17.3 Propeller Pitch Control

Question 1

Propellers are designed as either pusher or puller (tractor) propellers, which are then subdivided into fixed pitch propellers, adjustable pitch propellers, controllable and constant speeding propellers

Answer 1

Constant speed propellers are further categorised according to the method of pitch change used, for example hydraulic, mechanical, or electrical.

Question 2

In the case of multiple engine aircraft and motorised gliders, an engine must produce as little drag as possible when it is shut down. Therefore, their blades can also be moved into the feathering position (least drag)

Answer 2

.

Question 3

With large aircraft, the production of reverse thrust is intended to shorten the distance on landing. For this purpose, the propellers are turned into reverse pitch, where the air is accelerated forwards while the propellers continue to turn in the same direction. Thus, reverse thrust is produced

Answer 3

.

Question 4

the following types of propellers commonly have hydraulic pitch change mechanisms:

Answer 4

Constant speed propellers (pitch change from low (fine) to high (coarse) pitch)
Constant speed propellers with feathering position
Constant speed propellers with feathering and reverse (for turboprop engines)

Question 5

Disc Braking:

Answer 5

Some propellers can move their blades to a very fine (low) pitch on landing. This causes a negative angle of attack and thus a braking force. This force is proportional to the forward speed of the aircraft.

Question 6

Feathering:

In the event of an engine failure, Centrifugal Turning Moments (CTMs) tend to move the propeller blades into a fine (low) pitch. Eventually, this results in a negative angle of attack and causes a large drag force like “Discing Braking.” The torque force is reversed, and this causes the airflow to drive the engine in the normal direction of rotation.

Answer 6

To prevent this undesirable condition, the propeller blades are moved through maximum coarse pitch until the blade chord line is approximately 90º to the plane of rotation. This feature is known as FEATHERING and reduces the drag by placing the leading edge directly into the oncoming airflow.

Question 7

Blade ranges =

Answer 7

-BETA Range =Pilot controls the pitch on the ground

-Negative (Reverse) Pitch= Used for power-on braking and manoeuvring

Ground Fine Pitch = Used for engine starting and discing braking

ALPHA Range = Also known as the constant speed range - the propeller governor controls the pitch

Max Fine Pitch = Used for rapid acceleration during take-off, allowing a shorter take-off run

Coarse (High) Pitch = Used for high forward speed

Question 8

The propeller pitch cannot be selected between Max coarse pitch and feather. In the event of engine failure on a multiple engined aircraft, the propeller moves to feather in one step.

Answer 8

.

Question 9

The pitch change mechanism can be further categorised as:

Answer 9

moving piston
moving cylinder

Question 10

moving piston

Answer 10

Moving piston pitch change is a self-explanatory expression to indicate that a piston is attached to the propeller blade lugs and moves within a cylinder.

The motive power used is oil pressure directed to one side or the other of the piston, although in some cases spring pressure is used on one side of the piston.

Note: Spring pressure, used in the context of propeller pitch change, can mean mechanical springs, compressed gas or even a combination of the two.

Question 11

moving cylinder =

Answer 11

Moving cylinder pitch change is, again, a self-explanatory expression to indicate that the cylinder is attached externally to the propeller blade and moves in response to the pressure felt on one side of a fixed piston. A balance weight or oil can be used to turn the properller blade in the opposite direction.

Moving cylinder applications are generally found on smaller aircraft propellers.

Question 12

Single Acting Propellers

Some propeller systems operate in such a way that oil pressure changes the pitch in one direction only. Movement in the opposite direction is the result of spring force and the torsion moments of the blades themselves.

Answer 12

Propellers that use such a pitch change mechanism are called single acting propellers.

Question 13

Single Acting Propellers for Single Engine Aircraft

Answer 13

With these propellers, the oil pressure moves the blades in the direction of high (coarse) pitch and the spring moves it towards low pitch. After engine shutdown, the blades are in the lowest (fine) pitch stop position, which is optimal for restarting the engine. Should the engine fail during flight, this blade position is favourable for windmilling, which makes it easier to restart the engine.

Question 14

Single Acting Propellers for Multi-Engine Aircraft

Answer 14

If single acting propellers are used on multi-engine aircraft, oil pressure moves the blades in the direction of low (fine) pitch. The springs and torsional moments of the blades, move the blades towards high pitch. If engine failure occurs during flight with decreasing oil pressure the blades move in the high (coarse) pitch direction. In this way they have already covered part of the transition to the feather position.

