17.3 Flashcards
What are the three reasons that a prop governor will go into overspeed?
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
What type of gearing is required for a turbo prop aircraft?
Usually some form of compound epicyclic design with reduction ratios of between approximately 6:1 and 20:1
What are the three main types of engine for a turboprop?
Direct Connected (Integrated).
Compounded.
Free Power Turbine
How does a direct connected engine power the prop?
The turbine of this type drives the compressor through the reduction gearbox, the propeller. A single power lever controls both the engine fuel flow and the
propeller pitch
Disadvantages of a direct connected engine?
Requires fine (low) pitch stop for starting.
Complex interconnection between throttle and propeller control.
May be unstable with sudden changes in load due to the attitude of the aircraft
How does a compound engine work?
A twin spool layout where the HP turbine drives the HP compressor and the LP turbine drives the LP compressor and propeller reduction gearbox.
Power is controlled by a lever which selects blade angle, and various automatic systems maintain RPM by adjusting fuel flow
What is the disadvantage of a compound engine?
Only for big engines
What are the advantages of a compound engine?
Engine free from surge.
Good flexibility.
Good acceleration
Low power starting due to twin spools
How does a free power turbine work?
The compressor and propeller are mounted on different shafts and driven by independent turbines. The power lever provides direct control of the gas
generator fuel flow while the conditioning lever controls the propeller RPM
Advantages of a free power turbine?
Easier starting
Independent control of engine and propeller
Engine and propeller can both operate at optimum speed which gives lower fuel consumption
What is ESHP?
Although most of the energy of the gasses are used to drive the compressor and propeller, there will still be some residual thrust available from the exhaust. The sum of the SHP and thrust is termed Equivalent Shaft
Horsepower. This indicates total power available from the engine
What are the 4 types of control arrangements for a turboprop?
Integrated Control of both Propeller RPM and Fuel Flow
Direct Control of Fuel Flow
Direct Control of Blade Angle
Twin Lever Control
Explain Integrated Control of both Propeller RPM and Fuel Flow
Suitable for direct coupled and compound engine types. Propeller RPM is selected by the power lever which simultaneously adjusts fuel flow thereby
ensuring the correct fuel flow for a selected propeller RPM. At maximum RPM, further increases in power are achieved by increasing the fuel flow. The Constant Speed Unit (CSU) increases the blade angle to
absorb the extra power to keep the propeller RPM constant
Explain Direct Control of Fuel Flow
Suitable for use on a free power turbine. The gas generator is controlled in the same manner as a turbojet. The propeller is maintained at a constant
selected RPM by the CSU
Explain Direct Control of Blade Angle
Can be used on any type of turboprop engine. The power lever selects a blade angle and various automatic systems adjust the fuel flow to keep the
propeller RPM constant
The direct control of blade angle in a free power turbine system is also used on some helicopters. Operation of the collective pitch lever adjusts
fuel flow/engine RPM to suit the selected requirements
Explain twin lever control
Installed on aircraft with single acting propellers. The engine is controlled by the power lever and the propeller RPM is dependently selected using
the propeller conditioning lever
Explain ground fine pitch stop
A fixed abutment stop on the operation piston which allows the propeller to take up a zero degree position. This is known as the Discing position and
is used for starting and initial acceleration. The blades move to this position when the power lever in the cockpit is moved to ground idle
Explain FFPS
This is a retractable stop used to limit the fine (low) pitch available during flight. It is always in the engaged position during take off and cruise power
settings. The FFPS is locked in position automatically as the power lever is advanced to take off. This stop is operated by oil pressure but controlled electrically via a
cockpit located FFPS lever with two positions, ENGAGED and WITHDRAWN.
Requirements
• To energise both engine lock withdrawal solenoids.
• To provide cockpit indication that the circuit is energised.
• To de-energise the electro-hydraulic stop circuit when the FFPS circuit is
energised
Explain the electro-hydraulic stop
A pitch coarsening mechanism to prevent the propeller drifting below flight fine pitch should a failure of the FFPS occur. The system is activated by a micro
switch which will energise the system if the propeller blade angle should decrease below the FFPS by 2 degrees. The system is ARMED when the FFPS lever is ENGAGED and DISARMED when the FFPS lever is WITHDRAWN. Pitch coarsening is controlled by a Piston Valve Lift Solenoid
Explain the hydraulic pitch lock?
A spring operated valve which is designed to prevent the propeller from moving to a finer (lower) pitch in the event of an operating oil supply failure. The valve is positioned in the coarse (high) pitch oil line and in the event of an oil pressure failure, the spring will overcome and close the valve thereby
trapping the oil in the coarse (high) pitch oil line
What does manual feathering do to to the PCU?
Selection of the feathering lever overrides the propeller control unit (PCU) centrifugal flyweights by lifting the pilot valve to direct oil, supplied by an
electric feathering pump, to the coarse (high) pitch line and driving the propeller to the feather position
When is up-trim automatically carried out?
- Torque of the local engine falls below 25%
- NP (as indicated by the torque sensor) falls below 80%
- PLA is in the rating detent
- MTOP is not set
When is autofeather engaged?
Autofeather 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. Following a 3-second delay, an overriding drive coarse signal is input on both active and standby Control Lanes; the servo valve then select coarse
pitch. When one propeller is autofeathered, the autofeather function of the other propeller is automatically disarmed. The auxiliary feathering pump is also activated for approximately 30 seconds. This makes sure adequate oil pressure is available for propeller feathering
