Fighter Combat - Tactics and Maneuvering

398 APPENDIX
Figure A-8. Origin of Induced Drag
an effect by altering the fighter's apparent weight. Figure A-9 shows how
this occurs.
This figure depicts a fighter of a given weight (W) in steady-state flight
supported by its lift (L). In this condition the vertical forces must balance
each other as well as the pitching moments (torques), which tend to cause
the aircraft to rotate ("pitch") nose-up or nose-down. Since weight acts
through the CG and lift through the "aerodynamic center" (AC), these two
forces cause a nose-down pitching moment with the CG forward as shown.
In order to maintain a level flight attitude under these conditions, the tail
must produce a downward lift (Lt) large enough to offset the nose-down
moment of the wings (i.e., XWL = XtLt). This download on the tail, however,
must also be supported by the wings, just like dead weight, so induced
drag will increase for a given load factor. This additional induced drag is
known as "trim drag."
If the CG was moved rearward (by fuel distribution, ordnance loading,
etc.) until it coincided with the AC, the tail lift requirement and resulting
trim drag would be eliminated. Further rearward movement in the CG
would require an upward lift from the tail, which essentially adds additional
wing area to the aircraft, thereby reducing induced drag. In practice,
however, there is a limit to rearward CG location because of aircraft
controllability considerations.
Conventional fighters (i.e., those with rear-mounted pitch-control surfaces)
benefit from aft CGs. Canard-configured fighters (i.e., those with
pitch controls located forward of the wings), however, normally benefit
from forward CG location.
As the speeds of fighters increased through the years, the phenomenon
of "compressibility" was encountered. As aircraft move through the air,
pressure disturbances are created which propagate outward in all directions
at the speed of sound. Those pressure waves that move ahead of the
aircraft tend to provide an "advance warning" to the air that the aircraft is
approaching, thus giving the air molecules time to begin moving out of the
way. The air then begins to part even before the aircraft arrives, which