Professor Von Kliptip AnswersYour Questions About The

McCAULEY FULL FEATHERINGCONSTANT SPEED PROPELLERGOVERNING SYSTEMFOR COUNTERWEIGHTED PRESSURE-TO-DECREASE PITCH PROPELLERS ONRECIPROCATING ENGINES

2

Professor, what do youmean by ”constant speed“system?

EDITOR‘S NOTE: Don‘t let the professor‘s looksfool you. He knows his stuff.

We mean a constant RPM system that permitsthe pilot to select the propeller - and engine -speed he wants for any situation andautomatically maintain that RPM under varyingconditions of airspeed and power.

How do you control the RPM?

We do it by varying the pitch of the propellerblades. In the sense that we’re talking about it,the pitch is the angle of the blades with relationto the plane of rotation. As blade angle isreduced, the torque required to spin thepropeller is reduced and, for any given powersetting, the airspeed and RPM of the engine willtend to increase. Conversely, if the blade angleincreases, the required torque increases. Then,the engine — and the propeller — will tend toslow down. Thus, by varying the blade angle orpitch of the propeller, we can control the RPM.

3

OK, Professor, what is afull feathering propeller?I’m glad you asked that question. It reminds meto tell you something important: This systemnormally is used only on twin-engine aircraft. Ifone of the engines fails in flight, you losepower. But worse than that, you could losecontrol of the aircraft if the propeller on theidle engine is permitted to windmill. To preventthis, we turn the propeller to a very high pitch,so that its blades are almost parallel to theairstream. This prevents windmilling and cutsdrag to a minimum. A propeller that can bepitched to this position is known as a fullfeathering propeller.

4

By constant speed, do youmean that the propelleroperates at the same RPMall of the time?Heavens, no! That would never do.

Remember, we said earlier that the systempermits the pilot to select the RPM he wants.He has a control for each engine in the cockpitthat permits him to do this. When he wantsmaximum power at low air speed, such as fortake-off, he pushes the controls full forward.With full throttle, this gives him low pitch andmaximum RPM. This is great for getting off theground, but it’s normally not desirable forcruising at high airspeeds. So for cruising, he canease back on the throttle and propellercontrols. This increases the pitch, and the speedsettles into the desired RPM for cruiseconditions. The RPM automatically stays setuntil he moves the controls.

In flight, it is poor practice to put the propellersin feathering position with the engine running— it overloads the engine and really shakesthings apart. The controls have detents at thelow RPM (high pitch) end to prevent this fromhappening. The pilot must move the controllever laterally or lift up to get around thedetent.

5

How do you change thepitch of the blades?

We do it hydraulically in a single acting system,using engine oil from the propeller governor todecrease the pitch of the propeller blades. Inthe propeller, oil pressure acting on the pistonproduces a force that is opposed by the twistingmoment of the blade counterweights and largesprings. To decrease the pitch, we direct highpressure oil to the propeller. This moves thepiston back. Motion of the piston is transmittedto the blades through the actuating links andpins, moving the blades toward low pitch.

When the opposing forces are equal, oil flow tothe propeller stops, and the piston will stopalso. The piston will remain in this position,holding the pitch of the blades constant, untiloil flow to or from the propeller is establishedby the governor.

6

Pitch is increased by allowing oil to flow out ofthe propeller to be returned to the enginesump. When the governor initiates thisprocedure, hydraulic pressure is decreased, andthe piston moves forward, turning the bladestoward high pitch. The piston will continue tomove forward until the opposing forces areagain equal. If all oil flows out of the propeller,the piston will move forward to the featherstop, and the blades will go to feather pitch.This will happen when the pilot moves thecontrol to the feather position or if oil pressureis lost for any reason.

Although they aren’t shown in our illustrations,mechanical stops are installed in the propellerto limit travel in both low pitch and featherdirections.

7

How do you prevent thepropellers from beingfeathered when you shutdown the engines on theground?Don’t park near the chicken coop! (Excuse thefunny, but I just couldn’t resist.)

Seriously, you’re showing how alert you are byrealizing that you do lose oil pressurewhen youshut the engines down. So, I’ll tell you how wedo it.

We use a centrifugal latch inside each propeller.This little spring loaded latch is held out ofengagement by centrifugal force. When anengine shuts down in the air, the windmillingpropeller holds the latch out, and the propellermoves into feather pitch. When the engines areshut down on the ground, the propellers are inlow pitch. After shutdown, there is nowindmilling. As soon as the propeller speedreduces to about 600 RPM, the spring pulls thelatch into engagement. This happens before oilpressure is lost. The latch holds the blades at anangle a few degrees above low pitch, so thatthe engine can be started easily.

8

So you do it with oil.How, Professor?

A look at the total system will help to explainthis. Besides the propellers, the other majorcomponents of the system are the governors,one for each propeller. Each governor mountson, and is geared to, the engine. This drives thegovernor gear pump and the flyweightassembly. The gear pump boosts engine oilpressure to provide quick and positive responseby the propeller. The rotational speed of theflyweight assembly varies directly with enginespeed and controls the position of the pilot

9

NOTE:The Governor isrepresentedschematically forclarity. In actualconstruction, thesump return isdown throughthe center of thepilot valve.

valve. Depending on its position, the pilot valvewill allow oil to flow back from the propeller,direct oil flow to the propeller, or assume aneutral position with no oil flow. As we sawearlier, these oil flow conditions correspond toincreasing pitch, decreasing pitch, or constantpitch of the propeller blades.

10

How do theflyweightschange theposition of thepilot valve?By utilizing centrifugal force.

