15.16 TURBOPROP ENGINES👌🏼 Flashcards
Explain feathering system
- propeller can be feathered when engine is shut down in flight
- prevents propeller spinning if engine fails
What do modern turboprop engines utilise to control both engine and propeller
- dual full authority digital engine control (FADEC)
Can a turboprop aircraft be unfeathered
Yes
Explain single-lever control
- gear coupled engines
-Single lever that controls both fuel and propeller control
Explain dual-lever turbines
- used on turboprop engines with free turbine
- one lever is power lever
- other is a propeller control lever
Explain thrust lever
- schedule fuel flow from idle to takeoff range.
- schedules blade angle too
What are the 2 main types of reduction gears
- parallel spur gear type
- epicyclical type
What is ‘BETA’ or ‘ground operating range’
- when thrust lever is retarded from idle to reverse, thrust lever schedules fuel and blade angle
Explain propeller lever
- connected directly to PCU and selects required rpm
- by a governor control spring
Explain control lock
- ensure aircraft control surfaces do not flutter excessively when engines are running on ground
What is the function of the reduction gear assembly
Reduce high rpm from the engine to a propeller tip speed (speed of sound)
Smaller reductions are achieved by what gears
- spur gears
Larger high torque reductions will use what gear
Epicyclic gears
What must occur if the oil supply to the propeller fails
- feathered quickly to avoid windmilling
If the torque indication system senses negative torque, a signal is sent where
Auto-feather system
How many locking mechanisms are there in pitch locking
2
(One is mechanical) ( other is hydraulic)
What keeps both locks open in pitch locking
Fine-pitch oil line pressure
If engine (oil pressure) fails what occurs
- shuttle valve in mechanical pitch lock will move under spring force to drain the oil holding the locks out of engagement
How are the locks engaged in pitch locking
- engage with ratchet teeth on the pitch change piston
Simply explain pitch locking
- prevents overspeeding of propeller
If a propeller speed exceeds 105% what occurs
- separate governor dumps oil pressure to return
- causes blade angle to increase until overspeed ceases
What does the mechanical system do if the power turbine detaches from shaft
immediately shut off the engine metered fuel supply in the even of a turbine shaft failure
Why is the viscosity of oil higher in a turbo prop engine
Due to heavier loads imposed on the gears
Explain parallel spur gears
- mechanically simple
- straight-cut or helical
Explain straight cut gears
- easier to cut, have lowest frictional losses
- very noisy
Explain helical engagement of gear teeth
- quieter but less efficient transmission of power
Explain epicyclic reduction gear
- incorporates sun gear wheel on power turbine shaft that drives serve real planet gear wheels
- engaged with fixed or rotating ring gear or annulus
Turboprop is the most efficient power source for aircraft operating between what speed
- 483 to 725km/h
(300 - 450mph)
The engine can extract as much as —% of the total power produced
95
How is the gas-coupled/free power turbine designed
- no mechanical connection between gas generator and free power turbine
What are the advantages of free power turbine engine
- Less vibration
- engine easier to start (especially in cold)
- propeller can be held at low rpm (e.g. during taxiing) (gives low noise)
- propeller brake can be used to prevent propeller movement
Explain gear/direct-coupled turbines
- Have extra turbines which remain mechanically connected
In the direct-coupled turbines, at normal operating conditions, both propeller speed and engine speed are constant.
True
