Posted for Kyrilian Dyer <kyrilian_av@yahoo.com>:

From the point of view of the whole airplane, the details of what loads the
top and bottom wing skins see is irrelevant.  Keep in mind that Paul was
responding to the original question, "Why does the inboard gear door get
pulled open during flight?"
 
 In engineering terms, one considers a 'control volume' that depends on what
problem you are trying to solve.  For the aerodynamicist, the difference in
pressure between the upper and lower surface of the top and bottom skins
individually is irrelevant, and his/her control volume is simply the external
surface over which the air flows.  But in the case where we are trying to
understand the loads that are being applied to the skin (including the gear
doors), one has to break the wing into appropriate physical parts based on the
details of the design.  For a solid propeller, one could argue that the
aerodynamic loads imparted push and pull on the surface, and thus try to
squish and pull it apart.  You would consider thin layers within the propeller
and figure out how the forces on the exterior try to push together or pull
these layers apart.  On a solid propeller I'd think these forces would be
minor and may not be considered, but they're still there.
 
 Regardless of the control volume you pick (the top surface, the volume
between and the bottom surface, or the whole thing as one), the total net
force will be the same.  In other words, if you have three pieces encompassing
the whole wing; if you change the air pressure of the internal cavity it will
influence the forces that the skins see, but there will be no net change in
lift that the wing produces.  If you fill the wing with fuel that has mass,
the weight will act on the top of the lower skin and will cause it (as well as
the whole wing) to see a greater downward force, equivalent to the weight of
the fuel.  Overpressurizing the wing tanks during leak testing may split your
wing as the skins see a lot of pressure from within, but the wing obviously
won't lift off your work bench or become heavier (by much).  The net force
over the whole wing is virtually unchanged.
 
 Similarly, if you have air being blown into the wing and pressurizing it a
bit, or if the external skin surface (or gear door) otherwise sees a pressure
lower than inside the wing, you'll have a greater problem keeping the gear
doors closed but you won't change the overall lift that the wing produces.
 
 Cheers,
 - Kyrilian
 
 H & J Johnson <hjjohnson@sasktel.net> wrote:
"""
 Or if your flying a a/c with a foam core wing?
 Interesting prespective.. what about propellers which are of solid
construction [not hollow]?
 JjJ
"""