Engine Intake Designs for High Speed Flight

Question 1

What are the four characteristics an ideal intake should provide?

Answer 1

1) Provide the engine with the amount of air required for the full range of operation
2) Airflow at all altitudes distributed evenly across the fan/compressor face
3) Acceleration or deceleration of air velocity to meet engine requirements.
4) Optimum initial air compression to augment compressor pressure rise and minimize drag

Question 2

Where do shockwaves typically form on the intake?

Answer 2

On the outside surface leading edge which generates drag.

Question 3

Do modern turbines require subsonic or supersonic airflow at the compressor?

Answer 3

Subsonic, thus intake system must slow it down.

Question 4

Why must flow be slowed down for compression?

Answer 4

To minimize the total pressure drop.

Question 5

On average, between what Mach range do modern turbine engines require the inlet flow to be decelerate to?

Answer 5

Between M 0.4 and M 0.7 at the compressor face.

Question 6

What is the pressure recovery ratio (P1/P0)?

Answer 6

P1 is total pressure at compressor face
P0 is free stream total pressure

It is a measure of intake efficiency

Question 7

As the aircraft gains speed and aerodynamic drag increases, what 3 intake design features allow the aircraft to achieve greater and more efficient engine power?

Answer 7

1) Lengthy ducting, allowing air that has entered at supersonic speeds to be slowed down gradually by oblique shock waves
2) Gap between intake and fuselage side (with diverter plate) prevents low energy boundary layer air from entering the engine
3) Flow’s static pressure inside the ducting increases, which creates a forward reaction against the inlet structure, which adds to overall thrust.

Question 8

What is the typical subsonic pitot intake?

Answer 8

• Characterized by well rounded lips which prevent boundary layer separation during maneuvers or high angles of attack
• Intake has a divergent duct which slows the incoming air to a suitable velocity whilst increasing the static pressure.

Question 9

What is a subsonic bellmouth intake?

Answer 9

• Used on slow subsonic aircraft and helicopters.

- Bellmouth acts as a convergent duct which increases the flow velocity to a speed suitable for the engine.

Question 10

What are the 2 geometrical supersonic intake designs?

Answer 10

1 ) Axisymmetric

2) Two dimensional

Question 11

What are axisymmetric intake designs?

Answer 11

Tend to be round and create cone shaped shockwaves.

Question 12

What are two dimensional intakes?

Answer 12

Tend to have flat sides and create wedge shapes shockwaves.

Question 13

What is the supersonic pitot intake?

Answer 13

• Simplest of the supersonic intake designs
• Flow is slowed down through 1 normal shock and the intake lips.
• Pressure recovery is through a divergent duct

Question 14

What speeds are the basic supersonic pitot intake suited for? Why not above this?

Answer 14

• Up to Mach 1.5

- Above this, pressure loses through normal shock are significant.

Question 15

What is the purpose of multiple shock intakes?

Answer 15

To reduce the pressure losses at high supersonic speeds by compressing the flow through a series of oblique shockwaves.

Question 16

What is the most basic type of two shock intake? How does it work? Why is the cone movable?

Answer 16

• Two shock intake
• Cone/wedge ahead of the intake generate an oblique shockwave in addition to the normal shockwave at the intake mouth.
• Pressure/temperature losses are less than a single normal shock intake, thus producing a more efficient intake design.
• Movable cone is used to position the oblique shock at the intake lips throughout the aircraft’s speed range.

Question 17

How are multiple shockwaves generated through an axisymmetric intake?

Answer 17

Multiple oblique shocks are generated by a more complex cone that can converge at the designed point.
- Shockwave can be positioned by using a variable geometry cone.

Question 18

How are multiple shockwaves generated on a two-dimensional intake?

Answer 18

Generated by a series of ramps and wedges which form compressive corners.

Question 19

REVISION: At supersonic speeds, what occurs to flow through a divergent duct with regards to Pressure, density and velocity?

Answer 19

• Pressure and density reduce

- Velocity increase

Question 20

REVISION: At supersonic speeds, what occurs to flow through a convergent duct with regards to Pressure, density and Velocity? What about subsonic speeds?

Answer 20

• Pressure and density increase
• Velocity reduces
• Opposite occurs at supersonic flow.

Question 21

How are convergent and divergent ducts used in an intake for supersonic speeds?

Answer 21

Convergent ducts used to slow flow down to subsonic and then divergent ducts used to slow flow down even further and increase static pressure.

Question 22

What is intake buzz? What can eliminated intake buzz?

Answer 22

• Intake buzz occurs when oblique and normal shockwaves are incorrectly positioned on the intake surface
• Additional shockwaves are created which causes disruption to the flow.
• Variable geometry can influence the optimal placement of shockwaves.

Question 23

What is used on variable geometry intakes to increase efficiency over the whole speed range of a supersonic aircraft?

Answer 23

Ramps/doors that are extended to change the profile of the intake.