Propellers and Propeller Control Systems
Domain 5 tests propeller geometry, control, and inspection.
Geometry and forces
- Blade angle is measured from the plane of rotation; angle of attack is blade angle minus relative wind angle.
- Geometric pitch is theoretical advance per revolution; effective pitch is actual advance; slip is the difference.
- Centrifugal twisting moment tends to flatten blade angle; aerodynamic twisting moment varies with lift.
- Constant-speed propellers use a governor that senses rpm and ports oil to change blade angle.
Control and limits
- Feathering aligns blades with airflow to reduce drag; reversing changes blade angle past flat pitch.
- Overspeed, underspeed, and governor failure have distinct indications.
- Propeller tracking, balance, and damage limits are inspection items; a prop strike requires engine inspection.
Worked example
A propeller has 72 in geometric pitch and turns 2400 rpm. Theoretical speed = (72 × 2400) ÷ 1056 = 163.6 mph. If actual speed is 150 mph, slip = 13.6 mph, or 8.3%. The reasoning: the 1056 constant converts inch-pounds per minute to miles per hour.
Traps
Believing the governor controls manifold pressure; using static rpm as cruise rpm; ignoring slip direction; treating a nicked blade as cosmetic; adjusting propeller balance without tracking.