M15.16 Turboprop Engines Flashcards
What is the speed range in which turboprop engines are most efficient?
483 to 725 km/h (approximately 300 to 450 mph)
This range is where turboprop engines provide the best specific fuel consumption.
How does the propulsive efficiency of a turboprop compare to that of a turbojet?
The turboprop’s efficiency remains constant, while the turbojet’s increases rapidly with airspeed
This characteristic makes turboprops more efficient at lower speeds.
What is the maximum percentage of total power produced by a turboprop engine that can be used to drive the propeller?
95%
The remaining energy is utilized as residual thrust.
What is commonly referred to as the gas generator in a turboprop engine?
Core engine
It produces the high-velocity gases needed to drive the power turbine.
What is the main feature of a gas-coupled/free-power turbine engine?
No mechanical connection between the gas generator and the free-power turbine
The only connection is through gas flow.
What is one advantage of a free-power turbine engine?
Easier to start, especially in cold weather
Other advantages include reduced vibration transmission and the ability to hold the propeller at low RPM.
What distinguishes gear/direct-coupled turbines from free-power turbines?
They have extra turbine discs mechanically connected to the compressor and reduction gearbox
This setup transmits increased power forwards.
What is the function of the reduction gear assembly in a turboprop engine?
To reduce the high RPM from the engine to a suitable propeller RPM
This prevents exceeding maximum propeller tip speed.
What is the reduction ratio of the Pratt & Whitney PT6A-34 engine?
15:1
At 33,000 RPM of the power turbine, the propeller rotates at only 2,200 RPM.
What are the two main types of reduction gears available?
- Parallel spur gear type
- Epicyclical type
What is a disadvantage of straight-cut spur gears?
They are very noisy
However, they are easier to cut and have lower frictional losses.
Describe the function of epicyclic reduction gears.
They incorporate a sun gear that drives several planet gears engaged with a fixed or rotating ring gear
The reduction gear ratio is calculated based on the number of driven teeth over driving teeth.
What is the function of the propeller electronic control in modern turboprop engines?
To control both the engine and propeller
This system ensures efficient operation and management of thrust.
What happens when the thrust lever is moved into the Beta range?
The control unit selects oil pressure to adjust the blade angle below flight fine pitch
This allows for ground handling and reversing.
What is the purpose of the Automatic Drag Limiting System (ADLS)?
To feather the propeller quickly to avoid windmilling
It activates if negative torque is detected.
What is the role of the propeller overspeed governor?
To dump oil pressure if propeller speed exceeds 105%
This increases the blade angle to reduce speed and prevent overspeed conditions.
What can result from a turbine shaft failure?
Catastrophic breakup of the engine
A mechanical system is designed to shut off the fuel supply in such events.
