30 Questions
What is the function of the spill valve in a movable spike inlet?
To dump unwanted ram air overboard at high speed
At what speed does the compression provided by the engine become relatively minor?
Twice the speed of sound
What happens to the air velocity when it enters the divergent area of the inlet?
It decreases
In subsonic flight, what is the position of the wedge and the dump valve?
The wedge is fully retracted and the dump valve is reversed
What is the purpose of creating a controlled sonic shock wave at the inlet during supersonic flight?
To slow air velocity to subsonic
What is the advantage of having a high-pressure ratio at high speeds?
It reduces the need for anti-surge devices
What is the main advantage of using a bellmouth inlet?
Its efficiency in conditions with little or no ram pressure
Why are mesh screens often used with bellmouth ducts during ground testing?
To protect technicians from ingestion hazards and provide FOD protection
What is a limitation of using inlet screens on aircraft?
They cause icing and fatigue failure, leading to maintenance problems
What type of installations often use compressor inlet screens?
Rotorcraft, turboprops, and ground turbine installations
What is a potential issue with using inlet screens on aircraft?
They cause fatigue failure and maintenance problems
What is a unique feature of some turboprop aircraft inlet separators?
A movable vane in the inlet airstream
What is the advantage of engine-mounted inlet ducts?
They allow for short, efficient inlet ducts with minimal internal skin friction
What materials are commonly used to construct inlets?
Aluminium alloy and/or composites such as carbon fibre and Kevlar
Where are inlet ducts typically mounted on multi-engine jet aircraft?
On the front of the engine or as part of the fuselage, engine pylon, or stub wing structure
What is the ideal air inlet configuration for a turbojet engine at subsonic or low supersonic speeds?
Single-entry, pitot type circular inlet
What happens to the efficiency of a single-entry, pitot type circular inlet as sonic speed is approached?
It begins to fall due to the formation of a shock wave at the inlet lip
Where are inlet ducts typically mounted on single-engine and some twin-engine military aircraft?
In the nose, under the fuselage, or on both sides of the fuselage
What is the primary purpose of ensuring the engine inlet duct is smooth and clean?
To reduce intake drag or resistance
What should be done with curves or bends in the intake duct?
They should be minimal and carefully blended
What is a potential consequence of bird strikes or hail on the inlet duct leading edge?
Damage to the duct
What is a potential cause of damage to internal acoustic lining?
Bird strikes, stones, and mishandling
Under what conditions are icing conditions most prevalent?
When operating the engine at high speeds on the ground
What is the importance of a gentle curve in the intake duct?
To reduce turbulence to a minimum
What is the purpose of turning on the anti-icing system before entering icing conditions?
To minimize the impact forces of ice on compressor blades and vanes
What type of systems are used in some smaller turboprop and turboshaft engines for anti-icing?
Electro-thermal anti-icing systems
What is a possible location for electro-thermal anti-icing systems?
Engine inlet case
What happens if the anti-ice system is turned on after compressor stalls occur from ice formation?
The engine will be severely damaged or fail completely
When is anti-icing heat required?
When visible moisture is present in the form of clouds
What is a limitation of electro-thermal anti-icing systems?
They should not be operated without air passing over them
Study Notes
Inlet Construction
- Inlet ducts are constructed from aluminum alloy and/or composites like carbon fiber and Kevlar.
- Inlets forming part of the aft fuselage may have a long "S" shaped duct.
Inlet Configurations
- Single-engine and some twin-engine military aircraft have their engines mounted within the fuselage.
- Inlet ducts may be mounted in the nose, under the fuselage, or on both sides of the fuselage.
Effects of Inlet Configurations
- A single, short, pitot type circular inlet is the ideal air inlet for a turbojet engine at subsonic or low supersonic speeds.
- This type of inlet makes full use of ram effect on the air due to forward speed and suffers the minimum loss of ram pressure with changes in aircraft attitude.
Inlet Types
- Moveable spike inlet: has a movable wedge that provides a similar function of convergence, divergence, and shock wave formation.
- Bellmouth inlet: increases aerodynamic drag on the airframe, but it is the most efficient option when there is little or no ram pressure available to force air into the compressor.
Inlet Screens
- The use of compressor inlet screens is usually limited to rotorcraft, turboprops, and ground turbine installations.
- Screens have been tried in high subsonic flight engines, but icing and screen fatigue failure caused so many maintenance problems that the use of inlet screens has for the most part been avoided.
Intake Airflow Distortion
- The engine inlet duct must provide a uniform supply of air to the compressor if the engine is to perform at optimum efficiency.
- Curves or bends in the duct must be minimal and carefully blended to prevent intake drag or resistance.
Inlet Duct Damage
- The inlet duct leading edge is susceptible to damage by bird strikes or hail.
- Damage to internal acoustic lining may be caused by bird strike, stones, and mishandling.
Inlet Duct Anti-ice Systems
- Icing of the engine and the leading edges of the intake duct can occur during flight through clouds containing supercooled water droplets or during ground operation in freezing fog.
- Anti-icing heat is required when visible moisture is present in the form of clouds.
- Some smaller turboprop and turboshaft engines use electric heat strip systems classed as electro-thermal anti-icing systems.
This quiz covers the design and construction of inlet ducts in gas turbine engines, including their mounting on engines or fuselage.
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