Gas Turbine Engine Inlet Construction

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Questions and Answers

What is the function of the spill valve in a movable spike inlet?

To increase air velocity

To provide a sonic shock wave

To control fuel flow

To dump unwanted ram air overboard at high speed (correct)

At what speed does the compression provided by the engine become relatively minor?

Three times the speed of sound

Supersonic speed

Subsonic speed

Twice the speed of sound (correct)

What happens to the air velocity when it enters the divergent area of the inlet?

It remains the same

It decreases (correct)

It increases

It becomes supersonic

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 Signup and view all the answers

What is the purpose of creating a controlled sonic shock wave at the inlet during supersonic flight?

To slow air velocity to subsonic Signup and view all the answers

What is the advantage of having a high-pressure ratio at high speeds?

It reduces the need for anti-surge devices Signup and view all the answers

What is the main advantage of using a bellmouth inlet?

Its efficiency in conditions with little or no ram pressure Signup and view all the answers

Why are mesh screens often used with bellmouth ducts during ground testing?

To protect technicians from ingestion hazards and provide FOD protection Signup and view all the answers

What is a limitation of using inlet screens on aircraft?

They cause icing and fatigue failure, leading to maintenance problems Signup and view all the answers

What type of installations often use compressor inlet screens?

Rotorcraft, turboprops, and ground turbine installations Signup and view all the answers

What is a potential issue with using inlet screens on aircraft?

They cause fatigue failure and maintenance problems Signup and view all the answers

What is a unique feature of some turboprop aircraft inlet separators?

A movable vane in the inlet airstream Signup and view all the answers

What is the advantage of engine-mounted inlet ducts?

They allow for short, efficient inlet ducts with minimal internal skin friction Signup and view all the answers

What materials are commonly used to construct inlets?

Aluminium alloy and/or composites such as carbon fibre and Kevlar Signup and view all the answers

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 Signup and view all the answers

What is the ideal air inlet configuration for a turbojet engine at subsonic or low supersonic speeds?

Single-entry, pitot type circular inlet Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

What is the primary purpose of ensuring the engine inlet duct is smooth and clean?

To reduce intake drag or resistance Signup and view all the answers

What should be done with curves or bends in the intake duct?

They should be minimal and carefully blended Signup and view all the answers

What is a potential consequence of bird strikes or hail on the inlet duct leading edge?

Damage to the duct Signup and view all the answers

What is a potential cause of damage to internal acoustic lining?

Bird strikes, stones, and mishandling Signup and view all the answers

Under what conditions are icing conditions most prevalent?

When operating the engine at high speeds on the ground Signup and view all the answers

What is the importance of a gentle curve in the intake duct?

To reduce turbulence to a minimum Signup and view all the answers

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 Signup and view all the answers

What type of systems are used in some smaller turboprop and turboshaft engines for anti-icing?

Electro-thermal anti-icing systems Signup and view all the answers

What is a possible location for electro-thermal anti-icing systems?

Engine inlet case Signup and view all the answers

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 Signup and view all the answers

When is anti-icing heat required?

When visible moisture is present in the form of clouds Signup and view all the answers

What is a limitation of electro-thermal anti-icing systems?

They should not be operated without air passing over them Signup and view all the answers

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.

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Description

This quiz covers the design and construction of inlet ducts in gas turbine engines, including their mounting on engines or fuselage.