Piston Engines and Propellers
Piston engines and propellers tests the four-stroke cycle, mixture and ignition, engine cooling, and how propeller pitch and RPM are controlled.
Engine operation
- Four strokes: induction, compression, power, exhaust. Light aircraft engines are usually air-cooled horizontally opposed types, direct-drive or geared.
- Mixture is rich for take-off and high power, and leaned for cruise and high density altitude, within POH limits.
- Detonation is uncontrolled rapid combustion: caused by low octane, over-leaning, high manifold pressure with low RPM, or high inlet temperature. Pre-ignition is ignition by a hot spot before the spark; it can destroy an engine very quickly.
- Two independent magnetos provide ignition; the engine runs on one, so a dead magneto is not a no-go item to ignore.
- Oil lubricates, cools, seals and cleans; cowl flaps and fins control cylinder temperature.
Propellers
- Fixed pitch: throttle sets RPM. Constant speed: the governor sets blade angle to hold RPM, and throttle sets manifold pressure.
- Increasing pitch increases angle of attack and engine load; decreasing pitch lets RPM rise. Feather for shutdown, reverse only where approved.
- Asymmetric blade effect (P-factor) is strongest at high angle of attack; gyroscopic precession and slipstream also affect handling.
Worked example
Geometric pitch 70 in, 2,400 RPM, true airspeed 120 kt. Theoretical advance = 70 × 2,400 = 168,000 in/min = 14,000 ft/min. One knot is 101.3 ft/min, so 14,000/101.3 = 138 kt. Slip = 138 − 120 = 18 kt, or 13% (18/138). Coarser pitch would reduce slip but increase engine load.
Traps
- Confusing detonation with pre-ignition.
- Ignoring a magneto drop because the engine still runs.
- Using RPM to control a constant-speed propeller - the governor does that.
- Operating over-square without POH approval.