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Part 4 Aerodynamics > Module 6 - Supersonic Flight > Flashcards

Module 6 - Supersonic Flight Flashcards

1
Q

What direct effect does the shape of a supersonic aerofoil have on the production of lift?

A

Almost no appreciable affect on lift - as long as the surface deflects towards or away from the airflow

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2
Q

What determines the pattern and shape of the waves around an aerofoil in supersonic flight?

A

Shape

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3
Q

Describe the pattern of expansion and compression waves that are formed around a flat plate set at a small AoA in supersonic flight?

A
  • Expansion fan on upper leading edge
  • Compression wave on upper trailing edge
  • Compression wave on lower leading edge
  • Expansion fan on lower trailing edge
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4
Q

What factors determine the first and last mach wave in an expansion fan?

A
  • First determined by the incoming mach number

- Last determined by the outgoing mach number

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5
Q

How does the pressure change over the upper surface of a flat plate in supersonic flight?

A

Pressure decreases as it passes through expansion waves

Pressure increases behind the oblique shockwave at the trailing edge

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6
Q

How does a supersonic aerofoil produce lift?

A

By creating pressure differences between the upper and lower surfaces by the pattern of compression and expansion shockwaves

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7
Q

What does Ackeret’s rule state? (6)

A

For thin sharp edged wings of small camber at low incidence in 2D frictionless, shock free supersonic flow the pressure coefficient is:

C(p) = 2ε / √(M^2 - 1)

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8
Q

In Ackeret’s Rule, what does ε represent?

A

The angle between the local airflow and the tangent to the surface

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9
Q

What does it mean if ε is negative or positive in Ackeret’s rule?

A

-ε: the surface deflects away from the incoming local airflow
+ε: the surface deflects towards the incoming local airflow

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10
Q

Describe the pattern of expansion and compression waves that from around a double wedge shaped aerofoil in supersonic flight, when set at a small AoA?

A
  • No change at LE on upper surface
  • Expansion Fan on middle upper surface
  • Oblique shockwave on upper TE
  • Oblique shockwave on lower LE
  • Expansion fan on middle lower surface
  • No change at TE on lower surface
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11
Q

Describe the pressure distribution around a double wedge aerofoil set at a small AoA in a supersonic flow?

A

Upper

  • First half: P1 = P(fs)
  • Second half: P2 < P(fs)

Lower

  • First half: P1 > P(fs)
  • Second half: P2 = P(fs)
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12
Q

What effect does location of maximum thickness on a symmetrical double wedge aerofoil have on lift and CoP and it location?

A

No appreciable affect

No effect on the CoP, it remains at 50% of the chord

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13
Q

What is the relationship between the location of the max thickness/camber of a cambered double wedge aerofoil and the CoP and CL?

A
  • As the max camber/thickness moves forward the CoP will move forward and the CL will increase
  • As max camber/thickness moves rearward the CoP will move rearward and the CL will decrease
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14
Q

When does the effect of the location max camber/thickness of a double wedge aerofoil have no real effect on drag?

A

When the max camber/ thickness remains at a location between 40%- 60% of chord

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15
Q

Describe the pattern of expansion and compression waves around a symmetrical biconvex aerofoil in supersonic flight when set at a small AoA? What difference may be present in a cambered aerofoil?

A
  • Expansion wave forms over entire upper surface
  • Oblique shockwave at trailing edge of upper surface
  • Oblique shockwave at leading edge of lower surface
  • Expansion waves over remained of lower surface

Camberd aerofoil may have a much weaker oblique shockwave on the upper leading edge also

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16
Q

Describe the pressure distribution around a bi-convex aerofoil set at a small AoA in supersonic flight?

A

Upper:

  • Pressure decreases from the LE to TE
  • Pressure increases at the TE due to the compression wave

Lower:

  • Pressure increases at the LE due to compression wave
  • Pressure decreases from behind LE shock to be equal to P(fs) at the TE
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17
Q

What is unique about the stall of a bi-convex aerofoil section?

A
  • There is no lift reduction when AoA increases to 30˚

- There is non stall when AoA is less than 30˚

18
Q

How does the L/D ratio of supersonic aerofoils compare to low subsonic aerofoils?

A
  • L/D very poor due to low CL and high CD
  • Supersonic L/D = 4 - 6
  • Subsonic L/D = 12 - 24
19
Q

Why is the performance of a bi-convex aerofoil more predictable than other wing sections?

A

Because the practical data collected from tests match the theoretical data very well and so it is predictable

20
Q

List the platform shapes of supersonic wings? (4)

A
  • Unswept wings
  • Swept wings
  • Delta wings
  • Variable sweep/geometry wing
21
Q

What are the advantages of an unswept supersonic wing design?

