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xxxFOF > Chap F: High Speed Flight > Flashcards

Chap F: High Speed Flight Flashcards

1
Q

what is transonic flight?

A

flight range between high subsonic and low supersonic

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2
Q

definition of mach number

A

ratio of true airspeed to local speed of sound

VTAS/a

a - local speed of sound

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3
Q

speed ranges in terms of M

A

low subsonic (incompressible): below M0.3
subsonic (compressible): M0.3 ≤ M ≤ M0.75
transonic: M0.75 ≤ M ≤ M1.2
supersonic: M1.2 ≤ M < M5.0
hypersonic: ≥ M5.0

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4
Q

what is sound created?

A

propagation of weak pressure waves

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5
Q

what happens when the sound emitter increases the speed?

A

pressure waves cannot outrace the soundwaves at M1.0 causing shock waves to occur

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6
Q

is maximum local speed greater or lower than flight speed?

A

greater than

some parts of the wing may experience M1.0 locally if flying at neat sonic speed

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7
Q

formation of incipient shock wave is due to?

A

rapid increase in pressure and density

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8
Q

definition of critical mach number

A

Mach number at which airflow at any part of the wing attains a velocity equal to local speed of sound

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9
Q

where is airflow fastest at?

A

maximum thickness

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10
Q

why is there a region of separated flow immediately aft of shock waves?

A

presence of large adverse pressure gradient

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11
Q

what is shock stall?

A

shock waves forms at the aft boundary of the supersonic flow causing the airflow to separate behind the shock waves, leading to loss of lift

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12
Q

difference between shock stall and high AOA stall

A

shock stall can occur at low AOA

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13
Q

how does shock stall happen?

A

flow after shock is turbulent and separated, causing region after shock to lose capability to produce pressure reduction for lift generation

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14
Q

behaviour of aircraft at shock stall

A

1) loss of lift
2) large increase in drag
3) rearward movement of CP
4) increased nose down pitching moments

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15
Q

reason for loss of lift at shock stall

A

due to boundary layer separation on upper surface

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16
Q

does lift coefficient increase or decrease after flow is fully supersonic?

A

decrease

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17
Q

what is transonic drag rise?

A

wave drag on highspeed aircraft increase substantially as flight speed approaches M1.0

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18
Q

what is the reason of transonic drag rise?

A

formation of shock waves around M0.8

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19
Q

how does drag coefficient increase/decrease in supersonic speed?

A

increase until bow wave attaches to the leading edge

decreases further once flow is fully supersonic

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20
Q

what is the force divergence mach number?

A

mach number where wave drag becomes very significant

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21
Q

where does CP move due to shock waves?

A

from 1/4 chord to 1/2 chord once flow is fully supersonic

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22
Q

how does movement of CP due to shock waves affect aircraft performance?

A

1) increase nose down pitching moment
2) increase longitudinal stability
3) more difficult to manoeuvre aircraft

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23
Q

what is buffeting?

A

result of unsteady flow caused by separation, shock waves move back and forth rapidly

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24
Q

what will buffeting cause?

A

structural failure if buffet frequency is the same w resonance frequency

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25
Q

where does the first shock wave appear first on the wing?

A

at the thickness part of the aerofoil on the upper surface as it is where velocity is sonic

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26
Q

what is tuck under effect?

A

shock waves appear at the wing root, resulting in flow separation and shock stall at the wing root

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27
Q

where does CP move during a shock stall? and what does it cause

A

rearwards and outwards, towards the wing tip

rearwards movement results in nose-down pitching moments

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28
Q

what is control reversal?

A

adverse effect on the controllability of aircraft (aircraft move in the opposite direction that the pilot chose)

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29
Q

when does control reversal occur?

A

when hinged trailing edge device type control surfaces deflect downwards resulting in CP moving aft

when amount of airflow over wing is large enough for forces generated by aileron twist the wing by itself

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30
Q

ways to raise critical mach number

A

1) slimness - use thin wing sections
2) sweepback - leading edge sweepback
3) area rule - coke bottle shape
4) boundary layer control

31
Q

how does little mean camber raise Mcrit?

A

lesser curvature means reduced acceleration over upper surface of wing

32
Q

disadvantages of thin wings

A

1) low CL max
2) higher stalling speed
3) higher take-off & landing speed
4) reduced fuel carrying capacity

33
Q

what is a supercritical wing?

A
  • flatter upper surface, rounder lower surface
  • has downwards curve on trailing edge
34
Q

how does supercritical wing increase Mcrit?

A

1) delay shock wave formation
2) reduce strength of shock wave over wing
3) lesser drag
4) more rigid but lighter

35
Q

advantages and disadvantages of supercritical wing

A

adv:
1) thicker section
2) improved structural strength
3) more capacity for fuel
4) lighter wing structure

disadv:
increased pitching moment at low speed
thinner trailing edge (harder to install TE devices)

36
Q

how does sweepback increase Mcrit?

