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Cat B- Mod 11 Aircraft Systems > 11.1.2 High Speed Flight > Flashcards

11.1.2 High Speed Flight Flashcards

1
Q

What is the speed of sound?

A

The speed at which pressure waves travel through a medium.

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2
Q

What is the speed of sound at sea level (15 degrees c)?

A

340m/s or 760mph

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3
Q

What is the speed of sound at 36,000 feet?

A

295m/s or 660mph

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4
Q

What happens when an aircraft approaches the speed of sound?

A

It experiences compressibility effects which become shockwaves, sudden increase in drag and lift buffeting.

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5
Q

What phase a typical engines optimised for?

A

Subsonic

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6
Q

What design adaptations does supersonic flight require?

A

Advanced engine and inlet designs as well as specialised propulsion systems.

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7
Q

How is Mach number worked out?

A

Velocity of object divided by speed of sound

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8
Q

What are the three ranges of aircraft speed?

A

Subsonic
Transonic
Supersonic

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9
Q

What happens as an aircraft approaches the speed of sound?

A

A compression wave is formed at the leading edge.

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10
Q

What is subsonic mach number?

A

Less than 0.8

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11
Q

What is transonic mach number?

A

0.8 to 1.2

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12
Q

What is supersonic mach number?

A

1.2 to 5.0

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13
Q

What is the behaviour of the air in the subsonic region?

A

Incompressible.

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14
Q

What happens to the air in transonic flight as it hits the upper surface of the wing?

A

It reaches mach 1, and forms a shockwave at 90 degrees to the airflow.

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15
Q

What is the name of the shockwave formed in transonic flight?

A

Normal

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16
Q

What challenges can transonic flight lead too?

A

Stability challenges due to the shock wave which can lead to flow separation.

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17
Q

What other effect can a normal shockwave have on the aircraft in transonic flight?

A

CL moves rearward causing nose down pitch.

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18
Q

What is the speed region above mach 5 called?

A

Hypersonic

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19
Q

In supersonic flight where does the shockwave move too?

A

The trailing edge of the aircraft.

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20
Q

What is the result of the air being compressible in the supersonic region?

A

Reduction in the coefficient of lift and increased coefficient of drag.

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21
Q

The coefficient of drag is higher in supersonic flight than which region?

A

Subsonic, but les than the transonic region.

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22
Q

If the mach number is 0.6 what percentage of the speed of sound is this?

A

60%

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23
Q

What is the critical mach number of an aircraft?

A

The lowest mach number where airflow over the aircraft at some point reaches the speed of sound.

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24
Q

What is the incipient shock wave?

A

Sharp increase in density at the point of maximum thickness where the airflow over the wing reaches mach 1.

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25
Q

What happens to the air as it passes through the shockwave?

A

Decelerates, kinetic energy is dissipated as heat and temp rise, pressure increases.

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26
Q

As speed of airflow increases what happens to the supersonic flow?

A

It grows larger and forms another shockwave.

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27
Q

What happens to the boundary layer behind the shockwave?

A

It thickens, becomes turbulent and loses kinetic energy.

28
Q

How is an oblique shock wave formed?

A

Bow wave that attaches to the leading edge and slopes backwards.

29
Q

What are expansion waves?

A

Not a shockwave.

30
Q

How is an expansion wave formed?

A

When supersonic airflow changes direction around a curve.

31
Q

What two typical supersonic profiles are there?

A

Double wedge and circular arc.

32
Q

On a double wedge profile at 0 degrees AoA what shockwaves are experienced?

A

Oblique @ LE
Expansion @ Mid
Oblique @ TE

33
Q

Where is the centre of lift on a circular arc profile when supersonic?

A

50% chord

34
Q

What is wave drag?

A

Portion of total drag which is due to the shock waves.

35
Q

What two ways are used to reduce wave drag?

A

Using vortex generators
Applying the area rule

36
Q

What are vortex generators designed to do?

A

Create vortex’s that reenergise the boundary layer.

37
Q

What benefit does a vortex generator have in supersonic airflow?

A

Creates an oblique shockwave, this reduces the airspeed behind the shockwave producing a smaller normal wave than without the vortex gen.

38
Q

What is a disadvantage of vortex generators?

A

They increase parasite drag slightly.

39
Q

What is area rule sometimes referred too as?

A

Transonic area rule.

40
Q

What is area rule?

A

A design rule used to reduce an aircrafts drag at transonic speeds.

41
Q

What is the Sears-Haack body shape?

A

The profile with the lowest theoretical wave drag in supersonic flow

42
Q

What does the area rule state?

A

Two aircraft with the same longitudinal cross-sectional area distribution have the same wave drag.

43
Q

What is aerodynamic heating also known as?

A

Friction heating.

44
Q

What is aerodynamic heating?

A

Heating of the airframe caused by the friction of the air.

45
Q

When is the only time that friction heating is significant?

A

Supersonic flight.

46
Q

Where does the greatest reduction in velocity and increase in temp occur?

A

Stagnation point.

47
Q

How much strength is lost in aluminium when the temp increases too 250 degrees c?

A

80%

48
Q

What is the max axial velocity for GTE?

A

Mach 0.4

49
Q

Why can GTE only handle a certain range of axial velocity?

A

It can change the angle of attack of the compressor blades.

50
Q

For supersonic aircraft what must the air entering the compressor be?

A

Slowed to subsonic.

51
Q

What is the least complicated inlet design?

A

Normal shock diffuser inlet.

52
Q

How does a normal shock diffuser inlet work?

A

Single normal shockwave at the inlet to slow the air to subsonic velocity.

53
Q

Why is a normal shock diffuser inlet not suitable for high supersonic speeds?

A

Because the shockwave is really strong and causes a great reduction in the total pressure recovered.

54
Q

What happens to ‘normal airflow” in a converging duct?

A

The velocity increases and the static pressure decreases.

55
Q

What does supersonic air do in a converging duct?

A

The air chokes at the narrowest point and actually starts to back up, decelerate and compress.

56
Q

Behaviours of subsonic flow to supersonic flow in a convergent or divergent duct are what?

A

Opposite. Subsonic air is the classic rules of venturi.

57
Q

How does a single oblique shock inlet work?

A

It forms an external oblique shock to slow the supersonic flow before the normal shock.

58
Q

How does a multiple oblique inlet work?

A

Uses a series of weak obliques to slow the air before the normal shock

59
Q

What is the benefit of a multiple oblique inlet?

A

It leads to less wasting of energy and the highest pressure recovery.

60
Q

What is the point of a variable supersonic inlet?

A

Allows the optimum shape of the inlet for varying angles of attack and different speeds.

61
Q

What are the difficulties of transonic flight?

A

Shockwave formation and an associated flow separation.

62
Q

How can the adverse effects of transonic flight be reduced?

A

If the shockwave induced separation and the intensity of the shock waves can be delayed and reduced.`

63
Q

What is one way of reducing the profile thickness and delay the critical mach number?

A

Sweep back of the wing.

64
Q

What is the sweep angle on most modern aircraft?

A

30 degrees

65
Q

What 3 major benefits do sweepback wings offer?

A

Reduce frontal area of the aircraft
Reduce the profile drag
Improve lateral stability

66
Q

What is the disadvantage of swept wings?

A

Increased span wise flow which can cause stagnation in the boundary layer.

Cat B- Mod 11 Aircraft Systems (28 decks)

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