# Constant-Speed Props

## Constant-speed props are usually found on high-performance aircraft, but most people don't realize the concept of varying the pitch of a propeller was suggested 30 years before the Wright Brothers flew. John Ruley discusses the history, repair, and alternatives to this staple of aviation.

0

Editor’s Note: This article originally appeared in Cessna Owner and Pipers Magazines.

“It was apparent that a propeller was simply an aeroplane travelling in a spiral course. As we could calculate the effect of an aeroplane travelling in a straight course, why should we not be able to calculate the effect of one travelling in a spiral course? At first glance this does not appear difficult but on further consideration it is hard to find even a point from which to make a start… The thrust depends upon the speed and the angle at which the blade strikes the air; the angle at which the blade strikes the air depends upon the speed at which the propeller is turning, the speed the machine is travelling forward and the speed at which the air is slipping backward… When any one of these changes, it changes all the rest, as they are all interdependent upon one another.” Orville Wright

Of all the problems the Wright Brothers faced in designing the first successful powered aircraft, they considered the propeller the most difficult. Amazingly, they weren’t the first to face up to the complex relationship between blade angle, speed of rotation, speed through the air and thrust — as early as 1871, a Frenchman named J. Croce-Spinelli suggested that a variable-pitch propeller could improve overall efficiency. He also suggested varying the pitch using hydraulic pressure, and that the ability to change pitch would be most important on takeoff.Croce-Spinelli was right, as you can see in the graph below, which shows propeller efficiency as a function of speed. Fixed-pitch propellers, which remain common on lower-performance airplanes, can be optimized either for best takeoff and climb performance (referred to as a “climb prop”), or for best speed (a “cruise prop”), but not both. A “constant speed” prop, by contrast, can be adjusted for maximum efficiency regardless of speed or engine power.

## Variable Pitch Or Constant Speed?

While early variable-pitch props (the first actually used in flight dates from 1917 at Britain’s Royal Aircraft Factory) were mechanically actuated, increasing performance (and resulting wear on complex mechanical linkages) led to the development of electrically and hydraulically adjustable props before World War II. The term “constant speed” refers to a refinement in which the pilot, rather than directly adjusting the blade angle, sets a rotational speed at which he wants the prop to operate; a “governor” operates to maintain this speed.In modern constant-speed props, this is done hydraulically (kudos to M. Croce-Spinelli!) using engine oil. The diagram below shows a cross-section of a typical constant-speed prop. Engine oil pressure against a piston in the hub causes the blades to move back from flat pitch. The pressure is regulated by a propeller governor (not shown). When the prop reaches the desired speed, the governor opens a valve, reducing oil pressure to the piston. With proper adjustment, this system will automatically adjust blade angle to maintain the speed selected by the pilot.

 Constant-Speed Propeller Mechanism (click for larger view)