Page "Aerodynamics" Paragraph 58

In solving a subsonic problem, one decision to be made by the aerodynamicist is whether to incorporate the effects of compressibility.

Compressibility is a description of the amount of change of density in the problem.

When the effects of compressibility on the solution are small, the aerodynamicist may choose to assume that density is constant.

The problem is then an incompressible low-speed aerodynamics problem.

When the density is allowed to vary, the problem is called a compressible problem.

In air, compressibility effects are usually ignored when the Mach number in the flow does not exceed 0. 3 ( about 335 feet ( 102m ) per second or 228 miles ( 366 km ) per hour at 60 ° F ).

Above 0. 3, the problem should be solved by using compressible aerodynamics.