15.16 Flashcards
Where is a turboprop engine the most efficient power source?
From aircraft operating from 483 to 725 km/h (300 to 400 mph)
What is the downside of the turboprop engine compared to the turbojet?
They are more complicated and heavier
What type of engine has the best fuel consumption out of any turbine engine?
Turboprop
How much of the total power produced is used?
95%
In a turboprop engine what is the core engine commonly known as?
A gas generator or gas producer
What does the core engine produce?
High-velocity gases that provide the energy to drive the power turbine
What is special in regards to the gas generator and the free-power turbine?
There is no mechanical connection (Gas flow is the inly connection)
What are the 4 advantages of a free-power turbine engine?
Vibration is not directly transmitted to the gas generator from the propeller/reduction gearbox
Engine is easier to start (especially in cold weather)
Propeller can be held at low rpm during taxi (low noise and blade erosion)
Propeller brake can be used to prevent propeller from turning so engine can be used as an APU
How to gear/direct coupled turbines differ from free-power turbines?
The addition of extra turbine discs which remain mechanically connected to the compressor and reduction gearbox
What adjusts the propeller pitch for the power requirements of the engine?
A complex integrated propeller control system
What is the same at normal operating conditions?
The propeller speed and engine speeds are constant
What must be coordinated to maintain a constant speed condition?
Propeller pitch and fuel flow
What must change about the propeller when the fuel flow changes and why?
The propeller pitch to maintain a constant speed
What is the function of the reduction gear assembly?
Reduce the high rpm from the engine to a propeller rpm
What is important about the propeller rpm?
It must not exceed the maximum propeller tip speed (speed of sound)
What is reduction achieved using?
Through a series of both spur gears and epicylic gears
What will smaller and larger reductions be done by?
Smaller - Spur gears
Larger - Epicyclic gears prevent torque couples forming
What does the reduction ratio depend on?
The relationship between the size of the driving and fixed gear
What are the gears used in engine reduction gearboxes made of and why?
High-quality alloy-steel forgings because of the high stresses they endure
What do ball bearings do in larger shafts?
Absorb and transmit all loads imposed on them to the casing
What are the 2 ways reduction gearboxes are lubricated?
By the engine lubrication systems
Their own self-contained system
Why is the viscosity of oil higher than that for a turbo-jet engine?
Because of the heavier loads imposed on the gears
What are the 2 types of reduction gears?
Parallel spur gear type
Epicyclical type
What is an advantage of a parallel spur gear?
They are mechanically simple
What type of gears can a parallel spur gear be?
Straight-cut
Helical
What are the 2 advantages and 1 disadvantage of straight-cut gears?
They are easier to cut and have the lowest friction losses
They are very noisy
What do torque changes result in and what does this require?
High loading on the individual gear teeth either high sideways loads, requiring large heavy-duty bearings
What is an advantage and disadvantage of helical gear?
Quieter
But less efficient transmission
How is an axial component on the helical gear handled?
Either a ball or tapered roller bearing
What does an epicyclic arrangement incorporate?
A sun gear wheel on the power turbine shaft, driving several planet gears that are engaged with either a fixed or rotating ring gear or annulus
Is the reduction gear ratio affected by the number of planet gears or number of teeth on them?
No
What is a fixed annulus?
When the outer fixed gear ring or annulus is fixed stationary
What is a rotating annulus?
When the arrangement rotates on fixed pinion shafts
What direction does the ring gear connected to the propeller shaft turn?
The opposite direction of rotation to the power turbine shaft
What is alpha range?
When the aircraft is in flight
What is beta range?
When the aircraft is on ground
What can the propeller provide when in beta range?
Zero or negative thrust
What do modern turboprop engines utilise to control the engine and propeller?
A duel FADEC system
What does FADEC stand for?
Full Authority Digital engine Control
What is the spinner assembly and what does it do?
A cone-shaped configuration that mounts on the propeller and encloses the dome and barrel to reduce drag
What does a turboprop engine have when it’s shut down in flight?
A feathering system
Can the propeller be unfeathered during flight if the engine needs to be started again?
Yes
Where is the throttle quadrant located and why?
On the central pedestal in the flight deck, so both flight crew members can operate the levers
What is on the throttle quadrant?
Integrated engine and propeller controls
What is a single-lever control?
Both the fuel and propeller control units are interconnected (forward flow increases fuel flow and selects a higher rpm)
What does the Propeller Control Unit (PCU) govern?
The maximum rpm of the engine
What takes over from the PCU when the rpm has risen to s predetermined level?
The fuel control unit, which will inhibit the rise of rpm
Where are duel-lever controls used?
Turbopropeller engine with a free turbine
What are the 2 levers used for in a duel-lever control?
One is a power lever
One is a propeller control lever
What does a thrust lever do?
Schedule fuel flow from idle to take-off
What does the thrust lever do when put into reverse?
Schedules fuel and the blade angle from +20 to -20 degrees
What moves the blade angle?
Oil pressure
What does a propeller lever do and what is it connected to?
Selects the required rpm by applying or releasing pressure on the governor control spring, it is connected directly to the PCU
When does the propeller level operate/not operate the propeller?
Operates it in the normal constant speed range, but doesn’t operate it when the propeller id in the beta range
Where is a control lock located?
On the control quadrant
What does a control lock ensure?
That the surfaces do not flutter excessively on the ground with the engines running
What does overspeed cause?
Extreme stress to the propeller and the engine that can lead to rapid and total failure in flight
How does the propeller balance engine power output?
By absorbing it by presenting the blades at an angle to the airflow (blades are not aligned with the plane of the propeller rotation)
How does the blade angle change mechanism work?
It is hydraulically operated by the engine oil
What would happen if the engine failed during flight due to centrifugal twisting motion?
The blades will try to return to a zero angle
What does the blades being at a zero angle do?
Presents the blades as a negative angle of attack and the propeller will windmill to uncontrollable rpm, the propeller is now driving the engine
What can the reduction gearing be as low as?
1:22, for every 1 spin of the propeller the engine will rotate 22 times
What do we do to avoid the propeller making the aircraft uncontrollable?
The blades are set at 90 degrees (feathering angle) so there is no angle of attack so the propellers will stop rotating
What does the solenoid do in a feathering system?
It lifts a valve to pass oil pressure to the control valve to force it fully up
Where is the oil supply for the feathering pump drawn from?
The engine oil tank which remains filled even if the engine oil pressure is lost
What are the 2 backup locking mechanisms?
Mechanical
Hydraulic
What happens if the oil pressure fails?
A shuttle valve in the mechanical pitch look will move under spring force ti drain the oil holding the locks out of engagement
What do the locks engage to?
The racket teeth on the pitch change position which means the piston will be unable to move towards fine pitch but can move towards course pitch under the pump pressure
What does the shuttle valve in the hydraulic pitch do?
Moves under spring pressure to block the oil return from the front of the piston
What happens if the propeller speed exceeds 105% ?
A separate governor will dump oil pressure to the return causing the blade angle to increase until the overspeed ceases
What is the governor?
A safety device designed to operate if the normal propeller governor and the fuel control overspeed governor fail to contain engine speed
What does a mechanical system do if the turbine detaches from its shaft?
Immediately shuts off the engine-metered fuel supply
