How does a propeller produce thrust and what affects its efficiency?

A propeller is a rotating airfoil that accelerates air rearward, producing thrust by the momentum change. Each blade section operates at an angle of attack determined by blade pitch, rotational speed, and forward velocity. Efficiency factors include: advance ratio (V/nD), blade design (airfoil, twist, taper), number of blades, tip speed (limited by compressibility), and propeller diameter. Maximum propeller efficiency (~90%) occurs at optimal advance ratio. Variable-pitch propellers maintain efficiency across speeds. Propellers are most efficient below 450 knots; higher speeds favor turbofans. Modern turboprops achieve excellent fuel efficiency for regional aircraft.