Professor Von Kliptip AnswersYour Questions About The

McCAULEY CONSTANTSPEED PROPELLERGOVERNING SYSTEMFOR NON-COUNTERWEIGHTED PRESSURETO INCREASE PITCH PROPELLERS ONTYPICAL SINGLE ENGINE AIRCRAFT

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Professor, what do youmean by a ”constantspeed“ 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, and thenautomatically 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 the rotation. As the blade angleis reduced, 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. Thenthe engine and the propeller will tend to slowdown. Thus, by varying the blade angle or pitchof the propeller, we can control the RPM.

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Do you mean that thepropeller operates at thesame speed all the time?Heavens. No! That would never do.

Remember, we said that the system permits thepilot to select the RPM he wants. He has acontrol in the cockpit for this. When he wantsmaximum power at low airspeed, such as fortake-off, he pushes this low pitch andmaximum RPM. This is great for getting off theground, but it’s normally not desirable forcruising at high airspeeds. So, for cruising, hecan ease back on the throttle and the propellercontrol. This increases the pitch and the speedsettles into the desired RPM for cruiseconditions. The RPM automatically stays setuntil he moves the control.

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How do you change thepitch of the blades?We do it hydraulically in a single acting system,using oil from the propeller governor toincrease the pitch of the propeller blades.

In the propeller, oil pressure acting on thepiston produces a force that is opposed by thenatural centrifugal twisting moment of theblades and a spring. To increase the pitch orblade angle, we direct high pressure oil to thepropeller. This moves the piston back. Motion ofthe piston is transmitted to the blades throughthe actuating link and pins, moving the bladestoward high pitch.

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

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Pitch is decreased by allowing oil to flow out ofthe propeller and be returned to the enginesump.When the governor initiates this procedure,hydraulic pressure is decreased and the pistonmoves forward, moving the blades towards lowpitch. The piston will continue to move forwarduntil the opposing forces are again equal.

Although they aren’t shown in our illustrations,mechanical stops are installed in the propeller tolimit travel in both the high and low pitchdirections.

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So you do it with oil.How, Professor?

A look at the total system will help to explainthis. Besides the propeller, the other majorcomponent of the system is the governor. Thegovernor mounts on, and is geared to, theengine. This drives the governor gear pump andflyweight assembly. The gear pump boostsengine oil pressure to provide quick andpositive response by the propeller. Therotational speed of the flyweight assemblyvaries directly with engine speed and controls

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the position of the pilot valve. Depending on itsposition, the pilot valve will direct oil flow tothe propeller, allow to flow back from thepropeller, or assume a neutral position with nooil flow. As we saw earlier, these oil flowconditions correspond to increasing pitch,decreasing pitch or constant pitch of thepropeller blades.

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

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

The “L” shaped flyweightsare installed with theirlower legs projecting undera bearing on the pilot valve.When the engine RPM islower than the propellercontrol setting, the speederspring holds the pilot valvedown and oil flows fromthe 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, so thereis no oil flow to or fromthe propeller.

The faster the flyweightsspin, the further out theyare thrown, causing thepilot valve to be raised andallowingmore oil to flow tothe propeller.

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How does the aircraft pilotcontrol this governor action?The cockpit control is connected to the governorcontrol lever. The lever is attached to the threadedshaft. As the lever is moved, the threaded shaft turnsand moves up or down to increase or decreasecompression on the speeder spring.

For example, when the cockpit control movedforward, the governor control shaft is screwed down,increasing compression on the spring. This meansthat the flyweights must spin faster to raise the pilotvalve, which results in a higher RPM setting.

When the cockpit control is pulled back, the governorcontrol lever and shaft turn in the opposite direction,relaxing compression on the spring. This reduces thespeed necessary for the flyweights to move the pilotvalve and produces a lower RPM setting.

Thus, with the cockpit control, the aircraft pilot canshift the range of governor operation from high RPMto low RPM or any area in between.

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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 control has set the speederspring at the desired RPM. The flyweights havepositioned the pilot valve to direct oil to or from thepropeller. This, in turn, has positioned the propellerblades at the pitch that absorbs the engine power atthe RPM selected. When the moment the RPMbalance occurs, the force of the flyweights equalsthe speeder spring load. This positions the pilotvalve in the constant RPM position, with no oilflowing to or from the propeller.

OK, so we’re flying alongat constant RPM. Whathappens if the airplanebegins to climb or enginepower is decreased?This results in an UNDERSPEED condition. Airspeed isreduced and, since the pitch of the propeller bladesis too high, the engine starts to slow down.However, the instant this happens the flyweightswill drop, causing the pilot valve to move down.Then oil flows from the propeller, reducing the pitchof the blades. This automatically increases the speedof the engine to maintain the former RPM setting.

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What happens if theairplane goes into adescent or engine poweris increased?This causes an OVERSPEED condition. Airspeedincreases. Since the pitch of the propeller blades istoo low to absorb engine power, the engine RPMbegins to increase. But the instant this happens, theflyweights move out and raise the pilot valve. This,in turn, causes oil to flow to the propeller, increasingthe pitch of the blades. Engine speed then slowsdown to maintain the original RPM setting.

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 Fax: 316-831-3858

www.mccauley.textron.com

Form No. MPC-3B Printed in the U.S.A.