UPDATE: I’ve totally changed my view in the light of what David said in the comments and having re-read Mr. Kottke’s post, the following is simply an archive of my idiocy, scroll down for current views.
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Jason Kottke made a recent blog post about a plane which is going on a conveyor belt designed to keep it stationary in relation to the ground. He claims that the aeroplane will take off as usual.
In my opinion a better way to think of the problem is to simplify it by removing the conveyor belt. Imagine a plane which has a (frictionless) vertical metal pole going up through it. This means that it can move up and down but not forwards and backwards.
A plane generates lift, by the Bernoulli Principle(and Coanda Effect, see update), by air moving over its wings: the only factor affecting whether the plane takes off is therefore the movement of the air over the wings. The propellor/turbine of the plane is not designed to directly ‘suck’ air over the wings. Instead its purpose is to make the plane move through the air indeed with some forms of propulsion no air would be sucked over the wings.
The propellor is designed to push against air to accelerate the plane in relation the air. The result of this is that air rushes over the wings faster. However if there is a large metal spike going through the plane it will not accelerate in relation to the air. The confusion arises because people think that accelerating the plane in relation to the air is the same as accelerating the air in relation to the plane. Propellors and turbines are designed to do the former and not the other.
The only air moving over the wings will be possibly a little generated by the propellor, and the wind. So without ridiculously powerful propellors mounted in front of the wings the plane will not take off. Obviously the bits of wing without a propellor on them will generate no wind at all.
In any practical circumstance the plane would not take off.
Update:
I have seen the error of my ways. My ‘simplification’ of the problem was incorrect because it is not the situation that actually occurs. I can justify it in my own mind best by imagining the converyor belt made of ice, very slippery, and the wheels as skids or something else with very little friction.
Now imagine the converyor belt going backwards (the plane is not running an engine), if it was made of tarmac the plane would move backwards fairly fast, pulled along by the tarmac. If it was ice the plane would move backwards less fast because their would be less friction between the skis and the ice. If the surface was totally frictionless the plane would not move backwards at all. It could then start its engines and take off as usual, pushing against the air which would be still.
Many thanks to David for pointing out my errors in the comments.
Update:
Someone in the comments points to this from Jef Raskin which explains the Coanda Effect in addition to the Bernoulli Principle.
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David · February 27, 2006 at 4:56 pm
plane which is going on a conveyor belt designed to keep it stationary in relation to the ground
Your analysis assumes that the conveyor belt keeps the plane stationary with respect to the ground. If you read the original statement of the problem, it says: “The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction).”
This does not mean that the plane would be stationary. Imagine a bullet travelling from left to right at 2000 mph. Now, imagine a conveyor belt a few feet below it, travelling right to left at 2000 mph. This wouldn’t make the bullet stop.
Now imagine the bullet is a jet plane (that took off somewhere else and is just skimming a few feet over the belt). Same story. Now imagine the jet lowers it’s landing gear so that the wheels touch the conveyor belt. The wheels will spin like crazy (in the idealized, no friction scenario, they won’t burn up), but the jet will still be travelling at 2000 mph from left to right.
-David
Admin comment by splasho · February 28, 2006 at 3:32 pm
David,
You are exceedingly correct, I have updated post accordingly, thank you!
I do feel foolish!!
Anonymous · March 1, 2006 at 12:59 pm
you are wrong about what makes a plane fly. If it depended on the Bernoulli Principle then planes could not fly upside down.
see http://jef.raskincenter.org/published/coanda_effect.html
Admin comment by splasho · March 1, 2006 at 2:25 pm
Thanks Anonymous you are right. It does come to the same thing in that airspeed relative to the wings is what creates lift but nevertheless you are correct, and I did realise there was a little more at wok. But thank you for a very interesting link which broadened my knowledge