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Old 12th May 2002 | 19:31
  #15 (permalink)  
John Farley

Do a Hover - it avoids G
 
Joined: Oct 1999
Posts: 2,201
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From: Chichester West Sussex UK
Oxford

Sorry if it seems to cut across your ideas, but Mr Bernoulli actually has no trouble with generating lift regardless of the wing cross section or any airframe related angle of incidence. It may help you to visualise why if you consider the task of carrying a sheet of 8x4 chipboard (or MDF if you are richer than some) from the B&Q door to your car in a strong wind. Edge on it is not very aerodynamic and easy to carry. At an angle of attack (AOA) of say 20 deg you might just find it leaves your sweaty grasp.

The reason that Mr Bernoulli’s explanation applies to this flat uncambered surface is thanks to something called the stagnation point (SP). The SP is that point (looking at a cross section of any wing) above which all air goes over the top surface and below which all air goes underneath.

If you are still bothering to read this, consider holding your board at a 90 deg AOA to the wind (no lift but plenty of drag) the SP is right in the middle of the board (this would be much easier with a pencil and paper) if you now rotate the board slowly back towards 0 deg AOA the SP moves slowly towards the leading edge (LE) but only gets there at 0 deg. At say 10 deg the SP is still a little way back from the LE (and of course on the undersurface of your board) and so the air that goes over the top actually has a longer journey than the air that goes underneath because it starts its journey from a little way back on the bottom surface.

Regards
John Farley is offline