Question 15

Centrifugal Force Pitch Change Moments

Answer 15

The centrifugal force of the propeller blade mass (CTM) produces a pitch change moment which turns the blade in the direction of low (fine) pitch. The creation of this natural pitch change moment (or fly-moment) is due to the distribution of the propeller blade mass

If the propeller blade is to turn towards low (fine) pitch because of centrifugal force, then a counterweight must be attached to the blade root. The natural forces on a propeller blade want to drive the propeller blades to a low pitch. In flight, this is an unsafe failure condition.

At low pitch in flight, a propeller can windmill, overspeed, and produce high drag

Counterweights can be attached to the blade which will cause the centrifugal loads on the counterweights to drive the blades towards a higher pitch (towards the feather).

In this way, a loss of hydraulic pressure will cause the pitch to increase to a safe setting.

Question 16

Double Acting Propellers

Answer 16

Large propellers are generally constructed with pitch change mechanisms where oil pressure leads to pitch change in both directions. These are called double acting propellers.
.
If the control valve is mounted behind the gearbox in the constant speed unit (CSU), the propeller shaft must have two oil transfer tubes, one for the front and one for the back of the piston. These oil tubes are constructed as coaxial tubes

Question 17

The valve for controlling the flow of oil to the two ends of the piston is mounted either behind the gearbox or in the propeller hub.

Answer 17

.

Question 18

A double-acting propeller typically uses oil pressure, either from the normal high-pressure pump or from an auxiliary pump,

Answer 18

.

Question 19

propeller governnor

Answer 19

A governor is an engine RPM-sensing device and high-pressure oil pump. In a constant-speed propeller system, the governor responds to a change in engine RPM by directing oil under pressure to the propeller hydraulic cylinder or by releasing oil from the hydraulic cylinder. The change in oil volume in the hydraulic cylinder changes the blade angle and maintains the propeller system RPM.

Question 20

On turboprop and reduction geared piston engines, a propeller governor is mounted on and driven by the reduction gearbox.

Answer 20

Direct drive piston engines can have the governor mounted on the front of the engine crankcase and driven by the camshaft or mounted on and driven by the accessory gearbox.

Question 21

Depending on the manufacturer, the propeller governor can be classified as a Constant Speed Unit (CSU), a Propeller Control Unit (PCU) or a Propeller governor.

Answer 21

.

Question 22

he CSU is a mechanical device that senses changes in engine speed caused by a change in power output, by means of a flyweight governor. This governor controls a hydraulic servo system feeding high-pressure oil, via a pilot valve, to a piston in the pitch change mechanism (PCM).

Answer 22

.

Question 23

The governor assembly consists of:

Answer 23

A spur gear type pump to provide a positive operating pressure for the system

A pilot valve to control the oil flow to and from the pitch change mechanism

a safety spring is fitted at the top of the governor so that in the event of a control lever failure, the spring drives the governor to the low (fine) pitch position thereby enabling thrust to be maintained.

Question 24

Pilot inputs to the governor can be by either the power lever linkage (single lever control) or a separate propeller conditioning lever (twin lever control).

Answer 24

.

Question 25

On-speed Condition

Answer 25

When the engine is operating at the RPM set by the pilot, using the cockpit control, the governor is operating at a neutral “on speed” condition

While the governor assembly is in this neutral position (flyweights in the vertical position

Question 26

Propeller RPM is determined by the balance between the engine power produced and the braking moment of the propeller blades.

Answer 26

.

Question 27

Under-speed Condition

Answer 27

If the pilot requires a higher propeller RPM the propeller control lever (conditioning lever) is moved forwards to the new selected speed. This exerts more tension on the speeder spring overcoming the centrifugal force on the flyweights and forcing the pilot valve down.

Question 28

Over-speed Condition

Answer 28

If the pilot requires a lower propeller RPM the propeller control lever (conditioning lever) is moved rearwards to the new selected speed. This reduces the tension on the speeder spring allowing the centrifugal force on the flyweights to lift the pilot valve up.

Question 29

Engine Stopped (Governor Functions)

Answer 29

No centrifugal force is acting on the flyweights, therefore the speeder spring loading pushes the landed valve down. Oil is trapped in the cylinder between the piston and the oil pump.

Question 30

Start Up to Idle Plus

(governor functions)

Answer 30

As engine RPM is increased it causes the flyweights to move outwards until the constant speed condition is reached. i.e., Flyweight load equals speeder spring load, pilot valve in a central position. Propeller is still in fine pitch.

