Rotor/Wing lift

I know It's picky but the high pressure under a wing dosen't push it into the
air, the low pressure on top pulls it up.

Buck

On Wed, 01 Dec 1999 02:54:26 PST Geoffery R <geofferyr@hotmail.com> wrote:

> I know it's picky, but the high pressure under a wing dosen't push it

Wrong! The pressure on the upper surface of a wing is low, but still
positive, so it actually pushes _downwards_. The pressure on the lower
surface is greater, so there is an imbalance between the low pressure on the
upper surface and the higher pressure on the underside, and this is what
produces the lift. Or, alternatively, you can consider that lift is an equal
and opposite reaction to the force produced by the wing which deflects the
flow downwards as it passes over the wing (downwash).

Getting back to the pressure explanation, confusion sometimes arises because
areas of a wing surface on which the local static pressure is
less than the ambient free-stream value (most of the surface) are
sometimes referred to as "suction regions". This is misleading, because
true suction (negative pressure -- a pull on a surface rather than a
push) cannot occur on a wing surface. However, the aerofoil _acts_ as
though there is a suction on those areas due to the imbalance of pressure
between the surface and the corresponding points on the other
side of the section -- but, in that case, we have to realise that there
is a "suction" on both the upper _and_ lower surfaces of a wing; it's
just that the "suction" on the upper surface is more powerful that that on the
lower surface, and again there is this imbalance that produces lift.

Sorry to go on about this, but I spend a lot of time trying to get my students
to understand what's really going on in this case. I won't even mention the
joys of explaining separation and transition.... <g>

Phil