Propellers and Engine Controls
Core concepts
Understand propeller theory: blade angle, pitch, angle of attack, slip, and the difference between fixed-pitch and constant-speed propellers. Learn how a governor controls RPM by varying blade angle, and how manifold pressure and RPM are managed together. Know the terms: feathering, reverse thrust, beta range, and propeller synchronization. Study engine controls such as throttle, mixture, propeller control, and condition levers. Understand how a propeller governor senses RPM and adjusts oil pressure to change blade angle. Know the purpose of propeller balance and tracking.
Traps to expect
Questions often test the relationship between blade angle and RPM: increasing blade angle increases load and reduces RPM, while decreasing blade angle increases RPM. Do not confuse manifold pressure with RPM; in a constant-speed propeller, throttle controls manifold pressure and the propeller control sets RPM. Watch for feathering (blades edge-on to reduce drag) versus reverse (blades angled to produce backward thrust). Also know that high blade angle at low RPM can cause detonation if overboosted.
Worked example
A propeller with a 72-inch diameter has a radius of 36 inches. If the tip speed is limited, increasing diameter increases tip speed proportionally. Rule of thumb: tip speed is proportional to diameter times RPM, so larger propellers must turn slower to avoid excessive tip speed and noise.
Study habit
Practice describing the cause-and-effect chain: control input → governor or fuel control response → blade angle or fuel flow change → RPM or manifold pressure change.