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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: 996
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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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   Trace   Private Reply  

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

And I'm optimistic. See, I think you can be realistic and optimistic at the same time. I'm optimistic we'll achieve -- I know we won't achieve if we send mixed signals. I know we're not going to achieve our objective if we send mixed signals - gwbush

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

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

Why should we hear about body bags and deaths. Oh, I mean, it's not relevant. So why should I waste my beautiful mind on something like that? -- Big Mama Bush

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

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

And I'm optimistic. See, I think you can be realistic and optimistic at the same time. I'm optimistic we'll achieve -- I know we won't achieve if we send mixed signals. I know we're not going to achieve our objective if we send mixed signals - gwbush

Dakmar  posted on  2005-12-01   20:08:35 ET  Reply   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   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   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.

And I'm optimistic. See, I think you can be realistic and optimistic at the same time. I'm optimistic we'll achieve -- I know we won't achieve if we send mixed signals. I know we're not going to achieve our objective if we send mixed signals - gwbush

Dakmar  posted on  2005-12-01   20:17:16 ET  Reply   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.

Why should we hear about body bags and deaths. Oh, I mean, it's not relevant. So why should I waste my beautiful mind on something like that? -- Big Mama Bush

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

On a conveyor belt, the plane would not move.

Bull, you should watch me take a airboat up a particularly wide escalator sometime. It's all about horsepower vs weight. Don't cheap out on landing gear too, I guess.

And I'm optimistic. See, I think you can be realistic and optimistic at the same time. I'm optimistic we'll achieve -- I know we won't achieve if we send mixed signals. I know we're not going to achieve our objective if we send mixed signals - gwbush

Dakmar  posted on  2005-12-01   20:52:19 ET  Reply   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.

And I'm optimistic. See, I think you can be realistic and optimistic at the same time. I'm optimistic we'll achieve -- I know we won't achieve if we send mixed signals. I know we're not going to achieve our objective if we send mixed signals - gwbush

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

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.

Do not--I repeat--DO NOT--try this at home.

Why should we hear about body bags and deaths. Oh, I mean, it's not relevant. So why should I waste my beautiful mind on something like that? -- Big Mama Bush

wbales  posted on  2005-12-01   21:06:37 ET  Reply   Trace   Private Reply  

I should have stopped at shooting bottle rockets at grocery clerks, but what do I know?

And I'm optimistic. See, I think you can be realistic and optimistic at the same time. I'm optimistic we'll achieve -- I know we won't achieve if we send mixed signals. I know we're not going to achieve our objective if we send mixed signals - gwbush

Dakmar  posted on  2005-12-01   21:09:54 ET  Reply   Trace   Private Reply  

In my opinion, the plane will move forward and can take off. If I'm wrong I'm going to have to think twice about the worth of a physics degree, lol. Here's my reasoning.

There is a force applied by the thrust of the engines to propel the plane forward. Where is the opposite force to counter it and hold the plane motionless? There is only one possible source in this brainteaser: the wheels, or more specifically the axles the wheels are mounted on. How are the "as frictionless as we can engineer them" axles going to stop the plane from moving forward if they are allowed to freely spin? Yes, there is friction between the wheels and the ground, but where's the friction in the axles? Without that it does not matter what the wheels are doing. Without friction in the axles none of the "conveyer counter force" can be transferred to the plane.

One more attempt to explain the importance of the axle. When you apply brakes in your car, in an attempt to stop, you are forcing the axle to stop spinning. Unfortunately, your wheels also get a vote. On ice, they often choose to disagree. However, if you don't apply the brakes, at least as far as going forwards is concerned, your car is on a near frictionless surface no matter what surface you are on: pavement, ice, or conveyer belts. If the axle is allowed to freely spin you can pretty much rule out the wheels.

And lastly, why the conveyer belt? Just find a plane that can take off with the brakes fully applied (leaving long black skid marks until it gets airborne). Put a coyote in the cockpit, put a roadrunner at the end of the runway, and make sure the manufacturer of the engine is Acme! Beep! Beep! Hehehe :)

When prosperity comes, do not use all of it. - Confucious
The nation is prosperous on the whole, but how much prosperity is there in a hole? - Will Rogers
There are 9,000 hedge funds out there. There aren't that many smart people in the world. - Michael Driscoll, a trader at Bear Stearns & Co. in New York
Some days you just want to pull out the Bonehead Stick and beat people senseless. - mirage

markm0722  posted on  2005-12-01   21:24:23 ET  Reply   Trace   Private Reply  

It is irrelevant what the tires or the conveyer belt are doing.

