30 Questions
What is the main purpose of allowing air to diffuse in the front portion of the duct?
To ensure a uniform pressure and minimal turbulence at the compressor inlet
What happens to the inlet pressure as the aircraft reaches its desired cruising speed?
It increases significantly, adding to the mass airflow
What is the design point of the compressor during cruising speed?
Optimum compression and fuel economy
What happens to engine performance if one section of the engine does not match the others?
Engine performance is affected, due to mismatch
What happens to the inlet pressure when the aircraft is operated on the ground?
There is a negative pressure in the inlet
At what speed range does ram recovery normally begin to occur in most aircraft?
Between Mach 0.1 and Mach 0.2
What is the primary function of a moveable spike inlet?
To provide a controlled sonic shock wave at supersonic flight
What happens to the pressure ratio as the aircraft speed increases?
It increases
What is the purpose of the dump valve in a moveable spike inlet?
To permit the escape of excessive pressure
Why is there no need for complicated anti-surge devices at high speeds?
Because control of fuel flow alone provides a sufficient safeguard against surge
What happens to the wedge in subsonic flight?
It is fully retracted
What is the function of the convergent and divergent areas in a moveable spike inlet?
To decrease the air velocity
What is the primary purpose of a convergent-divergent inlet duct in supersonic aircraft?
To slow the airflow to subsonic speed at the face of the engine
What happens to air at supersonic flow rates?
It is compressed to the point of creating shock waves
What is the benefit of using ram compression in engines?
It allows for more thrust with less fuel usage at a set altitude
What is the purpose of a movable restrictor in a convergent-divergent inlet duct?
To vary the geometry of the inlet duct
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 on the rotating aerofoils
What is the primary function of secondary air inlet doors in an aircraft engine?
To allow extra air into the inlet duct during excess negative pressure
What can occur if an aircraft exceeds expected flight attitudes, such as high AOAs or sideslipping?
Disrupted inlet airflow and potential compressor stall
Why is it important to face an aircraft into the wind before carrying out an engine ground run?
To prevent crosswind-induced compressor stall
What determines the pressure required to open secondary air inlet doors?
The tension of the door spring
Which of the following aircraft is particularly susceptible to crosswind-induced compressor stall?
B747
What must be done in accordance with the manufacturer's directions when adjusting secondary air inlet doors?
Any adjustments to the door spring
What is the primary reason for ensuring a smooth and clean intake duct?
To reduce turbulence to a minimum
What is the recommended design approach for curves or bends in the intake duct?
Use gentle curves to minimize turbulence
What is the primary concern with regard to the inlet duct leading edge?
Damage due to bird strikes or hail
What is the most common cause of damage to internal acoustic lining?
Mishandling or careless handling of fan blades
What is the primary purpose of an inlet duct anti-ice system?
To prevent ice accumulation during flight
What is the most critical condition for icing to occur on the engine and intake duct?
Low temperatures and high humidity
Study Notes
Subsonic Inlet Duct
- Air is allowed to diffuse and increase in static pressure in the front portion of the duct
- The air then progresses at a fairly constant pressure past the engine inlet fairing (also called the inlet centre body) to the compressor
- This ensures the engine receives air with minimal turbulence and at a more uniform pressure
Turbofan Inlet
- The turbofan inlet is similar in design to the turbojet except that it 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
- 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
- As aircraft speed continues to increase, ram compression increases, allowing the engine to use this effect to increase the compression ratio and create more thrust with less fuel usage at a set altitude
Supersonic Inlets
- A Convergent-Divergent (C-D) inlet duct (fixed or variable) is required on all supersonic aircraft
- The supersonic transport is configured with an inlet that slows the airflow to subsonic speed at the face of the engine, regardless of aircraft speed
- Subsonic airflow into the compressor is required to prevent shock wave accumulation, which is detrimental to the compression process
- A movable restrictor is often employed to form a C-D shape of variable proportion to vary the geometry of the inlet
Inlet Duct Losses
- Inlet duct losses can occur if the aircraft or conditions exceed expected flight attitudes, such as very high AOAs or sideslipping
- This can lead to a compressor stall
- Crosswinds can disrupt the inlet airflow and increase the AOA of the air into the compressor during ground running, causing compressor stall
- Some aircraft are more prone than others to this phenomenon, which is why it is important to face an aircraft into the wind before carrying out an engine ground run
Inlet Design
- The engine inlet duct must provide a uniform supply of air to the compressor if the engine is to perform at optimum efficiency
- The duct must create as little resistance as possible to achieve this
- To aid in the prevention of intake drag or resistance, the duct should be kept smooth and clean, and any damage in the intake area must be immediately repaired
- Curves or bends must be minimal and carefully blended, and the design of the intake should reduce turbulence to a minimum
- The walls of the duct must have flush rivets or fasteners if fitted, and the seal or joint between engine and duct must be as accurate as possible
Learn about the design and functionality of subsonic inlet ducts and turbofan inlets in aircraft engines, including air pressure and flow.
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