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plane question

#61
Geoff St. Germaine said:
I'm also an Air Navigator in the Air Force. ;)
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I'm ex-Air Force, my expetise is Air to Air Missles and fire control systems.
For the plane to take off under your senerio, the conveyor would be driving the wheels, having no effect on thrust therfore the plane still moves forward correct?

My understanding is the conveyor was keeping up w/ the wheels and the thrust making airspeed 0. Under those circumstances how does it take off?
 
#62
Another way to look at it:

If it were a submarine with wheels sitting on a conveyor belt on the bottom of the ocean, would it move forward? Sure it would, as it pulls itself through the water with its screws. Air is less dense than water, but the physics are the same. The wheels are irrelevant to the equation.

The wheels do not propel an airplane. It takes off by pulling itself through the air, not driving itself down the runway. The engine is still pulling air and generating thrust, regardless what the wheels are doing.

Jets don't fall out of the sky when they drop gear. ;)
 
#63
AxtoOx said:
I'm ex-Air Force, my expetise is Air to Air Missles and fire control systems.
For the plane to take off under your senerio, the conveyor would be driving the wheels, having no effect on thrust therfore the plane still moves forward correct?

My understanding is the conveyor was keeping up w/ the wheels and the thrust making airspeed 0. Under those circumstances how does it take off?
Click to expand...

You have to look at the force balance. The engines deliver quite a lot of thrust. The wheels on the other hand do not transfer very much force, and they do it through a very low friction. Think of how long it takes for something with wheels to stop when it doesn't use brakes. The force of the engines will be much larger than the force due to the wheels being spun.
 
#64
Geoff St. Germaine said:
You have to look at the force balance. The engines deliver quite a lot of thrust. The wheels on the other hand do not transfer very much force, and they do it through a very low friction. Think of how long it takes for something with wheels to stop when it doesn't use brakes. The force of the engines will be much larger than the force due to the wheels being spun.
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I'm not argueing that at all, the question states the conveyor keeps up w/ the "Planes Speed" that's where I ger air speed 0. I understand what you're saying, I just think we are reading the question different.

If it was just keeping up w/ the wheels, you're exactly right, but it says Planes Speed.
 
#65
lump said:
Another way to look at it:

If it were a submarine with wheels sitting on a conveyor belt on the bottom of the ocean, would it move forward? Sure it would, as it pulls itself through the water with its screws. Air is less dense than water, but the physics are the same. The wheels are irrelevant to the equation.

The wheels do not propel an airplane. It takes off by pulling itself through the air, not driving itself down the runway. The engine is still pulling air and generating thrust, regardless what the wheels are doing.

Jets don't fall out of the sky when they drop gear. ;)
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I understand you theory, but water being more dense, you would have the equivalant to air speed comming into play which would allow it to lift from the conveyor and go on it's way. The question states the conveyor is keeping up w/ the plane, not the wheels.
 
#67
AxtoOx said:
I'll bet the original question was meant to come up w/ the answer you 2 did and in it's present form, it's worded incorrectly.
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No, the way it is worded is fine, and the plane takes off.

The plane's engines accelerate the plane with respect to the air.

A little google reveals that this question has been brought up all over the place. I also found this video Link Removed.

Needless to say, by simply looking at the force balance it is clear that if the conveyor is tracking the speed of the aircraft and can only supply force to the aircraft through the friction in the wheels then there is no way that the conveyor can apply enough force to oppose the thrust. It is that simple.
 
#68
Geoff St. Germaine said:
No, the way it is worded is fine, and the plane takes off.

The plane's engines accelerate the plane with respect to the air.

A little google reveals that this question has been brought up all over the place. I also found this video Link Removed.

Needless to say, by simply looking at the force balance it is clear that if the conveyor is tracking the speed of the aircraft and can only supply force to the aircraft through the friction in the wheels then there is no way that the conveyor can apply enough force to oppose the thrust. It is that simple.
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OK, It's sinking in now. I was taken in just like the question intended me to be. The plane is still moving in respect to the air, not the ground. I got hung up on thinking the plane was staionary and unless you have hurricane force winds, you would not have air speed.
Oh well, didn't intend to fluster you any.
 
#69
AxtoOx said:
OK, It's sinking in now. I was taken in just like the question intended me to be. The plane is still moving in respect to the air, not the ground. I got hung up on thinking the plane was staionary and unless you have hurricane force winds, you would not have air speed.
Oh well, didn't intend to fluster you any.
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It's supposed to be a trick question I believe. Don't worry, I don't fluster that easy. :D
 
#71
AxtoOx said:
The question states the conveyor is keeping up w/ the plane, not the wheels.
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This is where people seem to get confused, but IMO the conveyor can keep up with the plane all it wants, it can even exceed the plane by 10 times if you can get it to do it. The basic fact is that in either scenario, the only net effect is a little increased drag on the wheels. It's being called a "treadmill" is what is throwing people off. The treadmill will affect a car or similar wheel propelled vehicle, but all it's going to do with an jet or propeller run vehicle is double the wheel speed, the treadmill is incapable of significantly affecting the net forward motion.
 
#72
LajoieT said:
This is where people seem to get confused, but IMO the conveyor can keep up with the plane all it wants, it can even exceed the plane by 10 times if you can get it to do it. The basic fact is that in either scenario, the only net effect is a little increased drag on the wheels. It's being called a "treadmill" is what is throwing people off. The treadmill will affect a car or similar wheel propelled vehicle, but all it's going to do with an jet or propeller run vehicle is double the wheel speed, the treadmill is incapable of significantly affecting the net forward motion.
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It got me and I used to work on the flightline. Like I said my area was Air to Air missles and firing and tracking systems,my concern was bringing them down, not getting them up.:)

USAF Worlds #1 distributor of MIG spare parts.
(Now I'll hear from the NAVY.)
 
#74
Brad Barker said:
i guess i don't understand how planes take off!


so it's the thrust, after all, that gets'em in the air?
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I see your point and the way the quetsion is worded I could see how people wouldn't think the plane would move forward at all.

But since the wheels on a plane arn't used to produce forward motion like they are on a car, how is the treadmill really going to stop the plane from moving forward when the only drag it puts on the plane is the slight amount of friction on its axels?
 
#75
Brad Barker said:
i guess i don't understand how planes take off!


so it's the thrust, after all, that gets'em in the air?
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Thrust, but also lift. Gliders don't have engines. However, forward thrust from a tow plane or a winch pulls them at a speed that generates lift which makes them fly.
 
#78
fr0me0 said:
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).

The question: Will the plane takeoff?
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NOPE, it needs wind going against it too.

KeithBMI said:
Here's one.

What would happen if an unstoppable object hit an immovable object?
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They can't exist in the same universe.
 
#79
Jazzin' said:
NOPE, it needs wind going against it too.
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There will be airflow over the wings and it will take off.

The simple thing to do is think of what would happen if you took a skateboard and held onto one end of it and dropped the other end (two wheels) onto a treadmill. It's easy to hold it in place, it doesn't get shot back, and you can easily accelerate it in the opposite direction that the treadmill is going in as the only force pulling it in the direction of the treadmill is the friction in the wheel bearings.
 

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