A Wednesday conundrum
Discussion
Moose. said:
A plane is standing on a runway that can move (some sort of giant conveyer belt). 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 the opposite direction). Can the plane take off?
AH I have worked it out.... the plane cannot take off.....and never will...
It's in 'reverse thust' going backwards...eh...well it would,if the runway was'nt moving at the same speed...
.... Ps, I win.
>> Edited by markbe on Wednesday 8th February 18:48
The Cecil article is all well and good, but read the first argument on the forum where this issue is discussed http://boards.straightdope.com/sdmb/showthread.php?s and your head, like mine, will in all probabilty start to hurt.
Well, anougther thought..If the plane was still, on the runway,brakes off, Mr Runway control man,could send the runway moveing under the plane,at any speed, and the plane Won't move.
I think I like this version..
ps,The plane also has frictionless bearings..
>> Edited by markbe on Thursday 9th February 14:29
I think I like this version..
ps,The plane also has frictionless bearings..
>> Edited by markbe on Thursday 9th February 14:29
xm5er said:
I Have a toy RC plane and when we dont have enough room to take off we simply hold it still in the air with the motors flat out and release it so with that in mind yes it would take off ?
Do us all a favour and pop down to a local gym and film it on a running machine.
Be sure to warn the assembled sweaters what's going to happen
(Or not...
get them to put their money/heads where their mouths are 
)scorp said:tinman0 said:scorp said:Wrong, planes at takeoff use thrust to get off the ground, the wings are ineffective at takeoff speeds.
so why do they need a runway then? put the engine to max and it should shoot stright up into the air by that reckoning.
Inertia.
If the wings are "ineffective at takeoff speeds" as you say, then why do 'planes have wings at all?
Why, in fact, is there any such thing as a "takeoff speed"?
Consider the fact that the takeoff ground speed is lower when taking off into a strong headwind (and for extra points, work out that the takeoff air speed is always the same regardless of wind)
I think you've somehow gotten yourself confused in a parallel universe where physics works in a completely different manner...
Okay folks - planes need wings. As a physicist I can say that there is really no one reason why planes fly but a combination of factors. The aerodynamic lift generated by the wing's asymetrical shape as stated by Bernoulli's principle will not exist at low speeds. But the fact that there is a PLANE surface at a steep angle of attack to the airflow will give an amount of aerodynamic lift as a result of the components of the air force acting on the wing. Part of the wind blows the wing backwards, part of it lifts the wing. Try driving along in your car and hold a small peice of plywood out of the window. Flat, it will cut through the airflow like a knife, at an angle then it will deflect the airflow and cause an upthrust or downthrust depending on the incline.
so why do they need a runway then? put the engine to max and it should shoot stright up into the air by that reckoning.
Inertia.
If the wings are "ineffective at takeoff speeds" as you say, then why do 'planes have wings at all?
Why, in fact, is there any such thing as a "takeoff speed"?
Consider the fact that the takeoff ground speed is lower when taking off into a strong headwind (and for extra points, work out that the takeoff air speed is always the same regardless of wind)
I think you've somehow gotten yourself confused in a parallel universe where physics works in a completely different manner...
Okay folks - planes need wings. As a physicist I can say that there is really no one reason why planes fly but a combination of factors. The aerodynamic lift generated by the wing's asymetrical shape as stated by Bernoulli's principle will not exist at low speeds. But the fact that there is a PLANE surface at a steep angle of attack to the airflow will give an amount of aerodynamic lift as a result of the components of the air force acting on the wing. Part of the wind blows the wing backwards, part of it lifts the wing. Try driving along in your car and hold a small peice of plywood out of the window. Flat, it will cut through the airflow like a knife, at an angle then it will deflect the airflow and cause an upthrust or downthrust depending on the incline.
percy flage said:
The Cecil article is all well and good, but read the first argument on the forum where this issue is discussed http://boards.straightdope.com/sdmb/showthread.php?s and your head, like mine, will in all probabilty start to hurt.
The 'expert' (Strafe) who doesn't make it clear in his diatribe whether he thinks it will or won't take off, then (3rd post) goes and says he's wrong FFS! Why is it so difficult? It is bleedin' obvious!
Eric Mc said:
Someone also fell into the trap of assuming a jet engine exerts its thrust against the "air" behind the engine exhaust pipe. NO IT DOESN'T. It exerts its thrust against ITSELF - thus moving the engine (and anything attached to it - such as a wing or airframe) in the opposite direction. This is a demonstration of Newton's Law "For every action there is an equal and opposite reaction".
The same holds true for any form of reaction engine - such as a rocket.
Propellor aircraft derive their thrust by "flying" forward through the air. Propellors are just a specialised form of wing at heart.
