Chapter IVFeathering Systems

Propeller feathering systems are used on mostmulti-engine aircraft to reduce the drag created bya windmilling propeller if an engine should fail. Tobe able to feather, the propeller must have a greatenough blade angle range to achieve an ap-proximate 90° blade angle. Also, the governormust be designed so that the constant-speedoperation can be overidden at anytime to featherthe propeller.

A means of unfeathering in flight is required inthe system and this may require additional systemcomponents. These additional components mayinclude accumulators, electric oil pumps. and oilflow control valves.

A. Basic Feathering SystemThe basic constant-speed operating com-

ponents of a feathering governor are the same asthose used in a constant-speed governor. In mostfeathering governors, the pilot valve and the gover-nor oil passages are designed so that oil pressureis directed to the propeller to decrease thepropeller blade angle (underspeed condition) andoil pressure is released from the propeller to in-crease the blade angles (overspeed condition).

The primary design difference between a con-stant-speed and a feathering governor is the addi-

Figure 4-1. The feather angle is 90°to the plane ofrotation whereas the constant-speed blade angle Isbetween 10 and 25° to the plane of rotation.

Figure 4-2. The lift rod engages the pilot valve tofeather the propeller.

tion of a lift rod connected to the pilot valve tomechanically lift the pilot valve to the overspeedposition when it is desired to feather the propeller.The lift rod raises the pilot valve when the cabinpropeller control is moved full aft. When the pilotvalve is raised, all of the oil pressure in thepropeller is released and the blades move to thefeather angle by the centrifugal force on the bladecounterweights and some other force, such assprings, depending on the propeller design.

The full aft movement of the cabin propellercontrol usually requires additional effort or asideways or outward movement to enter thefeather position. This is to prevent accidental

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TACHOMETERMANIFOLDPRESSURE (6 FUEL FLOW

MIXTURESPOWERPLANT

CONTROLPEDESTAL

THROTTLES

PROPELLERS

FEATHERING CONSTANT-SPEED

PISTON

CENTRIFUGALLATCH

HUBASSEMBLY

LATCHDISENGAGED

LATCHENGAGED

COUNTERWEIGHT

ENGINECYLINDER SHAFT

PISTOFIXEDLATCH (PLATE

Figure 4-3. The cabin propeller control may have to bemoved sideways to go Into the feather range on thecontrol quadrant.

feathering of the propellers during constant-speedadjustment.

When unfeathering the cabin propeller controlis moved forward into the constant-speed range.This lowers the lift rod far enough so that itdisengages the pilot valve and the governor re-turns to constant-speed operation (underspeedcondition before the engine restarts) and allowsthe engine to be restarted.

The engine is usually restarted by supplying oilpressure to the propeller through the governor,which causes the blade angle to decrease and the

Figure 4-4. A feathering McCauley propeller comparedto a McCauley constant-speed propeller.

Figure 4-5. A version of the centrifugal latchmechanism used to prevent a feathering propellerfrom feathering when the engine is shut down onthe gorund.

engine to windmill. Once the engine is windmilling,the ignition and fuel for the engine is turned onaccording to the particular aircraft operationsmanual and the system returns to normal opera-tion.

B. Feathering PropellersThe propellers used with feathering systems are

basically the same as constant-speed propellersexcept for the additional blade angle range and alonger nose cylinder to accommodate the greaterblade angle range. These propellers use oil pres-sure from the governor to decrease the propellerblade angles and counterweights, springs, and airpressure in various combinations to increase theblade angles and feather the propellers.

Most feathering propellers incorporate a cen-trifugally operated latch mechanism which locksthe blades at a low angle when the engine is at idle

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BRACKETNUTBRACKETBOLTACCUMULATORHOSE ASSEMBLYFILLER VALVESCREW

CLAMPSCREWCLAMPINTAKE MANIFOLDNUTLINE ASSEMBLYUNIONPROPELLER GOVERNOR

COURTESY OF WOODWARD GOVERNOR CO.

CYLINDER TYPE

Figure 4-6. The accumulator system In a Cessna 310.

on the ground. These latches prevent the bladesfrom feathering when the engine is shut down onthe ground. In flight, centrifugal force holds thelatches out and they do not interfere with theconstant-speed or feathering operation.

