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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: 875
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 # 119.

#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  

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  

If the plane is a prop, it might be able to take off. It would be an unusual plane for certain. The propeller air wash over the lifting surfaces could concievably generate enough lift to get the plane airborne, even as the plane had zero velocity.

If it used a high bypass turbo fan jet for it's propulsion, as is typical for jetliners, the answer is never, assumeing there is no headwind at all.

These engines are produce very little thrust, by their basic nature, until foreward velocity begins to cram air for combustion into the nacelle, where the air is slowed down, and hence increased in pressure for the first stage of the compressors to throw into the burners. When stationary, the compressors pull a vacuum in the nacelle and very little combustion can take place.

Then, and only then do you get the massive thrust these engines are renowned for.

BTW it is easy to tell what the design speed of a jet is. If the nacelle (Front intake of the engine) is 90 degrees to the air flow, it is a subsonic design. If the nacelle if angled, supersonic, with the angle matching the sonic shock wave angle generated by the speed of the plane. In other words, the greater the angle of the nacelle, the greater the design speed of the plane.

tom007  posted on  2005-12-01   19:48:04 ET  Reply   Untrace   Trace   Private Reply  

Yet another reason this plane wouldn't take off--the jet fan engine.

wbales  posted on  2005-12-01   20:02:28 ET  Reply   Untrace   Trace   Private Reply  

get some bottlerockets and visit your local supermarkey, see if bottle rockets care how fast conveyer belt is turning.

Dakmar  posted on  2005-12-01   20:08:35 ET  Reply   Untrace   Trace   Private Reply  

some bottlerockets and visit

The bottle rockets would fly, no matter what the conveyor was doing. They are a self contained control system. The propellent contains an oxident. Thrust here is independent of external conditions.

tom007  posted on  2005-12-01   20:13:25 ET  Reply   Untrace   Trace   Private Reply  

The bottle rockets would fly, no matter what the conveyor was doing. They are a self contained control system. The propellent contains an oxident. Thrust here is independent of external conditions.

oh, my God

wakeup  posted on  2005-12-02   2:10:32 ET  Reply   Untrace   Trace   Private Reply  

The bottle rockets would fly, no matter what the conveyor was doing. They are a self contained control system. The propellent contains an oxident. Thrust here is independent of external conditions.

oh, my God

Adolf Hitler... "What luck for rulers that men do not think."

John F. Kennedy... "Those who make peaceful revolution impossible will make violent revolution inevitable."

wakeup posted on 2005-12-02 02:10:32 ET Reply Trace Private Reply

Care to elaborate? Remember, we are talking about an air breathing engine here. By the way, I am using "Areospace Propulsion" by Dennis Sheppard, 1972, Cornell University, as my source.

tom007  posted on  2005-12-02   9:43:03 ET  Reply   Untrace   Trace   Private Reply  

I'm back to my original conclusion that the plane in this scenario could not fly since it would not be moving. If the conveyor belt is turning at exactly the same speed as the wheels, then by definition the plane is standing still and thus could not get airborn. Of course this could never happen, but that's the way the question is phrased.

Dakmar  posted on  2005-12-02   10:35:04 ET  Reply   Untrace   Trace   Private Reply  

I'm back to my original conclusion that the plane in this scenario could not fly since it would not be moving. If the conveyor belt is turning at exactly the same speed as the wheels, then by definition the plane is standing still and thus could not get airborn. Of course this could never happen, but that's the way the question is phrased.

But the plane WOULD move forward. Lets break it down:

1) The plane starts at a standstill. The conveyor is not moving.

2) The pilot lays into the throttles and the plane starts to move forward due to the thrust created by those, oh so inefficient, jet engines. Let's say it moves at 1 mph.

3) The conveyor now matches by moving at 1 mph in the opposite direction.

4) Whoa! That causes the wheels to rotate at a speed equivalent to 2 mph.

5) Which in turn causes the conveyor to double its speed ... which speeds up the wheels ... which speeds up the conveyor...

6) Meanwhile the turbines are spinning up and generating more thrust which moves the plane forward a bit faster -- relative to the surrounding environment and thereby adding even more speed to the feedback loop between the tires and conveyor.

7) The only question now is how long it will take for the conveyor and wheels to reach the speed of infinite or major component failure first.

8) Gawd, I hope the plane takes off before those suckers blow up!

Mr Nuke Buzzcut  posted on  2005-12-02   14:15:37 ET  Reply   Untrace   Trace   Private Reply  

#120. To: Mr Nuke Buzzcut (#119)

It doesn't say that the wheels/conveyor cannot reach infinite speed, so given the parameters of the question the plane cannot take off.

Dakmar  posted on  2005-12-02 14:28:16 ET  Reply   Untrace   Trace   Private Reply  

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