A
  • High CL in low speed range

- Relatively low CD at high supersonic mach number

22
Q

What are the disadvantages of an unswept supersonic wing design? Explain?

A
  • Wingtip effects: 3D flow around the wingtips can affect the pressure distribution
  • Wingtips outside Mach cone: If the wingtips are outside the mach cone, lift will be reduced and drag will be increased for the wing area outside the mach cone
23
Q

What are the advantages of a swept supersonic wing design?

A
  • Can produce relatively high L/D ratio for supersonic

- Can have relatively low drag if LE is inside the mach cone

24
Q

What are the disadvantages of a swept supersonic wing design?

A
  • Susceptible to torsion and twisting due to poor rigidity
  • At high supersonic mach numbers a series of oblique shockwaves form on the LE
  • Significantly higher shockdfag at high mach numbers
  • Tip effect at high mach number reduces lift and reduced L/D ratio
25
Q

What are the advantages of a supersonic delta wing design?

A
  • Aerodynamic characteristics of swept wings
  • Wing is rigid, and mechanically strong
  • Simple structure
  • Long chord and low t/c ratio
  • Relatively low wing loads due to large area
26
Q

What are the disadvantages of a supersonic delta wing design?

A

-Produces higher skin friction drag

27
Q

What is a variable geometry wing? Describe a common example?

A

A type of wing that can change its configuration in flight

-Often has straight wing for subsonic flight which can move to join the fuselage and becomes a swept or delta like wing in supersonic flight

28
Q

What rules should the body design of a supersonic aircraft follow?

A
  • Follow area rule

- Sharp nose/ leading edge to keep bow shockwave attached

29
Q

How does velocity change in the boundary layer around a supersonic object? What are the characteristics of the supersonic BL?

A
  • It is very thin
  • Speed increase can be assumed to be linear
  • Temperature in BL will be increased due to kinetic heating (friction) and so ρ and p will be different
30
Q

How does boundary layer separation play a part in the generation of lift on a supersonic aerofoil? What other consequences are there as a result of BL separation?

A
  • BL separation is a form of change in pressure (especially behind shockwaves)
  • If BL separates in the correct place it will assist in the production of lift by pressure difference

-BL separation will also increase the drag due to the adverse pressure gradient

31
Q

What are the effects of kinetic heating of the boundary layer?

A

Temperature increases

  • ρ decreases
  • p decreases
  • viscosity increases
  • C(D-skin) increases
32
Q

What are the 3/4 components of total drag when an aircraft is in supersonic flight?

A
  • Skin Drag
  • Shock drag
  • ‘Lift’ Drag
  • Vortex Drag

last 2 can be combined to be Induced drag

33
Q

What is kinetic heating?

A

When kinetic energy gets turned into thermal energy

-Air temperature increases due to the decrease of airspeed

34
Q

What are the sources of kinetic heating in flight? Briefly explain each?

A
  • Friction: In the BL work done against friction will be heat and will heat the surface the friction is coming from
  • Stagnation Points: Energy equation shows that as v=0, T will be maximum
  • Shockwaves: Energy is required to form the shockwave, the temp increases behind the shockwave. The stronger the shock, the larger the temperature change
35
Q

What state of Boundary Layer will produce a higher temperature increase?

A

Turbulent

36
Q

What points on the aircraft are most susceptible to kinetic heating?

A
  • Wing tips
  • Leading edge of body
  • Any stagnation points
37
Q

List the different methods to protect the aircraft from kinetic heating? (4)

A
  • Materials
  • Insulation
  • Surface Radiation
  • Surface Cooling
38
Q

Explain how different material can help protect against kinetic heating?

A
  • Different materials have different melting points, and become elastic before they reach the melting point
  • Aluminium=600˚C, Titanium=1600˚C
  • Ceramic composites are strong and good for use in high temperatures, but can be brittle
39
Q

Explain how insulation protect the aircraft from kinetic heating?

A
  • By covering heavily affected areas with high temperature resistant materials with low thermal conductivity
  • Heat will not penetrate easily and can protect the aircraft
40
Q

Explain how surface radiation can protect the aircraft from kinetic heating?

A
  • By designing highly affected areas to have larger surface areas heat can be dissipated out faster to prevent local overheating
  • Using high emissivity material will also radiate heat away from local structures
41
Q

Explain how surface cooling can protect the aircraft from kinetic heating?

A
  • Areas behind heated or susceptible surfaces are ducted or installed with jackets that have coolant running through to remove heat from the surface
  • Coolant must have high heat capacity or high latent heat
  • Requires a radiator for the coolant to cool
42
Q

What are some common features of controls of supersonic aircraft?

A
  • All moving slab type controls
  • Fully power operated
  • Synthetic and automatic controls
  • Almost certain to be fly by wire

Part 4 Aerodynamics (6 decks)

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