Implementing the Delta Planform

While swept wing adaptation in Europe, Russia, and America followed a generally similar pattern, the delta wing underwent markedly differ­ent international development. Generally, European designers initially emulated the Lippisch approach, resulting in designs with relatively thick wing sections (exemplified by the Avro Vulcan bomber and the "tailed” Gloster Javelin interceptor) that inhibited their ability to operate beyond the transonic. Only after the practical demonstration of Convair’s emerging family of thin-wing delta designs—the XF-92A research air­craft, the F-102 interceptor, the XF2Y-1 experimental naval fighter, the B-58 supersonic bomber, and the F-106 interceptor—did they conceptu­alize more "supersonic friendly” designs, typified by the Swedish Saab J35 Draken ("Dragon”), the British Fairey F. D.2 research airplane, the French Dassault Mirage I (progenitor of the Mirage fighter and bomber family). By the late 1950s, British and French aerodynamicists had so completely "closed” any "delta gap” that might have existed between Europe and America that they were already conceptualizing development of a Mach 2 supersonic transatlantic transport using a shapely "ogee” reflexive delta planform, a study effort that would, a decade later, spawn the Anglo-French Concorde.[88] Not so taken with the pure delta, Soviet designers joined American-like thin delta wings to the low-placed hori­zontal tail, generating advanced MiG and Sukhoi fighters and intercep­tors. These "tailed deltas” (particularly the MiG-21) possessed far better transonic and supersonic turning performance than could be attained by a conventional delta with its high induced drag onset at the increas­ing angles of attack characteristic of hard-maneuvering. (An American equivalent was the Douglas Company’s superlative A4D-1 Skyhawk,
a light attack bomber with maneuvering performance better than most fighters.)

Although it is commonly accepted that American delta aircraft owe their inspiration to the work of Lippisch—Convair’s delta aircraft repeatedly being cited as the products of his influence—in fact, they do not.[89] Unlike, say, the swept wing F-86 and B-47, which directly reflected German aerodynamic thought and example, America’s delta wing air­craft reflected indigenous, not foreign, research and inspiration. By the time that Lippisch first met with Allied technical intelligence experts, American aerodynamicists were already advancing along a very differ­ent path than the one he had followed. Jones had already enunciated his thin, sharply swept delta theory and undertaken his first tunnel tests of it. In June 1946—a full year after the German collapse—Convair engi­neers developing the experimental delta XP-92 interceptor had their chance to meet with Lippisch at Wright Field. By then, however, they had already independently decided upon a thin delta planform. "We had heard about Dr. Lippisch’s work and this gave us some moral support,” Convair designer Adolph Burstein recalled, adding: "but not much else. . . . We did not go along with many of his ideas, such as a very thick airfoil.”[90] Burstein and his colleagues arrived at their delta shape by beginning with a 45-degree swept wing, gradually increasing its sweepback angle, and then "filling in” the ever-closing trailing edges, until they arrived at the classic 60-degree triangular delta planform the company incorporated on all its subsequent delta aircraft. With a 6.5 thickness-chord ratio— less than half that of Lippisch’s DM-1—it was an altogether different – looking airplane.[91] Nor was Convair alone in going its own way; Douglas naval aircraft designer Edward Heinemann acknowledged that "At the close of World War II the work with delta planforms accomplished by

Dr. Lippisch in Germany became generally known and appreciated,” but that "Extensive wind tunnel tests showed there was no special merit to an equilateral triangle planform—especially those designed with thicker airfoils.”[92]

The chronology of American delta development, and the technical choices and paths followed by American engineers, supports both state­ments. At war’s end, advancing ground forces at Prien, Austria, had dis­covered a thick-wing wooden delta glider, the DM-1, which Lippisch had intended as a low-speed testbed for a proposed supersonic fighter, the P 13. At Army Air Forces’ request, it was shipped back to America in January 1946 for comprehensive testing in the Full-Scale Tunnel at the NACA’s Langley Aeronautical Laboratory. Had the tests gone well,
the possibility existed as that, as the Germans had intended, it might be flown as a glider. But the tunnel tests quickly disabused delta enthu­siasts of these hopes. As the AAF’s Langley liaison officer subsequently reported, the "Initial test results were very disappointing; the lift coef­ficient was low, the drag was high, the directional stability was unsatis­factory, and the craft was considered unsafe for flight tests.”[93]

Afterward, Langley engineers undertook a comprehensive study of the DM-1 configuration, not in the spirit of emulation but rather attempt­ing to find a way to fix it. After giving its wings sharp leading edges, seal­ing all slots and gaps around control surfaces, and removing the thick vertical fin and replacing it with a thin one (relocating the pilot under a streamlined bubble canopy), they had markedly improved its perfor­mance, doubling its lift coefficient, from 0.6 to over 1.2. But it remained an unsatisfactory design, proof enough that the Lippisch concept of del­tas was hardly one that could serve—or did serve—as a veritable template (as has been so often alleged) for the supersonic American, Swedish, and French delta fighters and bombers that flew over the next decade.[94] Subsequently, NACA engineers looked to far thinner and more streamlined configurations that, if not yet as extreme as Robert T. Jones’s original daggerlike concept, were even more amenable to the rigors of transonic and supersonic flight than the generously rounded contours of Lippisch’s thick wings and awkward pilot-enclosing vertical fins. By the beginning of 1947, they were already examining the technical requirements of slender, low aspect ratio delta configurations

to meet emerging military specifications for a Mach 1.5, 60,000-foot bomber interceptor.[95]