11.1.2 High Speed Flight

Question 1

What is the speed of sound in dry air at 20°C (68°F)?

Answer 1

343.2m/s

Question 2

In a stiff material, such as diamond, what is the speed of sound?

Answer 2

12,000m/s which is the maximum speed of sound in normal conditions

Question 3

What are the three sound regions?

Answer 3

• subsonic
• transonic
• supersonic

Question 4

In what sound region is the air said to be “incompressible”?

Answer 4

Subsonic region

Question 5

What happens when an aircraft travels supersonic?

Answer 5

Shockwaves are produced

• Large pressure changes
• Changes in density

Question 6

What are the Mach number zones for:

• subsonic
• transonic
• supersonic

Answer 6

Subsonic =  Mach <0.8
Transonic = Mach 0.8 - 1.2
Supersonic = Mach 1.2 - 5.0

Question 7

How do you find the Mach number?

Answer 7

The speed of the aircraft
———————————
The speed of sound

Question 8

What would a Mach number of 0.7 indicate?

Answer 8

70% of the speed of sound

Question 9

What is the critical Mach number?

Answer 9

The lowest forward aircraft speed before local Mach 1 can occur (over the wing)
It is the highest Mach number an aircraft can have without supersonic flow

Question 10

When does a normal shock wave occur?

Answer 10

At Mach 1.2

Question 11

What is the problem with a normal shockwave?

Answer 11

It increases the static pressure behind the shockwave and flow separation occurs as the air has little kinetic energy. This reduces the lift created

Question 12

What is a bow wave?

Answer 12

A wave that forms at the leading edge when an aircraft exceeds the speed of sound

Question 13

What is wave drag?

Answer 13

Drag caused by shockwaves

Question 14

How can wave drag be reduced?

Answer 14

• By the use of vortex generators (the reduced air speed behind the oblique shockwave reduces the normal shockwave)
• The ‘area rule’

Question 15

What is the disadvantage of using vortex generators?

Answer 15

It increases parasite drag slightly

Question 16

What is the ‘area rule’ diagram?

Answer 16

The cross sectional areas of the wing are plotted against the body length of the aircraft to show a very smooth curve

Question 17

Some aircraft have a waist fuselage. What is the advantage of this?

Answer 17

It reduces the drag and fulfils the area rule diagram of a smooth curve

Question 18

How do you work out the profile thickness?

Answer 18

Actual thickness
———————
Chord length

Question 19

Why do Aircraft have a thin wing profile rather than a thick one?

Answer 19

It reduces the shock wave created in transonic flight and also the wave drag of the profile

Question 20

If a profile is 1.5m thick and the chord length is 10m, what is the overall thickness?

Answer 20

15% or 0.15

Question 21

What is the purpose of a swept wing?

Answer 21

• reduced thickness as the chord length is longer

- increases the critical Mach number

Question 22

What is the sweep angle on a modern aircraft?

Answer 22

30°

Question 23

What is the purpose of a variable swept wing?

Answer 23

It improves the performance at both high and low speeds.

• high speeds = increased chord length by swept wings
• low speeds = decreased chord length

Question 24

What is a transonic profile?

Answer 24

A profile which performs better in the transonic range - no flow separation behind the shockwave

• flatter upper surface (decelerating the supersonic airflow creating a much smaller shockwave)
• more curved leading edge (immediately accelerates to supersonic)
• thinner trailing edge

Question 25

What is a wing with a transonic profile also known as?

Answer 25

A ‘rear loaded’ wing

Question 26

What is the cruising Mach speed for most airliners?

Answer 26

Mach 0.8

Question 27

What is the advantage of a transonic profile’s construction?

Answer 27

The entire outer skin is used as a torque box which allows for thinner and lighter material to be used

Question 28

What is the advantage of a greater wing chord?

Answer 28

Greater fuel capacity

Question 29

A transonic profile can have a smaller sweepback angle than a subsonic profile. What is the advantage to this?

Answer 29

- it improves the low speed characteristics | - it means simpler lift devices can be used

Question 30

What is a disadvantage of a transonic profile?

Answer 30

The drag on a transonic profile compared to a conventional profile is greater at a mid Mach number to below the critical Mach number.

Question 31

What is the ‘nose-down’ reaction called when an aircraft with a transonic profile stalls at the root as it passes the critical Mach number?

Answer 31

The ‘tuck-under’ effect

Question 32

What control surface counteracts the ‘tuck-under’ effect and what is the automated system called?

Answer 32

The horizontal stabiliser. | The automated system that does this is called the Mach trim system

Question 33

Why do shockwaves waste energy?

Answer 33

They convert useful energy into wasted heat energy

Question 34

What happens to temperature, density, pressure and velocity to supersonic airflow when it passes through a shockwave?

Answer 34

``` Temperature = increases Density = increases Pressure = increases Velocity = decreases ```

Question 35

What are the two main types of waves formed in supersonic airflow?

Answer 35

Shockwaves and expansion waves

Question 36

What are the two types of SHOCK waves?

Answer 36

Normal shockwaves | Oblique shockwaves

Question 37

What are normal shockwaves?

Answer 37

- It slows the velocity of the airflow down to subsonic - A lot of energy is wasted in the form of heat - The direction remains the same

Question 38

What is an oblique shockwave?

Answer 38

- it consumes less energy than a normal shockwave - the velocity of the airflow remains supersonic - the direction of the airflow is sharply changed however

Question 39

What is an expansion wave?

Answer 39

- supersonic airflow increases in velocity - no change in the energy - airflow follows the surface (as long as there is no flow separation)

Question 40

What are the two supersonic profiles?

Answer 40

- double wedge | - circular arc

Question 41

What is important about a double wedge profile in regards to the lift?

Answer 41

It produces zero net lift at a small angle of attack

Question 42

Where is the centre of lift on a circular arc profile if the airflow is SUPERSONIC?

Answer 42

50% chord position

Question 43

Where is the centre of lift on a circular arc profile if the airflow is SUBSONIC?

Answer 43

25% chord position

Question 44

What must be done to the air entering the compressor section of a supersonic aircraft?

Answer 44

It must be slowed down to subsonic airflow in the most efficient way (with the minimum energy loss)

Question 45

What type of supersonic jet engine compressor inlet is designed for speeds just in excess of the speed of sound?

Answer 45

A normal shock diffuser inlet | - creates a normal shock wave to slow the air down to subsonic

Question 46

What is a single or multiple oblique shock inlet?

Answer 46

A jet engine inlet that employs external oblique shockwave(s) to slow the supersonic airflow before a normal shockwave occurs. This reduces the energy loss

Question 47

What is a variable supersonic inlet?

Answer 47

Similar to a multiple oblique shockwave inlet it has actuators that move panels to create 3 oblique shockwaves to slow the supersonic airflow down before the normal shockwave occurs. This in turn, ensures minimum energy loss.

Question 48

What is aerodynamic heating?

Answer 48

The temperature rise that occurs when the Mach number increases. It increases slower at subsonic airflow but increases rapidly at supersonic airflow.

Question 49

What happens to an aluminium alloy’s strength if the temperature rises up to 250°C?

Answer 49

It loses strength by up to 80%

Question 50

What happens to stainless steel’s strength if the temperature rises up to 250°C?

Answer 50

It loses strength by up to 20%

Question 51

What happens to a titanium alloy’s strength if the temperature rises up to 250°C?

Answer 51

It loses strength by up to 45%