30 Questions
What is the main reason why in-wing design is no longer practical for modern engines?
High-bypass turbofan engines are not compatible with in-wing design
What is the primary material used in the construction of inlet ducts in wing design?
Aluminium alloy
Which of the following locations is NOT a common location for engine inlets?
On the landing gear
What is the typical method of attaching turbofan inlets to the engine?
Bolted to the forward flange of the inlet case
What is the primary consideration for air inlet duct design?
Operation at different airspeeds
What type of aircraft had engines installed in the wings?
Early commercial and military aircraft
What is a drawback of the single-entry pitot type duct?
Inlet turbulence at low airspeed and/or high AOA
Why are wing pylon-mounted engine inlets sometimes squared?
Due to proximity to the ground when the wing is flexed
What is the main advantage of the pitot type inlet in single-engine aircraft?
Achieves smooth airflow into the compressor
What is the effect of the divergent shape of subsonic inlet ducts?
Progressively increases in diameter from front to back, increasing static pressure
What is another term for subsonic inlet ducts due to their effect on pressure?
Inlet diffuser
What is the shape of the cross-section of pitot type inlets in some aircraft?
Squared
What happens to sand and ice particles when they enter the engine inlet with a movable vane?
They are separated from the air due to their inertia
What is the purpose of the helical vanes in the swirl chamber type of particle separator?
To impart a swirling motion to the incoming air
What happens to the foreign particles in the swirl chamber type of particle separator?
They are thrown to the outside of the chamber by centrifugal force
What is the main advantage of using divided entry inlets on single-engine aircraft?
To avoid using long inlet type ducts
What happens when an aircraft with divided entry inlets yaws?
A loss of ram pressure occurs on one side of the inlet
Where are the twin divided inlet ducts typically merged on single-engine aircraft?
At the wing leading edges
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 inlet ducts?
Aluminium alloy and/or composites such as carbon fibre and Kevlar
Where are the inlet ducts typically mounted on a single-engine or some twin-engine military aircraft?
Within the fuselage
What is the ideal air inlet for a turbojet engine fitted to an aircraft flying at subsonic or low supersonic speeds?
A single, short, pitot type circular inlet
What happens to the efficiency of a single-entry (pitot) type duct as an aircraft approaches sonic speed?
It begins to fall due to the formation of a shock wave at the inlet lip
What is the advantage of a pitot type inlet in terms of ram pressure?
It makes full use of ram effect on the air due to forward speed
What happens to ram compression as aircraft speed increases?
It increases
What is the primary purpose of a convergent-divergent inlet duct in supersonic aircraft?
To slow supersonic airflow to subsonic speeds
What happens to air at supersonic flow rates?
It compresses and creates shock waves
What is the approximate air velocity behind the final shock wave in a supersonic diffuser type inlet?
Mach 0.8
Why is subsonic airflow into the compressor required?
To prevent shock wave accumulation
What is the purpose of a movable restrictor in a convergent-divergent inlet duct?
To vary the geometry of the inlet duct
Study Notes
Inlet Types and Locations
- Engine inlets can be mounted in various locations, including:
- In the wing
- On the engine
- On the fuselage
- Within the fuselage
- Inlet designs vary to accommodate different airframe/engine combinations and engine mounting locations.
Inlet Construction in Wing
- Early commercial and military aircraft had engines installed in the wings, with inlet ducts constructed from aluminum alloy and built into the wing.
- This design is now impractical with high-bypass turbofan engines.
Inlet Construction on Engine
- Most multi-turbojet and turbofan aircraft have inlet ducts mounted directly onto the front of the engine.
- Inlets are constructed from aluminum alloy and/or composites such as carbon fiber and Kevlar.
- Turbofan inlets are typically bolted to the forward flange of the inlet case or fan case.
Inlet Construction on Fuselage
- Some multi-engine jet aircraft have engines mounted on the aft fuselage, with inlet ducts mounted directly onto the front of the engine or forming part of the fuselage, engine pylon, or stub wing structure.
- Inlets may have a long "S" shaped duct.
Inlet Construction Within Fuselage
- Single-engine and some twin-engine military aircraft have engines mounted within the fuselage, with inlet ducts forming part of the fuselage structure.
- 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
- Suffers from inlet turbulence, especially at low airspeed and/or high angles of attack
- Can be used for engines mounted in pods, wings, or other flying surfaces
- Squared inlets:
- Used for wing pylon-mounted engine inlets due to proximity to the ground when the wing is flexed
- Pitot type inlet on single-engine aircraft:
- Requires a long duct ahead of the compressor, resulting in a drop in pressure
- Achieves smooth airflow into the compressor
Subsonic Inlet Ducts
- Inlet ducts on business and commercial jet aircraft are of fixed geometry with a divergent shape.
- Air enters the inlet at ambient pressure and starts to diffuse, arriving at the compressor at a slightly increased static pressure.
- Ram compression increases with aircraft speed, allowing the engine to use this effect to increase the compression ratio and create more thrust with less fuel usage.
Supersonic Inlets
- Convergent-Divergent (C-D) inlet ducts (fixed or variable) are required for supersonic aircraft.
- Supersonic airflow is slowed to subsonic speed at the face of the engine using a movable restrictor to form a C-D shape of variable proportion.
- The C-D shaped duct reduces supersonic airflow to subsonic speeds, preventing shock wave accumulation.
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 it can cause an uneven distribution of airflow into the compressor when the aircraft yaws.
Learn about the different types and locations of air inlet ducts in gas turbine engines, designed to accommodate various airframe and engine combinations.
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