Page 174 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 161

High-Speed Flight 161

Power required Elbow

0.6 0.8 1 1.2 1.4
Mach number
Fig. 6.8. Power increases dramatically as the airplane
passes through Mach 1.

The reduced drag of a swept wing results from
reducing the effective Mach number of the wing.
The effective Mach number is the Mach number the
wing sees perpendicular to the leading edge, as
illustrated in Figure 6.9. A nonswept wing will
experience the sharp rise in the power requirement
(Figure 6.8) at a lower Mach number than a swept Effective
wing. At supersonic speeds wing sweep also helps Mach no.
to reduce the strength of the oblique shock wave
from the leading edge of the wing. Sweep angle
The increase in drag at Mach 1 was not Mach no.
understood at the time of the Rome conference. It
was later discovered that the presence of the shock
wave, shown in Figure 6.6, caused the airflow to
separate, increasing form drag. This is illustrated in
Figure 6.10. The three airfoils in Figure 6.10
represent three different flight Mach numbers. In
airfoil a, the flight Mach number is only enough to
accelerate the air to supersonic speed on the top of
Fig. 6.9. Effective Mach number on a swept
the airfoil. In airfoil b, the flight Mach number is wing.