....loading of a new aircraft must be chosen to satisfy
a number of other, often conflicting requirements. For example, landing
and takeoff performance, range, subsonic cruising speed, rate of climb,
and maneuverability all exert, in varying degrees, an influence on the final
choice of wing and thrust loading. Hence, the data defining the trend with
time of these quantities would be expected to show a good deal more scatter
than is evident in the speed correlation shown in figure 11.43.
The expected increase in data dispersion is indeed shown
in figures 11.45 and 11.46, which depict the variation in wing and thrust
loading with years. Nevertheless, successively increasing plateaus of these
quantities are shown to occur with the passage of time. As described previously
regarding the McDonnell Douglas F-15 Eagle, increased aircraft maneuverability
received great emphasis in the late 1960's. The corresponding reduction in
wing loading and increase in thrust loading are clearly shown by the trends
in figures 11.45 and 11.46 and indicate how new requirements can change these
two important aircraft design parameters.
As a matter of interest, the maximum wing loading shown
in figure 11.45 is about 157 pounds per square foot; this value for the General....
[351] Figure 11.46 - Trends in thrust-to-weight ratio for jet fighter
aircraft.
....Dynamics F-111D compares with about 49.2 for America's
first operational jet fighter, the Lockheed P-80 Shooting Star. The corresponding
thrust loading for the P-80 was 0.33; by comparison, the thrust loading of
the contemporary McDonnell Douglas F-15C is 1.07 at design gross weight. A
much higher thrust loading is usually available under combat conditions at
a reduced weight.
Maximum Subsonic Lift-Drag Ratio
Although aerodynamic data are not available for all
the aircraft discussed in this chapter, a key aerodynamic indicator of subsonic
cruising efficiency-the maximum lift-drag ratio-is shown in figure 11.47 as
a function of years for the aircraft for which this parameter is given in
table V.
For those aircraft designed solely for operation at
subsonic speeds, the maximum lift-drag ratio is higher than that of the North
American P-51. For example, the Lockheed P-80 has a value of (L/D)max of 17.6 as compared with
14.6 for the P-51; the swept-wing North American F-86 with its relatively
low-aspect-ratio wing still has a maximum lift-drag....
[352] Figure 11.47 - Trends in maximum lift-drag ratio for jet fighter
aircraft.
....ratio of about the same magnitude as that of the
P-51. The comparatively high efficiency of the subsonic jet fighters is
certainly due in part to the absence of large cooling drag increments and
adverse propeller interference effects that characterize propeller-driven
fighters.
Once an aircraft incorporates the features necessary
for even short-duration flight at supersonic Mach numbers, however, the maximum
subsonic lift-drag ratio is significantly reduced, as shown by figure 11.47.
The highly swept, thin, low-aspect-ratio wings characteristic of supersonic
aircraft are largely responsible for the low values of maximum subsonic lift-drag
ratio. Significantly, the General Dynamics F-111D with its variable-sweep
wing shows a maximum lift-drag ratio higher than that of the P-51 although
its maximum speed is in excess of Mach 2.0. At supersonic speeds, the values
of (L/D)max of fighter aircraft are usually less than Half of the subsonic
values. The simultaneous achievement of satisfactorily high values of the
maximum lift-drag ratio at both subsonic and supersonic speeds remains a major
challenge in aircraft design.
Concluding Remark
Five important aircraft design parameters have been
shown as a function of time in figures 11.43 to 11.47. Many significant fighter-aircraft
[353] improvements have been made
in the past 40 years that are not so easily shown in trend figures. For example,
fully powered controls together with sophisticated stability augmentation
systems make the flying and handling qualities of the modern jet fighter much
more tractable than those of its propeller-driven ancestors. Modern all-weather
navigation and attack systems were unknown in 1945. The great power and light
weight of the jet propulsion system combined with advanced airframe designs
give the modern jet fighter a broad range of mission capability that embraces
the spectrum from air-to-air interception to ground-attack operations. The
list of improvements could be extended almost endlessly but will be terminated
here with the following conclusion: The modern jet fighter is an outstanding
example of the development and application of modern technology by the cooperative
efforts of thousands of individuals in government, academia, and industry.