Would It Take Off?
Discussion
If the treadmill will allways match the aircrafts acceleration (as in the original question)then it can never move forward.
The engines can produce any power but this magic treadmill will either speed up or slow down to match it.
Grabrails are atached to the ground so not relevant to the question.
The engine force is not the same as using a grabrail would be.
The engine just pushes air out of the back of it. This will allways be cancelled by the treadmill/moving runways opposite speed.
The only way this aircraft will take off in this equilibrium is if a headwind of whatever the take off speed is passes over the wings.
The engines can produce any power but this magic treadmill will either speed up or slow down to match it.
Grabrails are atached to the ground so not relevant to the question.
The engine force is not the same as using a grabrail would be.
The engine just pushes air out of the back of it. This will allways be cancelled by the treadmill/moving runways opposite speed.
The only way this aircraft will take off in this equilibrium is if a headwind of whatever the take off speed is passes over the wings.
stovey said:
If the treadmill will allways match the aircrafts acceleration (as in the original question)then it can never move forward.
The engines can produce any power but this magic treadmill will either speed up or slow down to match it.
Grabrails are atached to the ground so not relevant to the question.
The engine force is not the same as using a grabrail would be.
The engine just pushes air out of the back of it. This will allways be cancelled by the treadmill/moving runways opposite speed.
The only way this aircraft will take off in this equilibrium is if a headwind of whatever the take off speed is passes over the wings.
WHY?
please think it through.
the engine dosn't "JUST" push Air out of the back, its the whole point.How the feck would it stop it from from taking off? Aircraft propulsion is provided via the jet motors, acting upon the air. Wheels have no say whatsoever in propulsion. The wheels and the treadmill would have their little battle, while the plane moves forward regardless, as it is not concerned in any way shape or form by how fast the wheels go.
stovey said:
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).
How can it ever move forward? The airpeed is always zero?
Just because the conveyer is trying to stop the plane moving doesn't mean it's succeeding.
Cheers,
Rob.
It funny reading this thread how many people seem to think planes are driven by there wheels!!
Thats the ONLY way the ground could efect a plane!!
In theory, a plane could take of when stationary if it had a 180mph head wind (taking 180 as the point of take of) and it could also fall out of the sky if it had a ground speed of 180mph and a tail wind of 100mph.
But thats nothing to do with this thread, The plane would take off, the conveyour would have to efect on the plane
Thats the ONLY way the ground could efect a plane!!
In theory, a plane could take of when stationary if it had a 180mph head wind (taking 180 as the point of take of) and it could also fall out of the sky if it had a ground speed of 180mph and a tail wind of 100mph.
But thats nothing to do with this thread, The plane would take off, the conveyour would have to efect on the plane
Rob_F said:
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).
How can it ever move forward? The airpeed is always zero?
Just because the conveyer is trying to stop the plane moving doesn't mean it's succeeding.
Cheers,
Rob.
The original question states that it will speed up to match the plane. There can never be any forward movement relative to the surrounding air.
Newtons laws please...
Thrust is an 'action'. The 're-action' is forward movement unless there is an opposite force to counter it.
Because the wheels spin the only countering force is caused by the rolling resistance of the wheels and tyres as they freewheel on the conveyor belt, and losses caused by friction in the bearings as the wheels rotate. Which aren't enought to counter the thrust from a propellor or Jet efflux.
All these energy losses go somewhere (heat into the tyres). The Wheels and tyres themselves will have some structural limitations.
Which in reality would more than likely be exceeded before the airframe had enough forward motion to actually get airborne.
>> Edited by bumblebee on Saturday 4th March 14:43
Thrust is an 'action'. The 're-action' is forward movement unless there is an opposite force to counter it.
Because the wheels spin the only countering force is caused by the rolling resistance of the wheels and tyres as they freewheel on the conveyor belt, and losses caused by friction in the bearings as the wheels rotate. Which aren't enought to counter the thrust from a propellor or Jet efflux.
