Module 6 Supersonic flight Flashcards
Explain a thin plate in supersonic airflow, with a aoa?
Upper surface of the plate is deflected away from the freestream airflow and a expansion fan is formed on the upper leading edge. Airspeed increases and air pressure decreases when airflow passes through a expansion fan.
The lower surface is deflected towards the freestream airflow, so an oblique shockwave is formed on the lower end of the trailing edge.
TE of the aerofoil is the opposite
What is the first and last Mach line of a expansion fan determined upon?
The first Mach line will be determined by the Mach of the incoming airflow
The last Mach line will be determined by the Prandtl Meyer function and equation.
How is lift produced on a thin plate aerofoil?
Pressure over the upper surface of the plate behind the expansion fan is lower than the air pressure behind the oblique shockwave on the lower surface. This creates a pressure difference which produces lift.
Pressure difference on the trailing edge does not effect the lift production.
What is the supersonic linearized theory?
can explain the air pressure around a supersonic aerofoil in a relatively simple eqaution. This theory defines a supersonic aerofoil: there is no shockwaves on the surface of a thin sharp edged aero foil with a small camber.
When is and e negative and positive?
- = if the surface tangent deflects away from the incoming airflow. \+ = If the surface tangent deflects to the incoming airflow.
Explain the double wedge aerofoil when the aoa to the airflow is 0?
The upper and lower surface are symmetirical and the pattern of the compressive oblique shockwaves and expansion waves formed on both surface are symmetrical: air pressure increases from the leading edge after the compressive oblique shockwave, then it decreases when air passes over the expansion region.
pressure distributions are identical therefore there is no lift production.
Explain the double wedge aerofoil at a aoa > 0?
The leading edge upper surface is parallel to the incoming airflow so there is no supersonic wave formed. The leading edge of the lower surface the airflow is deflected towards the airflow so an oblique shockwave is formed.
Expansion waves are formed at the middle of the upper and lower wedge
At the upper trailing edge the airflow is deflected towards the airflow and a oblique shockwave is formed.
Overall pressure over the upper surface and lower surface creates a lift production up.
How does location of maximum thickness and camber effect double wedge aerofoil CP and lift coefficient?
The location of maximum thickness has almost no effect on lift and CP for a symmetrical double wedge aerofoil. But the location of maximum thickness/camber does affect the lift coefficient and the center of pressure to a cambered double wedge aerofoil.
What is a bi convex aerofoil?
Two circular arc form the upper surface and lower surface of the aerofoil. Two arcs with a same curvature form a symmetrical bi convex. arcs with different curvature form a cambered aerofoil. Leading edge of the biconvex is relatively sharp and shockwaves will be formed and attached at the leading edge.
What is friction drag?
Drag caused by the viscous nature of air within he boundary layer, mainly considered as skin drag.
What is wave drag?
Drag caused by the formation of shockwaves around the whole body of the airplane.
What is induced drag?
Induced drag is associated with lift, which includes drag. The pressure distribution around a supersonic aerofoi/aircraft is formed by the sequence pattern of oblique shockwaves and expansion waves. The vortices behind the turbulent wake cause drag.
Dlift + Dlift = Dinduced
What is the dominant drag in supersonic aircraft?
Wave drag
What does a unswept wing produce?
High lift coeficient at low airspeed range, and relatively lower drag coefficient at high mach number number in supersonic speed range.
However a straight wings performance can be weakened by tip effects.
Explain swept wings?
Most common design, mach cone is formed from the nose of the supersonic aircraft and the swept wings are situated inside the cone/ Oblique shockwaves are formed of the leading edge of swept aircrafts therefore, shock drag would increase significantly at higher mach numbers.
Tip effect in a supersonic leading edge on a swept wing would reduce lift and make a low L/D ratio at high mach number
