Yes, the plane takes off because the wheels have nothing to do with the generation of thrust force on the plane.
The plane's engines will create thrust in exactly the usual manner. The change in momentum of the air passing through the engine creates a force on the axis of the engine.
The only effects of the conveyor belt are to change the local aerodynamics, the so called "ground effect". This will make the plane handle a little differently as it reaches take off speed (speed relative to the air, not the ground), and if the wheels are rotating at a different speed to normal, the effective mass of the aircraft will be a little different owing to the different amount of rotational kinetic energy stored in the spinning wheels.
The conveyor is, largely irrelevant.
Number_Cruncher
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Not sure what is meant by "moving in the opposite direction of rotation" however if the travelator is in effect transporting the plane along the runway it will assist the take off until the point where the wheels leave the ground, on the other hand if it moving in the opposite direction to the plane then it should not interfere with the take off but for the fact that the planes wheels will have to rotate at a much higher speed than normal which could cause tyre failure and will certainly create a small amount of added friction that the engines will have to counter.
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If the plane had enough magnetic levitation to prevent it scraping on the ground you could do away with wheels completely;.
The only question is, the plane will only take off with air movement over the wing.
So the plane has to move forward at 125 knots or the wind has to move at 125 knots.
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RF - currently 1 Renault short of a family
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If it's a Harrier then Yes as it doesn't need forward movement to get off the ground.
As for normal planes. The thrust from the engine is pushing against air and not tarmac, so it should take off. The wheels simply provide less resistance than the fuselage and the fact that they rotate cancels out most of the resistance anyway.
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Moving in opposite direction of rotation. Several interpretations.
1. Whilst one would normally imagine that an aeroplane would be moving forwards, the action of the travelator is to try to stop the aircraft from moving relative to the stationary ground so that it is stationary with respect to the air.
The wheels of an aeroplane don't provide any power to the plane. Therefore the engines will still push the aeroplane forward. So the motion of the travelator will never be fast enough (until the wheels melt and weld themselves to the travelator). So the aeroplane will just take off normally, at the far end of the travelator.
2. (That the wheels on the plane are said to 'rotate' suggests that this is not the correct scenario). The travelator is rushing the aeroplane towards the end of the runway, so that motion is provided by the travelator rather than the engines.
The aeroplane will have lift generated over its wings, so will take off... and then crash unless the engines are running. Are they?
I don't really understand the point of the question - but I suspect tat is the point of the question!
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If by "the opposite direction of rotation" you mean the wheels are prevented from rotating, then the plane can't take off, because it effectively has the brakes on--the conveyor actually becomes a giant brake pad acting on the tyre. If it's a rolling road, travelling from front to back of the plane, then the plane can take off.
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>>If it's a rolling road, travelling from front to back of the plane, then the plane can take off.
No it cannot as the movement of air over the wings is non-existent. But that scenario couldn't happen as the road could not move quickly enough to counter the thrust from the engines.
Remember, a plane is not powered from the engines.
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"Remember, a plane is not powered from the engines." As an ex-pilot I am bit confused by this! If a plane is not powered from the engines what are they there for? Why don't we dispense with them and stop all the greenhouse gases they cause
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>not powered from the engines
Errrr for 'engines' read 'wheels'. sorry!
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On the contrary, a plane is powered from the engines, not the wheels so as long as the wheels are allowed to rotate in the right direction it will be able to move forward. In this case the road is reacting to the wheels, not being driven by them. The wheels would have to spin infinitely fast, but the plane would move.
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>>If it's a rolling road, travelling from front to back of the plane, then the plane can take off. No it cannot as the movement of air over the wings is non-existent. But that scenario couldn't happen as the road could not move quickly enough to counter the thrust from the engines. Remember, a plane is not powered from the engines.
The speed of the aircraft relative to the air is *all* that matters. The speed of the wheels or ground is (besides some trivially small effects) irrelevant.
If the conveyor moves along with the plane, the wheels of the plane don't turn, but the speed of the plane relative to the air is unchanged when compared with a normal scenario, so the plane takes off.
If the conveyor is stationary, well, that's the normal case with terra firma, so that's obvious, the plane takes off.
If the conveyor moves against the direction of motion of the plane, the wheels turn twice as fast as usual. The speed of the plane relative to the air is unchanged when compared with the normal scenario, so the plane takes off.
