in the wheels. Planes rely on their engines for thrust, not the ground, and the conveyor belt is matching the plane's speed, not its thrust.

[CaptainChewbacca]

It also created the longest thread in the history of physics.org.

[Stark]

This is at least the third time.

Yes, the plane will take off, unless it has very high rolling friction in the wheels. Planes rely on their engines for thrust, not the ground, and the conveyor belt is matching the plane's speed, not its thrust.

Yes, the relevant part is whether the plane is held stationary or not. Different physics teachers would say different things.

[CaptainChewbacca]

OOh.. they're doing it on mythbusters tonight. I'm gonna watch.

[Joviwan]

*Reads thread. Reads links posted in posted thread*
Well, that was interesting. As someone who did terribly unwell with highschool physics, that was hard to wrap my head around. I have a much easier time understanding the complicated dynamics of a semi-colon and it's use around commas and ellipses in a formal essay then I do with physics equations of near any type that aren't that little three part triangle with S, T, and D scribbled in it.

I'm agreeing with the 'take off' crowd now.

[lPeregrine]

Dear god, let this problem die already! It seems like it comes back every few months to haunt me...


The answer, in very simple terms, that you can pretty easily try yourself: put a toy car on a treadmill. Set the belt speed to anything you feel like. Now push the car forward, and note how easy this is. Notice that with enough of a push, you can get it moving forward at any speed you feel like. Now just imagine wings on your toy car, and you see very clearly why the plane moves forward, generates lift, and takes off.

There are only three possible ways for the plane-on-a-belt to fail to take off, and none of them have anything to do with the usual answers given:

1) If the plane has a huge amount of friction in the wheels, allowing the belt to actually transfer a non-trivial force to the plane. But any halfway competent engineer will design the wheels to spin with as little friction as possible, so the only way this is going to happen is if the pilot holds down the brakes as hard as possible.

2) If the plane is so close to its load limits (overloaded WELL beyond the safe limit) or using up all available takeoff distance (the pilot is an idiot who is leaving no margin of error) that even a small amount of braking force from the belt is enough to hold it back. In theory, it is possible that the plane could fail to gain enough airspeed before reaching the end of the runway and crash, but only if the pilot is a complete idiot.

3) If doubling the rotation speed of the tires is enough to damage them. Finally a plausible answer... I don't know the exact safety margins on airplane tires, but it's pretty reasonable to think that subjecting them to double the rotation speed might be enough to blow the tires and bring the plane to a rather violent stop as the landing gear collapses.

[Stark]

Yah, not taking off requires the plane to actually be held stationary by the belt. Since it doesn't get thrust from it's wheels, this won't normally happen.

[JointStrikeFighter]

Theoreticly if the conveyor belt was moving fast enough it might generate so much wind as to overstress the planes airframe.

[Darth Servo]

Mike answered this two years ago

[Ghost Rider]

Fucking A, been there, done that.

Next time use the goddamn fucking search button, moron.