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Old 03-04-2006, 09:48 AM
zut zut is offline
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Join Date: Apr 2000
Location: Detroit, MI
Posts: 3,725
Originally Posted by enipla
Let me ask everyone this.

Put a toy car on your home treadmill. Tie a string to it. Start the tread mill at 1foot per second. Could your pull the car to you at 1fps? Having the toy cars wheels turn at 2fps Could you pull it off the tread mill? If you believe that you could pull the toy car off the tread mill, then the plane can take off.
Bad analogy, because the treadmill in this case isn't reacting to the plane's motion. That changes the problem considerably.

In any case, there are multiple interpretations of the problem, and the "answer" depends on your interpretation and your initial assumptions. flight lays out the two basic camps: The first is where the belt matches the plane speed with respect to the ground (that's what you're talking about). The second is where the belt matches the wheel rotation speed (or the belt matches the plane speed with respect to the belt). This requires the plane to remain stationary; otherwise the condition of the problem is violated. This second interpretation is (I think) what Paradoxic is talking about.

I've seen different wordings of the question (Cecil's original column had multiple versions, for example) that more or less strongly imply one interpretation or another. So I don't think it's necessarily conclusive to tie the "answer" to one wording or another, because other people have likely made conclusions based on alternate problem statements.

Originally Posted by jere7my
Agreed, with a quibble...
Agreed, also. I was objecting to Paradoxic's statement that "no matter how fast the conveyer moves, with or without anything on it, it won't make the air move any faster six feet above it. " My sense is that a runway-sized conveyor belt running at a few hundred miles per hour would move the air six feet above with some measurable velocity. Enough to create lift? I doubt it. Unless it's a huge belt going at ridiculous speeds for a long time, in which case you run into compressibility effects, and you get a supersonic shock wave around the wheels... That's getting pretty far afield from the original question, though.