The wheels are free to roll - their speed is irrelevant.
The behaviour of the conveyor makes no difference to how the aircraft generates thrust, and hence how it accelerates relative to terra firma, and the (still) air. As the wheels are free to roll, there's no way that the conveyor can apply any longitudinal force to the plane.
Number_Cruncher
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OK. We all agree here that if the aeroplane is moving with respect to the air then it takes off - and if it isn't, it doesn't. I think there is some difficulty in conceptualising the circumstances of the original (rather bizarre) post. I suspect we all have slightly different ideas as to this.
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we all agree that it matters not a hoot what the wheels are doing, so the whole premise of the orginal post is meaningless.
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RF - currently 1 Renault short of a family
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premise of the orginal post
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as i understood it, the premise of the original post was akin to walking up a down escalator so that you remained stationary to an observer on the ground. or using a treadmill - again remaining stationary to an observer on the ground.
in other words, the plane uses its energy just to stay where it was against the escalator trying to take it backwards. so it does not fly as the airflow over its wings is zero.
i posed a not too dissimilar question here a few days ago but in the context of wind drag on a car.
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but as the wheels do not provide the motive power, its not relevant.
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RF - currently 1 Renault short of a family
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i posed a not too dissimilar question here a few days ago but in the context of wind drag on a car.
I thought you were asking about heat loss due to convection, or was that another question?
Number_Cruncher
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The engines don't lift the plane off the ground - they provide thrust to move the plane forward through the air. As the plane gains speed the motion of air over the wings provides lift to raise the plane from the ground. The degree of lift is a function of the wing shape and its velocity through the air.
So it all depends on your interpretation of the conveyor belt. My assumption is that it's like a rolling road or running machine - no matter how fast you run you stay in the same place. If this is the case then no amount of thrust from the engines will lift the plane off the ground.
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It might be easier to visualise it like this. Imagine you are walking against the flow of a moving walkway so that you stay in one place relative to the scenery. That's how it would work if the plane was driven by its wheels. But if someone was walking right behind you and gave you a shove you would fall forwards. The shove is equivalent to the jet engines; it's completely unrelated to what your legs are doing.
Airspeed, not groundspeed is what gives the plane lift so it doesn't matter what the ground is doing, with one exception. If the conveyor is running backwards, turning the wheels in reverse and thus locking them, it would prevent the plane from moving forwards and gaining airspeed.
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with one exception. If the conveyor is running backwards, turning the wheels in reverse and thus locking them, it would prevent the plane from moving forwards and gaining airspeed.
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I don't agree with this bit.
Nothing is locking the wheels. There is no mechanism here that prevents the plane from gaining speed.
The only way to prevent the plane moving is to provide an equal and opposite force, which reacts the engine's thrust. You can't apply any significant longitudinal force through a freely rolling wheel - that's what wheels do!
Number_Cruncher
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am i missing something in the original question or am i reading too much in to it.
as i implied before: to me the question posed is simply that the plane is taxing for take off on a runway except that the runway is acting like a treadmill (travelletor) . so however fast the plane tries to move forward to try and reach take-off point, the travelettor keeps it stationary. assuming the plane manages to reach infinite forward speed, the travelettor does the same in the to ensure the plane does not move a millimetre. so there is no air movement over the wings, so the plane cannot fly.
the o.p. needs to come back here and specify exactly what his plane is doing.
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Physically, to keep the plane stationary, the simplest way to do that is to use a conventional runway, and hold the brakes of the plane full on.
Other ways to hold the plane still include fastening some rope to the back of the plane, and fastening the rope down. Or, put another identical plane nose to nose with the original plane, so that their thrusts cancel out.
I do agree, the OP does need to clarify the question.
Number_Cruncher
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You called..........
The question has just been cut and pasted, it seems it is appearing on various forums at the moment.
Yes you are reading too much into it - forget about brakes etc, you just need to concentrate on the facts. Thrust pushing the plane forward, and the wheels turning underneath it....thats it simple, plane takes off as NC has pointed out in the numerous posts he has made.
Many people seem unable to seperate Thrust from any other form of motive force that in a car would turn the wheels and keep it stationary on a rolling road. But as Thrust acts on the air around it and not on the physical ground the plane is standing on - people have problems.