With the odd exception, aircraft wheels are never driven. They just freewheel as the aeroplane moves across the ground. Forward motion is entirely down to thrust from either the jet or propellor. An exception was a 1973 aeroplane that was converted from a Ford Pinto and a Cessna 337 SuperSkymaster. Unfortunately, it wasn't bolted together very well and one day the Pinto and wings/tail assembly parted company. Even more unfortunately, the plane was a few thousand feet in the air when this happened - with predictable consequences for the occupants.
Newtons third law is commonly misquoted as above. A more accurate quote would be:
If object A pushes on object B with force F then object B will PUSH BACK on object A with force F.
The jet engine (or any other impulse engine eg propellor) pushes the air backwards very quickly. The momentum increase of the air as it is blown through and subsequently out of the rear of the engines very quickly is exactly balanced (due to conservation of momentum) by the momentum change of the aircraft.
The impulse generated = force * time it is pushing for.
the impulse also = change in momentum / time it is pushing for
This is from newtons second law:
Force=Mass*acceleration
and the fact that
acceleration = change in speed/ time taken to change speed.
ZR1cliff said:
I thought it was thrust versus height that kept it going forward and Kinetic energy is stored within the metal itself?
Kinetic energy is the energy you have by virtue of the velocity you are travelling at. Its not "stored" anywhere as such, its just an attribute of moving.
Ime confused here,I always thought Kinetic energy was a consequence of twisting a metal which stored up energy in the twist that was released when the pressure was off,is this not used in helicopters so the revs dont drop on gear change?
Stored energy is potential energy
potential energy can be in many forms
fuel is a store of chemical potential energy
kinetic energy is movement energy
kinetic energy can be rotational (turning), translational (linear movement) or both.
Energy stored in deformed objects is elastic potential energy eg a spring or twisted metal.
physicist steve said:
I thought it was thrust versus height that kept it going forward and Kinetic energy is stored within the metal itself?
Kinetic energy is the energy you have by virtue of the velocity you are travelling at. Its not "stored" anywhere as such, its just an attribute of moving.
Ime confused here
Stored energy is potential energy
potential energy can be in many forms
fuel is a store of chemical potential energy
kinetic energy is movement energy
kinetic energy can be rotational (turning), translational (linear movement) or both.
Energy stored in deformed objects is elastic potential energy eg a spring or twisted metal.
So when there is a gearchange resulting in loss of linear energy the metal untwists to take up the slack caused by the loss of torque momentarily therefore keeping the revs up?
I remember reading that they were looking to introduce this into F1 to stop power loss during gearchanges.
I've had another think about this...
Given that the wheel bearing is not perfect (i.e. friction free) then the conveyor CAN exert an acceleration on the plane. i.e. it can (and will) move it if the engines are off. Hence, the premise that the plane "free-wheels" is not entirely true.
So, if the engines are on a very low power then they can exactly balance the force generated by the conveyor through the bearings' friction, and remain stationary w.r.t. the air around it.
Now, the engines are presumably more than capable of overcoming this bearing friction, but this whole scenario is implausible anyway, it's not impossible to allow the conveyor to travel at any speed necessary (which is a given). And also to consider a bearing capable of rotating at such speeds.
So, if the conveyor can travel fast enough it will be able to convey enough force through the bearing's (presumably tiny) friction to balance the thrust of the jets. In this case it can balance the forces and maintain the craft at the same place, relative to the air surrounding it (the only factor affecting flight).
That bearing would get quite hot, I expect.
(My only doubt here is whether the force that can be applied through the bearing is fixed, i.e. whether it is independent of the conveyor speed or not.)
I think that I'm devoting too much time to this...
Given that the wheel bearing is not perfect (i.e. friction free) then the conveyor CAN exert an acceleration on the plane. i.e. it can (and will) move it if the engines are off. Hence, the premise that the plane "free-wheels" is not entirely true.
So, if the engines are on a very low power then they can exactly balance the force generated by the conveyor through the bearings' friction, and remain stationary w.r.t. the air around it.
Now, the engines are presumably more than capable of overcoming this bearing friction, but this whole scenario is implausible anyway, it's not impossible to allow the conveyor to travel at any speed necessary (which is a given). And also to consider a bearing capable of rotating at such speeds.
So, if the conveyor can travel fast enough it will be able to convey enough force through the bearing's (presumably tiny) friction to balance the thrust of the jets. In this case it can balance the forces and maintain the craft at the same place, relative to the air surrounding it (the only factor affecting flight).
That bearing would get quite hot, I expect.
(My only doubt here is whether the force that can be applied through the bearing is fixed, i.e. whether it is independent of the conveyor speed or not.)
I think that I'm devoting too much time to this...
