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Module 11 > 11.1.2 High Speed Flight > Flashcards

11.1.2 High Speed Flight Flashcards

1
Q

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

A

343.2m/s

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2
Q

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

A

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

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3
Q

What are the three sound regions?

A
  • subsonic
  • transonic
  • supersonic
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4
Q

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

A

Subsonic region

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5
Q

What happens when an aircraft travels supersonic?

A

Shockwaves are produced

  • Large pressure changes
  • Changes in density
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6
Q

What are the Mach number zones for:

  • subsonic
  • transonic
  • supersonic
A 
Subsonic =  Mach <0.8
Transonic = Mach 0.8 - 1.2
Supersonic = Mach 1.2 - 5.0
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7
Q

How do you find the Mach number?

A

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

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8
Q

What would a Mach number of 0.7 indicate?

A

70% of the speed of sound

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9
Q

What is the critical Mach number?

A

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

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10
Q

When does a normal shock wave occur?

A

At Mach 1.2

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11
Q

What is the problem with a normal shockwave?

A

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

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12
Q

What is a bow wave?

A

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

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13
Q

What is wave drag?

A

Drag caused by shockwaves

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14
Q

How can wave drag be reduced?

A
  • By the use of vortex generators (the reduced air speed behind the oblique shockwave reduces the normal shockwave)
  • The ‘area rule’
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15
Q

What is the disadvantage of using vortex generators?

A

It increases parasite drag slightly

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16
Q

What is the ‘area rule’ diagram?

A

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

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17
Q

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

A

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

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18
Q

How do you work out the profile thickness?

A

Actual thickness
———————
Chord length

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19
Q

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

A

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

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20
Q

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

A

15% or 0.15

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21
Q

What is the purpose of a swept wing?

A
  • reduced thickness as the chord length is longer

- increases the critical Mach number

22
Q

What is the sweep angle on a modern aircraft?

A

30°

23
Q

What is the purpose of a variable swept wing?

A

It improves the performance at both high and low speeds.

  • high speeds = increased chord length by swept wings
  • low speeds = decreased chord length
24
Q

What is a transonic profile?

A

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
25
Q

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

A

A ‘rear loaded’ wing

26
Q

What is the cruising Mach speed for most airliners?

A

Mach 0.8

27
Q

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

A

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

28
Q

What is the advantage of a greater wing chord?

A

Greater fuel capacity

29
Q

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

A
  • it improves the low speed characteristics

- it means simpler lift devices can be used

30
Q

What is a disadvantage of a transonic profile?

A

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

31
Q

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?

A

The ‘tuck-under’ effect

32
Q

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

A

The horizontal stabiliser.

The automated system that does this is called the Mach trim system

33
Q

Why do shockwaves waste energy?

A

They convert useful energy into wasted heat energy

34
Q

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

A 
Temperature = increases
Density = increases
Pressure = increases 
Velocity = decreases
35
Q

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

A

Shockwaves and expansion waves

36
Q

What are the two types of SHOCK waves?

A

Normal shockwaves

Oblique shockwaves

37
Q

What are normal shockwaves?

A
  • 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
38
Q

What is an oblique shockwave?

A
  • it consumes less energy than a normal shockwave
  • the velocity of the airflow remains supersonic
  • the direction of the airflow is sharply changed however
39
Q

What is an expansion wave?

A
  • supersonic airflow increases in velocity
  • no change in the energy
  • airflow follows the surface (as long as there is no flow separation)
40
Q

What are the two supersonic profiles?

A
  • double wedge

- circular arc

41
Q

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

A

It produces zero net lift at a small angle of attack

42
Q

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

A

50% chord position

43
Q

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

A

25% chord position

44
Q

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

A

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

45
Q

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

A

A normal shock diffuser inlet

- creates a normal shock wave to slow the air down to subsonic

46
Q

What is a single or multiple oblique shock inlet?

A

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

47
Q

What is a variable supersonic inlet?

A

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.

48
Q

What is aerodynamic heating?

A

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

49
Q

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

A

It loses strength by up to 80%

50
Q

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

A

It loses strength by up to 20%

51
Q

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

A

It loses strength by up to 45%

Module 11 (4 decks)

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