Subsonic aircraft

A subsonic aircraft is an aircraft with a maximum speed less than the speed of sound (Mach 1). The term technically describes an aircraft that flies below its critical Mach number, typically around Mach 0.8. All current civil aircraft, including airliners, helicopters and airships, as well as many military types, are subsonic.

Characteristics

Although high speeds are usually desirable in an aircraft, supersonic flight requires much bigger engines, higher fuel consumption and more advanced materials than subsonic flight. A subsonic type therefore costs far less than the equivalent supersonic design, has greater range and causes less harm to the environment.

The less harsh subsonic environment also allows a much wider range of aircraft types, such as balloons, airships and rotorcraft, allowing them to fill a much wider range of roles.

Subsonic aerodynamics

Subsonic flight is characterised aerodynamically by incompressible flow, where dynamic pressure changes due to motion through the air cause the air to flow away from areas of high dynamic pressure to areas of lower dynamic pressure, leaving the static pressure and density of the surrounding air constant. At high subsonic speeds, compressibility effects begin to appear.