|
In a message dated 2/1/2007 1:55:55 PM Eastern Standard Time, elippse@sbcglobal.net writes:
Since lift is a force due to a pressure difference across a surface, it may come as a shock to most of you that the pressure difference is across the top skin and the bottom skin. That's right, Ladies and germs, lift on a hollow wing is due to the air inside pushing up on the top skin and pushing down on the bottom skin, but the top skin wins, hopefully!
This is all just semantics. As long as the wing structure is built strong enough to prevent excessive deflection of the wing skin and to prevent it from departing the vehicle, what's inside is irrelevent to the calculation of lift.
Gas (air) pressure is assumed to act equally in all directions. Since the plan form from above is equal to the plan form from below, the summed lift forces from the internal gas will always be zero. Thus, once the airfoil geometry has become static, internal air pressure has no net effect on lift even if the wing is a baloon.
If the interior of the wing were vented to its upper suface at the point of minimum pressure, then the interior pressure would always be at or below the exterior pressure anywhere around the wing. The upper wing skin would then generate negative lift and the top skin would have to lose to the bottom skin in order to fly. It still wouldn't make any difference to the lift that supports the fuselage.
With regard to lift, all that is important is the distributed pressure differential between the top and bottom exterior surfaces.
Rob
|