Speeder spring Pilot valve Counterweight Counterweight Pilot control Speeder spring Pilot valve Pilot control Counterweight Counterweight Speeder spring Pilot valve Pilot control Counterweight Counterweight Figure 7-17.  Underspeed condition.  Figure 7-18.  Overspeed condition. Figure 7-19.  On-speed condition.  changing attitude. The governor, as a speed-sensing device,  causes the propeller to maintain a set rpm regardless of the  aircraft attitude. The speeder spring propeller governing range  is limited to about 200 rpm. Beyond this rpm, the governor  cannot maintain the correct rpm. Although many of the wood propellers were used on older  airplanes, some are still in use. The construction of a fixed- pitch, wooden propeller is such that its blade pitch cannot be  changed after manufacture.  [Figure 7-20]  The choice of the  blade angle is decided by the normal use of the propeller on  an aircraft during level flight when the engine performs at  maximum efficiency. The impossibility of changing the blade  pitch on the fixed-pitch propeller restricts its use to small  aircraft with low horsepower engines in which maximum  engine efficiency during all flight conditions is of lesser  importance than in larger aircraft. The wooden, fixed-pitch  An increasing number of light aircraft are designed for  operation with governor-regulated, constant-speed propellers.  Significant segments of general aviation aircraft are still  operated with fixed-pitch propellers. Light sport aircraft  (LSA) use multiblade fixed-pitch composite propellers on up  to medium size turbo prop aircraft with reversing propeller  systems. Larger transport and cargo turbo prop aircraft use  propeller systems with dual or double-acting governors  and differential oil pressure to change pitch. Some types of  propeller system are beyond the scope of this text, but several  propellers and their systems are described. If the engine speed drops below the rpm for which the  governor is set, the rotational force on the engine-driven  governor counterweights becomes less.  [Figure 7-17]  This  allows the speeder spring to move the pilot valve downward.  With the pilot valve in the downward position, oil from the  gear type pump flows through a passage to the propeller and  moves the cylinder outward. This in turn decreases the blade  angle and permits the engine to return to the on-speed setting.  If the engine speed increases above the rpm for which the  governor is set, the counterweights move against the force of  the speeder spring and raise the pilot valve. This permits the  oil in the propeller to drain out through the governor drive  shaft. As the oil leaves the propeller, the centrifugal force  acting on the counterweights turns the blades to a higher  angle, which decreases the engine rpm. When the engine is  exactly at the rpm set by the governor, the centrifugal reaction  of the counterweights balances the force of the speeder spring,  positioning the pilot valve so that oil is neither supplied to  nor drained from the propeller. With this condition, propeller  blade angle does not change. Note that the rpm setting is  made by varying the amount of compression in the speeder  spring. Positioning of the speeder rack is the only action  controlled manually. All others are controlled automatically  within the governor. Governor System Operation Propellers Used on General Aviation  Aircraft Fixed-Pitch Wooden Propellers 7-12