Inlets

Question 1

What is the purpose of the inlet in an aero-engine?

Answer 1

• Reduce air velocity by a diffuser shaped duct, to a suitable velocity for the compressor
• Capture the exact amount of needed air with minimal distortion of the flow

Question 2

How do subsonic inlets work and what is the most common type? What is its main requirement?

Answer 2

Pitot-inlet with fixed geometry.
Operate best at design point velocity.
The main requirement:
- Minimize pressure loss
- Ensure that the air enters the compressor with uniform pressure and velocity

Question 3

How do subsonic inlets ensure stable airflow at slow velocities?

Answer 3

By having thick inlet lips that the flow will stay attached to even at low velocities

Question 4

What are consequences of the flow being disturbed in subsonic inlets?

Answer 4

Compressor instabilities that may cause flameout or mechanical damage due to blade vibration

Question 5

How do the inlets in turboprops work?

Answer 5

• Thick inlet lips
• “Reversed” flow
• Air scooped in underneath or beside propeller
• At aft limit, turned 180 degrees and moves back toward front
• Air is reversed again in combustor

Question 6

What are advantages to the turboprop inlet?

Answer 6

• Allows shorter, compacter engines
• Robust against FOD
• Good for TO and landing on unpaved runways

Question 7

What are different types of supersonic inlets?

Answer 7

• Pitot types
• Bifurcated systems
• Variable-geometry
• Intake spikes
  Engine is normally buried inside fuselage

Question 8

What are requirements for a supersonic inlet? Why are thick inlet lips unfitting for these airspeeds?

Answer 8

• Ensure stable airflow at a suitable velocity for the compressor
• Low radar cross section
• Flow stability during rapid manoeuvering
• Thick inlet lips are unfitting because of normal shock waves yielding a very sharp static pressure rise without changing flow direction and correspondingly large velocity reduction

Question 9

How is intake of turbulent BL from the fuselage prevented in supersonic inlets?

Answer 9

• A gap between the inlet and the fuselage
• Bumper in front of inlet to redirect BL and slow down local airspeed
• Inlet positioned below the front section, so when rapid manoeuvering is performed the fuselage helps direct flow into inlet

Question 10

How does the conditions for a stationary gas turbine engine vary from the aero-engine? What are challenges and importances?

Answer 10

• Relatively constant aerodynamic operation conditions
• Challenges: hot and dusty environments
• Necessary to protect these turbines with filters/coolers

Question 11

What is a common technique for cooling stationary gas turbine inlets?

Answer 11

Spraying finely atomized water into the inlet

Question 12

Name the three most common pitot-type intakes

Answer 12

• Podded intakes, common for transport aircraft
• Integrated intakes, common for military aircraft
• Flush intakes, common in missiles