Fighter Combat - Tactics and Maneuvering

APPENDIX 397
Figure A-7. Engine Thrust Variation with Airspeed
very complex. The most important types of drag are discussed briefly here,
but no attempt will be made to qualify the reader as an aeronautical
engineer.
"Parasite drag" has many causes, but the most significant forms are skin
friction and pressure drag. Skin friction results from air molecules moving
across the surfaces of the aircraft. These molecules tend to stick to the
aircraft, and much air must be pulled along, adding resistance to the
aircraft's motion. Skin-friction drag is reduced by minimizing aircraft
surface area and maintaining the smoothest possible surfaces, and by
other, more esoteric, methods. Pressure drag results when high-impact air
pressure on the leading edges of the aircraft combine with reduced pressure
on trailing edges to produce a net rearward force. This form of drag is
reduced primarily by minimizing aircraft frontal area, and also by streamlining,
which tends to reduce air turbulence and decrease the size of the
low-pressure region that forms behind the moving aircraft.
Another type of drag, known as "induced drag," is actually a result of
lift. When a wing begins to produce lift, the actual resultant force is not
perfectly perpendicular to the relative wind, as lift is defined, but tends to
tilt backward somewhat. As illustrated in Figure A-8, this resultant force
(FR) has components both perpendicular to (lift) and parallel with (drag) the
relative wind.
In general, for a wing of a given size and shape, the greater the lift
produced under given conditions, the greater the induced drag will be.
Although this relationship is important for any aircraft, it is especially
critical for fighters, since their mission often involves high-load factors
requiring a great amount of lift. Induced drag is minimized by designing
wings of large area with long, thin planforms. The actual shape of the wing
is also very important. For subsonic flight an elliptical planform, made
famous by the Spitfire fighter of World War II, is theoretically optimum.
Other shapes, however, may be nearly as efficient from an induced-drag
standpoint and have other overriding advantages.
Reducing aircraft weight is another critical factor in minimizing induced
drag. Less weight requires less lift for a given turn performance,
resulting in less induced drag. The aircraft's center of gravity (CG) also has