Expert: Paul Soderman - 2/11/2009

Question
1.Could you please explain what a wave drag is ??
2.and what the inclination to flow (motion of the air) is?
3. Last question is airspeed the same as flight velocity?
Thank you

Answer
Wave drag is the drag induced on an aircraft (or other body) when it generates shock waves.  Shock waves radiate energy that is sensed by the aircraft as drag.  Shock waves occur at supersonic speeds, but on an aircraft the phenomenon can occur at speeds just below Mach 1 where the flow has accelerated around the wing, for example, and is locally supersonic.  A local shock wave is created.  The drag near Mach 1 is quite high.

Inclination to flow is angle of attack, which is the angle between the main axis of the body (e.g., wing chord) and the flight direction.  I'm not sure what you mean by motion of the air.

Flight velocity is the speed of the aircraft relative to the ground.  If we fix a coordinate system to the aircraft, we would experience airflow around the aircraft as in a wind tunnel.  The speed of the air is the airspeed.  Far from the aircraft it equals flight velocity, but as the flow accelerates over the wing, for example, it goes faster than flight velocity.  At other regions airspeed can be slower than flight velocity.  It is, in other words, the local flow speed relative to the aircraft.