T-34C AIRCRAFT SYSTEMS FAMILIARIZATION
Figure 7-3 Pitch Change Assembly
Blade Angle Forces
Control of propeller blade angle is achieved through the interaction of two opposing forces
within the pitch change assembly: feathering spring/counterweight versus high-pressure oil.
The "relaxed" state of the propeller is the low drag-maximum glide feather (FTHR) condition.
Without high-pressure oil, the feathering spring and counterweights will hold the piston aft and
the blades will remain in the feather position. When high-pressure oil flows into the pitch
change assembly via the hollow prop shaft and retainer cup, the servo piston moves forward and
the prop blades rotate to a low blade angle. Control of blade angle is then achieved by
controlling oil flow to and from the pitch change assembly: increase oil flow to decrease blade
angle and decrease oil flow to increase blade angle. With no change in oil flow (steady state),
equilibrium exists and blade angle remains constant. In flight, propeller blade angle will be
determined by such factors as power setting, altitude, and airspeed. The pilot will not know the
exact blade angle.
However, there are three times on the ground when the pilot knows the propeller blade angle:
feather (87.5°), idle low pitch stop (17.5°), and maximum beta (-5°). Spring and counterweight
sets 87.5°. High oil pressure sets both 17.5° and 5°.