Question 31

engine power increased to cruise

(governor functions)

Answer 31

Increased power tends to increase propeller RPM. This causes the centrifugal force to overcome spring force. Pilot valve moves up bleeding away fine (low) pitch oil. Propeller blade angle increases causing a decrease in propeller RPM until flyweights and speeder spring force are again in balance.

Question 32

Engine Power increased to Take-off

(governor functions)

Answer 32

when the power lever or propeller conditioning lever is moved to the take-off position, the input lever forces the speeder spring down against the flyweights. Fine (low) pitch oil pressure now moves the blades to a finer (lower) pitch for take-off (Still constant speeding).

Question 33

Take-Off (governor functions)

Answer 33

As forward speed increases the angle of attack decreases. Propeller load decreases and the propeller tries to over-speed. This is sensed by the flyweights which overcome the speeder spring, lifting the pilot valve and bleeding away fine pitch oil. (Still constant speeding).

Question 34

Feathering

(Governor functions)

In the event of an engine stopping in flight, the propeller must be feathered to prevent windmilling. This is achieved by the pilot selecting FEATHER by:

Answer 34

Twin Lever Cockpit - moving propeller conditioning lever to FEATHER.

Single Lever Cockpit - moving the HP cock through OFF to FEATHER.

This mechanically lifts the pilot valve, bleeding away fine (low) pitch oil pressure and allowing spring or gas pressure (assisted by counterweights) to move the blades to the feather position.

Question 35

Governor Over-speed and Under-speed

The propeller governor moves to the over-speed condition for three main reasons:

Answer 35

Aircraft goes into a dive increasing forwards speed; AOA is decreased.

When speeder spring force on flyweights is reduced.

When pilot increases fuel flow by moving the throttle forward. The under-speed condition results from the reverse of the above.

Question 36

Single Acting, Constant Speed, Counterweight Propeller Control

Answer 36

the pitch change mechanism in this case consists of a fixed cylinder that houses a sliding piston that is subjected to spring force on its front face and oil pressure on its rear face.

Question 37

Electrical/Electronic Operated Pitch Change Mechanisms

Answer 37

The propeller governor is like the constant speed unit governor used on oil operated pitch change mechanisms except that the governor pilot valve supplies pressure oil selectively to either side of a servo piston in the governor instead of the propeller. The servo piston positions a central contact inside an oscillating switch unit

Question 38

Some aircraft use a single propeller pitch lever that controls the speed of a master electric motor. Each engine drives a generator that produces three phase alternating current. The frequency of the current from each generator relates to the speed of its allied engine.

Answer 38

The frequencies are compared with the master and signals are then sent to the appropriate engine pitch change motors to synchronise the engine speeds to that commanded by the pitch change lever.

It is in reality an engine synchronisation
system, but the engine speeds can be chosen.

Question 39

Electronic Control

in an integrated digital electronic control for an engine and propeller, the mechanical propeller governor unit has been replaced by a two-stage servo valve

Answer 39

The servo valve is driven by a torque-motor that receives electrically generated signals from a solid-state digital electronic control unit (ECU).

Question 40

The ECU comprises two identical control lanes A and B. Each lane will have a supervisory and a limitation section. Each lane can exercise control and automatic or manual switch over is available if a lane fails

Answer 40

.

Question 41

ARINC 429 system is also known as

Answer 41

Mark 33 Digital Information Transfer System (DITS)

Question 42

The power source for the ECU is normally provided by an engine-driven, dedicated generator that produces three phase AC power.

Answer 42

The propeller phase signal received by the ECU is used to ‘synchro phase’ the propeller with the opposite engine propeller and this can be done with an accuracy within two degrees of one revolution.

Question 43

The latest integrated ECU will control all the static and transient engine operations using the propeller servo valve and the fuel control unit torque motors

Answer 43

In the eventuality that ECU fails, the entire system reverts back to manual control under a power lever.

Question 44

Basic propeller feathering systems consist of a feathering pump, reservoir, a feathering time-delay switch, and a propeller feathering light.

Answer 44

The propeller is feathered by moving the control in the cockpit against the low-speed stop. This causes the pilot valve lift rod in the governor to hold the pilot valve in the decrease RPM position regardless of the action of the governor flyweights. This causes the propeller blades to rotate through high pitch to the feathering position.

Question 45

Feathering may also be accomplished by pulling the engine emergency shutdown handle or switch to the shutdown position.

On some models this can be initiated by depressing the feathering button. This actions the auxiliary pump and feather solenoid, which positions the feathering valve to transfer oil to feather the propeller.

Answer 45

.

Question 46

Automatic (Auto-Coarsening) Feathering

This system is used to provide automatic operation of the feathering system during take-off and cruise.

