VARIABLE PITCH PROPELLERS
Full-Feathering vs. Constant Speed
A constant-speed (RPM) system permits the pilot to select the propeller and engine speed for any situation and automatically maintain that RPM under varying conditions of aircraft attitude and engine power. Thereby permitting operation of propeller and engine at most efficient RPMs. RPM is controlled by varying the pitch of the propeller blades - that is, the angle of the blades with relation to the plane of rotation. When the pilot increases power in flight, the blade angle is increased, the torque required to spin the propeller is increased and, for any given RPM setting, aircraft speed and torque on the engine will increase. For economy cruising, the pilot can throttle back to the desired manifold pressure for cruise conditions and decrease the pitch of the propeller, while maintaining the pilot-selected RPM.
A full-feathering propeller system is normally used only on twin-engine aircraft. If one of the engines fails in flight, the propeller on the idle engine can rotate or ³windmill,² causing increased drag. To prevent this, the propeller can be ³feathered² (turned to a very high pitch), with the blades almost parallel to the airstream. This eliminates asymmetric drag forces caused by windmilling when an engine is shut down. A propeller that can be pitched to this position is called a full-feathering propeller.
Changing Pitch
Pitch is changed hydraulically in a single-acting system, using engine oil controlled by the propeller governor to change the pitch of the propeller blades. In constant-speed systems, the pitch is increased with oil pressure. In full-feathering systems, the pitch is decreased with oil pressure. To prevent accidentally moving the propellers to the feathered position during powered flight, which would overload and damage an engine that is still running, the controls have detents at the low RPM (high pitch) end.
In a single-acting propeller system, oil pressure supplied by the governor, acting on the piston produces a force that is opposed by the natural centrifugal twisting moment of the blades in constant speed models or counterweights and large springs in full-feathering systems. To increase or decrease the pitch, high pressure oil is directed to the propeller, which moves the piston back. The motion of the piston is transmitted to the blades through actuating pins and links, moving the blades toward either high pitch for constant-speed systems or low pitch for full-feathering systems. (Figs. 1A & 1B)
When the opposing forces are equal, oil flow to the propeller stops and the piston also stops. The piston will remain in this position, maintaining the pitch of the blades until oil flow to or from the propeller is again established by the governor. (Figs. 2A & 2B) From this position, pitch is decreased for constant-speed systems or increased for full- feathering systems by allowing oil to flow out of the propeller and return to the engine sump. (Figs. 3A & 3B) When the governor initiates this procedure, hydraulic pressure is decreased and the piston moves forward, changing the pitch of the blades. The piston will continue to move forward until the opposing forces are once again equal. Mechanical stops are installed in the propeller to limit travel in both the high and low pitch directions.
