Module 17.3 Propeller pitch control Flashcards
What are the 3 methods pitch change?
- Mechanical
- Hydraulic
- Electrical
What is disc braking?
Moving blades to a very fine angle to get a negative angle of attack. Drag increases
What is power on braking? (reverse pitch)
Blade angle to a negative angle, allowing power to be applied and a controlled braking force
What is the purpose of feathering?
In the event of engine failure, feathering overcomes the CTM to put the blade to 90°
What is BETA Range?
Pilot controls the pitch on the ground
What is Negative (Reverse) Pitch used for?
Used for power-on braking and manoeuvring
What is Ground Fine Pitch used for?
Used for engine starting and discing braking
What is Max Fine Pitch used for?
Used for rapid acceleration during take-off, allowing a shorter take-off run
What Coarse (High) Pitch used for?
Used for high forward speed
The pitch change mechanism can be further categorised as what?
- Moving piston
- Moving cylinder
In Single Acting Propellers for Single Engine Aircraft how is the blade angle manipulated?
Oil pressure moves blade to coarse pitch
Springs move blades to fine pitch
In Single Acting Propellers for Single Engine Aircraft, why is the low blade angle desirable during engine failure?
Easier to restart the engine
In Single Acting Propellers for Multi-Engine Aircraft, how is the blade angle manipulated?
Oil pressure moves the blade angle to fine pitch
Springs move blade angle to coarse pitch
In Single Acting Propellers for Multi-Engine Aircraft, what occurs in the event of engine failure?
Propeller moved by spring top feathered position
What pitch do CTM’s take the blade to?
Fine pitch
What is a method of overcoming CTM?
Counterweights which because of the centrifugal force drive the blade to a coarser pitch
What are double acting propellers?
Propellers which use oil pressure to change pitch in both directions
What is a governor made up of?
RPM sensing device and a high pressure oil pump
Depending on the manufacturer, the propeller governor can be classified as what?
- Constant speed unit (CSU)
- Propeller control unit (PCU)
- Propeller governor
What does the governor assembly consist of?
- Spur gear type pump
- A pilot valve
What does the propeller control lever effect?
The tension on the speeder spring
In counterweight control, what does the counterweight moves the blade to?
Fixed feather stop, max coarse angle
In counterweight control, what does the oil pressure move the blade to?
Full reverse stop, finer angle
In electrical control, how is pitch change achieved?
Electric motor that drives a bevel gear through a high reduction ratio gear train.
How is electrical power supplied?
Slip ring and brush assembly
In the electrical control, what does the oil in the propeller governor actuate?
Servo pistons
In electronic control, what is the mechanical propellor governor unit replaced by?
Two-stage servo valve, driven by a torque a torque motor that receives signals from the ECU
What does the ECU receive inputs from?
The pilot, computed against EGT, N1 and torque
What happens if the ECU fails?
The entire system reverts back to manual control under a power lever
Why is feathering selected?
To prevent excessive drag and negative torque (Windmilling) in the event of engine failure
What does a basic propeller feathering system consist of?
- Pump
- Reservoir
- Time-delay switch
- Propeller feathering light
What effect does moving cockpit control against the low speed stop have?
Pilot valve lift rod in the governor to hold the pilot valve in the decrease RPM position irrespective of flyweight position
Causing the propeller to rotate through high pitch to feathered position
What effect does pressing the feathering button have?
Actions the auxiliary pump and feather solenoid, this positions the feathering valve to transfer oil to the propeller
How is the auxiliary pump turned off?
As pressure builds up it operates a pressure cut off switch
How may feathering also be achieved?
Pulling the engine emergency shutdown handle or switch to the shutdown position
When is auto feathering in use?
During take-off and cruise
In the event of engine failure what happens to the flyweights?
Overridden
What does the auto feathering system provide the operating engine?
Power up trim during engine loss at take off
When is auto feather selected to on?
Take off only
When is up trim triggered?
- Torque of local engine below 25%
- Propeller RPM falls below 80%
- PLA is in the rating detention
- Max take off power not set
What signals must be confirmed by both torque signals?
- Torque of local engine below 25%
- Propeller RPM falls below 80%
What accommodates the failure case of a propeller auto coarsening or inadvertently feathering, causing loss of thrust but not low torque?
The low speed condition
When is up trim directly signalled?
When an auto feather occurs
What is the purpose of sending dual up trim signals to the FADEC of the surviving engine?
To increase power by 10%
What is the purpose of increasing the power on the remaining engine?
Replace normal take off power with max take off power
As the piston moves under the feathering oil pressure what occurs to the fine pitch oil?
Displaced to the 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 what?
Relative airflow
How are propeller unfeathered on small engines?
Move the power lever to a high RPM and turn the engine on the starter motor
What are some systems equipped with to provide a reserve of pressure?
Unfeathering accumulator
What is the unfeathering accumulator charged through?
Non return valve
How is the propeller unfeathered in systems with an accumulator?
Power lever to a high RPM and accumulator pressure released by electrically operated solenoid valve or mechanical input to unseat the valve
What is the purpose of the centrifugal latches on single acting propeller systems?
If there is an oil leak, the propeller blades are forced to a coarse angle under spring pressure. Putting a heavy load on the starter
The centrifugal latches lock the piston in a fine pitch
How are the centrifugal latches engaged?
Spring pressure at low RPM
How are the centrifugal latches disengaged?
Centrifugal force
When positioned aft of flight idle, what does the power lever directly control?
Blade angle
In beta mode, what does the power lever work directly on?
Beta valve
Where is the first propeller overspeed backup?
Primary propeller governor
What does the primary propeller governor use to control speed?
Balance of speeder springs and flyweights
When does the primary propeller governor operate?
Between 102% to 103%
When does the overspeed governor operate?
104% to 106%
What does an oil overspeed governor house?
A set of flyweights connected to a control valve that is driven by a bevelled gear mounted on the propeller shaft
In the overspeed governor, what occurs when the propellers speed reaches a specified limit?
The governor flyweights lift the control valve and bleed off propeller servo oil.
Where does the propeller overspeed governor bleed oil too?
Reduction gearbox sump
To test the unit what solenoid is activated?
Speed reset
When the solenoid is activated, what occurs?
Servo oil pressure pushes against the reset piston to cancel the effect of the reset spring. The over speed governor can be tested at speeds lower than maximum.
Where does py pressure come from?
Fuel Control Unit
How does the Py overspeed generator work?
As pressure in the FCU reduces, this reduces fuel flow to the engine
