Propeller Systems
Blade angle and twist, constant-speed governing, fine/coarse/feather/reverse, pitch-change mechanisms and pitch locks, synchronising and synchrophasing, electrical de-icing and propeller indication.
Propeller systems, summary notes
Free- Sub-section 14.3 covers propeller systems, examined at the higher knowledge level for B2 because of their electrical control, indication and protection.
- A propeller converts engine torque into thrust. Blade angle (pitch) is measured between the blade chord and the plane of rotation. Because the blade section moves faster at the tip than at the root, blades are twisted so the angle of attack stays efficient along the span.
- Fixed-pitch propellers are efficient at only one condition. Constant-speed propellers use a governor to vary blade pitch so the engine holds a selected RPM: as load or airspeed changes, the governor increases pitch (coarse) to prevent overspeed or decreases pitch (fine) to prevent underspeed.
- Blade angle positions: fine or low pitch for take-off and high power, coarse or high pitch for cruise, feathered (blades aligned with the airflow) after a failure to minimise drag and stop windmilling, and reverse (negative pitch) for braking after landing on types so equipped.
- Pitch change is normally hydraulic, using engine oil boosted by the propeller control unit acting on a piston in the hub, opposed by counterweights and springs; electrically actuated pitch change is used on some types. Feathering is commanded by the flight crew or automatically on some installations, and unfeathering uses an accumulator or an unfeathering pump.
- Protections and interfaces a B2 engineer must know: overspeed governors, pitch locks to prevent inadvertent fine pitch or reverse in flight, synchronising (matching RPM) and synchrophasing (matching blade position between engines to reduce noise and vibration), electrical de-icing of blades using cyclic heater elements through slip rings and brushes, and indication of propeller RPM, blade angle and out-of-balance to the flight deck.
- Maintenance and safety: propellers are balanced statically and dynamically, blade damage is assessed against the manual limits, and the danger area around a propeller must be respected with the ignition and start systems made safe before any work.
- Blade angle
- Angle between the blade chord line and the plane of rotation
- Constant speed
- Governor varies pitch to hold selected RPM (coarse to slow, fine to speed up)
- Pitch positions
- Fine (take-off) · Coarse (cruise) · Feather (failure) · Reverse (braking)
- Synchrophasing
- Matches blade angular position between engines to cut noise and vibration
An engine fails in flight on a twin turboprop. Why is the propeller feathered?
Feathering turns the blades edge-on to the airflow, which minimises drag and stops the propeller windmilling. That reduces asymmetric drag and yaw, improves single-engine performance, and prevents further damage being driven through a failed engine.
How does a constant-speed unit respond when the aircraft accelerates in level flight?
The propeller tends to overspeed, so the governor senses the RPM rise and increases blade angle to a coarser pitch. The extra aerodynamic load restores the selected RPM, so the engine continues to run at the commanded speed while the propeller absorbs more power.
Why are propeller de-icing heater elements supplied through slip rings?
The blades rotate while the electrical supply is fixed to the airframe, so slip rings and brushes carry the current to the rotating spinner and blade elements. Supply is normally cycled between blade groups so the available power sheds ice progressively rather than heating everything at once.
Propeller systems quiz
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