Aircraft Propellers: Constant Speed, Feathering, and Critical Engine

The propeller converts engine brake horsepower into thrust.

Propeller Types

• Fixed Pitch: Blade angle is set by manufacture. Compromise between climb and cruise performance.
• Variable Pitch (Constant Speed): The blade angle can be changed in flight to maintain a selected RPM. This allows the engine to operate efficiently across different airspeeds.

Constant Speed Unit (CSU)

A Constant Speed Propeller uses a governor (CSU) to automatically adjust blade pitch to maintain the RPM selected by the pilot.

• Cockpit Control: Blue lever.
  • Forward: Fine Pitch / High RPM (Take-off, Landing).
  • Aft: Coarse Pitch / Low RPM (Cruise).
• Operation:
  • If airspeed increases or throttle is added -> RPM tends to rise -> CSU increases pitch (coarser) to increase drag and maintain RPM.
  • If airspeed decreases or throttle is reduced -> RPM tends to fall -> CSU decreases pitch (finer) to maintain RPM.

Operational Procedures

• Take-off / Landing: Propeller lever Full Forward (Fine pitch). This allows the engine to reach maximum RPM and power.
• Cruise: Propeller lever set to a lower RPM (Coarser pitch) for fuel efficiency and engine longevity.
• Pre-flight Check: Cycle the propeller lever.
  • Action: Move lever from High to Low RPM and back.
  • Indication: RPM should drop and recover. Manifold pressure (MAP) may rise slightly. Oil pressure should remain stable.
  • Purpose: Check governor function and circulate warm oil into the hub.

Feathering

Featured on multi-engine aircraft.

• Feathered Position: Blades turned approx. 90° to airflow (edge-on).
• Purpose: Minimize drag when an engine fails and stops. Prevents "windmilling" which causes immense drag and can damage the engine.
• Control: Pulling the prop lever fully aft (past a detent) feathers the prop.
• Unfeathering: Moving lever forward returns blades to fine pitch to allow windmilling for restart.

Reduction Gearing

Many engines use a reduction gearbox between the crankshaft and the propeller.

• Purpose: To keep propeller tip speeds below the speed of sound (efficient operation and noise reduction) while allowing the engine to run at higher, more efficient RPM.

Critical Engine (Multi-Engine)

The Critical Engine is the one whose failure results in the most adverse effects on handling (yaw).

• P-Factor: The descending blade produces more thrust than the ascending blade.
• Clockwise Rotating Propellers:
  • Right Engine: Thrust line is further from the CG (Longer arm).
  • Left Engine: Thrust line is closer to the CG (Shorter arm).
• Critical Scenario: If the Left Engine fails, the Right engine (with the longer arm) produces a strong yawing moment. Therefore, the Left Engine is the Critical Engine.