Quest for Performance: The Evolution of Modern Aircraft
Chapter 11: Early Jet Fighters
Design Trends
[347] Described above are a few of the many jet-fighter aircraft developed in the past four decades. Major increases in performance and capability have taken place since the end of World War II. A quick overview of some of these changes is provided by the trends shown in figures 11.43 to 11.47. In these figures, several of the aircraft physical and performance characteristics tabulated in table V have been plotted as a function of years. The quantities shown are as follows:
        1. Maximum speed, figure 11.43
        2. Sea-level rate of climb, figure 11-44
        3. Wing loading, figure 11.45
        4. Thrust-to-weight ratio, figure 11.46
        5. Maximum subsonic lift-drag ratio, figure 11.47
An upper-bound, or envelope-type, curve enclosing all the data points is shown on each figure. As a reference mark, data are given on most of the figures for the North American P-51 propeller-driven fighter of World War II fame. The symbols used to identify the various aircraft are given in figure 11.42.
Maximum Speed and Sea-Level Rate of Climb
A major objective in fighter aircraft design over the years has been the achievement of ever higher maximum speeds. In figure 11.43, the upper-bound curve of maximum speed as a function of years clearly....

plane symbols used in charts
Figure 11.42 - Symbols used in figures 11.43 to 11.47.


development chart of jet fighter speeds from 1940 to 1982
[348] Figure 11. 43 - Trends in maximum speed for Jet fighter aircraft.


....shows this trend and is characterized by a series of ever higher plateaus that correspond to different levels of technical capability. As compared with the North American P-51, the straight-wing fighter incorporating a jet engine raised the maximum speed plateau by about 100 miles per hour; the use of sweepback in a purely subsonic airframe raised the plateau by another 100 miles per hour. In the early 1950's, the upperbound curve shows an increase in maximum speed of about 700 miles per hour, or a doubling of the speed achievable in an operational fighter aircraft. The afterburning engine together with the major aerodynamic innovations (discussed generally in chapter 10 and for individual aircraft in the present chapter) are responsible for this large increase in maximum-speed capability. Detailed airframe refinement and increased engine thrust are the ingredients in the upper-bound increments evident in 1958 and 1970.

Like the maximum-speed trend, the upper-bound curve for sealevel rate of climb shown in figure 11.44 is also characterized by increasing plateaus. In contrast with the maximum-speed trend, however, the introduction of sweepback in a subsonic airframe resulted in no....

development chart of jet fighter rates of climb from 1940 to 1982
[349] Figure 11.44 - Trends in sea-level rate of climb for jet fighter aircraft.


....increase in rate of climb. Unfortunately, rate-of-climb data are not available for the newer fighters.

Wing Loading and Thrust-to-Weight Ratio
The quest for ever increasing maximum speeds was a primary driver in jet-fighter development for many years. If maximum speed were the only requirement, wing loading and thrust loading (thrust-to-weight ratio) might be expected to increase with time in a fashion closely related to the increase in maximum speed. In addition to maximum speed, however, both the wing and thrust....

development chart of jet fighter trends in wing load from 1940 to 1982
[350] Figure 11.45 - Trends in wing loading for Jet fighter aircraft.


....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....

development chart of jet fighter trends in thrust-to-weight ratio from 1940 to 1982
[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....

development chart of jet fighter trends in maximum lift-drag ratio from 1940 to 1982
[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.