some single acting propellers are designed both types · pdf filesome single acting propellers...

Propellers 9-5

POW

ERPLAN

T

Some single acting propellers are designed to let oil flow into the propeller hub to increase blade angle, while others are designed to let oilflow into the hub to decrease blade angle. The single acting constant speed propellers are either:• Counterweight • Non-counterweight

In counterweight propellers, counterweights are attached to the propeller blades in such a manner that the centrifugal force created by the counterweights when the propeller rotates, tend to either increase or decrease the propeller pitch, depending on the propeller design. In fig. PO 9.7 is illustrated a counterweight installation which tries to increase blade angle (move to coarse pitch).

Both types may have internal springs opposing the oil pressure.

Light and medium twin-engined aircraft are often fitted with a constant-speed, feathering propeller of the single acting type, see fig. PO 9.6. It is called a single-acting propeller because the pitch change mechanism is similar to a single-acting hydraulic actuator, that is, oil pressure moves the piston in one direction and mechanical forces move in the opposite direction. A common arrangement is to use oil pressure working with CTM (Centrifugal Turning Moment) to move the blades towards fine pitch, and use a feathering spring and/or gas pressure, and ATM (Aerodynamic Twisting

Fig. PO 9.6 Single acting propeller

Piston

Forwardmovement

Spring

Oil passage

Decreasing pitch

Fig. PO 9.7 Principles of operation of a counterweight single acting propeller with an internal spring

Engineshaft

Counterweight

Counterweight

Operating link

Hub

Cylinder

Piston

Blade Ferrule Retainingneedle

Bladebearing

Featheringspring

Oil tube(piston rod)

Powerplant E5 - Proof 2.indb 5 10.04.2008 12:03:51

9-6 Propellers

POW

ERPLAN

T

Moment) force from counterweights to move the blades towards coarse pitch.

In fig. PO 9.7 the oil pressure is fed to the front of the piston, and the counterweights and the feathering spring combine to act at the rear of the piston. The counterweights produce a centrifugal twisting movement but, as they are located at 90° to the chord line, they tend to move the blades towards coarse. The counterweights must be heavy enough and far enough away from the blade axis to overcome the inherent CTM of the blade; consequently counterweights are normally only used with narrow chord blades.

In fig. PO 9.6 is shown the principles for a non-counterweight propeller with an internal spring. The spring tries to decrease blade angle while oil-flow into the propeller increases blade angle. The unit has a piston on a hollow shaft. The hole in the shaft is for oil flow. A piston can slide axially on the shaft. The piston has mechanical linkages to the propeller blades. The linkages are arranged so that they decrease the blade angle when the piston moves forward.

9.7 Propeller GovernorThe information to the pitch change mechanism to change pitch comes from an engine driven mechanical governor. Historically the piston engine propeller governors were called CSUs (Constant Speed Unit), see fig. PO 9.8. On turbo-propeller engines they are called PCUs (Propeller Control Units). The governor has a number of main parts:

• A pair of fly-weights • A spring (sometimes referred to as a

speeder spring) • An oil control valve (referred to as a pilot

valve or governor valve)• An oil pump to boost engine oil pressure (a positive displacement gear type pump)

The governor fly-weights are driven by the engine. As the engine RPM increases the weights will tend to move outwards under

Flyweights

Booster gear pump

Engine oil inlet

Propeller line

Speeder spring

Relief valvePilot valve

Fig. PO 9.8 Governor speed unit for a single acting propeller


Propellers 9-7

POW

ERPLAN

T

centrifugal reaction. This movement will be opposed by the spring and with a reduction of engine RPM the reverse will occur.

At any time the propeller should absorb all the available engine power. If it does not absorb all the power exactly, then it will do one of two things: it will either overspeed or underspeed.

If, from a steady flight situation, a manoeuvre is performed that either increases or decreases the load on the propeller, it will tend to overspeed or underspeed which would require a correction to the resulting engine RPM by the pilot. To overcome this problem the governor mechanism detects the over or

underspeed condition and changes the pitch of the blades to maintain constant engine RPM.

The operation of the governor is shown in fig. PO 9.9. When the pilot’s CSU lever is set to max RPM (Full Fine), and the throttle is at a low power setting, the governor valve will be fully down, the propeller oil line will be open and the oil will be directed to the front of the piston to move the blades to the full fine position.

As the throttle is opened and the RPM increased, centrifugal force will raise the control valve, until a position is reached where maximum RPM are obtained and the oil line to the propeller is blanked-off.

