17.3 propellor pitch control

Question 1

what types of propellors typically have hydraulic pitch change mechanisms?

Answer 1

-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 2

what is disc braking?

Answer 2

moving blades to a very fine low pitch on landing causing a negative angle of attack and a braking force

Question 3

what is power on braking?

Answer 3

reverse pitch
propellors which can move their blades to a negative angle of attack and offer resistance to rotation

Question 4

what does power on braking allow for?

Answer 4

a proportional and controllable breaking force

Question 5

what is feathering?

Answer 5

in engine failure CTM tends to create a fine pitch causing a high drag/ disc braking
to avoid this propellor blades are moved to the 90 degree position reducing drag

Question 6

what is ground fine pitch used for?

Answer 6

Used for engine starting and discing braking

Question 7

what is max fine pitch used for?

Answer 7

Used for engine starting and discing braking

Question 8

what is the moving piston system for pitch adjustment?

Answer 8

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

Question 9

what is the moving cylinder system in terms of pitch change?

Answer 9

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.

Question 10

where is moving cylinder usually found?

Answer 10

smaller aircraft propellors

Question 11

what is a single acting propellor?

Answer 11

oil pressure changes the pitch in one direction only. the opposite direction is the result of spring force and the torsion moments of the blades

Question 12

what is the principle of Single Acting Propellers for Single Engine Aircraft?

Answer 12

oil pressure moves the blades in the direction of high (coarse) pitch and the spring moves it towards low pitch

Question 13

what occurs in engine shutdown for Single Acting Propellers for Single Engine Aircraft?

Answer 13

the blades are in the lowest (fine) pitch stop position, which is optimal for restarting the engine

Question 14

what will occur in engine failure for Single Acting Propellers for Single Engine Aircraft?

Answer 14

blade position is favourable for windmilling, which makes it easier to restart the engine.

Question 15

what is the principle of Single Acting Propellers for Multi-Engine Aircraft?

Answer 15

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.

Question 16

what occurs engine failure for Single Acting Propellers for Multi-Engine Aircraft?

Answer 16

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 17

why are counterweights attached to blades in terms of the effects of CTM?

Answer 17

they will cause the centrifugal loads on the counterweights to drive the blades towards a higher pitch (towards the feather). rather than a fine pitch which the CTM naturaly cause the blades to enter

Question 18

when counterweights are fitted to the blades what will a loss in hydraulic pressure lead to in terms of pitch?

Answer 18

the pitch to increase to a safe setting

Question 19

what is a double acting propellor?

Answer 19

pitch change mechanisms where oil pressure leads to pitch change in both directions

Question 20

what is the function of the propellor governor?

Answer 20

an RPM sensing device which relies on sensing oil pressure change in the propellor hydraulic cylinder or releasing oil pressure from the hydraulic cylinder in turn changing the blade angle

Question 21

what are the three names for a propellor governor?

Answer 21

constant speed unit (CSU)
propellor control unit. (PCU)
propellor governor

Question 22

what does the governor assembly consist of?

Answer 22

a spur gear type pump to provide pressure for the system
a pilot valve to control oil flow to and from the pitch change mechanism
safety spring to push to a fine position in failure

Question 23

what does an electronic control unit use to replace the mechanical propellor governor unit

Answer 23

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

Question 24

what will occur in an electronic control system in ECU failure?

Answer 24

the entire system reverts back to manual control under a power lever.

Question 25

how is electrical powered supplied in a electrical control unit?

Answer 25

a slip ring and brush assembly

Question 26

how does an electrical control differ from an electronic control

Answer 26

electrical control still relies on oil pressure and partially mechanical system

Question 27

how accurate is the electronic control typically?

Answer 27

within two propellor revolutions per minute

Question 28

how can feathering be activated?

Answer 28

either manually or through automatic means during certain stages of flight

Question 29

what are feathering system components?

Answer 29

feathering pump, reservoir, a feathering time-delay switch, and a propeller feathering light.

Question 30

how is feathering achieved?

Answer 30

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 31

how can feathering be manually activated?

Answer 31

-by depressing the feathering button
- by pulling the engine emergency shutdown handle or switch to the shutdown position.

Question 32

when can automatic feathering occur?

Answer 32

-in the event of an engine failure the centrifugal flyweights are automatically overridden and the electric feathering pump runs

Question 33

when is auto feathering set to on?

Answer 33

for take off only

Question 34

what is up trim?

Answer 34

the surviving engine to increase its power by 10%.

Question 35

when is up trim activated regardless of auto feather selection?

Answer 35

-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.

Question 36

how is feathering achieved?

Answer 36

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.
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 37

how is unfeathering achieved?

Answer 37

through the unfeathering accumulator

Question 38

what is the purpose of centrifugal latches?

Answer 38

-to prevent shut down engines oil from being trapped in the cylinder and forcing the blade angle to high pitch
-centrifugal latches force the propellor into a low pitch position when the engine is not spinning page

Question 39

what engages centrifugal latches?

Answer 39

by spring pressure at low RPM (typically 700 to 1000 RPM) and disengaged by centrifugal force as the RPM increases

Question 40

what is the BETA range?

Answer 40

the range of blade angles which can be obtained when the aircraft is operating on the ground

Question 41

what is reverse pitch?

Answer 41

the propeller blade angle is positioned to the negative side of ground fine (low) pitch.

Question 42

what are the disadvantages of reverse mode?

Answer 42

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

Question 43

how does moving the power lever rearwards put the propellors into reverse pitch?

Answer 43

-moving the lever rearwards moves the beta valve rearwards
-this allows more oil to flow into the dome
-causing the propellors to go into reverse pitch
-the dome pulls the carbon block slip ring forward and moves the beta valve outward again restricting oil flow

Question 44

what does the primary propeller governor/ CSU work to limit speeds up to?

Answer 44

102-103%

Question 45

what occurs if the overspeed reaches more 104-106%

Answer 45

propeller overspeed governor operates and returns oil from the propeller dome back into the reduction gearbox.

Question 46

which aircraft have another overspeed protection?

Answer 46

some particularly those powered by a PT6A engine