Propeller

Fixed-pitch propeller

• One blade angle (pitch), not adjustable in flight.
• Designer must choose between "climb" and "cruise" optimisation — usually a compromise.
• RPM rises with speed/altitude (less load); drops in climb (more load).
• Common trainers: C152, C172 (standard), PA-28 Cherokee.

Variable pitch / Constant-Speed Unit (CSU)

A governor automatically adjusts blade angle so the pilot-selected RPM is held constant.

Advantage of a variable-pitch propeller (in-flight adjustable)

An in-flight adjustable propeller achieves best propeller efficiency in all flight situations — take-off, climb, cruise, descent. Reason:

• The blade angle is optimised in every flight phase — e.g. fine on take-off, coarse in cruise.
• A fixed-pitch propeller is only optimal in a narrow band — either climb or cruise, not both.
• → CSU or variable-pitch propellers deliver consistently maximum efficiency across the whole flight regime.

Power management with CSU — order

If the flight manual (POH) provides no specific guidance, the standard order to change power on a CSU-equipped aircraft is:

→ Mnemonic: "First reduce the lever that goes toward damage; first advance the lever that goes away.".

Reason: at low RPM full throttle (high MAP) would cause the engine to run too high MAP at too low RPM → excessive cylinder pressure → detonation and damage.

Tachometer markings

The RPM indicator has standard markings, analogous to the airspeed indicator:

→ The caution range of engine RPM is marked by a yellow arc — the pilot operates in this range only in still air, because in turbulence structural stress or vibration may be critical.

RPM-indicator drive (tachometer drive)

The RPM indicator can be driven by a flexible shaft — a flexible cable that connects mechanically from engine to cockpit instrument:

• On simple fixed-pitch aircraft this is the standard construction (mechanical, robust, cheap).
• In modern aircraft RPM is often measured electrically or via a frequency sensor (magneto pulse or Hall sensor).
• → On flexible-shaft failure the tachometer no longer reads — see Subject 060 lesson "Cruise" for the response.

Propeller effects (relevant for Subject 080)

• Torque reaction — Newton III: engine turns prop → fuselage tries to turn opposite → roll tendency.
• P-factor / asymmetric thrust — at high AoA/low speed the descending blade produces more thrust → yaw tendency.
• Spiral slipstream — helical airflow behind the propeller strikes the vertical stabiliser → yaw tendency.
• Gyroscopic precession — on attitude change a 90°-offset reaction acts due to gyroscopic effect of the rotating prop.

All four effects produce typically a left tendency in climb for US/EU engines → counter with right rudder.

Sudden RPM rise on CSU — pilot reaction

If on a constant-speed propeller the RPM suddenly rises far beyond the permitted range (e.g. governor failure), the correct immediate reaction is:

• Pull back the power lever (throttle) until RPM returns to the green range.
• This protects the engine from structural damage by overspeed.

Critical engine

For multi-engine: the engine whose failure has the greatest negative effect on performance and control. On twin-engine aircraft with co-rotating propellers (both right-turning): the left engine is critical, because its P-factor contributes more to asymmetry. With counter-rotating props: no critical engine.