Jet Fighter Madness! Physics people come here!

[Teletubby]

A plane is standing on a runway that can move (some sort of conveyer belt of something). 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 opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

[big_black]

If you took a jet fighter, and sat it on a conveyor belt, which travels at the same rate as the spinning tyres, will the jet fighter ever be able to take off?

I'm pretty sure the answer is no. It's not actually going anywhere in that case.

"Wheels spinning" is not what helps a jet take off.

[D-EJ915]

Planes are like speakers, they push air to work.

[exnihilo]

I think we've discussed this. The answer is YES, the plane will take off. +1 to D-EJ915.

cg

[Ken]

I'm imagining this being equivalent to just turning on a really, ridiculously big fan. With no air actually moving over the wings to create a pressure difference (or a cushion of air beneath the wings in the case of a jet) you wouldn't (shouldn't?) get any lift.


Edit:
I say this because the plane is not moving at all in reference to the air. The only frame of reference in which it is moving is that of the conveyor belt.

[stevie_bees]

It would not take off as the plane does not have any forward motion, therefore no air flow over and under the wings. No air flow = no pressure difference = no lift.

Edit: Yes, we did do this before!!!

[ratherdashing]

We did this already, the answer is yes it will lift off.

[Mephis]

If it's VTOL, yea, otherwise no.

[St_Genesius]

Jesus fsck, not this sh!t again...

[Tor]

I would think that the jet engines transfer the energy to the wheels - jet fighters take off in a speed of around 160-180 knots, but even though the wheels move at that particular speed, the conveyer responds with the exact same speed in the opposite direction. And as said by others here, no significant air movement over the wings, thus no pressure to lift the jet. The jet engines will continue the acceleration, with the only result that the belt will speed up. Eventually the rubber tires will burn up due to friction (?) they're not constructed for, and the jet fighter will become one body with the conveyer belt.
What happens then depends on the force the conveyer belt applies versus the force of the jet engine, what will happen then....?

[garublador]

I would think that the jet engines transfer the energy to the wheels - jet fighters take off in a speed of around 160-180 knots, but even though the wheels move at that particular speed, the conveyer responds with the exact same speed in the opposite direction. And as said by others here, no significant air movement over the wings, thus no pressure to lift the jet. The jet engines will continue the acceleration, with the only result that the belt will speed up. Eventually the rubber tires will burn up due to friction (?) they're not constructed for, and the jet fighter will become one body with the conveyer belt.
What happens then depends on the force the conveyer belt applies versus the force of the jet engine, what will happen then....?

Do you really think the convyor can impart as much force on the plane as the jet engines, which is what is required for the plane to stay stationary? What part of the plane, exactly, is the belt pushing against?

[JacksonMIA]

As Zerb said in the last thread (and he's the only one who claimed any piloting experience), planes can take off even when moving backwards relative to the ground. It makes no difference what the wheels are doing - they're only to keep the plane from dragging along the ground.

Perhaps you don't need physics people (who tend to over analyze things, like I've been known to do), but you need pilots instead. They'll tell you the ground makes no difference. Wind speed is all that matters.

[MikeS]

We covered this before… why some don’t see that the obvious answer is that the plane can take off is beyond me.

[Guitar Toad]

This exercise is analogous to a person running on a treadmill, running on a conveyor belt.

If the next ground speed of the an airplane one a conveyor belt is zero like a person on running on the treadmill. The plane is not going to fly. One need not be a pilot to figure this one out. It's a pure physics problem, not a piloting problem. If the plane is an a conveyor belt, the plane will not fly.

On the conveyor belt the wheels of the plane are spinning but the wind velocity over the wings is not increasing. If the treadmill is moving at a low speed and the plane is moving at a very high speed such that the net forward velocity of the plane is such still sufficient to reach lift, then yes, the plane would fly. But, that is not "stationary" flight. Wave goodbye because the plane will be long gone.

If you were to put a giant fan, in front of the plane as in a wind tunnel, then the plane would "fly' but it wouldn't really go any where. It's net ground speed (if the air velocity of the moving air is fast enough for the plane aerodynamics) but it would still fly. Then a "stationary" flight could be accomplished.

[ksmith63]

planes fly because the shape of the wing creates a pressure difference (above and below the wing). this has nothing to do with the plane relative to the ground it is all about the 'velocity' of the air going over the plane + the angle of attack (the angle the centerline of the wing makes with the free stream air)

[MikeS]

This exercise is analogous to a person running on a treadmill, running on a conveyor belt.

No, it's not at all. A person running applies force to the ground to create forward movement. A plane applies force to the air.

[ksmith63]

from my aerospace engineering class

lift = (pressure above wing - pressure below wing)*cos(angle of wing)

notice this has nothing to do with wheels or treadmills.

[Guitar Toad]

No, it's not at all. A person running applies force to the ground to create forward movement. A plane applies force to the air.

A plane turns on its jet engine to create a substantial forward velocity relative to the ground. The purpose of the jet engine is to create forward movement. This forward movement creates airflow over the wings.

For a jet plane, high velocity airflow over the wings must occur to produce the lift effect. If the plane is on a conveyor belt/treadmill, then the relative ground speed is zero, and the air velocity over the wings is also zero under normal conditions.

[Guitar Toad]

from my aerospace engineering class

lift = (pressure above wing - pressure below wing)*cos(angle of wing)

notice this has nothing to do with wheels or treadmills.

Right, it has nothing to do with wheels or treadmills. Neither treadmills nor conveyor belts can produce lift.

Lift requires air movement over the wings of the plane. Some wing designs require a relatively low air speed...Wright Brothers at kitty hawk. Other wing designs require higher air speeds, ie, The B-52.

KSmith63, is there a special term for the minimum speed required to a wing to attain lift conditions?

[Pierre]

When it does take off though, it would stall right away no?