17.3 Flashcards
Two types of propellers?
Push or pull tractor propellers
Methods for changing pitch for controllable or constant speed propellers?
Hydraulic
Mechanical
Electrical
A shut down engine should be what to minimise drag?
Feathered
What is discing braking?
Moves props to a very low pitch on landing, causing negative angle of attack and thus a braking force
When is feathering done and why?
Done at engine failure to prevent engine windmilling
When is the BETA range used?
Pilot controls pitch on ground.
Pilot directly controls pitch of prop through throttle
When is ground fine pitch used?
Engine starting
When is reverse pitch used?
Braking and manoeuvring
What is ALPHA range?
Also known as the constant speed range.
The propeller governor controls the pitch.
When is max fine pitched used?
Used for rapid acceleration during takeoff
-allowing a shorter runway
When is Coarse (High) pitch used?
Used for high forward speed
How does a moving piston work?
A piston attached to the propeller blade lungs and moves within a cylinder
How does a moving cylinder work?
The cylinder is attached externally to the propeller blade
How do single acting propellers work?
Oil pressure changes the pitch in one direction only
Spring force and the torsion moments of the blades
How do single acting propellers for single engine aircraft work?
Oil pressure moves the blades in the direction of high (coarse) pitch and the spring moves it towards low pitch
How does single acting propellers work for multi engine aircraft?
-Oil pressure moves blades in direction of low (fine) pitch
-Springs and torsional moments of blades, move the blades towards high pitch
-Engine failure- decreasing oil pressure the blades move in the high (coarse) pitch direction
How does a double acting propeller work?
Pitch change are controlled both ways by oil pressure
Usually constructed as co-axial tubes, one containing oil for pitch and other for fine pitch
If control valve is mounted behind gear box, the propeller shaft must have what?
Must have two oil transfer tubes for each side of the piston
Where is propeller governer usually mounted?
On propeller reduction gearbox
Different names for governer?
Propeller governor
Constant speed unit
Propeller control unit
Different components of governor system?
-Spur gear pump to provide operating pressure
-Pilot valve which controls oil flow through the pitch change mechanism.
-Speeder spring pushes the valve downwards
-Centrifugal flyweights within the governor oppose the spring pressure and move the landed valve up to provide a coarser angle
-safety spring fitted to permanently set the blades to a fine pitch position in the event of control lever failure
What is on speed condition?
Flyweights are vertical and in position where no oil can flow in or out of governor assembly- creating a hydraulic lock
What is the under speed condition?
-Pilot requires higher RPM
-Lever forwards
-More tension on speeder spring
-Pilot valve down
-Piston move rearwards
-Engine RPM increased until new hydraulic lock
What is overspeed condition?
-Pilot selects lower speed,
-reduces tension on the speeder spring
-centrifugal force of flyweights to lift up the pilot valve
-Oil is now directed into pitch becomes coarser, slows RPM
-This happens until the speeder spring and centrifugal force tension is balanced again
-Creates new hydraulic lock
If engine is stopped what will be occurring in governing assembly?
No force on flyweights
Pilot valve in position
Governing system positions in start-up to idle plus?
Flyweights move outward with increasing engine RPM, until constant (on) speed is reached.
Landed valve/pilot valve in central position
The propeller rotates with a constant what?
RPM within its operating ranges
Changes in power are achieved by what?
Changes in torque
The propellers of turboprops have greater range of pitch change than those of what?
Piston engines
Propellers are only efficient a what RPM?
1000-1200 RPM
To match the propeller to the engine what is required?
A reduction gearbox
Reduction ratios are between approximately what?
6:1 and 20:1
Disadvantages of direct coupled engine arrangement?
Requires fine pitch stop for starting
Complex connection between throttle and prop control
Can be unstable with outside loads introduced with aircraft at different attitudes
Compounded engine arrangement advantages?
Due to engine operating at constant RPM, it is free from surge
Good acceleration
Low power starting as HP compressor on a different spool (twin spool engine)
Disadvantage of compound engine arrangement?
Used on large engines mainly
Advantages of free power turbine engine arrangement?
Easier starting
Independent control of engine and propeller
Engine and propeller operate at optimum speed
Lower fuel consumption
Disadvantages of free power turbine engine arrangement?
More Pilot intensive as he needs to know parameters and constantly maintain them
What is the engine stopped operation?
No centrifugal force on flyweight
Speeder spring pushes pilot valve down
Oil trapped in cylinder between piston and oil pump
What is start up to idle plus operation?
Flyweights move outwards
Increasing RPM until constant speed reached
Pilot valve central
Propeller in fine pitch
What is increased to take off operation?
Speeder spring forced down against flyweights
Fine pitch oil moves blades to a lower pitch for take off
What is take off operation?
As forward speed increases the angle of attack decreases
Propeller load decreases and propeller tries to overspeed
What is increased to cruise operation?
Propeller rpm increases
Centrifugal force overcomes spring force
Pilot valve up
Blades turn more coarse
RPM decreases until flyweights and speeder springs are in balance
What goes where during feathering condition?
Pilot valve lifts up
Counterweights and spring pressure move blades to feathered position
The propeller governor moves to the over speed condition for what three main reasons?
- 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
Under-speed condition results in reverse of above
Some systems have a what to provide a reserve of pressure to turn the blades?
Unfeathering accumulator
What do centrifugal latches do?
When engine shuts down they stop the piston moving and lock it into fine pitch
What is shaft horse power?
Power available from engine at propeller driveshaft
What is equivalent shaft horsepower?
Residual power from engine exhaust plus shaft horsepower
What type of turbines use direct control of fuel flow?
Free power turbines
What type of direct control is used when propeller follows engine?
Direct control of fuel flow
What type of direct control is used when engine follows propeller?
Direct control of blade angle
At max RPM more power is achieved by what?
By increasing fuel flow
Turbo prop pitch control system requirements?
Provide constant speed control
Minimum drag on failed engine
Minimum starting drag
Create high drag for A/C braking
Limit fine pitch during flight
What is ground fine pitch stop?
For starting, a stop on piston ensures the blades are at zero degree position
-causes least drag
What is flight fine pitch stop?
-Retractable stop to limit the amount of fine pitch available during flight
-This ensures thrust is given when needed
-Fine pitch would cause engine to overspeed
-Withdrawn for starting and ground operations
What is FFPS operated by and where is it controlled?
Operated by oil pressure
Controlled electronically in cockpit
Two positions of FFPS?
Engaged
Withdrawn
Requirements of FFPS?
-Energise both engine lock withdrawal solenoids
-Provide cockpit indication that circuit is energised
-De-energise electro hydraulic stop circuit when the FFPS circuit is energised
What does the electro-hydraulic stop do?
Coarsens the pitch of the blade preventing the propeller from drifting below flight fine pitch, should FFPS fail
What is automatic feathering?
During take off and cruise, if engine shuts down the feathering position system will take over
Advantages of beta range operation?
Reduce landing roll
Reduce break wear
Improved manoeuvrability on ground
Reversing of the aircraft during taxiing
Disadvantages of beta range operation?
Reduced engine cooling when in reverse pitch
Increased blade damage
Complicated control system
Resetting of the beta valve is done through what?
Beta ring
Propeller governor ineffective in what range?
BETA range
How is beta range controlled?
Power lever
