In aircraft with constant-speed propellers, a propeller control lever (prop lever) is available in the cockpit for the pilot. The pilot controls the propeller by varying its RPM by moving the prop levers. When the prop lever is moved forward, the RPM increases, and when it is pulled back, the propeller RPM decreases. A Pitch Change Mechanism (PCM) or also known as a Constant Speed Unit (CSU) then maintains the set RPM. Thus, a Constant Speed variable Pitch Propeller. The Pitch Change Mechanism (PCM) / Constant Speed Unit (CSU) uses oil pressure to either drive the propeller blade angle to a higher angle (coarse pitch) or a lower angle (fine pitch). The CSU is driven by the engines, and it can detect if the propeller is either in an over-speed or under-speed condition.

The Main Components of the PCM/CSU are:

• The Speeder Spring.
• Flyweights.
• Control Valve.

When the pilot moves the prop lever forwards or backward, he or she modifies the tension in the speeder spring. When it is moved backward, tension is lowered, and when moved forward, the tension is increased. The flyweights rotate with the engine, and it is the behavior of the flyweight that determines the positioning of the oil control valve. When the propeller is in an under-speed condition or if its blade angle is too high, the propeller RPM starts to decrease. This increases the propeller torque, and the speeder spring tension can overcome the flyweights and causes them to collapse. This causes the control valve to move down letting oil pass to the fine-pitch side of the propeller while the coarse side is connected to the oil return.

The RPM increases until the engine torque transferred to the speeder spring via the flyweights can no longer overcome the spring tension. At this point, the propeller starts spinning at the set RPM. When the propeller is in an over-speed condition, the blade angle becomes too low or too fine, and this causes RPM to increase. This causes the flyweights to spread due to the rise in centrifugal force.

This makes the control valve move up, letting oil pass to the coarse pitch side while the fine pitch is linked to the oil return causing the propeller blades to increase their blade angle. This increases the propeller torque, which exerts a force on the speeder spring and thus causes the flyweights to push down until the RPM reaches the pilot set value.

At MHD-ROCKLAND’s Northwest Propeller division we are ready to answer all your propeller related questions.

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