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I knew that post would start something! Let's take
three cases of a HOLLOW wing flying at sea
level, where the pressure is 2116 PSF. Case 1. Pressure in the wing, 2116
PSF, pressure on top of wing, 2076 PSF, pressure on the wings bottom, 2196 PSF.
The air inside the wing pushes up on the top surface at 40 PSF while it pushes
down on the bottom surface at 20 PSF. Net lift, 20 PSF up. Case 2. Same
pressures on the top and bottom of the wing, but the pressure in the wing is
reduced to 2076 PSF. No pressure differential across the top skin, so no force
on it, but there is 20 lb upward acting on the bottom skin. Net lift,
20PSF. Case 3. Same pressure on the outside, but 2196 PSF inside. No
differential across the bottom skin, but 20 PSF up across the top skin. Net
upward force, 20 PSF on the top skin! Sure, you can define the lift as the
pressure difference between the top and bottom surfaces, but that won't
tell you where the stresses are! If there was no upward force against the top
skin, it wouldn't be necessary to lace the fabric on the ribs of fabric-covered
wings. Look at the lift; it acts on the fabric, then passes to the ribs, and
from them to the spar, and then to the fuselage. Oh well! Old ideas die
hard!
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