T-34C AIRCRAFT SYSTEMS FAMILIARIZATION
Beta operation provides propeller reverse thrust. The beta valve, located on the front face of the
propeller governor (Figure 8-2), controls the flow of oil to the servo piston at 17.5° (low pitch
stop) and in the beta range to a maximum of -5°. The beta valve is controlled by the propeller
reversing lever, which is attached at one end to the beta feedback ring and at the other end to the
cam box assembly through a push-pull control. Movement at either end moves the beta valve.
At low engine power settings on the ground, the prop governor is not operating at sufficient
speed to control oil flow to the servo piston. The tendency of the propeller would be to stabilize
at 0° blade angle, an undesirable condition. The beta valve prevents this by maintaining a
positive blade angle (17.5°) until the pilot chooses to select propeller reversing. When the PCL
is moved into the BETA range, the beta valve is repositioned to allow oil to flow to the servo
piston and rotate the blades to an angle between 17.5 and 5 degrees. The beta feedback ring
repositions the beta valve to stop oil flow when the desired blade angle is reached. Selecting
BETA with the propeller feathered will result in damage to the pitch change assembly.
Additionally, do not select BETA when the engine is not operating.
Do not select BETA unless the engine is running and the condition lever is in the RPM range.
Selection of BETA while airborne is prevented by the beta lockout mechanism, which is located
on the left sidewall between the cockpits.
To select BETA three conditions must be met:
DC power available to operate a solenoid.
The nose landing gear strut compressed (aircraft on the ground).
The beta switch on top of the PCL depressed.
When BETA is selected, a pin, which blocks the rearward movement of the PCL, is removed by
The movement of the PCL within the BETA range progressively resets the low pitch stop and
allows the prop to hold any blade angle in the beta range to a maximum of -5º.