How a Propeller Generates Thrust
A propeller blade is a rotating airfoil. As it spins, each section encounters a combination of rotational velocity (tangential) and forward flight velocity (axial). The resultant velocity vector sets the local angle of attack. The lift component of the aerodynamic force on each blade element has a forward component (thrust) and a tangential component (torque). Integrating these along the blade length gives total propeller performance.
The Advance Ratio
The advance ratio J = V/(nD) is the key dimensionless parameter. At J = 0 (static), the propeller produces maximum thrust but with low efficiency because all the energy goes into accelerating air from rest. As airspeed increases, J rises and efficiency improves until reaching a peak — typically around J = 0.7-0.9 for a well-designed blade. Beyond this, angle of attack drops and thrust collapses.
Blade Element Integration
The simulation uses a simplified blade element approach. Each radial station has a local velocity triangle determined by RPM, airspeed, and blade pitch angle. The lift coefficient is estimated from the local angle of attack, and thrust is integrated across the blade. Real propeller design also accounts for induced velocity (the air accelerated by the propeller itself) through momentum theory coupling.
Efficiency and Design Trade-Offs
Propeller efficiency peaks when the blade pitch matches the advance ratio so that every element operates near its optimal lift-to-drag angle of attack. Fixed-pitch propellers sacrifice takeoff thrust for cruise efficiency, or vice versa. Constant-speed propellers adjust pitch in flight, maintaining 80-90% efficiency across the flight envelope — a key reason turboprops remain competitive with jets for short-haul routes.