High Speed Flight

Question 1

What is the only physical characteristic of the air that affects the speed of sound?

Answer 1

Temperature

Question 2

Explain the meaning of the term ‘Speed of sound.’

Answer 2

The speed at which sound waves travel.

Question 3

Explain what Mach number is.

Answer 3

Mach number is the ratio between the true speed and the local speed of sound of an object or aircraft.

Question 4

State what ‘Critical Mach Number’ (Mcrit) is.

Answer 4

It is the flight Mach number at which, at some point around the aircraft, the airflow reaches Mach One (the local speed of sound). It marks the start of the transonic speed range.

Question 5

Briefly explain the different flight speed ranges in terms of airflow velocity around the aircraft.

Answer 5

Subsonic: All airflow around the aircraft is below the local speed of sound.
Transonic: Some airflow around the aircraft is above, some below the local speed of sound.
Supersonic: All airflow around the aircraft is above the local speed of sound.

Question 6

Why does lift reduce and drag increase when a shockwave forms on the upper surface of the wing?

Answer 6

causes shock-induced separation, which reduces lift and increases drag.

Question 7

State 3 effects of ‘Shock Stall’ on an aircraft in the transonic speed range.

Answer 7

1. Increased Drag
2. Decreased Lift
3. Nose down pitching

Question 8

Explain the final effect on the center of pressure and aircraft attitude as a conventional aircraft passes through the transonic speed range.

Answer 8

The center of pressure moves rearwards, giving a nose-down attitude.

Question 9

What causes instability in the transonic speed range?

Answer 9

The shock wave moving rapidly backwards and forwards on aerofoils.

Question 10

Name 6 methods or devices used to overcome problems in the transonic speed range.

Answer 10

1. All moving tail plane
2. Spoilers
3. Vortex generators
4. Wing fences
5. Notched or saw-tooth leading edges
6. Swept/delta wings and sharp leading edges

Question 11

Briefly explain why flight controls (ailerons, elevators, rudder) could become ineffective in the transonic speed range.

Answer 11

Shock induced separation means less airflow going over control surfaces

Question 12

Identify a method used to overcome ineffective control in the transonic speed range.

Answer 12

All moving tail plane or spoilers.

Question 13

Name 3 devices used to reduce span-wise flow and shock-induced separation to stop tip stall.

Answer 13

1. Vortex generators
2. Wing fences
3. Notched or saw-tooth leading edges

Question 14

Explain what happens to the speed of the airflow as it passes through a normal shock wave.

Answer 14

It reduces, dropping to below the local speed of sound.

Question 15

Name 2 types of high-speed aerofoils and draw the shape of each.

Answer 15

1. Double Wedge
2. Biconvex
3. Supercritical (shapes drawn in image)

Question 16

Give 2 reasons why specialist materials might be used on a supersonic aircraft.

Answer 16

1. To reduce the weight of the aircraft
2. To reduce drag and cope with very high temperatures.

Question 17

Name 3 things a ‘High Speed Intake’ must control before the airflow enters the engine.

Answer 17

1. Airflow Velocity
2. Airflow Mass Flow Rate
3. The Boundary Layer or Stability of the Airflow

Question 18

Explain the devices used to control the mass flow rate of the intake.

Answer 18

Auxiliary intake doors allow more air to enter the intake, increasing mass flow rate. Spill valves open to reduce mass flow, spilling airflow.

Question 19

Name 3 devices that control the boundary layer to give a stable flow in the intake.

Answer 19

1. Boundary Layer Fences
2. Boundary Layer Diverters
3. Boundary Layer Bleed