A
  • increases all stability
  • delays onset of shock stall
37
Q

sweepback advantages and disadvantages

A

adv:
- higher Mcrit
- improved stabilities

disadv:
- wing tip stall due to twisting
- possible span-wise flow breakaway near wing tip
- possible loss of aileron control

38
Q

what is the area rule?

A

reduce volume of body when there is wing, tail surface, etc to avoid discontinuities in cross-sectional area distribution from nose to tail

39
Q

area rule advantages

A

1) reduced drag
2) pitch instabilites removed
3) higher acceleration

40
Q

area rule disadvantages

A

1) reduced cross sectional area
2) conflict with:
landing gear storage
passenger seating
headroom
lesser fuel tank area

41
Q

ways for boundary layer control in high speed flight

A

1) vortex generators
2) wing fences/stall wedges
3) LE notches/dogtooth

42
Q

what are vortex generators and what do they do?

A

small airfoils that are attached to the surface of the wing to catch airflow at high AOA

re-energise boundary layer thus preventing flow separation

43
Q

what do flow separation create at high speed fight?

A

loss of lift and increased drag

44
Q

what are wing fences?

A

fin-like vertical surfaces attached to the upper surface of the wing

45
Q

what do wing fences help with?

A

minimise span-wise flow separation & reduce wing tip stalling

46
Q

how does transonic speed affect control systems?

A

control reversal likely to happen

lose control over ailerons and elevators when shockwave disrupt airflow

control surfaces at the back has no effect on flow ahead of shock wave

47
Q

where is shock wave most likely to form?

A

at hinge line (between fixed and control surfaces)

48
Q

how to prevent stabilisers from promoting formation of shock waves?

A

adjust trim or aircraft using slab or all moving tail plane

49
Q

one way to have good control at high speed?

A

fuel transfer to front/back of aircraft

low speed - more forward CG
high speed - more aft CG

50
Q

definition of supersonic flight using Mach number

A

M ≥ M1.2

51
Q

mach angle relative to mach number

A

higher the mach number, small the mach angle = smaller mach cone

52
Q

definition of compressive flow

A

hitting against a sharp corner (shock waves) formed by mach lines

53
Q

converging-diverging duct for compressive flow properties

A

converging:
P increase, V decrease, density increasing

diverging:
P decrease, V increase, density decreasing

opposite of incompressible flow (for P & V)

54
Q

definition of expansive flow

A

flow going into divergent duct (bigger space, convex corner) forming expansion waves

55
Q

what do expansion waves do? are they gradual or shock change?

A

increase velocity

decrease: pressure, density, temp

gradual change

56
Q

what corner creates shockwaves? and how does the airflow change its properties?

A

concave corner

tighter space, so increase pressure, decrease velocity

57
Q

expansion wave effect on energy

A

no change as there is no shock

58
Q

typical section for high speed flight?

A

slender and with minimum camber

59
Q

pressure coefficient arrow direction on wing

A

-ve Cp point outwards
+ve Cp point inwards

60
Q

flat plate for supersonic flight

A

ideal pressure distribution but has no depth (no structural)

61
Q

types of section for supersonic flight

A

1) flat plate
2) double wedge
3) hexagonal wedge
4) circular arc

62
Q

double wedge section description

A

sharp LE and TE, max thickness at mid-chord

63
Q

hexagonal section description

A

upgrade from double wedge:
improved aerodynamic qualities
increased structural integrity

but cannot handle low speed flight

64
Q

law of forbidden signals meaning

A

disturbance originating from a point on wing can only affect area behind it

65
Q

supersonic wing planform

A

low AR swept wings

66
Q

cause of aerodynamic heating

A

caused by fluid friction due to high speed of airflow over it

friction between air molecules and surface slows it down

67
Q

aerodynamic heating process

A
  • airflow come to rest at various stagnation points (considerable change of energy)
  • total energy is constant (Bernoulli’s eqn)
  • KE decrease means pressure energy increase
  • pressure increase means temp increase
68
Q

where is aerodynamic heating lowest and highest?

A

lowest at low speed (negligible)

highest at high speed and in troposphere where density is greatest

69
Q

effect of aerodynamic heating on aircraft

A

strength of metal drops as temperature increases

70
Q

ways to minimise aerodynamic heating on aircraft

A

parts of aircraft subjected to high aerodynamic heating can be made with other type of high heat resistance materials (Titanium, stainless steel)

71
Q

actual surface temperature formula

A

t = T + ΔT

72
Q

similarities & differences of shock and conventional stall

A

similarities:
loss of lift
flow separation

differences:
shock is in high speed, low AOA
conventional is low speed, high AOA

73
Q

what happens when airflow through oblique shock waves?

A

increase in temp and static pressure, decrease in velocity and change in direction

74
Q

why do supersonic aircraft have sharp leading edge?

A

enables the bow wave to fully develop and attach to the LE therefore minimising the size of subsonic patch

xxxFOF (6 decks)

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