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 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.
The propeller is one of the most efficient sources of thrust available and is common on subsonic aeroplanes and airships. Sometimes it is enclosed in the form of a ducted fan. At higher subsonic speeds and at high altitudes, such as attained by most airliners, the high-bypass turbofan becomes necessary. Pure jets such as the turbojet and ramjet are inefficient at subsonic speeds and not often used.