Constant-Speed Propeller Systems by Derek W Beck June 2008

Constant Speed Propeller Adjusting propeller pitch allows for better efficiency (more engine power converted into thrust) in a variety of flight attitudes (climb, cruise, etc) On the C172RG, the range is 12.0º to 26.5º Greater pitch demands greater torque from the engine, thus for a given power setting, the result is lower RPM’s Source: FAA-H-8083-3A Airplane Flying Handbook p. 11-4

Propeller Control Pitch is controlled by the (blue) pitch control lever

Increasing Pitch Oil is supplied through the propeller piston to a piston head, which is pushed down due to the oil pressure, pushing it against a spring and holding it into a high-pitch position Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html

Maintaining Pitch The pressure is maintained in the system to hold the pitch on the propeller Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html

Decreasing Pitch Oil is allowed to drain from the system, allowing the spring to “win” against the oil, forcing the pitch to high pitch Spring Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html

The Governor Source: http://flash.aopa.org/asf/engine_prop/swf/flash.cfm?

The Governor – Controlling Pitch Governor controls the oil pressure to the propeller (thus, controlling pitch) Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html

Control of the Governor The propeller control lever tightens a threaded shaft that in turn tightens the speeder spring, which increases resistance against spinning flyweights, which are mechanically connected directly to the engine… To Prop Lever Speeder Spring Flyweights Prop Lever Forward Prop Lever Back Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html

Governor Underspeed When engine RPM is slower than the propeller control setting, the speeder spring holds the pilot valve down and oil flows from the propeller… This is called “underspeed” To Prop Lever Speeder Spring Spinning Flyweights Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html

Governor On Speed As engine RPM increases, the tops of the weights are thrown outward by centrifugal force; the lower legs then pivot up, raising the pilot valve against the force of the speeder spring so no oil can flow to or from the propeller… To Prop Lever Speeder Spring Spinning Flyweights Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html

Governor Overspeed The faster the flyweights spin, the further out they are thrown, causing the pilot valve to be raised (against the spring) and allowing more oil to flow to the propeller… This is called “overspeed” To Prop Lever Speeder Spring Spinning Flyweights Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html

Putting It All Together Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html

On Speed On Speed condition results when the RPM is constant. Movement of the cockpit controls have set the speeder springs at the desired RPM. The flyweights have positioned the pilot valves to direct oil to or from the propellers. This, in turn, has positioned the propeller blades at a pitch that absorbs the engine power or RPM selected. When the moment of RPM balance occurs, the force of the flyweights equals the speeder spring load. This positions the pilot valves in the constant RPM position with no oil flowing to or from the propellers. Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html

Overspeed At constant speed, an Overspeed condition results and airspeed increases when the airplane begins a descent or engine power is increased. Since the pitch of the propeller blades is too low to absorb engine power, the engine RPM begins to increase. At the instant this happens, however, the flyweights move out and raise the pilot valves, causing oil to flow to the propeller, increasing the pitch of the blades in both cases. Engine speed then slows to the original RPM setting. Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html

Underspeed If the airplane begins to climb or engine power is decreased, an Underspeed condition results. Airspeed is reduced and, since the pitch of the propeller blades is too high, the engines begin to slow down. At the instant this happens, the flyweights will droop, causing the pilot valves to move down. Simultaneously, oil flows from the propeller, reducing the pitch of the blades. This automatically increases the speed of the engines to maintain the original RPM setting. Source: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html

Other Notes The governor boosts the oil pressure sent to the propeller by 3 to 6 times (e.g. to 300 PSI) The governor range sets the limit on maintaining a constant speed propeller Beyond these limits, throttle has a direct control on the RPM’s Within these limits, the propeller pitch adjusts for any given throttle setting to maintain the RPM’s In general, maintain propeller setting (RPM) higher than throttle setting (manifold pressure)

Technical (Backup)

Propeller Efficiency Efficiency (nu) (1.0 = 100%) is a function of the advanced ratio (J), which is dependent on velocity (V), propeller speed (n), and propeller diameter (D) Thus, nu = f(V/nD) Source: http://web.mit.edu/16.unified/www/SPRING/propulsion/UnifiedPropulsion7/UnifiedPropulsion7.htm