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training airplane we also learned that the propeller speed would vary even when the throttle position was not changed: Pull up into a climb and the prop slows down; nose over into a dive and the prop speeds up. Technically, this is because the angle-of-attack of the fixed pitch propeller blades is increasing as airspeed decreases and vice versa. Non-technically, it’s the same effect as when the toy pinwheel held out of Dad’s car window rotates faster the faster the car goes. “Windmilling effect” is the name assigned to this phenomenon. The higher the airspeed, the more windmilling effect the propeller experiences.
When adjustable-pitch or variable-pitch propellers made their appearance they were not initially constant speed propellers. Yes, the pilot now had the ability to change the propeller blade angle and hence
the propeller’s angle of attack but no governor was installed that did this automatically. As power and airspeed increased, it was necessary for the pilot to move the propeller control (sometimes a lever, often times an electrical switch) to make the propeller’s angle-of-attack (“bite of air” in layman’s terms) increase, providing more resistance to rotation and hence keeping the RPM at the desired amount.
The addition of a Propeller Governor – the device that could change the propeller’s bite of air automatically – converted the variable-pitch propeller into the constant speed propeller. You realize this name is a lie, right?! The propeller can only keep the selected speed constant when sufficient power and airspeed exist to raise the speed of the propeller up to the desired RPM. Reduce
power and slow the airspeed down and the propeller will eventually slow down also. Vice versa – go into a full-power, redline airspeed dive and there is the possibility the propeller will speed past the selected speed. Why? Because the propeller designers pick low pitch and high pitch limits of blade angle travel. The low pitch limit is selected with the goal of preventing excessive propeller drag. The airplane would “fall out of the sky” in the flare for landing if the low pitch blade angle limit were set too close to flat pitch. Vice versa, why have the high pitch limit set above the value needed to get enough rotational resistance to prevent exceeding maximum propeller speed in a reasonably high-powered, high airspeed dive?
When variable pitch propellers began being installed on multi-engine airplanes it became immediately
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APRIL 2021
KING AIR MAGAZINE • 21