Constant Speed Governor Operation
Propeller Reference

The constant speed governor is based on standard centrifugal governor principles. The earliest application of this concept was made on a steam engine in 1788, by James Watt, the English inventor. The aircraft constant speed propeller governor is used to direct oil flow to and/or from the prop, thereby modulating blade angle (B.A.)

• A governor is an RPM sensing device
• It responds to changes in RPM by directing oil pressure to or from the propeller until the preset RPM is restored - governing RPM
• Basic configuration
  • Drive shaft - connected to engine; runs 80% to 110% of engine RPM, depending on gearing
  • Oil pump drive gear located on drive shaft, and meshes with driven gear (operates exactly like engine oil pump, is a gear type pump). It boosts engine oil pressure to approximately 270-310 psi.
  • Pressure relief valve - bypasses any unused oil to governor oil inlet.
  • Pilot valve (also called pilot spool) - located in the center of the drive shaft, directs oil through ports in the drive shaft, sending oil to or from the prop
  • Fly weights - adjust the position of the pilot valve. They are "L" shaped. During rotation centrifugal force exerts an outward force on the flyweights. As the fly weights tip out, the pilot valve is raised.
  • Speeder spring - located on top of the fly weights. It opposes the centrifugal force exerted on the flyweights.
• "On-speed condition"
  Fly weight and spring force are matched, and the pilot valve remains in the neutral position. Negligable oil pres. moves to or from the prop
• "Over-speed condition"
  • Engine RPM is higher than governing RPM, so the centrifugal force acting on the fly weights is stronger than the speeder spring force, and the fly weights tip out, raising the pilot valve.
  • Oil flows, either to the prop or from the prop as necessary to increase B.A. until engine RPM matches governing RPM
• "Under-speed condition"
  • Engine RPM is below governing RPM. Centrifugal foce, acting on fly weights, decreases and speeder spring force causes the fly weights to tip in lowering the pilot valve.
  • Oil flows to the prop or from the prop as necessary. This decreases B.A. until engine RPM matches governing RPM
• By increasing or decreasing speeder spring pressure being exerted on the fly weights, we can set the RPM at which the forces will balance each other, and thus center the pilot valve. This is called "governing RPM"
• Governor manufacturers:
  • McCauley - McCauley props
  • Woodward - Hartzell props
  • Edo Aire - least popular, but appears on certain aircraft, i.e. Bellanca Super Viking
  • Hamilton Standard - Used with Hydromatic prop, also some Hamilton Standard Counterweight props
• System configurations:
  • Cockpit controls - Prop control, usually located between throttle and mixture control.
  • Linked to the governor by
    1. Push-pull cable - most popular
    2. Control cable - used on large and old A/C
    3. Push-pull rod / torque tube combinations (rare)
  • Governor locations
    1. Continental 6 cyl. - left front engine case just in front and below cyl. no. 6
    2. Lycoming 6 cyl. - left front engine case above and in front of cyl. no. 2
    3. Lycoming - some 4 @ 6 cyl. lower righ taccessory case at nose bearing location.
  • Oil from the governor is transferred to the prop through the center of the crankshaft