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Title: Brainteaser: There's an airplane on the runway..
Source: Elsewhere
URL Source: http://www.someplaceelse.com
Published: Dec 1, 2005
Author: I have no idea
Post Date: 2005-12-01 01:10:02 by Jhoffa_
Keywords: Brainteaser:, airplane, runway..
Views: 2859
Comments: 202

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation. There is no wind.

Can the plane take off?

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Begin Trace Mode for Comment # 37.

#32. To: Jhoffa_ (#0)

Can the plane take off?

Of course! It doesn't matter what the ground is doing with respect to the wheels, because the wheels don't propel the craft down the runway. The propellor or jet engine cause the plane to move forward and they work just fine even if the ground is moving in the wrong direction. It will just cause the tires to rotate at twice the speed they would normally.

Now I'm going to read the thread and see what everyone else said and whether I've missed something. hehehe

Mr Nuke Buzzcut  posted on  2005-12-01   16:03:45 ET  Reply   Untrace   Trace   Private Reply  

I disagree. Forward motion is what provides the wing lift required for a plane to leave the ground. Forward motion keeps a plane aloft. All engine power would be merely transferred to the conveyor belt through the wheels and there would be no forward motion by the plane and its wings, ergo, no take off.

wbales  posted on  2005-12-01   16:16:23 ET  Reply   Untrace   Trace   Private Reply  

I disagree. Forward motion is what provides the wing lift required for a plane to leave the ground. Forward motion keeps a plane aloft. All engine power would be merely transferred to the conveyor belt through the wheels and there would be no forward motion by the plane and its wings, ergo, no take off.

You are wrong on two counts, one of which is significant to the question.

1) Wing lift is created by the pressure differential created by the air passing over the airfoil surface - the top of which is longer, causing the air to move move swiftly than it does across the bottom.

2) The plane would have forward motion thanks to the thrust caused by the engines. The wheels are irrelevant, neutral, just along for the ride. The conveyor could be run at a speed 100 times that of the forward motion of the aircraft and all it would accomplish is making the wheels spin faster before takeoff. Forward motion and thereby wind speed would in no way be affected.

Mr Nuke Buzzcut  posted on  2005-12-01   18:22:04 ET  Reply   Untrace   Trace   Private Reply  

#38. To: Mr Nuke Buzzcut, JHoffa_ (#37)

this hurts my brain. ;)

christine  posted on  2005-12-01 18:36:25 ET  Reply   Untrace   Trace   Private Reply  

The plane would have forward motion thanks to the thrust caused by the engines.

Not if it was sitting on a conveyor belt. When taking off, the thrust from the engines propel the plane down the runway overcoming the friction between the wheels and the runway. As speed increases and the plane attains take off speed, lift on the wings pulls the plane up off the ground.

Think of running on ice--there is no traction--no friction to create forward movement despite a lot of thrust. Or think of a car wheel spinning on ice-- lots of thrust--no forward motion.

In this hypothetical, increasing thrust of the engines would only translate into higher wheel and conveyor belt counter rotation. The plane would sit spinning its wheels and the conveyor belt no matter how much thrust was being produced by the engines. The plane would not move. The wheels and the conveyor belt would be doing all the moving--the work and energy of the engines being realized there. The plane would remain stationary and would not, therefore, be able to attain lift rquired for take off.

wbales  posted on  2005-12-01 19:14:02 ET  Reply   Untrace   Trace   Private Reply  

Another example:

Suppose I am on a treadmill going at at snails pace. I have no forward motion. There is ZERO relative wind speed on my face as I am stationary.

Now, I turn the treadmill up to 1000 mph and I am running at 1000 mph with it. I still have no forward motion. I would still be stationary and there is ZERO relative wind speed on my face as I remain stationary.

All the energy I am expending is between my feet and the treadmill. If I had wings in this situation, they would be useless.

wbales  posted on  2005-12-01 19:28:18 ET  Reply   Untrace   Trace   Private Reply  

The plane would have forward motion thanks to the thrust caused by the engines.

That is, BTW, very true ONCE AND ONLY AFTER a plane is airborne.

wbales  posted on  2005-12-01 19:39:35 ET  Reply   Untrace   Trace   Private Reply  

The plane would have forward motion thanks to the thrust caused by the engines.

there it is

wakeup  posted on  2005-12-02 01:31:53 ET  Reply   Untrace   Trace   Private Reply  

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