C. Unfeathering SystemsTo unfeather a propeller, oil pressure must be

supplied to the propeller so that the blades willrotate to a lower angle and start to windmill theengine. There are four methods which may be usedto accomplish unfeathering: developing oil pres-sure by rotating the engine with the engine starter;supplying oil pressure to the propeller from anaccumulator; using an electric pump to supply oilpressure to the governor; and, directing oil fromanother engine propeller system to apply pressureto the feathered propeller.

1. Engine StarterTo unfeather the propeller in some aircraft the

starter motor for the engine is used to rotate the

Figure 4-7. Two types of unfeathering accumulators.

engine and the governor to develop oil pressurewhich is directed to the propeller through theunderspeed condition of the governor anddecreases the propeller blade angle. As the bladeangles decrease, the airflow from the aircraftmoving through the air will cause the propeller tostart to rotate. As the engine and propeller start torotate, the oil pressure from the governor willincrease to the normal governor pressure and thepropeller-governor system will return to constantspeed operation.

2. Accumulator Systems

Many aircraft incorporate an accumulator inthe propeller system to be used when unfeather-ing the propeller. The accumulator is normallylocated in the engine compartment and is con-nected to the governor by a flexible or a rigid oilline.

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OIL LINE TO GOVERNOR

AIR VALVE SEALS

NITROGENAT

100 PSI

PISTONNO OIL CHARGE

NITROGENCOMPRESSED

TO 300 PSIOIL CHARGEOF 300 PSI

OIL CHARGED ACCUMULATOR

Figure 4-8. When the accumulator is charged with oilthe air in the accumulator Is compressed to thesame pressure as the oil.

The accumulator may be of the ball-dia-phragm or the piston-cylinder type. In eithercase, an air charge of about 100 psi will be onone side of the diaphragm or piston when theengine is shut down on the ground and the otherside will be connected to the governor oil pres-sure line for the propeller through the oil linementioned above.

During normal constant-speed operation, thegovernor oil pressure that is applied to thepropeller is also directed to the oil side of theaccumulator to charge the accumulator with oil atgovernor oil pressure. Since the governor oil pres-sure is two to three hundred psi, the oil pressurein the accumulator will compress the air in theaccumulator to the same pressure as the oil andthereby store a charge of oil at governor oil pres-sure.

If the propeller is feathered, the oil in the ac-cumulator is trapped by a check valve in thegovernor or by a control valve attached to thegovernor.

To unfeather the propeller, the cabin propellercontrol is moved forward. This sets the governorpilot valve to direct oil to the propeller (governorunderspeed condition) and the check valve orcontrol valve is opened and the oil pressure in theaccumulator is released into the propeller. The oilpressure moves the propeller to a lower angle, thepropeller starts to windmill, and the engine can berestarted.

Figure 4-9. The governor is set to decrease thepropeller blade angle when unfeathering with anaccumulator.

Once the system returns to constant-speedoperation, the accumulator is again charged andis set for the next unfeathering operation.

Unfeathering 011 PumpsA few aircraft use an electrically operated oil pump

to supply oil pressure for unfeathering the propeller.In this unfeathering system an oil line from theengine oil sump is connected to an electric oil pumplocated in the engine compartment. The output sideof the pump is connected to the propeller oil linebetween the governor and the propeller.

The oil pump operation is controlled from thecabin by a toggle switch or push button. When thepilot desires to unfeather the propeller, thepropeller control lever in the cabin is moved intothe constant-speed range and the cabinunfeathering switch is operated until the oil pres-sure from the electric oil pump causes the bladesto move to a low enough angle that the propellerstarts to windmill. The switch is then released andthe normal engine restart procedure is followed.During this operation, oil pressure from theelectric pump is also applied to the governor, butwith the governor in the underspeed condition. Nosignificant amount of oil pressure is lost throughthe governor.

Unfeathering Crossfeed SystemA few aircraft designs use a crossfeed device to

tap oil pressure from an operating constant-speedsystem and direct it to the feathered propeller.

To unfeather with a crossfeed system, thepropeller control lever for the feathered engine isplaced in the full forward position (this may varyfor some aircraft) and the crossfeed valve in the

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