Randazzo

So, you have a plane on a conveyor belt...

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2 hours ago, sevenperforce said:

thrust does not produce lift. Thrust moves the plane forward; the motion of the air over the airfoil produces lift.

It is the thrust which moves the wing (airfoil) though the air. I understand that it is the air moving across the surface of the wing which generates the lift, therefore flight. Even a glider receives "thrust" from some source, be it a giant rubber band, a sling-shot, catapult, or powered tow craft, to begin the process of air moving across the wing.

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36 minutes ago, cineboxandrew said:

Thats airspeed, not ground speed

Fairly sure they have some specs for touchdown speed.

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Have you ever seen a plane took off while standing still? I have, at least RC ones and against the wind :P

But bigger planes can also do that, some propeler plane can lift their tail while standing still, because the propeler is moving the air that passes through the elevator.
 

 

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If a treadmill was placed on a plane, and the treadmill was set to match the speed of the plane, would the plane be able to take off?

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8 minutes ago, sevenperforce said:

If a treadmill was placed on a plane, and the treadmill was set to match the speed of the plane, would the plane be able to take off?

This is an serious safety issue, the belt is likely to overheat moving at so high speed, it probably end up catching fire and break up. trowing burning pieces everywhere 
 

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I'd be more worried that the treadmill might fall off the plane and FOD the engine.

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5 hours ago, sevenperforce said:

Whenever the conveyor belt detects the wheels moving, it will immediately attempt to match their speed, which will increase their speed, which will cause the conveyor belt to accelerate faster and faster in an attempt to meet its designed performance criteria. Meanwhile, the plane merrily takes off without even noticing.

No, because the plane is held in place by the belt.  In either interpretation it doesn't take off.  

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6 minutes ago, DAL59 said:

No, because the plane is held in place by the belt.  In either interpretation it doesn't take off.  

The belt is not holding onto the plane because there is no way for the belt to hold onto the plane.

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Well, if it helps we know that a treadmill definitely works normally on a spaceship.

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10 minutes ago, DAL59 said:

No, because the plane is held in place by the belt.

Incorrect; @sevenperforce had it exactly correct.

The plane isn't "held in place by the belt" because there's nothing attaching the plane to the belt.  The belt doesn't touch the plane and therefore can't exert any force on it.

The only thing the belt is touching is the plane's wheels.  And it can't actually exert a forward-or-backward force on the wheels, to any significant degree, because they roll.

What the belt can do is make the wheels spin.  But that has nothing at all to do with the motion of the plane.

Here's how the physics work, here:

  • The conveyor belt can accelerate backward as hard as it likes, limited only by the power of its motor.  This does not affect the motion of the plane.
  • The airplane can accelerate forward as hard as it likes, limited only by the power of its engines.  This does not affect the motion of the conveyor belt.
  • Sandwiched in between these two things are the plane's wheels.  They move with the plane (since they're rigidly attached to it).  But they're free to spin.  The spin speed of the wheels is determined by the sum of, 1. how fast the conveyor belt is moving backwards, and 2. how fast the plane is moving forwards.

The spin speed of the wheels is the result of the plane's motion (and the conveyor belt's), not the determinant of it.  You appear to be mixing up cause and effect.

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1 minute ago, Snark said:

The spin speed of the wheels is the result of the plane's motion, not the determinant of it.  You appear to be mixing up cause and effect.

But, the plane's wheels need to spin in order for the plane to move forward.  

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No they don't. It depends on the relative motion of the plane and the belt. If the belt matches the speed of the plane, the plane can take off without the wheels spinning at all and the brakes fully applied.

If the belt is stationary, the wheels are turned by friction with the belt as the plane accelerates by the force of the engines.

If the belt goes backwards the wheels turn but the plane doesn't move. The wheel hubs are effectively frictionless and can't exert any force on the plane to accelerate it.

Edited by RCgothic

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12 minutes ago, DAL59 said:

But, the plane's wheels need to spin in order for the plane to move forward.  

If you have no friction with the surface, the wheels wont spin, but the plane would still take off.

EDIT: So if you have a treadmill without friction, it will be tha same as a ground without friction. If you add friction, it will only make the wheel spin twice as fast (as already told here)

Edited by VaPaL

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What if instead of the plane itself, the treadmill was configured to keep the passengers on the plane stationary by moving the plane back and forth around them? What happens when two passengers walk in opposite directions?

If you can have a magical treadmill of impossible so can I.

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29 minutes ago, DAL59 said:

But, the plane's wheels need to spin in order for the plane to move forward.  

You've got the tail wagging the dog, here.

The plane moves, because its engines make it move and there's no force stopping it from moving.  When the plane moves, that makes the wheels spin.

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2 minutes ago, Snark said:

there's no force stopping it from moving. 

Other than the treadmill pushing it backward.

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7 minutes ago, 5thHorseman said:

What if instead of the plane itself, the treadmill was configured to keep the passengers on the plane stationary by moving the plane back and forth around them? What happens when two passengers walk in opposite directions?

 

They would probably fall down and get hurt.

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6 minutes ago, DAL59 said:

Other than the treadmill pushing it backward.

Except that it doesn't.  There's no magical force connection between the conveyor belt and the airplane (unless you're positing that someone has glued one end of a rope to the conveyor belt's surface and the other end to the airplane's chassis).  The conveyor belt doesn't push it backwards.

The conveyor belt does push backwards against the lower surface of the plane's wheels.  But the result of that is simply to spin the wheels-- not to give the plane a hard shove backwards.

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13 minutes ago, DAL59 said:
16 minutes ago, Snark said:

The plane moves, because its engines make it move and there's no force stopping it from moving.

Other than the treadmill pushing it backward.

The treadmill cannot push the plane backward. It cannot push the plane because it is not touching the plane.

The treadmill is simply touching the plane's wheels. The underside of those wheels, to be precise. And no amount of pushing against the underside of those wheels will exert any significant force on the chassis of the plane.

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8 minutes ago, sevenperforce said:

The treadmill is simply touching the plane's wheels. The underside of those wheels, to be precise. And no amount of pushing against the underside of those wheels will exert any significant force on the chassis of the plane.

The treadmill is equivilent to brakes.  

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2 minutes ago, DAL59 said:

The treadmill is equivilent to brakes.  

Brakes act between the wheels and the plane, not the wheels and the ground

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14 minutes ago, DAL59 said:

The treadmill is equivilent to brakes.  

Typically, planes disengage their brakes before attempting to take off.

With the brakes disengaged, nothing the treadmill does can keep the plane from moving.

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