If a bike is placed on a moving floor (i.e. on a flat-bed truck) and the truck moves, the bike moves with it. It is a free-wheeling object, but there is nothing to say it will start moving to counteract the movement of the truck. Surely it's exactly thye same principle here:
The plane is parked, brakes off, on a runway. The runway then starts to move in a certain direction, and the plane would move with it, in the same direction. The plane then applies a forward force (via the engines)to take off (to get itself to a speed which enables sufficient life to take off). However, the movement of the plane is dependant on the wheels, as they are aiding it's movement. For whatever speed the thrust of the plane would push it forward, the runway is moving at that same speed in the oposite direction. They are totally cancelling each other out because the wheels are not being allowed to turn fast enough to make the plane move forward (i.e. faster than the opposing speed of the runway). The plane cannot make progress on it's geographical position, and as such the air is not moving around the wing enough to lift the plane.
I can see the argument from both sides, but ultimately the plane's progress is dependant on the ability for it to travel along the ground, and develop lift from its speed and the forces this air-movement speed generates on the wings. If the plane isn't making geographical movements forward, the air around it isn't moving enough to generate lift.
If the plane did not manifest it's movement PARTLY through the wheels, then the plane would take off. But in this case surely the wheels are not allowing the plane to physically, geographically move forward, and hence build airspeed and lift around the wings?! The air around the plane is only moving as it would around a parked plane, and a parked plane would not take off.
The plane is parked, brakes off, on a runway. The runway then starts to move in a certain direction, and the plane would move with it, in the same direction. The plane then applies a forward force (via the engines)to take off (to get itself to a speed which enables sufficient life to take off). However, the movement of the plane is dependant on the wheels, as they are aiding it's movement. For whatever speed the thrust of the plane would push it forward, the runway is moving at that same speed in the oposite direction. They are totally cancelling each other out because the wheels are not being allowed to turn fast enough to make the plane move forward (i.e. faster than the opposing speed of the runway). The plane cannot make progress on it's geographical position, and as such the air is not moving around the wing enough to lift the plane.
I can see the argument from both sides, but ultimately the plane's progress is dependant on the ability for it to travel along the ground, and develop lift from its speed and the forces this air-movement speed generates on the wings. If the plane isn't making geographical movements forward, the air around it isn't moving enough to generate lift.
If the plane did not manifest it's movement PARTLY through the wheels, then the plane would take off. But in this case surely the wheels are not allowing the plane to physically, geographically move forward, and hence build airspeed and lift around the wings?! The air around the plane is only moving as it would around a parked plane, and a parked plane would not take off.
whatever said:
I've had another think about this...
Given that the wheel bearing is not perfect (i.e. friction free) then the conveyor CAN exert an acceleration on the plane. i.e. it can (and will) move it if the engines are off. Hence, the premise that the plane "free-wheels" is not entirely true.
So, if the engines are on a very low power then they can exactly balance the force generated by the conveyor through the bearings' friction, and remain stationary w.r.t. the air around it.
Now, the engines are presumably more than capable of overcoming this bearing friction, but this whole scenario is implausible anyway, it's not impossible to allow the conveyor to travel at any speed necessary (which is a given). And also to consider a bearing capable of rotating at such speeds.
So, if the conveyor can travel fast enough it will be able to convey enough force through the bearing's (presumably tiny) friction to balance the thrust of the jets. In this case it can balance the forces and maintain the craft at the same place, relative to the air surrounding it (the only factor affecting flight).
That bearing would get quite hot, I expect.
(My only doubt here is whether the force that can be applied through the bearing is fixed, i.e. whether it is independent of the conveyor speed or not.)
I think that I'm devoting too much time to this...
Ok, so now go and find a bearing (a device whose sole job is to rotate freely) that can hold back 50,000lb of thrust
ehyouwhat said:
If a bike is placed on a moving floor (i.e. on a flat-bed truck) and the truck moves, the bike moves with it. It is a free-wheeling object, but there is nothing to say it will start moving to counteract the movement of the truck. Surely it's exactly thye same principle here:
The plane is parked, brakes off, on a runway. The runway then starts to move in a certain direction, and the plane would move with it, in the same direction. The plane then applies a forward force (via the engines)to take off (to get itself to a speed which enables sufficient life to take off). However, the movement of the plane is dependant on the wheels, as they are aiding it's movement. For whatever speed the thrust of the plane would push it forward, the runway is moving at that same speed in the oposite direction. They are totally cancelling each other out because the wheels are not being allowed to turn fast enough to make the plane move forward (i.e. faster than the opposing speed of the runway). The plane cannot make progress on it's geographical position, and as such the air is not moving around the wing enough to lift the plane.
I can see the argument from both sides, but ultimately the plane's progress is dependant on the ability for it to travel along the ground, and develop lift from its speed and the forces this air-movement speed generates on the wings. If the plane isn't making geographical movements forward, the air around it isn't moving enough to generate lift.
If the plane did not manifest it's movement PARTLY through the wheels, then the plane would take off. But in this case surely the wheels are not allowing the plane to physically, geographically move forward, and hence build airspeed and lift around the wings?! The air around the plane is only moving as it would around a parked plane, and a parked plane would not take off.
You are taking the piss now aren't you.
Have you noticed how items such as bicycles tend to be strapped down when transported like this?
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