Changing my answer. God-damn geniuses!!!
As sam and Joe pointed out (others too) The fact that the plane has wheels allows the belt to be an insignificant factor in this equation. So, the thrusters push the plane foward relative to the ground/air. The wheels spin backwards on the belt but that's irrevelant. The only thing that changes is that the wheels are spinning backwards (at an increased rate) as the plane accelerates down the runway. I'm going with pilot/engineer on this one. Plane takes off Quote FTW.
Originally Posted by Joe Perez
(Post 62864)
In order to keep the plane from moving forward, you'd have to move the whole atmosphere, not just the ground.
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Originally Posted by samnavy
(Post 62791)
Shit, could I be wrong about this? I've got a whole room full of pilots here talking about it and we're split 50/50.
You have to remember an airplane moves relative to the parcel of air that it's in, not the ground. |
Yes, Philip, but if the airplane is stationary, it's got no airflow...you have to move relative to the ground to get airflow, dude.
When you're jogging on a treadmill, do you feel wind rushing by your face as you move through the air? Not on any treadmill I've ever been on. |
okay, now strap a rocket on your back while your runnin' on the treadmill, light that puppy off, and see if you feel any wind on your face.
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Right, you won't. As long as the treadmill can keep up to maintain your lack of motion relative to the ground/Earth's surface, you'll not feel any wind in your face.
Are you guys all in on this together trying to pull my leg or something? :D |
The rocket wouldn't make your legs move faster. It would simply propel your body up the threadmill. Hence wind, and if you transform yourself into a plane, liftoff.
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Yes, but you'll need some air molecules moving over your wings to do anything...how are you achieving this when you're stationary?
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Originally Posted by bripab007
(Post 62900)
you have to move relative to the ground to get airflow, dude.
The wheels allow the ground to move "backwards" as the plane stays still. (ignoring friction since we're in make-believe mode) The speed the wheels are spinning is an insignificant factor. |
Originally Posted by Joe Perez
(Post 62854)
Don't ask a pilot, ask an engineer. :cool:
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Yes, but how're you moving relative to air when the plane is stationary? You guys are getting caught up in the wheels still...I don't even care about them...that's why I was giving the example of the reverse jet engines mounted to it.
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Originally Posted by bripab007
(Post 62900)
you have to move relative to the ground to get airflow, dude.
An airplane flying 45kts into a 55kts headwind will fly backwards relative to the ground. This isn't rocket science, it's very basic physics folks. |
Originally Posted by bripab007
(Post 62912)
Yes, but you'll need some air molecules moving over your wings to do anything...how are you achieving this when you're stationary?
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Originally Posted by bripab007
(Post 62917)
Yes, but how're you moving relative to air when the plane is stationary? You guys are getting caught up in the wheels still...I don't even care about them...that's why I was giving the example of the reverse jet engines mounted to it.
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Originally Posted by bripab007
(Post 62917)
Yes, but how're you moving relative to air when the plane is stationary? You guys are getting caught up in the wheels still...I don't even care about them...that's why I was giving the example of the reverse jet engines mounted to it.
If you take friction of wheel bearings into account, The engines just have to work a little harder before the plane starts moving. <-- 1st year engineering student, but this is stuff I learned last year in high school physics. |
Originally Posted by bripab007
(Post 62917)
Yes, but how're you moving relative to air when the plane is stationary? You guys are getting caught up in the wheels still...I don't even care about them...that's why I was giving the example of the reverse jet engines mounted to it.
It does not matter what speed or direction the wheels are travelling, thus the moving runway is a non-factor. |
Originally Posted by bripab007
(Post 62917)
Yes, but how're you moving relative to air when the plane is stationary? You guys are getting caught up in the wheels still...I don't even care about them...that's why I was giving the example of the reverse jet engines mounted to it.
F= M*A A = F/M The force in this situation is jet thrust (Ft) - frictional force of the tires to the treadmill (Ff) - wind resistance (Fr). making the equation: A = (Ft-Ff-Fr)/M Now. Ff = uN (mu * normal force) and airplane tires have a fairly low mu if I recall (hard compound). However, a very large mass so there is a signifigant Ff. The normal force is equal to Mass (M) * gravity(g) (or weight) Equation: A = (Ft-(mMg)-Fr)/M For now we will ignore the wind resistance because some of you seem to think the plane is sitting still (which it's not), and therefor has no wind resistance. A = (Ft - (mMg))/M A = Ft/M - mMg/M A = Ft/M - mg Notice that A (the acceleration of the aircraft) is NOT dependant on the velocity of the ground. It is only dependant on thrust, mass, the tire friction coefficient, and gravity. If you take into account wind resistance it is dependant on velocity, however this affects the craft the same regardless of the treadmill. |
Originally Posted by UofACATS
(Post 62929)
Think this way. The giant treadmill could be going reverse direction than the example (going foward) it would still have no effect.
It does not matter what speed or direction the wheels are travelling, thus the moving runway is a non-factor. |
Wait a second...does this this airplane have normal, circular wheels?
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Originally Posted by bripab007
(Post 62904)
Are you guys all in on this together trying to pull my leg or something? :D
The only thing the engine in the plane is doing relative to the treadmill is overcomming the friction of the bearing and tire once it does that the treadmill could be going mach 4 and it wouldn't matter. |
Yeah, I guess I got caught up in the idea of this magical conveyor belt that would be able to move fast enough to overcome what little amount of friction there is from the plane's wheels against the conveyor belt's surface.
But the conveyor belt would have to be going thousands of miles an hour in the opposite direction...it's pretty much impossible...but I thought that was a stipulation in the original questions...magical conveyor belt. I stand by the fact that if you have no airflow across the wings, you have no lift. |
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