25 Questions
What is the advantage of an engine-mounted inlet?
Reduced skin friction
What materials are inlets constructed from?
Aluminium alloy and composites
On which part of the fuselage are inlet ducts mounted in some multi-engine jet aircraft?
Aft fuselage
What type of inlet is ideal for an aircraft flying at subsonic or low supersonic speeds?
Single-entry pitot type
What happens to the efficiency of a single-entry pitot type inlet as sonic speed is approached?
It falls due to the formation of a shock wave
What is the primary purpose of allowing air to diffuse in the front portion of the duct?
To provide a uniform pressure and minimal turbulence to the engine
At what point does the compressor reach its aerodynamic design point and produces its optimum compression and fuel economy?
When the aircraft reaches its desired cruising speed
What happens to the inlet pressure when the aircraft begins to move forwards?
It increases due to the ramming of air into the inlet
What is the main difference between a turbofan inlet and a turbojet inlet?
The turbofan inlet discharges only a portion of its air into the gas generator
At what speed range does ram recovery normally begin to occur in most aircraft?
Mach 0.1 to Mach 0.2
What is the function of the spill valve in a movable spike inlet?
To dump unwanted ram air overboard at high speeds
What happens to the compression provided by the engine as aircraft speed increases?
It becomes relatively minor
What is the purpose of the wedge in a movable spike inlet?
To provide a controlled sonic shock wave
At what speed can a pressure ratio of 50:1 be achieved?
Three times the speed of sound
What happens to the dump valve in subsonic flight?
It is reversed to act as an air scoop
What is the effect of increasing aircraft speed on ram compression?
It increases the compression ratio
What is the purpose of a convergent-divergent inlet duct in a supersonic aircraft?
To slow down airflow to subsonic speed at the face of the engine
What happens to air at supersonic flow rates in a duct?
It is compressed to create a shock wave
What is the function of a movable restrictor in a supersonic inlet duct?
To vary the geometry of the inlet
What is the air velocity behind the final shock wave in a supersonic diffuser type inlet?
Mach 0.8
What happens to sand and ice particles when they enter the engine inlet with a movable vane?
They are separated from the combustion air due to their inertia
What is the purpose of the helical vanes in a particle separator?
To create a swirling motion and separate dirt particles
What happens to the foreign particles in a particle separator?
They are blown overboard by compressor bleed air
What is the advantage of using divided entry inlets on single-engine aircraft?
They avoid using long inlet type ducts
What happens when an aircraft with a divided entry inlet yaws?
A loss of ram pressure occurs on one side of the inlet
Study Notes
Inlet Construction
- Engine-mounted inlet ducts can be short and efficient with minimal internal skin friction.
- 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.
Inlet Construction on Fuselage
- Multi-engine jet aircraft may have inlet ducts mounted on the aft fuselage.
- Inlets forming part of the aft fuselage may have a long "S" shaped duct.
Inlet Construction Within Fuselage
- 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
- Single-Entry (Pitot) Type Duct: ideal for subsonic or low supersonic speeds, uses ram effect to increase air inlet pressure.
- Inlet pressure increases add significantly to the mass airflow as the aircraft reaches its cruising speed.
- Turbofan inlet is similar to the turbojet except that it discharges only a portion of its air into the gas generator.
Ram Pressure Recovery
- Ram pressure recovery occurs when the aircraft begins to move forward, air is rammed into the inlet, and ram recovery takes place.
- The resultant increase in inlet pressure cancels the drop in pressure in the inlet, and conditions return to ambient.
- Ram recovery normally begins to occur at speeds between Mach 0.1 and Mach 0.2 in most aircraft.
Supersonic Inlets
- Convergent-Divergent (C-D) inlet duct is required on all supersonic aircraft to slow the airflow to subsonic speed at the face of the engine.
- A movable restrictor is often employed to form a C-D shape of variable proportion.
- Supersonic diffuser type inlet provides a means of creating both a shock wave formation to reduce air velocity and a variable C-D shape.
Movable Spike Inlet
- The movable spike inlet provides a means of creating both a shock wave formation to reduce air velocity and a variable C-D shape.
Divided Entry Inlets
- Divided entry inlets are used on single-engine aircraft to avoid using long inlet type ducts.
- The airflow may remain divided until it reaches the engine compressor or merge smoothly before reaching the engine.
- The disadvantage of the divided type of inlet is that when the aircraft yaws, a loss of ram pressure occurs on one side of the inlet.
Explore the design and construction of inlet ducts in gas turbine engines, including engine-mounted inlet and inlet construction on fuselage.
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