Aerodynamics of Transonic Aerofoils Flashcards
When will a normal shockwave form on an aerofoil? (3)
- When free stream Mach number reaches the critical Mach number, the local Mach number over the aerofoil at the most cambered location will reach one (ie the airflow becomes sonic and Mach waves will occur at this location).
- When the local airspeed over the aerofoil is greater than the speed of sound, a normal shockwave will be formed due to the pressure disturbance, which could be caused as a result of the pressure difference between the free-stream pressure and local pressure.
- An initial normal shockwave will occur at the upper surface of an aerofoil
What happens to pressure, density and temperature following a shockwave? Why?
- Air pressure, density and temperature all will increase.
- This is because the airflow behind a shockwave becomes subsonic as the freestream airspeed is lower than the speed of sound.
Describe what a lambda foot is?
- The root of a shockwave on an aerofoil is much thicker than the top part of the shockwave.
- One side of the lambda foot is a part of the normal shockwave, and the other side of the lambda foot is a small oblique shockwave.
What is lambda foot caused by? Explain the pressure in the lambda foot? (3)
- Partially caused by “pressure leak”.
- Pressure change is not sudden through the boundary layer, where the local speed can be lower than speed of sound.
- Thus pressure in the lambda foot is greater than the pressure in front of the normal shockwave, but smaller than the pressure behind the normal shockwave.
What happens to the turbulent wake when the Mach number continues to increase?
Higher the Mach number is before the normal shockwave, the stronger the shockwave, the stronger the turbulent wake’s oscillation will be.
What happens to the separation point if the shockwave is relatively weak? What happens to the turbulent wake and separation point if the shockwave is strong? (4)
- Turbulent wake tends to separate from the surface.
- Separation can occur at the rear part of the aerofoil if the shockwave is relatively weak.
- When the shockwave gets stronger, separation point moves forward - gets closer to the shockwave.
- The turbulent wake can separate immediately if the shockwave is strong.
Explain in detail, the movement of a shockwave along the aerofoil when the free-stream Mach number increases from Mcrit (8)
- ) When Mfs reaches Mcrit, the local airflow becomes sonic (local Mach number ML = 1) and Mach waves occur at this location.
- ) When Mfs increases after reaching Mcrit, a normal shockwave forms on the upper surface of the aerofoil.
- ) As Mfs increases further, after reaching Mcrit, the normal shockwave moves rearwards along the upper surface. The intensity of the shockwave is greater than that in (2). Turbulent wake behind the shockwave oscillates more, and separation point moves forward.
- ) As Mfs continues to increase, a normal shockwave forms on the lower surface of the aerofoil. The upper shockwave gets stronger, and its turbulent wake separtes just behind the shockwave.
- ) Both the upper and lower shockwave move rearwards as Mfs increases. The normal shockwave on the lower surface moves faster than the upper shockwave and settles at the T.E. of the aerofoil first.
- ) As Mfs increases further close to 1, the upper shockwave moves to the T.E, thus both shockwaves settle at the T.E. Both upper and lower surfaces are covered in supersonic airflow, no separations on both surfaces of the aerofoil
- ) When Mfs becomes greater than 1, a bow shockwave forms at the leading edge, then detaches from the leading edge.
- ) For a symmetrical aerofoil at 0 AoA, shockwave formation and movement on upper and lower surfaces occurs simultaneously when the Mfs changes from Mcrit to Mdet.
What is Detachment Mach Number? What happens to airflow at this stage? (i.e before and after shockwave and behind an oblique shockwave). (3)
- The free-stream Mach number when the bow shockwave detaches from the L.E.
- At this stage, the airflow in the freestream is supersonic. - The Mach number of the airflow behind the bow shockwave in front of the leading edge is subsonic (behind the normal part of the bow shockwave).
- Behind the oblique parts of the bow shockwave, the airflow is still supersonic, meanwhile supersonic flow is both surfaces of the aerofoil.
What is Mach buffet and shock stall? (4)
- Vortices in the turbulent wake behind a normal shockwave detach from airflow, and produce airflow oscillation, and the shockwave oscillates.
- When the shockwave intensifies with the increase of Mfs, the detachment of turbulent wake vortices from the aerofoil surface vibrate the structure of the aerofoil with noise, known as Mach buffet.
- The pilot experiences it like in a low speed/high AoA buffeting, when the aircraft is in Mach buffet.
- Following the Mach buffet, the aerofoil will lose lift due to turbulent wake separation and get into a stall, known as shock stall.
What are the two types of shock drag?
Wave drag and boundary separation drag.
What is wave drag? (4)
- When a shockwave is formed, air pressure, temperature and density all increase.
- It requires energy to form the shockwave, and the energy is provided by the airflow.
- Higher the Mach number, more kinetic energy the airflow would lose to form the shockwave i.e. to overcome more “resistance” to flow past the shockwave.
- This “resistance” is known as Wave Drag, also called Energy drag.
What is boundary separation drag? (3)
- Turbulent wake detaches from the surface of an aerofoil behind the shockwave, when the Mfs increases, it affects the aerodynamic forces just like it is in subsonic boundary layer separation.
- The vortices in the separating turbulent wake causes fore-aft pressure differences.
- This pressure difference exerts on the aircraft to form a drag, known as boundary separation drag.
Explain what happens to Cp, CL and CD when travelling from Mcrit to Mdet in the first stage? (5)
- The Mfs reaches Mcrit (0.75), a shockwave is formed on the upper surface of a wing, and gets stronger with the increase of Mfs, but slowly moves rearwards.
- The shockwave is at the most cambered position, the pressure in front of the shockwave decreases due to the increase in local airspeed.
- CP moves forward.
- CL increases due to the lower pressure before the shockwave.
- CD increases, because the formation of the shockwave, and potential of the turbulent wake behind the shockwave.
Explain what happens to Cp, CL and CD when travelling from Mcrit to Mdet in the second stage? (4)
- Upper shockwave moves rearward, the turbulent wake starts separating, and as Mfs increases further, a shockwave forms on the lower surface of the aerofoil.
- After CoP moves to the front most position, CoP will move rearward.
- CL starts decreasing from its peak when the upper shockwave get stronger, then suddenly decreases sharply, due to the formation of lower shockwave and the wake separation.
- Cd increases significantly during this period due to the same reasons.
What is the drag-divergence mach number (Mdd)? What are its other names?
- The free-stream Mach number at which the Cd increases significantly is called drag-divergence Mach number.
- Drag rise Mach number Mdr, or critical drag rise Mach number Mcdr.