All these energy losses go somewhere (heat into the tyres). The Wheels and tyres themselves will have some structural limitations.
Which in reality would more than likely be exceeded before the airframe had enough forward motion to actually get airborne.
>> Edited by bumblebee on Saturday 4th March 14:43
I must be missing something here so i'll explain my thoughts. .
The wheels provide no forward motion, All of that comes from the engines.
As the power increases the treadwheel will speed up or slow down to match this force. The wheel's will just spin faster or slower matching the treadwheel.
The aircraft is stationary relative to the air so has no airpeed and no lift.
The wheels provide no forward motion, All of that comes from the engines.
As the power increases the treadwheel will speed up or slow down to match this force. The wheel's will just spin faster or slower matching the treadwheel.
The aircraft is stationary relative to the air so has no airpeed and no lift.
stovey said:
The original question states that it will speed up to match the plane. There can never be any forward movement relative to the surrounding air.
By your logic if there can never be any relative forward movement then the conveyor will never speed up or accellerate.
Ergo there has to be some forward movement for the conveyor to respond to in order to accelerate and match the speed. If the Airframe is accelerating then the conveyor has to accelerate quicker to 'catch up' the speed difference and then match it.
Unless there is some forward movement the conveyor won't even start turning....
Do you see the flaw in your reasoning?
stovey said:
I must be missing something here so i'll explain my thoughts. .
The wheels provide no forward motion, All of that comes from the engines.
As the power increases the treadwheel will speed up or slow down to match this force. The wheel's will just spin faster or slower matching the treadwheel.
The aircraft is stationary relative to the air so has no airpeed and no lift.
I think your missing something. All the wheels do is allow the plane to travel relatively friction-free (for the purposes of this scenario it's easier to thing totally frictionless) along the ground. They have nothing whatever to do with horizontal forced being applied to the plane. As such the horizontal propulsion by the jet engines will be the same as always, ie the plane will take off.
If we think of it more real-world, with there being friciton in the bearings of the wheels, then the plane just requires a little more thrust before it takes off to over come this added friction - but it still takes off!
Cheers,
Rob.
>> Edited by Rob_F on Saturday 4th March 14:53
stovey, imagine this:
An aircraft sits with its wheels on a set of rollers, just like a rolling road, they are free moving and will spin in the opposite direction as the aircraft is pointing in.
The engines start and thrust is supplied to the surrounding air.
Are you suggesting that the aircraft would be unable to propel itself off the rolling road?
An aircraft sits with its wheels on a set of rollers, just like a rolling road, they are free moving and will spin in the opposite direction as the aircraft is pointing in.
The engines start and thrust is supplied to the surrounding air.
Are you suggesting that the aircraft would be unable to propel itself off the rolling road?
The wheels are free to spin. Andy Mac said:Exactly. well put. but unfortunately some people will resist logic with all their being. The same ones who couldn't see the point in going to school. Fine by me. tediously repetative jobs do need to be done!!
How the feck would it stop it from from taking off? Aircraft propulsion is provided via the jet motors, acting upon the air. Wheels have no say whatsoever in propulsion. The wheels and the treadmill would have their little battle, while the plane moves forward regardless, as it is not concerned in any way shape or form by how fast the wheels go.
There has to be a force in equilibrium with the thrust in order to prevent the airframe moving. This can only be achieved when the countering forces from the rolling resistance of the wheels and bearing losses are equal to the force provided by the thrust reaction.
The wheels would have to be going bloody fast to provide enough countering force to balance things out and prevent the airframe from moving. By which time it might have taken off - if the tyres haven't exploded and it has crashed off the side in a big fireball...
>> Edited by bumblebee on Saturday 4th March 15:02
The wheels would have to be going bloody fast to provide enough countering force to balance things out and prevent the airframe from moving. By which time it might have taken off - if the tyres haven't exploded and it has crashed off the side in a big fireball...
>> Edited by bumblebee on Saturday 4th March 15:02
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