Fighter Combat - Tactics and Maneuvering

APPENDIX 401
Figure A-11. Typical H-M Diagram
The H-M diagram shows a fighter's Ps capability on a grid of altitude (H)
versus Mach number (M). Often lines of constant Es are also provided for
additional reference. Specific excess power is depicted by lines of constant
PS, in this case using ft/sec as the unit of measurement. For the given
conditions (weight, configuration, power, load factor), Ps can be related
directly with climb rate. For instance, at about 35,000 ft MSL and 1.0 M,
this aircraft shows a Ps of about 200 ft/sec. Therefore, under these conditions
its climb rate would be 200 ft/sec.
The Ps = 0 line on this diagram (the outermost contour) shows the
fighter's maximum 1-G steady-state performance. When operating at a
speed-altitude combination (energy state) which places it on this line, the
fighter cannot climb without losing airspeed and cannot accelerate without
losing altitude. Inside this "operating envelope" (region of positive Ps
values) the aircraft is free to climb and/or accelerate at will, subject to the
rate limitations specified by the Ps contours. Outside this steady-state
envelope is a region of negative Ps/ where the fighter may operate for short
periods of time by giving up energy (i.e., decelerating and/or diving).
Many important operating capabilities of a fighter may be found along
the Ps = 0 line. Assuming this plot is representative of a 1-G condition,
some of these capabilities are:
point a: minimum sustained Mach number at any altitude (.3 M at sea
level)
point b: maximum sustained subsonic altitude (56,000 ft at .9 M)
point c: maximum sustained altitude at any speed (67,000 ft at 1.95 M)
point d: maximum sustained Mach at any altitude (2.2 M at 55,000 ft)
point e: maximum sustained Mach at sea level (1.35 M)
Although this fighter may be capable of the performance depicted from a
strictly thrust-versus-drag standpoint, other factors, such as those illustrated
by the V-n diagram, may limit performance capabilities. Figure A-12
illustrates the possible impact of such limitations.
The H-M plots presented in Figures A-l 1 and A-12 have been representative
of H-M plots for supersonic fighters. Subsonic jets and prop-driven