Answer 46

In the event of an engine failure the centrifugal flyweights are automatically overridden and the electric feathering pump runs, driving the propeller to the feather position.

The auto feather system provides automatically initiated propeller feathering, and good operating engine power up trim following an engine failure during take-off.

Question 47

Auto feather is selected ‘ON’ for take-off only, using the Auto-feather switch light on the engine instrument panel

The ARM light will turn on when each engine torques exceed a minimum value of 50% and both power levers are advanced beyond 60° Power Level Angle (PLA).

Answer 47

.

Question 48

Up trim is triggered (regardless of Auto feather selection) when:

Answer 48

Torque of the local engine falls below 25%.
NP (rotational speed of the propeller) as indicated by the torque sensor falls below 80% (of maximum RPM).
PLA is in the rating detent.
Maximum Take-off Power (MTOP) is not set.

Either of the first two conditions must be confirmed by both torque sensor signals

Question 49

The low-speed condition accommodates the failure case of a propeller auto coarsening or inadvertently feathering, causing loss of thrust but not low torque. Up trim is also directly signalled when an auto feather occurs. Dual up trim signals are sent to the Full Authority Digital Engine Control (FADEC) of the surviving engine to increase its power by 10%. The effect of this is to replace Normal Take-off Power (NTOP) with Maximum Take-off Power (MTOP)

Answer 49

.

Question 50

Auto feather is triggered from the armed state when the torque of the local engine, as indicated by both torque signals, falls below 25% for at least three seconds

Answer 50

.

Question 51

The auxiliary feather pump provides a backup source of oil pressure to the propeller pitch-change mechanism.

Answer 51

the pump is supplied with oil from an auxiliary oil reservoir built into the propeller Reduction Gear Box (RGB) to permit auto feather in the event of loss of engine oil pressure.

Question 52

the auto feather system can be disarmed by:

Answer 52

Pushing OFF the auto feather switch light
Retarding one or both power levers to flight idle
Each engine torques dropping below approximately 50%

Question 53

The propeller speed (NP) under-speed cancel signal prevents the FADEC from raising engine speed (NH) (if the engine is running in the case of an unscheduled feather command) to maintain propeller RPM, as the feathered propeller decreases below 660 RPM.

Answer 53

Auto feather test is automatic on selection

Question 54

Provided the engine controls are set to high power, a low torque signal will complete the auto-feather circuit and energise the valve lift solenoid and feather pump. Oil pressure is delivered to the valve lift piston, which raises the landed valve to the coarse pitch position.

Answer 54

.The high-pressure oil supplied by the feathering pump is now fed to the coarse pitch side of the pitch change piston, pushing it onto the feathering stop. As the piston moves it displaces the fine pitch oil to return.

The position of the feathering stop within the pitch change mechanism allows the propeller blades to rotate so the chord line is parallel to the relative airflow and therefore prevents windmill.

Question 55

Unfeathering Accumulator

After feathering a propeller in flight, the pilot can attempt an engine restart. To carry this out, the propeller must be moved to a fine (low) pitch. On many small engines, this is done by moving the power lever to a higher RPM position and turning the engine on the starter motor to allow sufficient oil pressure to build up to move the blades to a fine (low) pitch.

Question 56

Centrifugal Latches

When an engine fitted with a single-acting propeller is shut down, oil pressure is trapped in the cylinder. If there is any leakage of oil within the governor, the propeller blades are forced towards coarse pitch by the spring or gas pressure in the pitch change cylinder. This would put an unacceptable load on the starter system.

Answer 56

Centrifugal latches prevent this from occurring by mechanically locking the piston in the fine (low) pitch position when the engine is not rotating. The latches are engaged by spring pressure at low RPM (typically 700 to 1000 RPM) and disengaged by centrifugal force as the RPM increases.

Question 57

reverse pitch propellers

Answer 57

The BETA range is used to describe the range of blade angles which can be obtained when the aircraft is operating on the ground. It is, generally, associated with propellers which have a reverse pitch capability.

Question 58

In reverse pitch the engine/propeller turns in the same direction as in the normal (forward) pitch position, but the propeller blade angle is positioned to the negative side of ground fine (low) pitch.

Answer 58

used for backing away from obstacles when taxiing, controlling taxi speed, or to aid in bringing the aircraft to a stop during the landing roll.

Question 59

Reverse pitch does not mean reverse rotation of the engine. The engine delivers power just the same, no matter which side of ground fine (low) pitch the propeller blades are positioned.

Answer 59

.

