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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: 2675
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 # 102.

#33. To: Jhoffa_ (#0)

Can the plane take off?

Without reading too many of the responses so far, if I understand right, at whatever speed the plane begins to move forward, the conveyer belt moves backwards.

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).

Neil McIver  posted on  2005-12-01   16:14:23 ET  Reply   Untrace   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.

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

But if they had jet engines powerful enough and planes able to withstand the skidding, then it would take off.

Therein lies a problem made by many: confusion and/or equating the behavior/actions of solid fuel or liquid fuel rocket engines WITH turbofan gas turbines. Jets have the latter. Two extremely different propulsion systems-- two totally different engines.

wbales  posted on  2005-12-02   8:03:31 ET  Reply   Untrace   Trace   Private Reply  

#124. To: wbales, RickyJ, tom007, Neil McIver, markm0722, wakeup, Dakmar, Mr Nuke Buzzcut, Soren (#102)

But if they had jet engines powerful enough and planes able to withstand the skidding, then it would take off.

Therein lies a problem made by many: confusion and/or equating the behavior/actions of solid fuel or liquid fuel rocket engines WITH turbofan gas turbines. Jets have the latter. Two extremely different propulsion systems-- two totally different engines.

All of these engines generate thrust. What is going to counter that thrust?

If you want my opinion, the problem lies with the "skidding".

What's skidding? The wheels are allowed to freely rotate. Freely rotating tires do not stop planes. In fact, they are designed to hinder the plane as little as possible. If engineers had their way they'd be frictionless (at least in the direction the plane is moving) to ease takeoff.

Time for another analogy I think.

Let's say I am standing on a frictionless treadmill (conveyer belt). As I try to run the treadmill slides out from under me. Each step I take merely makes the treadmill spin. I'm not going anywhere no matter how hard I try. The reason is I am using my feet (wheels) to provide forward "thrust" and all that energy is lost. It cannot be transferred to the surface I am standing on because it is frictionless. I might just as well be trying to walk on ice with while wearing icy shoes. Perhaps this is why many seem to think the plane will not move.

However, now let's say I mount a fan on my back (jet engine). I turn it on. I'm standing on a frictionless treadmill. Am I going to move forward? Most definitely. A force is being applied to me. What's going to stop me from moving? What is going to counter the thrust of that fan? My feet? Not a chance. I'm standing on the equivalent of ice! I can do anything I want with my feet and it isn't going to matter. I could try running backwards in place. I could try running forwards. My feet don't matter though. The more frantic I move them the more comedic the effect as I get pushed off the front of the treadmill. And believe me, if I was a coyote and mounted Acme's jet engine on my back, I'd be trying to maximize comedic effect. ;)

It does not matter how efficient or inefficient its fan is. As long as ANY force is being applied which is not countered by an opposing force the plane will accelerate. There is no opposing force to counter it. The engine is not the key to understanding this problem in my opinion, but the realization that the wheels of the plane cannot counter the thrust of the engines.

As a side note, on the off chance the wheels spin infinitely fast in this brainteaser (not saying they are, wasn't needed for a solution), there is one more issue with the plane taking off. It would have perfect gyroscopes mounted (Gyroscope). It would prevent the plane from performing "yaw" and "roll". Luckily only "pitch" is needed in takeoff (Pitch, Yaw and Roll). That could definitely pose a problem on landing though. There would be no circling back to the same airport, lol.

markm0722  posted on  2005-12-02 17:19:13 ET  Reply   Untrace   Trace   Private Reply  

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