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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: 2518
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 # 176.

#101. To: All (#0)

Interestingly, the place I first saw this question posted did not have a answer. Actually, that's part of the reason I posted it... because there was such a debate over it.

IMO, Yes.. The plane will fly.

As markm0722 pointed out above, it's generating thrust.

If you make 100,000lbs of thrust, where does it go? You can't destroy it, that's impossible.

You're not expending it on the runway, because the wheels don't drive. You're not burning it up with friction, either.. as that's not mentioned. Perfect hypotheticals here.

It's obviously going into forward motion. The wheels rotating (forcing the conveyor to match speed) is proof that the plane is indeed moving forward and regardless of what the conveyor is doing.

Jhoffa_  posted on  2005-12-02   7:20:27 ET  Reply   Untrace   Trace   Private Reply  

The wheels rotating (forcing the conveyor to match speed) is proof that the plane is indeed moving forward and regardless of what the conveyor is doing.

The wheels rotating (forcing the conveyor to match speed) is proof that the plane is indeed transferring engine thrust to the conveyor belt while the plane remains stationary. Forward movement in a jet on the ground is created by friction between the ground and the tires. Remove that friction and no forward movement.

Again, jet planes DO NOT have rocket engines. High bypass turbofans DO NOT behave like rocket engines.

wbales  posted on  2005-12-02   8:18:59 ET  Reply   Untrace   Trace   Private Reply  

The wheels rotating (forcing the conveyor to match speed) is proof that the plane is indeed transferring engine thrust to the conveyor belt while the plane remains stationary.

No it's not and no it doesn't.

I can't believe I got sucked into this again.

wakeup  posted on  2005-12-02   19:50:59 ET  Reply   Untrace   Trace   Private Reply  

In the real world a powerful plane would take off after wasting precious fuel and wear & tear countering a conveyor belt some idiot is running in the wrong direction. Steam catapults currently in use seem much more sensible for that task.

This is why I said there isn't enough detail, like are the wheels skidding. It seems clear (to me, anyway) that we are to assume that the tires of the airplane are in perfect contact with convbelt, so once again, no, the plane could not get airborn because an unknown force is causing the conveyer belt to negate any forward motion.

Dakmar  posted on  2005-12-02   20:04:14 ET  Reply   Untrace   Trace   Private Reply  

This shall be my last post on this subject (for a least 10 minutes).

Of the four aerodynamic forces, two are initially involved in a plane sitting for takeoff: weight and thrust. A high bypass gas turbine does not blast a plane down the runway like a bullet out of a .357 pistol. The pilot throttles and the thrust must overcome the weight of the plane to move the plane forward. As the plane--STARTING out SLOWLY at first--gains speed, lift and drag come into play as the plane approaches take off speed. Then, it is lift which eventually pulls the plane off the ground.

The weight of the plane sits on the tires. As thrust is applied, the weight of the plane is pushed along ON THE TIRES if on a STATIONARY surface. If the surface on which the tires (weight of the plane) sits is moving against the rotation of the tires while perfectly matching the speed of the tires, the plane cannot and shall not move. Intial momentum could NEVER be reached to make the plane move. Therefore, it could NEVER reach take off (lift) speed.

Again, high bypass gas turbines are NOT rocket engines.

wbales  posted on  2005-12-02   22:19:19 ET  Reply   Untrace   Trace   Private Reply  

You need to watch the fan on the skateboard movie I posted earlier. I don't know why you are so hung up on the type of engine. It either supplies thrust or it does not.

The axle the wheels are mounted on is designed to be as frictionless as possible. How is a frictionless device going to counter the forward thrust?

markm0722  posted on  2005-12-02   22:43:44 ET  Reply   Untrace   Trace   Private Reply  

It has nothing to do with thrust or friction since the wheels of the airplane would have to remain motionless in order to satisfy the requirement about the conveyor belt counter rotating in opposite direction...

Dakmar  posted on  2005-12-02   22:47:39 ET  Reply   Untrace   Trace   Private Reply  

It has nothing to do with thrust or friction since the wheels of the airplane would have to remain motionless in order to satisfy the requirement about the conveyor belt counter rotating in opposite direction...

I think I cover that in my last post. I caught on to why some seem to see a paradox here. The speed of rotation of the wheels must match the speed of the rotation of the conveyor belt, but that does not imply that speed is in any way related to the speed of the plane down the runway.

Part of the problem is that rotational speeds really shouldn't be measured in miles per hour, but rather in degrees of rotation per hour. That's what seemingly creates the paradox here. I'd say the brainteaser is a bit poorly worded in that regard.

markm0722  posted on  2005-12-02   22:54:42 ET  Reply   Untrace   Trace   Private Reply  

#180. To: markm0722 (#176)

The speed of rotation of the wheels must match the speed of the rotation of the conveyor belt, but that does not imply that speed is in any way related to the speed of the plane down the runway.

what else needs to be said

tires, wheels, conveyer belt... all irrelevant

wakeup  posted on  2005-12-02 23:01:26 ET  Reply   Untrace   Trace   Private Reply  

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