Page 163 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 150

150 CHAPTER SIX

Fig. 6.1. Transonic F-18 with shock wave over
wing. (Photo courtesy of the U.S.Air Force.)

their names imply, the categories are related to the speed of sound
(sonic). As stated before, the Mach number is the airplane’s speed in
units of the speed of sound. At Mach 1 the plane is going exactly at the
speed of sound. Subsonic refers to speeds below Mach 1. Transonic
refers to speeds approaching Mach 1. Commercial transports, most
military transports, bombers, and business jets fly at transonic
speeds. Supersonic refers to speeds above Mach 1, and is usu-
The North American X-15 (Figure
ally left to fighters and interceptors for short bursts. Hyper-
6.16) reached a speed of 4534
sonic refers to speeds of high Mach numbers. At present, the
mi/h (7295 km/h) or Mach 6.72.
only hypersonic vehicle is the Space Shuttle during reentry.
Mach number also relates to the apparent speed of the air as seen by
the airplane. The airplane travels at a single Mach number with respect
to the air at a distance. The airflow around the airplane is traveling at
different speeds and thus has a different local Mach number. In
transonic flight significant parts of the airflow, relative to the aircraft,
are both subsonic and supersonic. In the case of supersonic flight
virtually all of the air relative to the airplane is supersonic. Finally, in
hypersonic flight the vehicle is going so fast that certain additional