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Post by clockwork on Feb 3, 2006 3:41:00 GMT -5
That would be incorrect, Mike. Airplanes fly because of lift, not thrust. Lift is caused by the wing (airfoil) moving through the air at a speed great enough to generate enough lift to carry the weight of the airplane. Let's say an airplane needs to move at 60 mph to generate enough lift to fly. If the conveyor belt is moving at 60 mph, and the airplane's engine is generating enough thurst to keep it in a constant position on the conveyor belt, there is no air moving across the wing. The plane is sitting still relative to the surrounding air. The airplane needs to be pulled or pushed with enough thrust to move the wing through the air at 60 mph. Therefore, using the example above, the airplane would have to generate enough thrust to move at 60 mph just to stand still on the conveyor belt + an additional 60 mph to generate the air movement across the airfoil required to lift the airplane. Grouse Grouse, please take a breath and realise we all agree with you on the lift aspect, no movement, no lift. Why can't you see that the plane will move??? The conveyor going backwards does NOT keep it in a stationary position!! I thought fly fishing was a thinking man's sport?, there are more blinkered horses here than at the cheltenham gold cup! ;D
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Post by nicepix on Feb 3, 2006 3:44:51 GMT -5
If a train is going at 99 miles an hour west and hits a fly flying east at 1 mph. The two collide and the train continues west with the fly attached. The fly must have stopped as it changed direction(east to west) as it was in contact with the train the train must have stopped as well. I lost track with the plane. Malcolm, The train doesn't stop. It is merely slowed by the impact, though as Paul Daniels used to say - "Not a lot." The plane is not on the track silly, that's the train.
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Post by Exerod on Feb 3, 2006 4:46:56 GMT -5
If a train is going at 99 miles an hour west and hits a fly flying east at 1 mph. The two collide and the train continues west with the fly attached. The fly must have stopped as it changed direction(east to west) as it was in contact with the train the train must have stopped as well. I lost track with the plane. Malcolm, The train doesn't stop. It is merely slowed by the impact, though as Paul Daniels used to say - "Not a lot." The plane is not on the track silly, that's the train. It was bad enough with leaves on the line and the wrong kind of snow, how long before we hear train delayed by 'flies on the windscreen'?
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Post by elwyman on Feb 3, 2006 5:08:29 GMT -5
I think the answer is "yes and no" - it would depend on the type of plane. Take the two extreme cases: -
1) A very powerful jet plane would behave like a missile. The thrust is so poweful that the wheels on the conveyor don't prevent take off. The wheels would spin fast initially, but at some point, the forward thrust force exceeds the wheel friction and they just skid along the belt as the plane gathers speed and takes off.
2) A small single prop plane would probably (?) struggle to generate enough forward thrust to overcome the friction between the wheels and the belt. The wheels would spin and the plane would not move forward.
Can we borrow Grouses's plane and try this out? ;D
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Post by mikeconnor on Feb 3, 2006 5:26:35 GMT -5
[2) A small single prop plane would probably (?) struggle to generate enough forward thrust to overcome the friction between the wheels and the belt. The wheels would spin and the plane would not move forward. UNQUOTE
The amount of friction between the wheels and the belt is negligible. That´s what the wheels are for.
It is immaterial which type of plane.
The only relevant criteria is that the thrust comes from the plane´s engines.
The belt, its speed, direction, or anything else except the plane´s engines, and the surrounding air, have nothing at all to do with the matter.
TL MC
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Post by nicepix on Feb 3, 2006 7:16:32 GMT -5
I think that the key here is that the planes moves. If it moves in relation to its position on the earth's surface and gathers enough speed for lift to occur then it will fly. It's just that the wheels will be rotating twice as fast as if it were a normal take off.
Of course if its a carpenter's plane, then it won't.
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Post by elwyman on Feb 3, 2006 7:19:35 GMT -5
Small planes have to build up speed gradually along the runway. I don't see how a small plane could generate sufficient forward momentum to physically move forward against a conveyor belt, from a standing start.
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Post by mikeconnor on Feb 3, 2006 7:23:29 GMT -5
Small planes have to build up speed gradually along the runway. I don't see how a small plane could generate sufficient forward momentum to physically move forward against a conveyor belt, from a standing start. The wheels simply spin faster. Even the slightest thrust will move the plane forward relative to the air surrounding it, and that is airspeed. The conveyor belt is completely irrelevant. TL MC
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Post by Gander on Feb 3, 2006 7:25:23 GMT -5
It may be useful for some if you look at this from a different angle.
Let us say the aircraft is travelling down the conveyor belt at 200 mph, but the conveyor belt is stationary. Let us also say that 200 miles an hour is the point this plane achieves lift off. You would agree that the plane will take off as from a normal runway.
Now imagine that as the plane hits 200 mph on the belt/runway someone switches the conveyor belt on. It instantly moves at 200 mph. Will the plane stop dead?
