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BCF related

Dinogeorge@aol.com wrote:

>> Airfoils must be asymmetric, because it is the difference between the
airflow over the top and bottom surfaces that provides the lift; a   
symmetric wing would have the same kind of airflow over top and bottom   
and thus generate near-zero lift.<<

To which James Cunningham replied (in part):

> This is false.<

I interpreted George as simply saying that an airfoil, by definition, is   
asymmetrical in cross section.

> Competition aerobatic airplanes have perfectly symmetrical wings so   
that they can fly equally well rightside up or upside down.<

Flight control in stunt planes is achieved almost exclusively by using   
flaps to deflect the airstream in various directions, thereby pushing the   
aircraft in various directions. Even level flight in such planes is   
achieved with flaps partially down. This is necessary because a plank or   
any type of symmetrical wing generates no lift at a zero angle of attack.   
 This type of flight creates a great deal of drag and is extremely   
inefficient--as demonstrated by the substantially larger engine   
requirements in stunt planes. In fact, an asymmetric wing would be a   
distinct _disadvantage_ in an up-side-down attitude since the pilot would   
need to use flaps to compensate for the downward force created by the   
Bernoulli effect.

But an asymmetric wing design is the _only_ way to get aerodynamic lift   
at a zero angle of attack. The advantage of an asymmetric wing moving   
through an airstream at a zero angle of attack is that it minimizes drag   
while providing lift. This is energetically the most efficient mode of   
flight and it seems reasonable that such a design would offer some   
adaptive advantages during the evolution of flight.