Aircraft Engine Inlet Design

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Questions and Answers

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 (correct)

To reduce the mass airflow as the aircraft reaches its cruising speed

To increase turbulence and pressure past the engine inlet fairing

To provide an additional source of air for the fan

What happens to the inlet pressure as the aircraft reaches its desired cruising speed?

It decreases significantly, reducing mass airflow

It remains constant, unaffected by the aircraft's speed

It becomes negative, reducing engine performance

It increases significantly, adding to the mass airflow (correct)

What is the design point of the compressor during cruising speed?

Maximum compression and minimum fuel economy

Optimum compression and fuel economy (correct)

Minimum compression and maximum fuel economy

None of the above

What happens to engine performance if one section of the engine does not match the others?

Engine performance is affected, due to mismatch Signup and view all the answers

What happens to the inlet pressure when the aircraft is operated on the ground?

There is a negative pressure in the inlet Signup and view all the answers

At what speed range does ram recovery normally begin to occur in most aircraft?

Between Mach 0.1 and Mach 0.2 Signup and view all the answers

What is the primary function of a moveable spike inlet?

To provide a controlled sonic shock wave at supersonic flight Signup and view all the answers

What happens to the pressure ratio as the aircraft speed increases?

It increases Signup and view all the answers

What is the purpose of the dump valve in a moveable spike inlet?

To permit the escape of excessive pressure Signup and view all the answers

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 Signup and view all the answers

What happens to the wedge in subsonic flight?

It is fully retracted Signup and view all the answers

What is the function of the convergent and divergent areas in a moveable spike inlet?

To decrease the air velocity Signup and view all the answers

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 Signup and view all the answers

What happens to air at supersonic flow rates?

It is compressed to the point of creating shock waves Signup and view all the answers

What is the benefit of using ram compression in engines?

It allows for more thrust with less fuel usage at a set altitude Signup and view all the answers

What is the purpose of a movable restrictor in a convergent-divergent inlet duct?

To vary the geometry of the inlet duct Signup and view all the answers

What is the approximate air velocity behind the final shock wave in a supersonic diffuser type inlet?

Mach 0.8 Signup and view all the answers

Why is subsonic airflow into the compressor required?

To prevent shock wave accumulation on the rotating aerofoils Signup and view all the answers

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 Signup and view all the answers

What can occur if an aircraft exceeds expected flight attitudes, such as high AOAs or sideslipping?

Disrupted inlet airflow and potential compressor stall Signup and view all the answers

Why is it important to face an aircraft into the wind before carrying out an engine ground run?

To prevent crosswind-induced compressor stall Signup and view all the answers

What determines the pressure required to open secondary air inlet doors?

The tension of the door spring Signup and view all the answers

Which of the following aircraft is particularly susceptible to crosswind-induced compressor stall?

B747 Signup and view all the answers

What must be done in accordance with the manufacturer's directions when adjusting secondary air inlet doors?

Any adjustments to the door spring Signup and view all the answers

What is the primary reason for ensuring a smooth and clean intake duct?

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

What is the recommended design approach for curves or bends in the intake duct?

Use gentle curves to minimize turbulence Signup and view all the answers

What is the primary concern with regard to the inlet duct leading edge?

Damage due to bird strikes or hail Signup and view all the answers

What is the most common cause of damage to internal acoustic lining?

Mishandling or careless handling of fan blades Signup and view all the answers

What is the primary purpose of an inlet duct anti-ice system?

To prevent ice accumulation during flight Signup and view all the answers

What is the most critical condition for icing to occur on the engine and intake duct?

Low temperatures and high humidity Signup and view all the answers

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

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Description

Learn about the design and functionality of subsonic inlet ducts and turbofan inlets in aircraft engines, including air pressure and flow.