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Old 12-08-2006, 10:00 AM   #41
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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


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Quote:
Originally Posted by Joe Perez View Post
In order to keep the plane from moving forward, you'd have to move the whole atmosphere, not just the ground.

Last edited by UofACATS; 12-08-2006 at 10:22 AM.
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Old 12-08-2006, 10:33 AM   #42
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Originally Posted by samnavy View Post
****, 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.
No, you're not wrong. Let them think about it a little. They're stuck in "car mode" instead of airplane mode.
You have to remember an airplane moves relative to the parcel of air that it's in, not the ground.
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Old 12-08-2006, 11:06 AM   #43
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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.
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Old 12-08-2006, 11:09 AM   #44
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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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Old 12-08-2006, 11:26 AM   #45
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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?
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Old 12-08-2006, 11:36 AM   #46
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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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Old 12-08-2006, 12:02 PM   #47
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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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Old 12-08-2006, 12:03 PM   #48
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Quote:
Originally Posted by bripab007 View Post
you have to move relative to the ground to get airflow, dude.
you have to move relative to the air, not ground.

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.
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Old 12-08-2006, 12:10 PM   #49
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Originally Posted by Joe Perez View Post
Don't ask a pilot, ask an engineer.
soon as one shows up we'll ask him.
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Old 12-08-2006, 12:10 PM   #50
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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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Old 12-08-2006, 12:11 PM   #51
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Quote:
Originally Posted by bripab007 View Post
you have to move relative to the ground to get airflow, dude.
no, you don't. An airplane that can fly at 45kts into a 45kts headwind will stay stationary relative to the ground.

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.
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Old 12-08-2006, 12:11 PM   #52
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Quote:
Originally Posted by bripab007 View Post
Yes, but you'll need some air molecules moving over your wings to do anything...how are you achieving this when you're stationary?
You wouldn't be stationary, the forward thrust of the engine would overcome the friction of the wheel bearings/your legs/whatever we're talking about now and propel you forward relative to the stationary air around the conveyor belt.
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Old 12-08-2006, 12:11 PM   #53
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Quote:
Originally Posted by bripab007 View Post
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.
because it isn't ******* stationary.
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Old 12-08-2006, 12:15 PM   #54
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Quote:
Originally Posted by bripab007 View Post
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.
The plane is only stationary as long as the engines are off. (disregarding friction) As soon as the engines start up, the plane moves forward through the air.

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.
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Old 12-08-2006, 12:18 PM   #55
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Quote:
Originally Posted by bripab007 View Post
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.
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.
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Old 12-08-2006, 12:38 PM   #56
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Quote:
Originally Posted by bripab007 View Post
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.
you're wrong.

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.
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Old 12-08-2006, 12:40 PM   #57
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Quote:
Originally Posted by UofACATS View Post
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.
If the treadmill is moving in the same direction as the plane and at the same speed than in theory it will negate the friction in the tires and allow to plane to take off in a shorter distance. Assuming all else is the same.
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Old 12-08-2006, 12:43 PM   #58
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Wait a second...does this this airplane have normal, circular wheels?
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Old 12-08-2006, 12:47 PM   #59
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Quote:
Originally Posted by bripab007 View Post
Are you guys all in on this together trying to pull my leg or something?
Funny, this is what I keep thinking

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.
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Old 12-08-2006, 12:48 PM   #60
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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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