As I recall someone pointed out that airplanes go faster than
their theoretical prop pitch indicates. I think it was Tracy but cannot
find the message. I gave a too vague answer and my mind just kicked in
(perhaps a month later) with a  better answer. The answer is the
lift coefficient for a Clark Y airfoil is about .3 at zero angle of attack
due to camber of the blade. So at less than zero angle of attack, down to 
minus five degrees, the blade is still generating thrust.

Paul Lamar
 
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