Propeller Systems Flashcards
What is the purpose of a reduction gearbox
On powerful designs used to translate large amounts of power generated at high engine rpm into large amounts of torque for a relatively slow turning propellor
What are propellor blades made off
Propellor hubs are metal. Blades are made from aluminium alloy or composites
What is the aerodynamic force in line with drive shaft
Thrust
What aerodynamic force acts parallel to and in the opposite direction to plane of rotation
Torque Drag.
How is thrust increased on a fixed pitch propeller
Increasing engine RPM.
What are the disadvantages to a fixed pitch propeller system
Only operates efficiently in a narrow range of RPM. Below or above this speed engine becomes less efficient.
What are the advantages of a variable pitch propeller system
Adjustable pitch angle to maintain the blade at the optimum AOA through speed range.
How is thrust increased on a variable pitch propeller
Thrust is increased by increasing blade AOA rather than propellor rpm.
What is the Power control unit (PCU)
Mechanism responsible for changing blade pitch angle.
How does the PCU work
Inside Prop hub a piston is connected tot pins/links on root of propeller blade. The linear motion of the piston is translated into rotational movement by the pins.
What drives the PCU
Electric motors or more commonly by oil pressure or combo or oil/spring pressure.
How does a single acting PCU work on a single engine aircraft
Pressurised oil is fed to one side of the piston only driving pitch angle towards coarse.
Role of single acting PCU on twin engine aircraft
Coarsen off blade to feather position in event of engine failure.
Counterweights on root of blade creates a couple to twist blade towards feathered position.
How does a double acting PCU work
Oil pressure supplied to both sides of piston. Switching system directs high pressure oil to appropriate side to either coarsen or fine-off propeller.
Why are double acting PCU’s needed
Used on larger propellor systems to overcome greater centrifugal and aerodynamic forces produced by the blades.
What is the purpose of a centrifugal latch
Minimise propeller drag during engine start to reduce load on engine. Stops the blades from moving to fully feathered position on engine shut-down.
How does a centrifugal latch work
At low rpms (>1000) pins emerge and stop the piston from moving back towards feather position.
What happens if engine torque exceeds torque drag and vice versa.
If it exceeds propeller will accelerate and if its less propellor will decelerate.
What does a Control speed unit do
Match torque drag to engine torque.
If propellor rpm falls moves blades to finer pitch.
If propellor rpm rises moves blades to coarser pitch.
What is CSU overspeed
As engine rpm increases the governors rotating flyweights are thrown outwards lifting valve against spring pressure. In lifted position landed valve directs pressurised oil to coarse side of PCU. Blades coarsen, propeller torque drag increases and engine rpm reduces.
What is CSU underspeed
Governor flyweights move inwards and landed valve goes down. Valve directs high pressure oil to fine pitch side. Blade moves finer, propellor drag reduces below engine torque and propellor accelerates back to equilibriums.
What is CSU on speed
Propeller torque=engine output. Governor moves inwards and landed vale lowered which shuts off supply of high pressure oil to valve chamber. Hydraulic lock created which holds propeller at selected pitch angle
What does the propellor rpm lever do
Allows to select different RPM’s.
How is the feathering system backed up
Key requirement is supply of high pressure oil for the PCU which comes from engine.
Backup system is a separate oil reservoir with sufficient oil to allow prop to be feathered. Supplemented by electric driven pump.
What is the role of auto-feathering system
Take over engine shut down and prevent loss of control
What is the beta range
Range of (v.fine) pitch angles available below flight idle. Ie reverse thrust
What is the role of the power lever in Beta Range
Power lever controls blade pitch.
What is cabin noise
Propellors in line with cabin. Noise created by pressure waves from blade tips hitting fuselage.
What is the synchronising unit
Eliminates rhythmic amplitude by synchronising the propellors by rotating all at same RPM. One engine made a master and others slaves. Synchronising unit compares RPM of slaves and adjusts them so that they are same as Master.
What is a synchrophasing unit
Once’s synchronised SP unit makes a very brief and temporary rpm adjustment to slave engines to adjust the position of the blades of each propeller. Reduces cabin nose levels.
What is used as an indication of engine power for a constant speed propeller and a fixed speed propeller
Map gauge for constant speed. (Pressure of air passing through the inlet manifold)
RPM for fixed.
What power measurement does a turboprop engine use.
Torque gauge which indicates amount of torque being applied by the engine to the propeller shaft.
Torque sensing can be done electronically or by measuring oil pressure.
What does the fully fine setting do
Minimises torque drag allowing engine fo respond very quickly to changing power requirements.
How can overboosting be prevented at low rpm
Rev up and throttle back.
What are the CSU checks
Start- rpm lever opens to encourage oil flow. Look to see rise n fall rpm
Feathering check- momentarily set to feather, see rpm drop quickly before removing the rpm.
Rpm lever Fully forward for taxi and takeoff
