Page 171 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 158

158 CHAPTER SIX

Supersonic Shock Wave
air flow

Mach 0.8

Fig. 6.6. A transonic airfoil accelerates the air to supersonic
speeds and then forms a shock wave.

supersonic

1 subsonic

Air speed (Mach no.) free stream velocity

shock wave

Distance (chord length) 1
Fig. 6.7. The airspeed over a transonic and a subsonic airfoil.

Figure 6.7 shows the airspeed, in units of Mach number, of the air
over a subsonic and a transonic wing. The subsonic airfoil is traveling
at a speed just below the critical Mach number such that the air never
reaches supersonic. The transonic airfoil is just above the critical
Mach number so the air becomes supersonic. The Mach number of
the subsonic airfoil decreases after the peak while on the transonic
airfoil it increases, until the shock wave.
So how does the wing know where to put the normal shock wave?
Let us first assume that the normal shock wave forms at the trailing
edge in order to meet the trailing-edge condition. What we would find