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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: 936
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 # 95.

#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  

A jet would throttle up and not give a damn about what the tires were thinking.

Dakmar  posted on  2005-12-01   19:51:13 ET  Reply   Untrace   Trace   Private Reply  

Yes, the engine could care less what the tires are doing, but the fan jets must have O2 to perform . At zero air intake velocity, ie. no foreward motion of the plane, the engine is starved for O2. Remember the nacelle actually slows down the air intake velocity and thus increases the air pressure so the compressor blades can get ahold of it.

One of my Aerospace engineering professors stated that you could stand in front of a fighter jet and keep it from moving with your hands - but is it got a little foreward motion..... you are flying.

He did not mean this literally, but was trying to demonstrate how these engines work. - they are designed to have plenty of high velocity air entering the nacelle.

tom007  posted on  2005-12-01   20:10:39 ET  Reply   Untrace   Trace   Private Reply  

How would sitting on conveter belt with wheels spinning be different from idling on tarmac, they still sucking in air. I know I've been on jets that were able to utilise their engines for taxiing, don't tell me they are helpless if they come to a complete stop, I know better.

Dakmar  posted on  2005-12-01   20:17:16 ET  Reply   Untrace   Trace   Private Reply  

I know I've been on jets that were able to utilise their engines for taxiing, don't tell me they are helpless if they come to a complete stop, I know better.

They are rendered helpless if sitting on a conveyor belt. That is why so few modern airports have conveyor belt runways.

And anyway, back to the question: if this plane could take off on a moving conveyor belt, we shouldn't need runways at all.

Nope, runways are required for a jet to attain take off speed--that critical moment when airspeed produces enough lift to get the plane off the ground.

On a conveyor belt, the plane would not move. An observer in the control tower would just be watching a stationary jet revving its engines. No forward movement = no lift = no take off.

wbales  posted on  2005-12-01   20:47:48 ET  Reply   Untrace   Trace   Private Reply  

On a conveyor belt, the plane would not move. An observer in the control tower would just be watching a stationary jet revving its engines. No forward movement = no lift = no take off.

blast up those jets and unless the conveyer belt is physically restraining the airplane wheels with chains or something the airplane is gonna scoot across a scrolling hanna-barbara inspired runway like it was some sort of joke, it all makes perfect sense now.

Dakmar  posted on  2005-12-01   20:57:21 ET  Reply   Untrace   Trace   Private Reply  

it all makes perfect sense

what patience and restraint you show, I'm impressed

wakeup  posted on  2005-12-02   2:23:24 ET  Reply   Untrace   Trace   Private Reply  

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