Question

Why do we may avoid normal shocks when designing aircraft geometries?

Answer 1

Ans. A shock wave is a very thin region of energy in a supersonic flow across which pressure , temperature, density, and entropy increases.

Mach no , flow velocity, total pressure decreases.

Total enthalpy remain same.

When shock wave occurs perpendicular to velocity it is called normal shock .

It is analysed with 1 dimensional equation.

Across normal shock wave Mach no , stagnation pressure, flow velocity decreases.

While static pressure, static temperature, density increases and stagnation temperature remain the same.

Strength of normal shock wave is more.

So During design phase of aircraft geometries we avoid to have normal shock wave because this wave will produce more interference drag and wave drag. The losses will be more and the results will drastically change the condition of the flight.

As per designers view we avoid formation of shock anywhere on aircraft geometries by applying sweep on wings, reducing critical Mach no and drag divergence number to a lower value.

Drag divergence number is that free stream Mach no at which large rise in drag coefficient occurs.

We prefer streamlined pattern of flow and avoid the flow to be supersonic over the airfoil surface by selecting appropriate type of airfoil.

Value of Mcr.( Critical Mach no) depends on design of airfoil but it's value also varies with incidence.

Mcr increases as thickness increases or camber increases.

Hence we avoid formation of normal shock wave when designing aircraft geometries.