Fig. PO 9.9 Propeller Governor conditions on a single acting constant speed propeller

Propellerlever

Higher rpm

Lower rpm

Pump

Smallbladeangle

Fly weights

Drive

Speeder spring

Set rpm ismaintained

Rpm too highblade angle set to asmaller value


9-8 Propellers

POW

ERPLAN

T

Further increase in power would raise the valve allowing the oil to drain. The propeller coarse pitch spring and the blade counterweights would move the blades to a coarser pitch condition. When the engine RPM are stabilised the CSU governor flyweights are vertical, just balancing the force of the CSU governor spring. The propeller is said to be on-speed.

When the pilot wishes to select a particular engine RPM he moves the RPM lever, which

sets the datum for the governor control spring. This is why it is sometimes called a speeder spring.

9.8 FeatheringThe RPM lever positions are typically MAX RPM - MIN RPM and FEATHERED going from fine pitch to fully coarse. Normally the RPM lever has to be moved through a “positive gate” to select feather. This action manually raises the governor oil control

Engine oil inletDrainPropelleroil line

Non-return valve

Solenoid valve

Drive shaft

C.S.U. oil pump

Relief valve

Non-return valve

Governor valve

Drive gears

Governor

Control spring

To pilots control leverUnfeathering button

Unfeatheringaccumulator

Fig. PO 9.10 Constant speed unit for a single acting propeller showing feathering


Propellers 9-9

POW

ERPLAN

T

valve overriding any other signals on the CSU. This will drain the oil from the fine pitch side of the pitch change piston. The feathering spring plus the force of the blade counterweights will move the propeller into feather, see fig. PO 9.10.

9.9 UnfeatheringTo move the propeller out of feather requires a supply of oil pressure and an accumulator provides this, which is charged

during normal operation. The RPM lever is moved out of feathered into the constant speed range; these actions releases the oil control valve and it will move fully down under the action of the governor control spring. The propeller oil line is open to the fine pitch side of the pitch change piston, the unfeathering button is pressed, releasing the oil from the accumulator, see fig. PO 9.11. The propeller begins to unfeather and, as it windmills, engine oil pressure will increase and complete the unfeathering operation.

Engine oil inletDrainPropelleroil line

Non-return valve

Solenoid valve

Drive shaft

C.S.U. oil pump

Relief valve

Non-return valve

Governor valve

Drive gears

Governor

Control spring

To pilots control leverUnfeathering button

Unfeatheringaccumulator

Fig. PO 9.11 Constant speed unit for a single acting propeller showing unfeathering


9-10 Propellers

POW

ERPLAN

T

9.10 Centrifugal LatchOn the ground with the engine stationary, oil pressure will gradually reduce due to leakage through the CSU causing the propeller to feather, under the action of the feathering spring. This would place an unacceptable load on the engine when restarting. To prevent the propeller feathering under this condition, stops in the form of latches are fitted. The latches are called centrifugal latches. The centrifugal latch is disengaged by centrifugal force at all engine speeds above ground idling, see fig. PO 9.12.

Below ground idle RPM the return spring overcomes the centrifugal force and insert the latch stops. With the stops inserted,

the propeller blades can only coarsen approximately 5° of blade angle. When the engine is started at engine speeds above idle, oil pressure moves the propeller to fully fine and centrifugal force removes the latches.

This feature also means that it may not be possible to feather the blades below a certain minimum low RPM in flight and the Flight Manual or equivalent will point this out. They are nor required in the case of free turbine engines, such as the PT-6, where the compressor/turbine gas producer will start without having to drive the propeller.

A gas turbine that has the compressor/turbine spool driving the propeller through

Fig. PO 9.12 Centrifugal Latch

Piston

Piston rod

Cylinder

Fine pitchstop plate

Latch weighttension springs

Latch weights

Latch stops


Propellers 9-11

POW

ERPLAN

T

a gearbox will require the blades to be in the ground fine pitch position to minimise propeller torque load during ground start and will be held in this position by a centrifugal latch.

9.11 Double Acting PropellerNormally the more powerful piston and gas turbine engines are fitted with double-

acting propellers. The basic principle of operation is similar to the single acting type. The main difference is that there are no feathering springs or blade counter-weights. Oil is used on both sides of the pitch change mechanism piston, so two pipes are required between the control CSU lines and the hub. Fig. PO 9.13 shows a constant speed unit for a double acting propeller “on speed”.

Fig. PO 9.13 Constant speed unit for a double acting propeller showing on speed.

Propeller control

Governordrive gears

Governorvalve

Governorweights

Governorspring

Piston lift valvesolenoid

P.C.U. oil pump

Pitch locksolenoid

Non-returnvalve

Pitch lock

Feather/unfeatheroil supply

Engineoil inlet

Fine pitch

Reliefvalve

Coarsepitch

Valve liftpiston

Constant speed

H.P. fuel cock

Feather

Max

Min

Drive shaft


9-12 Propellers

POW

ERPLAN

T

If an overspeed is detected and the control valve is moved up, pressure is applied to the “increase-pitch” line while the “decrease-pitch” line is opened to the engine sump. Vice-versa for underspeed, see fig. PO 9.14.

