Supersonic Flight Flashcards
What speed range is supersonic flight?
Mfs>Mdet
How do supersonic a/c produce lift?
Rely on the surface deflecting to/from airflow, forming SW/EW for which the resulting change in air properties creates lift.
There is no direct relation between the shape of the aerofoil and its ability to produce lift, however the shape will affect the pattern of SW/EW.
What is the pressure coefficient rule?
When is E +/-
Ackerets rule: for a thin, sharp LE wing of a small camber at a low incidence angle in 2D frictionless shockfree flow,
Cp=2E/ SWRT(M^2-1).
E is the angle between the local airflow and the surface
- if the surface deflects away from the airflow E is -ve
- if the surface deflects toward the airflow E is +ve
Does a thin plate produce lift ( 0* AoA/small AoA)?
Why?
At 0* AoA no lift is produced.
At a small AoA lift is produced.
This is because the LE of the upper surface of the thin plate is deflected away from the airflow so expansion waves are formed and Cp+. The lower surface LE deflects toward the airflow so a shockwave is formed and Cp-. What occurs at the TE is not relevant for lift production but will affect drag. Pressure in the expansion region will be decreasing while pressure after the shockwave will increase. This means a higher pressure under the wing so lift is produced, confirmed by the Cp diagram. Although simple, they have poor rigidity so are not practical.
Does a double wedge aerofoil produce lift (0* AoA/small AoA)?
Why?
At 0* AoA, no as the pattern of SW/EW is the same on the upper/lower surface so no pressure differential exists.
At an AoA yes. The upper surface will initially lie parallel to the airflow so nothing occurs, but at the middle the surface deflects away from the surface so Cp- and EW form. On the lower surface at the LE the surface deflects toward the airflow so Cp+ and SW forms. From the middle the surface deflects away from the surface so Cp-, however goes to neutral as Cp+ + Cp- = Cp0. The resulting Cp diagram shows the lift production.
How does the location of max thickness affect Cop and Cl?
Where is the optimum place for max thickness/camber?
On a symmetrical DW aerofoil, the location of max thickness has little effect on CoP/lift, but will have an effect for a cambered aerofoil. It only affects the distribution of Cp.
40-60% of the chord is ideal as little effect on drag for both symmetrical/cambered DW aerofoil.
Describe the shape of a bi convex wing
Can be symmetrical (two circular arcs with same curvature) or cambered (different curvature)… both with a sharp LE.
Describe how a symmetrical bi convex wing produces lift
At a small AoA, an oblique shock will form attached on the under side of the LE and at the TE of the upper surface. Both surfaces are covered in gradually deflected EW as the tangent of the surface is deflecting away from the airflow. As pressure decreases after the EW pressure decreases on both surfaces but is however higher on the lower surface due to the oblique shock at the LE. So lift is produced and can be proved using the Cp diagram.
Describe how a cambered biconvex wing produced lift
Will usually have a slight positive camber (curvy upper surface). They work on the same principal as the symmetrical biconvex aerofoil with the exception that there is an attached oblique shock to both the upper/lower LE, with the upper being much weaker. So will produce slightly less lift at the LE and this can be see on the Cp graph.
What is Cl/Cd like for supersonic aerofoils?
Is there up/down wash?
What other wing shape is there?
Low Cl, high Cd so low L/D ratio.
No up/down wash.
Hexagonal.
What platform shapes are there for supersonic a/c?
Unswept/swept/variable sweep wing
Delta wing
Describe the advantages/disadvantages of unswept wings
A: higher Cl at low air speeds and lower Cd at high air speeds.
D: decrease in performance due to “tip effects”. This is as airflow over straight wing is approx 2D however complex 3D formations occur at the wingtip which affect the pressure distribution. The wingtips can sit outside the Mach cone if they are long and this increases drag/decreases lift.
Describe the shockwave formation around a swept wing.
Describe the advantages/disadvantages of swept wings
A detached bow shock forms at the nose when supersonic. If the wings lie within the Mach cone, flow at the LE will likely be subsonic which decreases drag. This phenomenon disappears at high Mach speeds and a series oblique shock can form at the root of the LE as flow remains supersonic increasing drag.
A: work effectively and can have a higher L/D ratio.
D: have rigidity issues and bend/twist. High shock drag at high M so reduced L/D. Tip stalls can occur.
Describe the advantages/disadvantages of delta wings
They join the fuselage and wings together, has the characteristics of a sweet wing with more rigidity.
A: improved rigidity and loading on the wing is small due to larger surface area
D: relatively low t/c and produces a lot of skin drag.
When do variable sweep wings change the sweep angle?
What is a common feature of swept wings?
Straight at low air speeds and more sweet at high air speeds.
Sweepback angle is less than the Mach angle so wings lie within Mach cone meaning subsonic LE and less drag.
