61A Propeller Construction

    

TTP	B1-L3	ATA 61A
Beech 90 Series	B2-L1	Propeller Construction

 The propellers on C90A/SE aircraft are Hartzell three-bladed propellers of the steel hub design that require periodic lubrication. 

   The propellers on the C90B aircraft through LJ-1541 are McCauley™ four-bladed  propellers that  require no periodic lubrication. The propellers on the C90B/GT/GTi aircraft from LJ-1542 are Hartzell four-blade propellers of the split/aluminum hub design that require periodic lubrication.

    All the propellers are constant speed, full feathering, and reversible. Normal prop operating blade angle ranges are flight (+85.8° to +12°), beta (+12° to +3°), and reverse (+3° to –10°) (blade angles given are for the four-bladed Hartzell propellers).

 

  The propeller control lever is connected, through a linkage and cable assembly, to the primary governor. The primary governor is mounted at the 12 o’clock position on the reduction gearbox assem- bly. The propeller lever is used to select propeller RPM by changing spring and fly weight forces within the primary governor.

 

    The condition lever is connected to the top of the fuel control unit. It is used to select fuel cutoff, low idle, or high idle by controlling fuel flow to the combustion chamber. The condition lever does not affect propeller operation.

    The power lever is connected to the cam  box assembly via the power lever arm. This arrange- ment allows the power lever to increase fuel flow by actuating the fuel control unit (FCU) control arm in the reverse or beta range via the FCU interconnect rod.

 

     The primary governor, driven from the reduction gearbox, controls the propeller rpm (Np) within a specific rpm range depending on the aircraft type and provides for constant speed operation of the propeller.

 

     The overspeed governor, also driven from the reduction gearbox, acts as a backup governor in the event the primary governor fails. It controls the propeller rpm (Np) to a fixed value. Since it is a backup unit and is normally never used, it can be tested on the ground using the PROP GOV TEST OFF switch on the pilot outboard subpanel. The fixed propeller test value varies depending on the serial number of the aircraft being operated.

 

     A third governor (Nf or fuel topping governor) is within the primary governor housing. It is used to prevent propeller overspeed if the primary or overspeed governors do not, by trimming  fuel from the fuel control unit. In the flight range, this governor operates in the overspeed mode to limit Np to 106% of the selected propeller rpm on the propeller control lever. In the reverse range, the Nf governor operates in the underspeed mode to limit Np to 95% of the selected propeller rpm.

 

    Propeller blade angle is controlled through a sin- gle-acting piston within the propeller. The primary governor contains a gear-type oil pump that takes the normal engine oil pres- sure and raises it to approximately 385 psi.  This pressure is controlled using an oil pressure regu- lator within the primary governor assembly. The primary governor ports this high oil pressure to the piston to decrease the blade angle to a lesser or finer pitch or releases the piston oil pressure and external blade counter weights and internal springs within the propeller piston increase blade angle to a greater or coarser pitch.

 

    For constant speed operations, the governor regu- lates the oil pressure to maintain the propeller rpm selected on the propeller lever.

      For beta or reverse operation, the beta valve on the primary governor is opened wider to move the blades to an angle below the normal flight range.

 

      Feathering is accomplished by relieving oil pres- sure within the propeller and allowing the external blade counter weights and internal springs within the propeller to drive the blades to a feather posi- tion. If the engine is shut down on the ground, and not manually feathered, the propeller slowly drives into feather as the oil pressure within the piston bleeds down. If an engine fails in flight, the propeller continues to windmill at a rpm and blade angle relative to the speed of the aircraft as the primary governor oil pump provides enough oil pressure to keep the propeller from feathering. An automatic feathering system is provided to feather the propeller in flight if an abnormal low torque occurs.

PTP  Beech 90 Series	B1	LOC		SGH
	B2