# planes shouldn't fly then!!

Discussion in 'Physics & Math' started by scifes, May 19, 2009.

1. ### leopoldValued Senior Member

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17,455
oops, heh heh.

3. ### Blender3d777Registered Member

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19
yeah, but thats why they dont use boat propellers on planes.

5. ### OliHeute der Enteteich...Registered Senior Member

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11,888
Look at the speeds of rotation.
That's what rpm means.
A fan rotates much more slowly than a propeller.
Plus, of course, the fact that water, being a denser medium, requires fewer rpm than an air screw.

7. ### Blender3d777Registered Member

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I know what rpms are...but the speed has nothing to do with it, If your talking about newtons law, that means the more air you move backwards the object would move forward by that much...and the big industrial fans that we use move a lot of air...but they dont go anywhere.

And the design is wrong...my Point was that if you put a ship propeller on an airplane it would not move, or very little. Thats why airplane propellers are shaped the way they are, to take advantage of that low air pressure zone created in front of them

8. ### D HSome other guyValued Senior Member

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2,257
The problem with using Bernoulli is misusing it. The problem with the invoking Newton's second law is that it doesn't say how a wing makes the airflow turn. Both explanations have their place.

This, on the other hand, is nonsense:
You are confusing conservation of vorticity with conservation of vortexes (whatever that is).

The nice thing about the Newtonian explanation is that it nicely shoots down false explanations of lift. Ignoring rockets and balloons, which generate lift by completely different mechanisms than do aerosurfaces, a lift-generating aerosurface must generate a downward in order to create lift. Airplanes do indeed leave a mass of down-flowing air behind them.

A thousand words on this subject:

9. ### tsmidRegistered Senior Member

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368
No, it shoudn't produce a net lift at a zero angle of attack, as the area of the leading half deflecting the air upwards (and thus pushing the airfoil down) is equal to the area of the trailing half deflecting the air downwards (and thus pushing the airfoil up). So it would produce merely a torque.

See my pages regarding Bernoulli's Principle and Airplane Aerodynamics and Aerodynamic Drag and Lift and d'Alembert's Paradox for more in this respect.

Thomas