When an aircraft flies at subsonic speed (0 M to 0.8 M) the velocity of the air over the wing will be in the subsonic limit.
But an aircraft flies at transonic speed (0.8 M to 1.2 M) the velocity of the air over upper surface of the wing will reach supersonic speed (1.2 m to 5 m).
This supersonic speed will cause shock waves over the upper surface of the wing, the shock wave causes flow separation on the upper surface of the wing , This flow separation will lead to turbulence, This turbulence result in stall of the aircraft.
This condition is known as super critical condition and the certain speed that causes supersonic flow over the airfoil is known as critical speed or critical mach number.
So to fly the aircraft under this super critical condition we use super critical airfoil. The design parameters of the super critical airfoil resulting in delay the flow separation over the upper surface of the wing to avoid stall and let the aircraft fly continuously under this super critical condition.
DESIGN OF SUPER CRITICAL AIRFOIL
The design of super critical airfoil is similar to the inverted ordinary (conventional) airfoil. Super critical airfoil having flat upper surface, curved lower surface and downward curved upper trailing edge.
IMPORTANT FACTORS TO BE CONSIDERED WHILE DESIGNING SUPER CRITICAL AIRFOIL
- TRAILING EDGE THICKNESS
- MAXIMUM THICKNESS
- AFT UPPER SURFACE CURVATURE
Trailing Edge Thickness
The design philosophy of the super critical airfoil need that the trailing edge slopes of both upper surface and lower surface should be same. This will result in delaying the flow separation on the upper surface of the airfoil by reducing the pressure gradient (change in pressure) on the upper surface.
The thickness of the airfoil plays a important role in delaying flow separation.When the thickness increases the delay time of separation also increases.
Aft Upper Surface Curvature
The aft upper surface of the super critical airfoil is shaped to accelerate the flow that come out of the shock wave in order to produce lift. This lift compensate the lift that loose by flat upper surface.
Advantages of Super Critical Aircraft
- Higher subsonic cruise speed
- Less drag
- Less fuel consumption
- High Range and Long Endurance
- Increase in flight efficiency
- Reduction in flight Cost.