You might want to rethink that. The key part you are overlooking is highlighted in bold.

"The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation."

God is always good!
"It was an interesting day." - President Bush, recalling 9/11 [White House, 1/5/02]

RickyJ  posted on  2005-12-01   22:24:30 ET  Reply   Trace   Private Reply  

Just find a plane that can take off with the brakes fully applied

Well if there are such planes then of course it would fly, but since there aren't, it's a mute point.

God is always good!
"It was an interesting day." - President Bush, recalling 9/11 [White House, 1/5/02]

RickyJ  posted on  2005-12-01   22:37:59 ET  Reply   Trace   Private Reply  

If I'm wrong I'm going to have to think twice about the worth of a physics degree, lol.

Wow, you are putting a lot on the line there aren't you? I would have kept that info to myself.

God is always good!
"It was an interesting day." - President Bush, recalling 9/11 [White House, 1/5/02]

RickyJ  posted on  2005-12-01   22:39:31 ET  Reply   Trace   Private Reply  

Just find a plane that can take off with the brakes fully applied

Well if there are such planes then of course it would fly, but since there aren't, it's a mute point.

That was the joke part of my post by the way. Hence the reference to the Wile E. Coyote and Road Runner.

When prosperity comes, do not use all of it. - Confucious
The nation is prosperous on the whole, but how much prosperity is there in a hole? - Will Rogers
There are 9,000 hedge funds out there. There aren't that many smart people in the world. - Michael Driscoll, a trader at Bear Stearns & Co. in New York
Some days you just want to pull out the Bonehead Stick and beat people senseless. - mirage

markm0722  posted on  2005-12-01   22:42:19 ET  Reply   Trace   Private Reply  

Assuming the plane is propelled forward by jet or prop engines and not through a drive transmission to the wheels (which would not be a particularly well designed aircraft), then it won't stop the plane from going airborne. The only difference it would make is that the wheels would be spinning twice as fast at the airspeed when it takes off (which I'm assuming won't cause them to blow out, causing the plane to crash).

The only way the plane could take off is if it actually did move forward or up. Since the force of the engines is basically parallel with the ground then the only option is moving forward to become airborne. How does a multi-ton object become air born when it has no upward force being applied? The only way it could move forward is if it skidded down the runway and took off that way. I don't think they make planes capable of doing that, so it wouldn't take off. But if they had jet engines powerful enough and planes able to withstand the skidding, then it would take off.

God is always good!
"It was an interesting day." - President Bush, recalling 9/11 [White House, 1/5/02]

RickyJ  posted on  2005-12-01   22:55:42 ET  Reply   Trace   Private Reply  

The only way it could move forward is if it skidded down the runway and took off that way.

Why would it skid? The axle is freely rotating. The wheels are free to turn. The brakes aren't being applied.

When prosperity comes, do not use all of it. - Confucious
The nation is prosperous on the whole, but how much prosperity is there in a hole? - Will Rogers
There are 9,000 hedge funds out there. There aren't that many smart people in the world. - Michael Driscoll, a trader at Bear Stearns & Co. in New York
Some days you just want to pull out the Bonehead Stick and beat people senseless. - mirage

markm0722  posted on  2005-12-01   22:57:38 ET  Reply   Trace   Private Reply  

Since the conveyor belt must turn at exactly the opposite speed of the airplane wheels, and since there is nothing preventing the airplane from accelerating, I think we've discovered some form of perpetual motion, how can we patend this?

And I'm optimistic. See, I think you can be realistic and optimistic at the same time. I'm optimistic we'll achieve -- I know we won't achieve if we send mixed signals. I know we're not going to achieve our objective if we send mixed signals - gwbush

Dakmar  posted on  2005-12-01   23:11:45 ET  Reply   Trace   Private Reply  

Since the conveyor belt must turn at exactly the opposite speed of the airplane wheels, and since there is nothing preventing the airplane from accelerating, I think we've discovered some form of perpetual motion, how can we patend this?

A patent would be good. We can sell it to Acme. We'll need a warning label though for the coyote though, just so we don't get sued.

"A conveyer belt will not counter the force of this back-mounted rocket, ESPECIALLY if wearing roller skates."

