Re: Rotor/Wing lift

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 
into the air, the low pressure on top pulls it up. <

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
------------------------------------------
(Dr) P.A. Atcliffe
Senior Lecturer
Faculty of Engineering
University of the West of England, Bristol
Phone: +44 (0)117 344 2496
Fax:   +44 (0)117 976 3873
Email: Phillip.Atcliffe@uwe.ac.uk