What direct effect does the shape of a supersonic aerofoil have on the production of lift?
Almost no appreciable affect on lift - as long as the surface deflects towards or away from the airflow
What determines the pattern and shape of the waves around an aerofoil in supersonic flight?
Shape
Describe the pattern of expansion and compression waves that are formed around a flat plate set at a small AoA in supersonic flight?
- Expansion fan on upper leading edge
- Compression wave on upper trailing edge
- Compression wave on lower leading edge
- Expansion fan on lower trailing edge
What factors determine the first and last mach wave in an expansion fan?
- First determined by the incoming mach number
- Last determined by the outgoing mach number
How does the pressure change over the upper surface of a flat plate in supersonic flight?
Pressure decreases as it passes through expansion waves
Pressure increases behind the oblique shockwave at the trailing edge
How does a supersonic aerofoil produce lift?
By creating pressure differences between the upper and lower surfaces by the pattern of compression and expansion shockwaves
What does Ackeret’s rule state? (6)
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)
In Ackeret’s Rule, what does ε represent?
The angle between the local airflow and the tangent to the surface
What does it mean if ε is negative or positive in Ackeret’s rule?
-ε: the surface deflects away from the incoming local airflow
+ε: the surface deflects towards the incoming local airflow
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?
- 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
Describe the pressure distribution around a double wedge aerofoil set at a small AoA in a supersonic flow?
Upper
- First half: P1 = P(fs)
- Second half: P2 < P(fs)
Lower
- First half: P1 > P(fs)
- Second half: P2 = P(fs)
What effect does location of maximum thickness on a symmetrical double wedge aerofoil have on lift and CoP and it location?
No appreciable affect
No effect on the CoP, it remains at 50% of the chord
What is the relationship between the location of the max thickness/camber of a cambered double wedge aerofoil and the CoP and CL?
- 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
When does the effect of the location max camber/thickness of a double wedge aerofoil have no real effect on drag?
When the max camber/ thickness remains at a location between 40%- 60% of chord
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?
- 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
Describe the pressure distribution around a bi-convex aerofoil set at a small AoA in supersonic flight?
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
What is unique about the stall of a bi-convex aerofoil section?
- There is no lift reduction when AoA increases to 30˚
- There is non stall when AoA is less than 30˚
How does the L/D ratio of supersonic aerofoils compare to low subsonic aerofoils?
- L/D very poor due to low CL and high CD
- Supersonic L/D = 4 - 6
- Subsonic L/D = 12 - 24
Why is the performance of a bi-convex aerofoil more predictable than other wing sections?
Because the practical data collected from tests match the theoretical data very well and so it is predictable
List the platform shapes of supersonic wings? (4)
- Unswept wings
- Swept wings
- Delta wings
- Variable sweep/geometry wing
What are the advantages of an unswept supersonic wing design?
- High CL in low speed range
- Relatively low CD at high supersonic mach number
What are the disadvantages of an unswept supersonic wing design? Explain?
- 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
What are the advantages of a swept supersonic wing design?
- Can produce relatively high L/D ratio for supersonic
- Can have relatively low drag if LE is inside the mach cone
What are the disadvantages of a swept supersonic wing design?
- 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