9.12 Feathering (Double Acting Prop)To feather the propeller a separate pump driven by an electrical motor, just simply called a feathering pump, draws oil from a feathering oil reserve supply. The feathering reserve is designed into the oil tank, in a similar way to hydraulic reservoirs as used for hand pump operation, using the stack pipe arrangement. The pump may have a separate electrical selector switch of the press button or single pole type.

Press button switches are normally held in by a solenoid which will “pop-out” when feathering is completed, a pressure switch sensing the increasing oil pressure at the end of the feathering operation de-energises the solenoid, it is called a Pressure Operated Cut-Out Switch (POCOS).

Single pole switches are sprung to OFF and must be held to ON to operate the pump motor. A warning light may also be installed adjacent to the switch to indicate that the feathering pump is being operated. Unfeathering uses the same feathering pump, controls and indicators.

9.13 Propeller: Normal OperationGround CheckAfter the engine has been started and warmed up in accordance with the relevant

Fig. PO 9.14 Constant speed unit operation

Condition : Constant speeding

Propeller lever

Governor flyweights

Pressure relief valve

Fromengine

Control valve

To engine sump

From feathering motor

Decreasepitch

Increasepitch

Controlpiston

Condition : Overspeeding

Returnoil

Increasepitch

Condition : Underspeeding

Decreasingpitch

Return oil

To engine sump


Propellers 9-13

POW

ERPLAN

T

Ops Manuals, the CSU lever is moved a couple of times between Maximum and Minimum RPM to allow warm oil to circulate through the propeller and to “exercise” the propeller. These are the steps:• Set the CSU lever to maximum RPM • Set the power (boost) to not more than the

maximum permitted • Move the lever from max towards min and

note the decrease in RPM (the propeller is coarsening away from the fine pitch stop).

• After the initial decrease in RPM (about mid-position of the gate) slightly open and close the throttle and check that the RPM remains constant, confirming the propeller is constant speeding

• Return CSU lever to maximum and check that RPM increases

Flight ControlFor take-off, select maximum RPM on the CSU lever and open the throttle to take-off power (boost). For other conditions of flight the method of operation will be in the relevant Ops Manuals.

FeatheringA typical feathering drill is:• Close the throttle• Operate the appropriate feathering

mechanism• Turn off the fuel supply

In the case of fire, operate the fire extinguisher when the propeller has feathered.

On twin engine and multi-engined aircraft when feathering and unfeathering on training flights, it is essential to keep the generators of the live engines on line as the feathering pump motor imposes heavy loads on the aircraft electrical system. Usually, feathering pump motors have a time limit of so many operations for a fixed time period to prevent them from overheating.


9-14 Propellers

POW

ERPLAN

T

QUESTIONS

1 A propeller blade is twisted, so as to:

a) Allow a higher mechanical stress b) Avoid the appearance of sonic phenomena c) Decrease the blade tangential velocity from the blade root to the tipd) Keep the local Angle of Attack constant along the blade

2 As speed increases the thrust of a fixed pitch propeller will:

a) Decrease to a constant value b) Increase to a constant value c) Decrease initially and then increase d) Eventually decrease to zero

3 Which way do the governor flyweights move when the propeller overspeeds:

a) Outwards, causing the propeller to change to a coarser pitch b) Inwards, causing the propeller to change to a finer pitch c) Outwards, causing the blades to pitch lock d) Inwards, causing the propeller to change to a coarser pitch

4 The constant speed unit (CSU) governor works on the principle of:

a) Spring pressure acting against centrifugal force b) Manual operation c) Centrifugal twisting moment d) Newtons first law of motion


Propellers 9-15

POW

ERPLAN

T

QUESTIONS5 A reverse-pitch propeller is:

a) One where the blade pitch has been reduced to a negative pitch to reduce the landing run b) One where the blade pitch has been increased to increase forward thrustc) Used to counteract wind milling drag d) A propeller which varies from coarse to fine pitch automatically

6 Some propellers are fitted with accumulators for the purpose of:

a) Unfeathering the propeller b) Providing standby fine pitch control c) Feathering the propeller d) Assisting in the unfeathering and feathering operation

7 In reverse pitch mode, the thrust is most effective:

a) With high forward speed b) With low forward speed c) On high altitude airfields d) On ground taxi only

8 On a turbo-prop aircraft the ground fine pitch stops:

a) Control the range of blade angle in reverse pitch b) Avoid excessive overloading and high turbine gas temperature (TGTs) of the engine at low RPM’sc) Are removed prior to flight to allow the blade angles to coarsend) Are removed prior to flight to allow the blades to fine off

The answers can be found at the end of the book.


Intentionally blank


some single acting propellers are designed both types · pdf filesome single acting propellers...

Documents