Page 175 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 162

162 CHAPTER SIX

a b

c

Fig. 6.10. Shock waves on a transonic airfoil at three different
Mach numbers.

The B-47 was the United States’ such that the airflow becomes supersonic on both the top and
first jet-powered, swept-wing bottom surfaces, although to a lesser speed on the bottom
bomber. The swept wing was a surface. Airfoil c shows a Mach number near Mach 1 where
last-minute change based on the shock waves on both top and bottom surfaces have
data gathered from the German become strong enough to move to the trailing edge and
research during WWII. produce a substantial, drag-producing wake. This is the
situation of the jet fighter in Figure 6.1. The discovery of the
airflow separation led to better airfoil designs for transonic and
supersonic flight. The wake due to separation of the air is indicated on
all three of the wings in the figure.
For supersonic aircraft, there is a simple relationship relating the
angle of sweep to the design supersonic speed of the airplane. This is
illustrated in Figure 6.11, which shows the sweep angle as a function
of Mach number. The purpose of sweep for supersonic aircraft is to
keep the effective Mach number at the wing leading edge at or below
Mach 1 (Figure 6.9). It does not take sophisticated military
intelligence to determine the supersonic operating conditions of an
adversary’s airplane. All one has to do is look at the sweep angle. For
example, a Mach 2 airplane will have a sweep of 60 degrees.

Area Rule

The amount of wave drag on supersonic aircraft is a function of the
size of the aircraft. To illustrate this point, imagine throwing a small