Beep! Beep! ;)

When prosperity comes, do not use all of it. - Confucious
The nation is prosperous on the whole, but how much prosperity is there in a hole? - Will Rogers
There are 9,000 hedge funds out there. There aren't that many smart people in the world. - Michael Driscoll, a trader at Bear Stearns & Co. in New York
Some days you just want to pull out the Bonehead Stick and beat people senseless. - mirage

markm0722  posted on  2005-12-01   23:19:10 ET  Reply   Trace   Private Reply  

The brakes aren't being applied.

The original post said nothing about the brakes not being applied.

God is always good!
"It was an interesting day." - President Bush, recalling 9/11 [White House, 1/5/02]

RickyJ  posted on  2005-12-01   23:20:58 ET  Reply   Trace   Private Reply  

I have no idea

So some kid in college wants all of us to do their homework for them? Figures.

God is always good!
"It was an interesting day." - President Bush, recalling 9/11 [White House, 1/5/02]

RickyJ  posted on  2005-12-01   23:23:48 ET  Reply   Trace   Private Reply  

Elsewhere

Exactly where is this source, and more importantly, who are they?

God is always good!
"It was an interesting day." - President Bush, recalling 9/11 [White House, 1/5/02]

RickyJ  posted on  2005-12-01   23:26:38 ET  Reply   Trace   Private Reply  

"A conveyer belt will not counter the force of this back-mounted rocket, ESPECIALLY if wearing roller skates."

Perfect! A+ w/extra credit for the Loony Tunes reference.

Esso  posted on  2005-12-01   23:42:22 ET  Reply   Trace   Private Reply  

The brakes aren't being applied.

The original post said nothing about the brakes not being applied.

Perhaps the pilot made a hefty sidebet back at the terminal.

That plane will takeoff over my dead body! I'll keep the brakes on! - Pilot

Top Story - Passengers Felt Bumpy Takeoff, Co-Pilot Arrested for Murder over Conveyer Belt Wager

(I'm just being silly, please forgive me. :))

When prosperity comes, do not use all of it. - Confucious
The nation is prosperous on the whole, but how much prosperity is there in a hole? - Will Rogers
There are 9,000 hedge funds out there. There aren't that many smart people in the world. - Michael Driscoll, a trader at Bear Stearns & Co. in New York
Some days you just want to pull out the Bonehead Stick and beat people senseless. - mirage

markm0722  posted on  2005-12-01   23:58:55 ET  Reply   Trace   Private Reply  

I stand by what I have written. I have confidence in my understanding.

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   1:15:49 ET  Reply   Trace   Private Reply  

"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."

Bingo. Give that man a happy face.

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   1:22:23 ET  Reply   Trace   Private Reply  

"...All engine power would be merely transferred to the conveyor belt through the wheels ..."

What? How do you figure?

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   1:27:33 ET  Reply   Trace   Private Reply  

does the weight of the fly count in the total weight of the aircraft?

yep

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   1:28:47 ET  Reply   Trace   Private Reply  

I doubt that would be much of a factor.

me too

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   1:30:02 ET  Reply   Trace   Private Reply  

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

there it is

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   1:31:53 ET  Reply   Trace   Private Reply  

I misread that earlier, the plane would be stationary

and, you've misread it again

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   1:34:17 ET  Reply   Trace   Private Reply  

Where's this runway at.

that's not near as important as the color of the plane and whether it is being hijacked

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   1:38:21 ET  Reply   Trace   Private Reply  

In this hypothetical, increasing thrust of the engines would only translate into higher wheel and conveyor belt counter rotation.

nope, you missed it entirely, embarrassing, very embarrassing

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   1:41:12 ET  Reply   Trace   Private Reply  

he plane wouldn't care about little spinny wheels and would shoot off into the air

and now the plane has feelings

what if the plane just said, I DON'T WANT TO FLY

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   1:45:45 ET  Reply   Trace   Private Reply  

There are too many unaccounted-for factors for this to be a classic brain teaser.

Nope. The question is complete.

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   1:50:20 ET  Reply   Trace   Private Reply  

he 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.

wrong, thrust moves it forward and the plane lifts off once air speed is sufficient

give it up, it's starting to hurt

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   1:54:36 ET  Reply   Trace   Private Reply  

if it used a high bypass turbo fan jet for it's propulsion, as is typical for jetliners, the answer is never,

wrong

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   2:00:20 ET  Reply   Trace   Private Reply  

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

tires were thinking

as you all know, tires don't think until they reach terminal velocity

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   2:03:28 ET  Reply   Trace   Private Reply  

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