The answer is no. It will not stop of slow as the belt speeds up because the thrust of the airplane is not being transferred through the wheels. The wheels are turning because they are in contact with the ground while the plane is already moving through the air even if it has not overcome gravity yet i.e. reaached lift of point.
Look at the original question. It does not say the aircraft is not moving forward (i.e. has no velocity). It tells us that the belt speed matches the aircraft speed. The force of the belt motion will only have an effect on the free-wheeling wheels of the plane. There is no mechanism to transfer that force to counteract the force of the engine drive.
Hope that clears it up. (The answer is yes, the plane will fly because it can still achieve forward motion independant of the belt speed).
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Post by mikeconnor on Feb 3, 2006 7:28:35 GMT -5
I think that the key here is that the planes moves. If it moves in relation to its position on the earth's surface and gathers enough speed for lift to occur then it will fly. It's just that the wheels will be rotating twice as fast as if it were a normal take off.
Of course if its a carpenter's plane, then it won't. [/quote]
It moves in relation to the air surrounding it. That is airspeed.
The earths surface, or any other surface is irrelevant. Movement in relation to that is groundspeed.
If you do the same experiment with a car, then the groundspeed in relation to the conveyor belt surface is zero. The car does not move because the belt is moving backwards at the same speed as the wheels are driving the car forward.
The wheels on a plane dont drive anything at all. They merely spin.
TL MC
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Post by mikeconnor on Feb 3, 2006 7:36:43 GMT -5
It may be useful for some if you look at this from a different angle. Let us say the aircraft is travelling down the conveyor belt at 200 mph, but the conveyor belt is stationary. Let us also say that 200 miles an hour is the point this plane achieves lift off. You would agree that the plane will take off as from a normal runway. Now imagine that as the plane hits 200 mph on the belt/runway someone switches the conveyor belt on. It instantly moves at 200 mph. Will the plane stop dead? The answer is no. It will not stop of slow as the belt speeds up because the thrust of the airplane is not being transferred through the wheels. The wheels are turning because they are in contact with the ground while the plane is already moving through the air even if it has not overcome gravity yet i.e. reaached lift of point. Look at the original question. It does not say the aircraft is not moving forward (i.e. has no velocity). It tells us that the belt speed matches the aircraft speed. The force of the belt motion will only have an effect on the free-wheeling wheels of the plane. There is no mechanism to transfer that force to counteract the force of the engine drive. Hope that clears it up. (The answer is yes, the plane will fly because it can still achieve forward motion independant of the belt speed). You are basically correct. The belt speed can never match the aircraft speed, for the simple reason that the belt is moving at groundspeed, and the plane is moving at airspeed. THERE IS NO CORRELATION BETWEEN THESE TWO DEFINITIONS Indeed, with regard to the airspeed of the plane, it is quite immaterial whether the belt moves at all or at what speed it does so. Apart from the freely spinning wheels, there is no other physical connection between the belt and the plane. Airspeed is the speed of an object relative to the surrounding air. Groundspeed is the speed of an object relative to a fixed point on earth. TL MC
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Post by leisuredgentleman on Feb 3, 2006 7:37:43 GMT -5
I agree with Mike Connor in everything he says, except that: The answer is 'NO'. For, as soon as the plane starts to move forwards the wheels start to turn. And we are told that the conveyor belt then turns in the opposite direction. Therefore (because the plane is still moving forward - because no matter what we do with the conveyor the plane will continue to move forward) the wheels on the plane are made to move more quickly. For the conveyor belt will still detect that the plane is moving relative to the air, so the conveyor will speed up a bit more. But still the plane is moving, so the conveyor belt moves more quickly so the wheels move more quickly so the conveyor moves more quickly. And so on. So the moment the plane starts to move forward - even before it reaches 1mph - the wheels will be accelerated to an extraordinarily high speed. And they will melt. And the plane will then be welded to the conveyor belt. So the wheels will not be moving, so the plane will come to a halt, stuck to the belt. If it is a truly frictionless system, the conveyor belt will soon approach - and pass - the speed of light, which will lead to some different, alarming conclusions. The problem lies in the original statement that the conveyor moves in the opposite direction to the wheels at equal speed. This makes no sense for our scenario. I've just re-read the question and changed my mind. For this version of the question refers to the conveyor belt matching the speed of the PLANE, not its wheels. So the answer is Yes. I'm not getting into a plane with the Grouse.
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Post by mikeconnor on Feb 3, 2006 7:49:11 GMT -5
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Post by mikeconnor on Feb 3, 2006 8:13:16 GMT -5
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Post by MarkH on Feb 3, 2006 8:41:13 GMT -5
OK I'm convinced and have gone from a no to a yes. The plane will move forward, generate lift and take off.
The backward motion of the conveyor will just spin the wheels of the plane faster and will have no effect on the forward motion of the plane itself.
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