Fighter Combat - Tactics and Maneuvering

402 APPENDIX
Figure A-12, Possible Limitations to Fighter Performance
fighters can be expected to exhibit fundamentally different operating
envelopes. Figure A-13 shows some typical comparisons. Piston-powered
fighters suffer from rapid loss of engine performance above critical altitude,
thrust decay with increasing airspeed, and possibly propcompressibility
losses at moderate subsonic Mach. These effects can be
seen to limit severely the operating envelopes of such aircraft. By contrast,
the better high-altitude performance of the jet engine, the typically cleaner
aerodynamics of the airframe, and the usually increasing thrust with speed
provide the jet fighter with a greater range of operating speed and altitude.
The subsonic fighter, however, typically lacks sufficient thrust to overcome
the dramatic rise in wave drag above critical Mach, so its performance
degenerates badly above that speed. Since the rise in wave drag is
usually greater than the corresponding increase in jet thrust with speed,
even the supersonic fighter generally exhibits a loss of performance in the
transonic region. This aircraft, however, has sufficient excess thrust to
carry it through the transonic range so that it may take advantage of the
slower rate of drag rise at higher Mach, particularly at high altitude.
As stated in the note to Figure A-11, the performance capabilities
depicted in the H-M plot are good for only one condition of weight,
configuration, power setting, and load factor. A change in any of these
parameters affects the Ps contours and the sustained operating envelope
(PS = 0 line), as shown in Figure A-14. To illustrate this effect, assume a
fighter is operating at the energy state represented by point /. In the first
plot (assume 1 G) the fighter is in an area of positive Ps (inside the
steady-state envelope) and can therefore climb or accelerate under these
conditions. But if load factor (for example) is increased substantially, as
depicted in the second plot, the Ps = 0 contour shrinks, placing the fighter
outside the steady-state envelope and into a region of negative Ps. This
negative Ps, which is probably the result of increased induced drag and
wave drag at the higher load factor, will cause the aircraft to lose speed
and/or altitude. Obviously there must be some load factor that would