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The conveyor moves at the speed of the wheels rotating. So if the plane begins to move and the conveyor is moving from back to front it will push the wheels against the way they want to go (there's your equal but opposite part) and stop them. Since gravity is pushing the plane down onto the ground (airspeed is nil), the friction between the tyres and the conveyor operates exactly like a brake on the wheels. When a plane's brakes are on it doesn't move until the force from the engines overcomes the friction between tyres and runway. Is there any conventional plane that can take off with all the brakes fully applied?
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Ignoring the bit about brakes which I think is not material - If the conveyor moves from the back of the plane towards the front of the plane *and* the wheels aren't rotating, the plane must be moving along with the conveyor. So, it can take off because although the plane isn't moving relative to the conveyor, it is moving relative to the air.
I explain exactly this case in an earlier post above.
The conveyor isn't relevant - it can't stop the plane. With the brakes off, the conveyor can't apply any lonitudinal force to the plane. If the conveyor can't apply longitudinal force, it can't affect the longitudinal motion of the plane. If you draw a free body diagram of the plane, and write out Newton's Second Law of motion for the plane, it should all become clear.
I suspect this exchange could continue for ages, with neither of us being able to convince the other of his error. Perhaps we ought to be civil, and agree to disagree?
Number_Cruncher
Number_Cruncher
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Hmm the question is. would the plane be moving forward relative to air. if the runway was slipping away at exactly the same speed as the wheels were turning would it be moving forwards relative to airflow?
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RF - currently 1 Renault short of a family
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i have just spoken to a colleague with a ph.d in aeronautical engineering and aerodynamics from imperial college. he confirms that if the question is as i understood it, the plane would remain stationary realtive to the air around it and to a stationary observer on the ground.
the friend also condirmed what i had said in an earlier thread about convection and the flow of air around a car depending on the relaive speed and direction of the wind to the car's speed and direction.
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Your friend confirmed that if the plane remained stationary then it remained stationary. Bravo to him! How has that moved this forward?
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I think we all agree on these two points.
1. If the plane moves forwards relative to the air, it takes off. If it doesn't, it doesn't.
2. The question is so badly worded that it makes no sense. We can explain '1' to each other until we are blue in the face. We cannot, however, conclude what the wheels and moving runway are doing as 'rotating in opposite directions' does not make any sense!
Given '2', we all have different visualisations of the problem, so cannot draw any concord.
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About the only way I can see to make the plane not take off hinges upon the definition of still air.
If the air is defined as being still with respect to the earth, then the plane must take off. The speed of the conveyor is irrelevant.
If the air is defined as being still with respect to the conveyor, then I can envisage a scenario where the relative velocity between the air and aeroplane can be kept at zero, preventing take off, i.e., the conveyor and air move along with the aeroplane.
I've just been laughing at our kitten chasing its tail, thinking "silly little thing". Having re-read my many posts on this monumentally important topic today, I bow to the sense and wisdom of the small cat!
Number_Cruncher
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I really cannot believe this exchange is still going on. Unless I am missing something it is really quite simple, if the travelator is transporting the plane along the runway it will assist with the take off until the point where the wheels leave the ground, otherwise if it is moving in the opposite direction to the plane then it should not interfere with the take off but for the fact that the planes wheels will have to rotate at a much higher speed than normal which could cause tyre failure and will certainly create a small amount of added friction that the engines will have to counter.
The only point of clarification is that the travelator will not remove or reduce the need for the engines in any respect.
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Agreed this puzzle is badly worded as it implies that the conveyor matches the speed of the wheels but with opposite rotation, therefore the plane must be stationary therefore it cannot take off. But, the thrust from the engines acts on the air the push the plane forward it doesn't drive the wheels so the plane must move forward......but it can't move forward because the conveyor will match the wheel speed and there's the paradox.
Consider this more logical version...
"A plane is standing on 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 opposite direction)."
This clearly states that the plane is moving, let's say 10mph. The conveyor therefore moves at 10mph in the opposite direction. So the plane is moving at 20mph relative to the conveyor (and so the wheels are spinning at twice the speed of the conveyor). So, if the plane needs to reach 100mph to take off, it will be travelling at 200mph relative to the conveyor...but it will take off.
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It's all conjecture. Does it really matter?
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What\'s for you won\'t pass you by
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Does it really matter?
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Frankly, not in the least!
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