A plane on a conveyor belt

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semi-pro waster said:
You aren't taking off in a vacumn (you'd also need to use engines that don't require oxygen) and therefore there is still air movement.
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But if the plane is not moving then the air movement is subject only to wind, and if wind could lift planes... well the industry would be in trouble :p

OK, so if the conveyer is only turning the wheels, but its turning them at exactly the same speed as to counter forward movement, then the plane will not be moving... How does the plane move forward if there is a conveyer moving it backwards via the wheels? While the plane is in contact with the ground the conveyer will be directly affecting its motion... it doesn't leave its wheels behind.
 
Helium_Junkie said:
But if the plane is not moving then the air movement is subject only to wind, and if wind could lift planes... well the industry would be in trouble :p

OK, so if the conveyer is only turning the wheels, but its turning them at exactly the same speed as to counter forward movement, then the plane will not be moving... How does the plane move forward if there is a conveyer moving it backwards via the wheels? While the plane is in contact with the ground the conveyer will be directly affecting its motion... it doesn't leave its wheels behind.
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Simple answer. The conveyor does not counter the forward movement of the aircraft. It only spins the wheels.



andyp said:
The plane isn't moving.

There's no pressure difference.

There's no lift.

Plane doesn't take off.
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Wrong. It does move.
 
Helium_Junkie said:
If the thrust that is generated is countered by a conveyer belt
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This is the bit people can't get their head around. The conveyor belt doesn't counter the engines' thrust.

Conveyor belt pushes against wheels -> These forces are equal.

Engines push against air -> A forward-moving force is generated.

It's two independent systems.
 
Helium_Junkie said:
OK, so if the conveyer is only turning the wheels, but its turning them at exactly the same speed as to counter forward movement, then the plane will not be moving... How does the plane move forward if there is a conveyer moving it backwards via the wheels? While the plane is in contact with the ground the conveyer will be directly affecting its motion... it doesn't leave its wheels behind.
Click to expand...

It doesn't counter the forward movement. The wheels rotate but that rotation has no effect on the plane fusealage. If the plane had no thrust it would still stay stationary. Therefore add thrust and it will move forward.

That is basic physics...
 
Helium_Junkie said:
But if the plane is not moving then the air movement is subject only to wind, and if wind could lift planes... well the industry would be in trouble :p

OK, so if the conveyer is only turning the wheels, but its turning them at exactly the same speed as to counter forward movement, then the plane will not be moving... How does the plane move forward if there is a conveyer moving it backwards via the wheels? While the plane is in contact with the ground the conveyer will be directly affecting its motion... it doesn't leave its wheels behind.
Click to expand...

Because the conveyor belt isn't moving at the same speed as the wheels. The conveyor belt moves at the same speed at the plane, not the wheels.
 
andyp said:
The plane isn't moving.

There's no pressure difference.

There's no lift.

Plane doesn't take off.
Click to expand...

The engines move the plane forward. The wheels will counter whatever speed the conveyor belt is moving. Making the conveyor belt and wheels completely pointless in the equation.
 
OK, from the original post, the one which we are debating.

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.
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There we have it. The plane its NOT stationary. It IS moving. It says so. Up there. In really big letters.
 
Scuzi said:
Simple answer. The conveyor does not counter the forward movement of the aircraft. It only spins the wheels.
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So even though the wheels will be spinning at twice the speed they should be (or some other such hypothetical nonsense), the plane will still move forward and take the wheels with it, because the conveyer is only moving at the same speed as the plane - not the spinning speed of the wheels...

So if the conveyer were to be moving at a speed to directly counter the spin of the wheels as the engines pushed the plane, then I think what I was picturing before would be true...

But this conveyer isnt doing that, this conveyer is moving at the speed of the plane.

I think I get it... I misunderstood the scenario.
Unless I'm still not quite right?

Either way, thanks Scuzi :p That helped.
 
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Belmit said:
This is the bit people can't get their head around. The conveyor belt doesn't counter the engines' thrust.

Conveyor belt pushes against wheels -> These forces are equal.

Engines push against air -> A forward-moving force is generated.

It's two independent systems.
Click to expand...

Bingo!
 
I love this problem, it takes some lateral thinking to understand it but once it sinks in it makes sense. Its a bit harsh to say people are stupid for not 'getting' it straight away, most peoples intuition (including mine :o) is that the plane can't be moving. And most people refuse to have their intuition challenged.
 
Scuzi said:
Simple answer. The conveyor does not counter the forward movement of the aircraft. It only spins the wheels.




Wrong. It does move.
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Oh - the OP makes it look like the plane is standing still.

So the concept is flawed in it's intent to deceive from the very beginning.

If the plane accelerates to V2 of course it will take off.

If the plane doesn't accelerate enough to get to V2, it won't take off.

Simple as.
 
OK, let's do this another way, by re-wording the original question, removing all the bits that aren't needed to answer the question correctly.

A plane is standing on a runway that can't move. The plane moves in one direction. Can the plane take off?"
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