Page 162 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 149

6
CHAPTER

High-Speed Flight

upersonic fighters, commercial transports, bombers, and the Con-
corde fly at speeds greater than most small airplanes. The speeds at
Swhich they fly introduce some additional physics that are not pre-
sent in low-speed flight. In particular, the compressibility of the air
becomes significant. We have said that in the understanding of lift air
is to be considered an incompressible fluid because of the low forces
involved with low-speed flight. But air is compressible. All fluids, even
water, are compressible on some level. When air compresses, its den-
sity changes. This is a phenomenon inherently different than in low-
speed flight, where the air’s density remains essentially unchanged as
it passes over a wing, fuselage, or other parts of the airplane.
Figure 6.1 shows a fighter flying at just below the sound speed. The
air flowing over and under the wing has expanded, lowering the
density and temperature to cause water vapor to condense. In high-
speed flight one must understand where, when, and how the density
of the air changes.

Mach Number

Low-speed flight is subsonic flight. High-speed flight can be broken
into three basic categories: transonic, supersonic, and hypersonic. As

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