What does Ackeret’s rule state? What is the formula
- For thin, sharp-edged wings of small camber at low incidence angles in 2-D frictionless shock-free supersonic flow the pressure coefficient is:
Cp = 2ε / √M^2-1
What is the angle ε when the surface deflects away from the incoming airflow?
Negative.
What is the angle ε when the surface deflects toward the incoming airflow?
Positive.
What effects does location of maximum thickness have on lift and CoP for a symmetrical double wedge aerofoil?
No effect on lift and CoP for symmetrical.
Does location of maximum thickness/camber effect lift and CoP for a cambered double wedge aerofoil?
- Location of max thickness/camber does affect CL and CoP for cambered.
Where is lift and CoP located on a symmetrical double wedged aerofoil?
- CoP remains at the centre of the chord.
- Lift/CL remains the same.
What happens to lift/CL and CoP when location of max camber on a cambered double wedge aerofoil moves forward?
- CoP moves forward
- CL/Lift increases
What effect does location of max thickness on drag? What is the condition for this?
Little effect on drag when the location of maximum thickness is 40-60% of chord.
What are the different types of supersonic aerofoils? (4)
- Thin plate
- Double wedged aerofoil
- Biconvex aerofoil
- Hexagonal
What are the different types of supersonic wings and planforms? (4)
- Unswept wings
- Swept wings
- Delta wings
- Variable geometry wings
What are the advantages and disadvantages of a thin plate aerofoil?
Advantages:
- Simple and economical
Disadvantages:
- Poor rigidity which would affect the stability of the flight
What are is the advantage of using a double-wedged aerofoil?
Advantages:
- Popular
- Manufactured easily
- Strong
What is the advantage of using a biconvex aerofoil?
Advantage:
- Increasingly used in modern supersonic aircraft
- Can perform better in subsonic speed.
- Design can be optimised, because the practical aerodynamic data of biconvex aerofoil match with the aerodynamic theory well, making performance predictable.
What design of a body of a supersonic aircraft should follow? Why? What is the preferred design?
- Area rule in order to reduce drag.
- Slender shape with a relatively sharp LE/nose and T.E. is always preferred.`
What is the advantage of using a sharp leading edge for a supersonic aircraft?
- Allows bow shockwave formed at the L.E. attached to the surface.
- Would cause greater drag if the shockwave detaches from the L.E.
What is the disadvantage of using a sharp leading edge for supersonic aircraft?
It is a source of kinetic heating.
Is there a boundary layer when an aircraft is in supersonic flight? How thick is it?
Yes there is a boundary layer and it is very thin.
Describe how speed increases in the boundary layer of a supersonic aircraft?
- Speed of air particles on the surface under the boundary layer is “0” and increases very quickly to reach the free-stream speed.
- Assumed that airflow speed in the boundary layer increases linearly in the direction perpendicular to the surface.
Why is the friction in the boundary layer a complex issue? (3)
- Part of KE of airflow would be consumed to overcome skin drag, this part of KE is transferred into heat energy.
- Airflow within the boundary layer will be heated up and temperature of air within the boundary layer increases, and density and pressure will be changed.
- Due to the increases in temperature, viscosity increases, causing the increase of Cdskin and increased friction of the airflow.
What are the four different drags associated to supersonic flight?
- Dfr = drag due to friction because of viscous airflow, mainly skin drag.
- Dwave = Wave drag caused by formation of shockwaves around the whole body of the aircraft.
- Dvortex + Din = Drag associated with lift, including drag caused by vortex-induced drag, and the drag derived from the pressure distribution around the aircraft which produces lift.
What is the definition of kinetic heating?
The fact that kinetic energy turns into heat energy or the increase of temperature of air is the result of the decrease in airspeed.
List the 5 different sources of kinetic heating on supersonic flight?
- Sharp-leading edge
- Wave drag
- Friction within boundary layer
- Turbulent boundary layer
- Stagnation point
How does friction within boundary layer cause kinetic heating? (3)
- Kinetic energy of the airflow would be consumed to overcome the friction for air to keep flow forward.
- Work done against friction will change into thermal energy - heat.
- The surface that friction exerts on will be heated, therefore temperature will increase.
Which boundary layer produces a higher temperature?
Turbulent boundary layer produces higher temperature increase than laminar boundary layer.
