...wings.) For the ferry mission at Mach
0.6 and altitude of 30 000 feet, a wing sweep of 20° is
optimum, whereas a wing sweep of about 75° is best for Mach
2.0 flight at 60 000 feet. If the sweep angle was to be fixed at
75° however, the subsonic lift-drag ratio and consequently
the range in this speed regime would be cut in half. For Mach 1.2
at sea level, a sweep angle of about 110° appears best. Thus,
by varying the sweepback angle, a single aircraft can be optimized
for several widely different flight conditions.
In addition to the important effects on
aircraft performance just described, variable sweep can be used to
control, at least to some extent, the magnitude of the gust loads
imposed on the aircraft. As anyone who has flown in a transport
aircraft knows, rough air is usually encountered more frequently
at low rather than high altitudes. Mach 1.2 flight at sea level
constitutes a particularly severe gust-loads environment, not only
because of the frequency of gust encounter but because the
magnitude of the load imposed on the aircraft for a given
 size gust increases with speed. Minimizing
the magnitude of these loads reduces structural weight and pilot
fatigue and increases the accuracy of weapons delivery.
Fortunately, the large sweep angles needed to reduce drag at
supersonic speeds also reduce the magnitude of the gust loads.
Finally, the low sweep position is useful in obtaining the maximum
lift coefficients needed for STOL field lengths. The field lengths
listed in reference 162 for the F-111 are less than 3000 feet.
The F-111 first entered operational
service In 1967 and saw action in the Vietnam conflict. This very
versatile aircraft, though failing to meet some of its original
performance objectives, is still an outstanding machine and is
likely to remain in the USAF inventory for many years. An
interesting account of the aerodynamic research that led to the
F-111 concept is contained in reference 184.