The “L” shaped flyweightsare installed with theirlower legs projectingunder a bearing on thepilot valve. When engineRPM is slower than thepropeller control setting,the speeder spring holdsthe pilot valve down, andoil flows to the propeller.

As engine RPM increases,the tops of the weightsare thrown outward bycentrifugal force. Thelower legs then pivot up,raising the pilot valveagainst the force of thespeeder spring, and thereis no oil flow to or fromthe propeller.

The faster the flyweightsspin, the further out theyare thrown, causing thepilot valve to be raisedand allowing more oil toflow from the propeller.

11

How does the aircraft pilotcontrol this governor action?The cockpit control lever for each propeller isconnected to the control lever on its governor. Thelever is attached to a threaded shaft. As the lever ismoved, the threaded shaft turns and runs up ordown to increase or decrease compression on thespeeder spring.

For example, when the cockpit control is movedforward, the governor control shaft is screweddown, increasing compression on the spring. Thismeans that the flyweights must spin faster to raisethe pilot valve and results in a higher RPM setting.

When the cockpit control is pulled back, thegovernor control lever and shaft turn in theopposite direction, relaxing compression on thespring. This reduces the speed necessary for theflyweights to move the pilot valve and produces alower RPM setting.

Thus, with the cockpit controls, the aircraft pilot canshift the range of governor operation from highRPM to low RPM or any area in between.

12

So how does all this resultin constant speed?By producing what is known as an ON SPEEDcondition. This exists when the RPM is constant.Movement of the cockpit controls have set thespeeder springs at the desired RPM. The flyweightshave positioned the pilot valves to direct oil from orto the propellers. This, in turn, has positioned thepropeller blades at a pitch that absorbs the enginepower at the RPM selected.When themovement ofRPM balance occurs, the force of the flyweightsequals the speeder spring load. This positions thepilot valves in the constant RPM position with no oilflowing to or from the propellers.

OK, so we’re flying alongat constant RPM. Whathappens if the airplanebegins to climb or enginepower is decreased?This causes an UNDERSPEED condition. Airspeedis reduced and, since the pitch of the propellerblades is too high, the engines start to slowdown. But the instant this happens, theflyweights will droop, causing the pilot valves tomove down. Simultaneously, oil flows to thepropellers, reducing the pitch of the blades. Thisautomatically increases the speed of theengines to maintain the original RPM setting.

13

What happens ifthe airplane goes into adescent or engine poweris increased?This results in an OVERSPEED condition. Airspeedincreases. Since the pitch of the propeller blades istoo low to absorb engine power, the engine RPMbegins to increase. However, the instant thishappens, the flyweights move out and raise thepilot valves. This, in turn, causes oil flow to flowfrom the propellers, increasing the pitch of theblades. Engine speed then slows down to maintainthe original RPM setting.

14

If governor action tends todecrease the pitch withdecreasing RPM, how doyou manage to feather apropeller?

We do it by providing a mechanical linkage thatoverrides the flyweights and speeder spring. When thecockpit control is moved to feather, the governor leverand shaft are turned beyondnormal lowRPMoperatinglimits. As the threaded shaft backs out, the shaft lift rodengages the pilot valve spindle and lifts the pilot valve.This causes oil to flow out of the propeller, and it movesto feather pitch.

How do you unfeather apropeller?Unless the airplane is equipped with the unfeatheringaccumulator option (I’ll tell you about options later),you move the propeller control to high RPM (low pitch)and engage the engine starter. When the engine isturning over fast enough, sufficient oil pressure will bedeveloped to force the propeller blades out of feather.

What about those options,Professor?UNFEATHERING ACCUMULATOR OPTIONThe unfeathering accumulator option will permit afeathered propeller to be unfeathered in flight for airstarting the engine. The governor ismodified to providean external high-pressure oil outlet through a checkvalve and a device for unseating the check valve. Theexternal outlet is connected to an accumulator. One sideof the accumulator is filled with compressed air (drynitrogen) and the other side with oil so that oil can be

15

stored under high pressure. This is what happens whenthe aircraft is in normal flight. When the propeller isfeathered, the check valve maintains oil pressure in theaccumulator. When the propeller control is moved fromfeather to low pitch, the check valve is unseated,permitting the high-pressure oil in the accumulator toflow to the governor pilot valve. With the governorcontrol lever and shaft in low pitch, the speeder springforces the pilot valve down so that the oil flows to thepropeller and moves the blades to low pitch.

This booklet is presented in the interest ofhappier, safer and wiser flying by

PROPELLERS • SPINNERS • DEICERSGOVERNORS & ACCUMULATORS

5800 E. Pawnee, Wichita, KS 67218Telephone: 800-621-7767 • 316-831-4021

Fax: 316-206-9948 • www.mccauley.textron.com

Form No. MPC-4 (Rev. 6/2008) Printed in the U.S.A.

SYNCHRONIZING/SYNCHROPHASING OPTIONS

The governor mechanism provides the means for synchronizing andsynchrophasing the two propellers on twin-engine aircraft. Thesynchronizing option adjusts propeller RPM so that both props are turningat the same speed. McCauley installs a slotted disc on each governor shaftalong with a transducer that sends a frequency signal to an electroniccontrol. This control compares the signals fromboth governors and adjustsone of them to bring it into “synch” with the other.

Once the props are synchronized, the synchrophaser option adjusts theposition of the blades on one propeller with respect to the blades of thesecond prop for reduced noise and vibration. McCauley synchrophasersare solid state units that automatically synchronize prop speed with aphasing control operated by the pilot. This phasing control manuallyadjusts the difference between the two propellers to minimize the “beat”of the props.