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Aviation History
1949
1949 - 0616.PDF
420 FLIGHT APRIL 14TH, 1*549 Fighter Requirements demonstrations by American jet bombers have sharply challenged the curiously widespread assumption that jet intercepters will possess a comfortable inherent superiority in speed. Two American medium bombers have lately established new standards of performance on transcon- _ tinental flights, the Northrop XY-49 having averaged 502 m.p.h. and the Boeing XB-47 607 m.p.h. Moreover, the belief, so frequently expressed, that such machines, though fast, are hopelessly deficient in range, is surely disproved by the fact that these high-speed, high-altitude flights involved distances of well over 2,000 miles. The potential military load for these distances was not declared, but the ranges demonstrated are believed to be well within the capacity of the two aircraft. In order to attain its range.the jet bomber must fly high. Over-the-target altitudes of 35,000 to 40,ocoft are already attainable, and the jet intercepter must therefore be developed to achieve the greatest possible speed at height. This involves not only increased thrusts but, to en- sure economy and controllability, aerodynamic advances also. Rapid improvement in sea-level performance, as wit- nessed by the successive raising of the world's absolute speed record from 606.25 m.p.h. in 1945 to 670,981 m.p.h. in 1948, has tended to divert atten- tion from the sharp falling-off in speed with in- creasing height observed in cur- rent jet fighters. This loss may result from de- creased thrust or, where thrust is sufficient, from Mach limitations. Apropos of thrust, a typical British turbojet, the Rolls-Royce Denvent 5, de- livers 2,880 lb at " 500 m.p.h. at sea level, and 1,425 lb at the same speed at 30,000 ft. By taking fuller advantage of ram effect, the Goblin, with its single-sided impeller, maintains its thrust to a greater degree. Makers' figures for the Meteor 4 (two Derwent 5s), flying at combat rating with no external tanks, show a top speed of 585 m.p.h. at sea level ; 569 m.p.h. at 20,000ft; 545 m.p.h. at 30,000ft; 518 m.p.h. at 40-oooft; and 493 m.p.h. at 45,000ft. The corresponding values for the Vampire 6 (export version of the Vampire 5, with Goblin 3) are 548 m.p.h. at sea level ; 534 m.p.h. at 20,000ft : Believed to be one of the fastest American prototypes, the twin-jet McDonnell "penetra- tion" fighter has a high degree of sweep-back, well shown in this view. The two-seat Northrop F-89 Scorpion, one of the heaviest fighters yet butit, is intended for all-weather interception. The North American F-86 Sabre, now in production, can carry two drop-tanks under the thin swept-back wing. 513 m.p.h. at 30,000ft; 489 m.p.h. at 40,000ft; and 48c m.p.h. at 45,000ft. It will be seen that at extreme altitudes the Vampire, though still the slower machine, has tangibly reduced the Meteor's margin of superiority—a gain attributable to the characteristics of the Vampire airframe and to the high "installed efficiency" of the Goblin. Developed for the attainment of higher Mach Numbers, and with increased thrust, the Vampire promises speeds which, if not, perhaps, commens urate with the im- proved climbing powers already exemplified by the performance of the Ghost- Vampire, should be ample to maintain its en- viable position among high-alti- tude fighters. The world's fastest fighters at present are the North American F-86 and later prototypes for the U.S.A.F., all with highly loaded swept-back wings. Low-level speeds approaching 700 m.p.h. have been claimed for the F-86, but specific data, especially concerning performance at height, are withheld. With sufficient thrust the type is probably capable of level-flight speeds greater than that of sound, and has already been dived at Mach .Numbers above unity. The value of a high limiting Mach Number B; not, however, to be assessed solely in terms *of ultimate speed. Implied freedom from compressibility trouble means more accurate shooting and less worry for the pilot who, fighting at great heights, may be seriously handi- capped by the narrow margin between the limiting speed f-f his aircraft and its stalling speed. Given sufficient speed, the interc-pter must be manoeuvrable enough to be quickly and accurately positioned for its attack. Manoeuvrability is a function of wing loading, span loading and power loading. A low, wing-loading is desirable to permit high accelerations in turns, and low span and power loadings are beneficial both to rate of turn and rate of climb. Here, in fact, is rooted one of the designer's most intractable problems.J Should he provide too small a wing area his fighter may; prove powerless to administer the coup de grace before- the bomber has discharged its load ; on the other hand,:' should he allow too generous an area, top speed will be; affected and structural strength may suffer. The delta' wing-form may lighten his worry on this score. With the bomber flying at, say, 550 m.p.h. at 40,000ft, the fighter is likely to be severely restricted in its tactics. Attack from astern may be the only practicable method. If flying near its ceiling, at a coarse angle of incidence, I! 6
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