Sunrise Aviation

Last month, I advanced the argument for the variable pitch propeller--one capable of changing pitch angle to suit varying flight conditions. The first props that used this approach were quite simple--variable pitch props with two possible pitch settings: fine, or low pitch, for takeoff and climb; coarse, or high pitch, for cruise. The first setting provided high RPM for high horsepower production at low airspeeds and the second ensured good efficiency at cruising airspeed.
Although the two position prop has been replaced in current usage, it remains a generally effective solution to the problems of prop design. Its limitations stem from the fact that it has only two optimum airspeed and RPM combinations--a big improvement over fixed pitch props, but still restrictive to the pilot interested in a wide choice of operating speeds and fuel burn rates.
The next development remains the most common: the "constant speed" prop; one that is easily adjustable through a wide range of positions from the lowest possible pitch angle to the highest. Transition to this prop is an important part of "complex" airplane checkouts, giving pilots an additional mechanism to deal with and a host of possible errors to avoid.

THE MECHANISM
Before tackling the techniques of flying with a constant speed prop, the mechanism itself requires some understanding. Unlike its fixed pitch cousin--a single cast piece of aluminum--the constant speed version consists of three major components: the BLADES, the HUB, and the PROP GOVERNOR.

The BLADES are movable and may be twisted freely between two extremes: high pitch and low pitch, corresponding to low and high RPM respectively.

The HUB, normally concealed by the prop spinner, contains the mechanism for changing the angle of the blades. Inside is a hydraulic piston attached by mechanical links to the blades; oil pressure is applied to this piston through a passage drilled down the center of the crankshaft; when changes in oil pressure cause the piston to move, the prop blades are twisted to higher and lower pitch angles.

When the airplane is at rest, a spring in the hub assembly helps to hold the blades firmly at one of their two extremes--in single engine airplanes, this is typically LOW pitch, so that when you walk out on the ramp before engine start, the prop blades should be at their lowest, least angled, pitch setting.

It is common to refer to this blade angle as the “default position.” In flight, centrifugal force and air resistance work to push the blades toward default; oil pressure on the piston works in the opposite direction.

Next comes the GOVERNOR, which serves as the interface between pilot and prop.

The governor is a combination high pressure oil pump and hydraulic valve. When a command is sent to change RPM, the governor alters oil pressure to the hub, causing the hydraulic piston there to move and change prop blade angle.
As an example, if the command is for lower RPM, the governor opens its valve to allow increased oil pressure to act on the hub piston, twisting the prop blades to higher pitch angles so that they take a larger “bite.” This results in an increased load on the engine that immediately lowers RPM.

Conversely, if the command is for higher engine speed, the governor reduces hydraulic pressure to the hub, with the result that the blades naturally twist closer to their default setting. This reduction in blade angle lowers the load on the engine, and RPM increases.

In either case, the governor then works to maintain the chosen engine speed. Because it is engine driven, it immediately senses RPM changes and can respond by altering oil pressure to the hub, thereby twisting more or less pitch angle into the prop blades so as to keep engine speed constant. If RPM tends to increase, perhaps because of an increase in power, the governor responds by increasing prop blade angle to increase the load on the engine and prevent the increase. Conversely, if RPM starts to lessen, prop blade angle is lowered, thus reducing engine load to maintain the original RPM.

ENGINE START & TAKEOFF
At its top end, the governor is pre-set to limit RPM to the redline limit established by the engine manufacturer; whenever the prop control in the cabin is pushed fully forward, the governor attempts to achieve this RPM. Directly after engine start and during taxi, the low power settings are not enough to lift RPM anywhere close to the redline limit, so the governor has no cause to act: the blades remain at their default--low pitch--position, and for all intents and purposes prop pitch is “fixed.”

We’ll pick it up at that point next time.