Question

State the difference between Supersonic Aerofoil and Supercritical Aerofoil.

Hint:

To remember: Supersonic aerofoils are thin and sharp like a knife to slice through supersonic shockwaves. Supercritical aerofoils are flat on top to delay shockwaves and save fuel on commercial passenger flights.

Answer 1

Step 1: Defining the Role of an Aerofoil:
An aerofoil is the cross-sectional shape of a wing. Its shape is carefully designed to manipulate airflow to generate lift efficiently while minimizing aerodynamic drag. Different speed regimes require entirely different aerofoil designs.

Step 2: Analyzing Supersonic Aerofoils:
Supersonic aerofoils are designed for aircraft flying faster than the speed of sound ($\text{Mach} > 1.0$), such as fighter jets.
• Shape: They are extremely thin with sharp leading and trailing edges. They often have symmetrical, diamond, or double-wedge cross-sections.
• Aerodynamic Principle: At supersonic speeds, shock waves form at the wing's front. A sharp leading edge is required to keep this shock wave attached to the wing, preventing severe turbulence and reducing massive wave drag.

Step 3: Analyzing Supercritical Aerofoils:
Supercritical aerofoils are designed for aircraft flying at high subsonic/transonic speeds ($\text{Mach } 0.7 \text{ to } 0.85$), which includes almost all modern commercial airliners.
• Shape: They feature a flattened upper surface, a rounded leading edge, and a highly curved (under-cambered) trailing edge.
• Aerodynamic Principle: As air flows over a normal wing, it accelerates. At high speeds, the air on the upper surface can reach supersonic speeds even though the plane is subsonic, creating a drag-inducing shock wave. The flat top of a supercritical aerofoil prevents this local acceleration, delaying the shock wave and reducing drag. This allows passenger planes to fly faster and save fuel.