Constant Speed Propeller Operation

Principle of Operation

A constant speed propeller uses a governor to sense propeller RPM and adjust blade pitch hydraulically. When the RPM rises above the selected value, the governor increases blade pitch (coarser), absorbing more engine power and reducing RPM. If RPM falls, the governor decreases pitch (finer), reducing load and increasing RPM. The pilot selects the desired RPM using the propeller control lever, which sets the governor spring tension.

Power Setting and MAP Indicator

With a constant speed propeller, engine power is set using manifold pressure (MAP) and RPM. The MAP gauge is essential because the throttle controls engine power, while the propeller lever sets RPM. Pilots must monitor both to avoid exceeding engine limitations, especially during high-power, low-RPM operations.

Operation at Different Airspeeds

As true airspeed (TAS) increases, the propeller's blade angle must increase to maintain a constant angle of attack and RPM. The governor automatically coarsens the pitch as speed increases and fines it as speed decreases. This ensures optimal efficiency over a broad speed range.

Propeller Overspeed

If the propeller RPM exceeds the selected value (overspeed), the governor responds by increasing blade pitch, absorbing more power and bringing RPM back to the set point. If the governor cannot correct the overspeed (e.g., due to mechanical failure), RPM may remain dangerously high.

Low-Pitch Stop (Centrifugal Latch)

The low-pitch stop prevents the blades from moving below a certain fine pitch during flight, protecting against excessive RPM. On some systems, a centrifugal latch holds the blades in fine pitch during engine shutdown to ensure safe engine restarts.

Single-Acting vs Double-Acting Systems

Single-acting propellers use oil pressure to move blades in one direction (usually towards coarse), with springs or counterweights returning them to fine. Double-acting systems use oil pressure to move blades both ways, offering more precise control, especially in multi-engine aircraft.

Pre-Flight and After-Start Checks

After engine start, pilots must check the constant speed propeller system by cycling the propeller control lever. This ensures oil flows through the hub and verifies proper pitch change response before flight.