Question 60

With a turboprop engine, to obtain enough power for flight, the power lever is placed somewhere between flight idle and maximum. The power lever directs signals to a fuel control unit to manually select fuel

The propeller governor selects the propeller pitch needed to keep the propeller/ engine on speed. This is referred to as the propeller governing or alpha mode of operation.

Answer 60

.When positioned aft of flight idle, however, the power lever directly controls propeller blade angle. This is known as the beta range of operation.

Question 61

the beta range of operation consists of power lever positions from

Answer 61

flight idle to maximum reverse

Question 62

Beginning at power lever positions just αft of flight idle, propeller blade pitch angles become progressively finer with aft movement of the power lever until they go beyond maximum ground fine (low) pitch and into negative pitch, resulting in reverse thrust

Answer 62

.

Question 63

While in a direct coupled/ constant-speed engine, the engine speed remains unchanged as the propeller blade angles achieve their negative values.

Answer 63

On a free power turbine engine, as the negative 5° position is reached, further αft movement of the power lever also result in a progressive increase in engine RPM until a maximum value of about negative 15° of blade angle and 85% N1 are achieved.

Question 64

Advantages of reverse pitch propellers

Answer 64

Reduce Landing Roll
Reduce Brake wear
Improve manoeuvrability on the ground
Reversing of the aircraft during taxiing

Question 65

Disadvantages of reverse pitch propellers

Answer 65

Reduced engine cooling when in reverse pitch
Increased blade damage
Complicated control system

Question 66

Beta Mode

Answer 66

In beta mode the blade angle is changed directly with the power lever, so that any angle between zero thrust and flight idle (or full reverse) can be selected. Here control of the blade angle works in the form of follow up control. For this purpose, the power lever works directly on the beta valve.

To obtain more reverse thrust, move the power lever back more to set the beta valve rearward again, and the process repeats.

The RPM is selected for the governor inside the control unit with the condition lever and then remains constant. In this operational range the prop governor is ineffective.

Question 67

Overspeed Protection

The first prop overspeed backup is in the primary propeller governor, using the balance of speeder springs and flyweights. The primary propeller governor, also referred to as the CSU (Constant Speed Unit), works to control any propeller overspeed up to approximately 102 to 103% Np (prop speed)

Answer 67

Should the propeller speed exceed that value, from approximately 104 to 106% Np, the propeller overspeed governor operates and returns oil from the propeller dome back into the reduction gearbox.

Question 68

When the propeller speed reaches a specified limit (four percent over maximum propeller speed), the governor flyweights lift the control valve and bleed off propeller servo oil into the reduction gearbox sump, causing the blade angle to increase. An increase in blade pitch puts more load on the engine and slows down the propeller.

Answer 68

.

Question 69

the flyweights inside the main propeller governor open a bleed air link allowing pressurised air, referred to as ‘Py’ pressure, to leak into ambient air. Py pressure comes from the Fuel Control Unit (FCU). As the pressure drops in the FCU, fuel flow is reduced to the engine slowing it down and therefore propeller speed also.

Answer 69

.

Question 70

As P3 air enters the FCU, it is divided into two components, Px and Py. They have different pressures and between the two they allow a balance to be obtained in evacuated bellows inside the FCU. Evacuated bellows are the “brain” of the FCU. Anything that upsets the air pressure balance will cause the FCU to react.

Answer 70

.

Question 71

An oil over speed governor houses a set of flyweights connected to a control valve that is driven by a bevelled gear mounted on the propeller shaft.

Answer 71

.

Question 72

The flyweight’s centrifugal force is acting against two springs: a speeder spring and a reset spring. When the propeller speed reaches a specified limit (four percent over maximum propeller speed), the governor flyweights lift the control valve and bleed off propeller servo oil into the reduction gearbox sump, causing the blade angle to increase. An increase in blade pitch puts more load on the engine and slows down the propeller.

Answer 72

.

Question 73

To test the unit, the speed reset solenoid is activated, and servo oil pressure pushes against the reset piston to cancel the effect of the reset spring. With less spring tension acting on the flyweights, the over speed governor can be tested at speeds lower than maximum.

Answer 73

.

Question 74

On twin installations, a second solenoid valve is mounted on the over speed governor and is used in conjunction with the aircraft auto feather system

Answer 74

.

Question 75

how are double acting propeller pitch change mechanisms operated

Answer 75

normally by oil pressure but by fly weights in an emergency

Question 76

how are single acting propeller pitch change mechanisms operated

Answer 76

by oil pressure and spring force

Question 77

how can a propeller automatic feathering system be selected off

Answer 77

by retarding a power lever to flight idle