Page 164 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 151

High-Speed Flight 151

physical phenomena creep in. This is discussed in a later section.
Why is the compressibility of the air important? In Chapter 2 the idea
that air is being diverted down to create lift was emphasized.
The lift on the wing was shown to be proportional to the
A supersonic bomber, being
amount of air diverted down times the vertical velocity of that
developed by the United States
air. The amount of air that is diverted per second was
in the 1960s, was lost in a
determined by considering the volume diverted. The analogy of
midair collision when it was
a scoop was introduced to demonstrate this. But this assumes
flying formation for a publicity
that the air density is constant. If density changes, then the
photo.
amount of air in a given volume changes. This can confuse our
intuition of the behavior of air.

Lift Is Still a Reaction Force

High-speed flight, like low-speed flight, requires that air be diverted
to create lift. This basic principle does not change with the introduc-
tion of compressibility. Ultimately, as an airplane flies overhead, it
diverts air down as it passes, regardless of its speed. But there are
some changes as to how this occurs at high speeds. Recall that in
low-speed flight there is upwash in front of the wing due to circula-
tion. As speed increases, upwash begins to disappear. Upwash was
possible because air transfers information at the speed of sound,
which is faster than the wing at low speeds. Thus the air is able to
transfer information in front of the wing and the air is able to adjust
for the oncoming wing. There is upwash and the airflow separates
before the arrival of the wing. As the wing moves faster, there
is less time for the air ahead to move out of the way of the
During WWII the United States
wing. Once the airplane becomes supersonic, upwash ceases.
was producing 5500 aircraft per
Also, an almost instantaneous compression wave known as a
month.
shock wave forms in front of the wing. Shock waves are dis-
cussed in detail a little later.
Understanding shock waves and their drag and power
consequences is somewhat complicated. It helps to first understand
some of the fundamental properties of supersonic air. During the next
few sections a basic primer on supersonic airflow will be presented.