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How should a power increase be executed on a constant-speed propeller, provided that no other procedure is described in the flight manual?

  • A

    Decrease manifold pressure; Increase RPM

  • B

    Decrease RPM; Increase manifold pressure

  • C

    Increase manifold pressure; Increase RPM

  • D

    Increase RPM; Increase manifold pressure

A Constant Speed propeller is a common type of adjustable pitch propeller in which the pitch of the blades is constantly adjusted by a governor to keep a steady RPM speed. Even if the load on the engine varies, the governor ensures that the RPMs stay the same. Aeroplanes with a Constant Speed propeller have two controls, the Throttle and the Propeller Control.

The Throttle controls engine power output, while the Propeller Control sets the propeller RPM.

Once the RPMs are set, the propeller governor automatically adjusts the propeller blade pitch to maintain the selected RPM.

Once set and in cruise, a decrease in propeller load or an increase in airspeed will cause the blade angle to increase, making the blades “bite” more air, slowing down rotation. An increase in propeller load or a decrease in airspeed will cause the blade angle to decrease, taking smaller “bites” of air, and increasing propeller speed.

Constant Speed Propeller equipped aeroplanes have Manifold Pressure gauges which indicate the power output of the engine and is controlled by the Throttle. The Manifold Pressure gauge measures the Manifold Absolute Pressure, or the absolute pressure of the fuel/air charge inside the intake manifold. The operating range of the engine is indicated on the Manifold Pressure gauge by a green arc, with a red line indicating maximum pressure. Operating beyond this limit can put stress on the engine cylinders and lead to a catastrophic engine failure.

When both RPM and Manifold Pressure need to be adjusted, it is essential to modify their settings in the correct order.

If the power needs to be decreased, reduce Manifold Pressure before reducing the RPM. Reducing RPM first could make the Manifold Pressure automatically increase above redline, causing damage.

If the power needs to be increased, the opposite order must be followed, increase RPM and then Manifold Pressure.

The governor is operated hydraulically by engine oil pressure, which is directed through a hollow crankshaft and into the propeller. The oil pushes on a piston, which is connected to a lever, and then the propeller blades, adjusting the pitch. It is not recommended to run the engine at low RPM at high Manifold Pressure. Doing so greatly increases stress on the engine, and could lead to a situation where detonation could occur, which results in engine damage.

A power increase on a constant-speed propeller, provided that no other procedure is described in the flight manual, must be executed first by increasing RPM and after by increasing Manifold Pressure.

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