Fighter Combat - Tactics and Maneuvering

396 APPENDIX
only about half its sea-level power at 18,000 ft above mean sea level (MSL).
Since the 1930s, however, most first-line fighters have been equipped with
superchargers or turbochargers which can allow the engine to maintain its
rated power to near 30,000 ft MSL. Jet engines, on the other hand, suffer a
loss in thrust which is slightly less than the rate of air density reduction
with altitude, and significantly less than that for normally aspirated piston
engines. Jets typically lose about half their thrust by 25,000 ft. However,
there is no common device similar to the turbocharger for maintaining
jet-engine performance at altitude. Figure A-6 shows the typical thrust
variations.
The altitude labelled "tropopause" in this plot denotes the level at
which atmospheric air temperature ceases to fall. Above this height, typically
about 36,000 ft MSL, air temperature is constant. Jet engines in
particular benefit from lower air temperatures as altitude increases. Above
the tropopause this benefit no longer exists, resulting in a faster rate of
jet-thrust decay. Consequently the tropopause is an important altitude in
jet-fighter performance. "Critical altitude" is of great significance for
turbocharged fighters. This is the highest level at which the turbocharger
can maintain full-rated engine power.
The effect of speed on engine thrust is illustrated in Figure A-7. Propeller
thrust is usually greatest in the static condition (i.e., zero airspeed) and
falls rather rapidly with increasing airspeed. Jet thrust also may be expected
to diminish slightly as speed increases above the static condition.
As airspeed rises farther, however, ram compression in the engine inlet
generally results in significant increases in thrust until engine and inlet
design limits are approached. It is quite obvious from this plot why jet
fighters exhibit superior high-speed performance.
Drag
As shown by Equation 4, engine thrust is only part of the energymaneuverability
story; aircraft drag characteristics are equally important.
Many phenomena contribute to total aircraft drag, some of which can be
Figure A-6. Engine Thrust Variation with Altitude