30 Questions
What is the benefit of engine-mounted inlet ducts?
Short, efficient inlet ducts with minimal internal skin friction
What materials are inlets constructed from?
Aluminium alloy and/or composites such as carbon fibre and Kevlar
Where are the inlet ducts mounted on some multi-engine jet aircraft?
Directly onto the front of the engine or part of the fuselage, engine pylon, or stub wing structure
What is the ideal air inlet for a turbojet engine at subsonic or low supersonic speeds?
A single, short, pitot type circular inlet
Why does the efficiency of the single-entry pitot type inlet begin to fall at sonic speed?
Due to the formation of a shock wave at the inlet lip
Where are the inlet ducts 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 purpose of allowing air to diffuse in the front portion of the duct?
To receive air with minimal turbulence and uniform pressure
What happens to inlet pressure as the aircraft reaches its desired cruising speed?
It increases significantly
What is the result when the flight inlet, compressor, combustor, turbine, and tailpipe are not in match with each other?
Engine performance is affected
What is the primary difference between a turbofan inlet and a turbojet inlet?
The turbofan inlet discharges only a portion of its air into the gas generator
What occurs when the aircraft begins to move forwards on the ground?
Ram pressure recovery takes place
At what speed range does ram recovery normally begin to occur in most aircraft?
Mach 0.1 and Mach 0.2
What is the primary function of the spill valve in a moveable spike inlet?
To dump unwanted ram air overboard at high speed
At twice the speed of sound, what pressure ratio is possible in a moveable spike inlet?
30:1
What is the purpose of the wedge in a moveable spike inlet during supersonic flight?
To force a controlled sonic shock wave
What happens to the compression provided by the engine as aircraft speed increases?
It becomes relatively minor
What is the purpose of the divergent area in a moveable spike inlet?
To reduce air velocity
What is the function of the dump valve in a moveable spike inlet during subsonic flight?
To act as an air scoop
What can happen if the anti-ice system is used to de-ice the inlet area after compressor stalls occur from ice formation?
The impact forces of ice on compressor blades and vanes can severely damage the engine
When is anti-icing heat typically required?
When visible moisture is present in the form of clouds
What type of anti-icing system is used in some smaller turboprop and turboshaft engines?
Electro-thermal anti-icing system
Why do electro-thermal anti-icing systems need to be cycled on and off?
To prevent overheating of the strip and the part of the engine it is attached to
What is used to anti-ice the engine and inlet in one type of system?
Hot engine or reduction gearbox oil
When should electro-thermal anti-icing systems be operated?
Only when the engine is running
What is the purpose of the movable spike in a moveable wedge inlet?
To produce a normal shock wave during transonic flight
What type of shock waves develop at the lip of the inlet as airspeed increases to supersonic?
Multiple oblique shock waves
What is the primary function of a bellmouth inlet?
To eliminate the 'necking down' effect of an airstream
What shape is the bellmouth inlet convergent to?
Compressor inlet circumference
What is the typical location of bellmouth inlets?
Helicopters and engine test cells
What happens to the 'mouse' as airspeed increases to supersonic?
It expands to almost fill the intake
Study Notes
Inlet Construction
- Inlet ducts can be mounted directly onto the front of the engine or form part of the fuselage, engine pylon, or stub wing structure.
- Inlets are constructed from aluminium alloy and/or composites such as carbon fibre and Kevlar.
- Inlets forming part of the aft fuselage may have a long “S” shaped duct.
Inlet Configurations
- Single-Entry (Pitot) Type Duct: ideal for subsonic or low supersonic speeds, makes full use of ram effect, and suffers minimum loss of ram pressure with changes in aircraft attitude.
- Single-Entry Type Duct: as sonic speed is approached, its efficiency begins to fall due to the formation of a shock wave at the inlet lip.
Effects of Inlet Configurations
- Inlet pressure increases significantly add to the mass airflow as the aircraft reaches its desired cruising speed.
- The compressor reaches its aerodynamic design point and produces its optimum compression and fuel economy at cruising speed.
- If any section of the engine does not match the others for whatever reason, engine performance will be affected.
Turbofan Inlet
- Similar in design to the turbojet inlet, but discharges only a portion of its air into the gas generator, with the remainder passing into the fan.
Ram Pressure Recovery
- When the aircraft engine is operated on the ground, there is a negative pressure in the inlet due to the high velocity of the mass airflow being drawn into the inlet by the compressor.
- As the aircraft begins to move forwards, air is rammed into the inlet and ram recovery takes place, cancelling the drop in pressure in the inlet.
- Ram recovery normally begins to occur at speeds between Mach 0.1 and Mach 0.2 in most aircraft.
Moveable Spike Inlet
- Provides a similar function of convergence, divergence, and shock wave formation.
- Has a spill valve to dump unwanted ram air overboard at high speed.
- Allows for full advantage of energy recovery at high speeds.
Bellmouth Inlets
- Convergent in shape, commonly found on helicopters and engine test cells.
- Present a mouth considerably wider in circumference than the engine compressor inlet and smoothly converge, funnelling air down to compressor inlet circumference.
- Eliminate the ‘necking down’ effect of an airstream passing through a plain orifice, and allow the engine to draw all the air it can use.
Inlet Anti-ice
- Anti-ice systems are used to prevent ice formation on the inlet.
- Turbofan: uses hot air from the compressor to anti-ice the inlet.
- Turboprop: uses electric heat strip systems or hot engine or reduction gearbox oil to anti-ice the engine and inlet.
This quiz covers the construction of inlet ducts in gas turbine engines, including engine-mounted inlet and inlet construction on fuselage.
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