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Collin’s Aerodyne mock-up. Built around 1959, Lippisch stands next to this incredibly unattractive concept. via Bill Rose Collin’s Aerodyne mock-up. Built around 1959, Lippisch stands next to this incredibly unattractive concept. via Bill Rose was to be fitted with a new 7,000 Ib (31.13kN) static thrust XJ-40 Westinghouse turbojet and wind tunnel testing of D-571 models indi- cated that the engine would provide the potential for supersonic speed in level flight. Initial trials exceeded expectations and the US Navy commissioned the construction of two full-sized D-751 prototypes, allocating the designation XF4D-1 during December 1948. Later the name Skyray was adopted which reflected the aircraft's very unusual sleek manta-ray shape. The Navy specified the Westinghouse XJ40-WE-6 engine for this air- craft but, aware of the engine’s troublesome reputation, Heinemann rather thoughtfully designed the airframe to take any jet engine up to 42 inches (1.06m) in diameter, which allowed the use of several alternatives. With a length of 45ft 5in (13.84m), a wingspan of 33ft 6in (10.21m) and a wing area of 557sq/ft (51.8sq/m), the Skyray was more or less the same size as the troublesome Vought Cutlass and it could be folded up into much the same space for hangar storage. Proposed armament for the Navy’s new Skyray was four 20mm can- nons, which would be supplemented by air-to- air missiles as they became available. By June 1950 the Westinghouse XJ40 was still undergoing development and a decision was taken to fit the XF4D-1 prototypes with less powerful Allison J35-A-17 turbojets rated at 5,000 Ib (22kN) static thrust. The first XF4D-1 flight took place on 23rd January 1951 at Edwards AFB and, although there were some technical problems, the aircraft reportedly handled fairly well. Westinghouse finally pro- vided XJ40-WE-6 turbojets for the pro- gramme but serious problems persisted and in March 1953 a decision was made to switch to the more powerful newly developed Pratt & Whitney J57-P-2 axial flow afterburning engine, which had an anticipated rating of 10,000Ib (44.48kN) static thrust. As things worked out the ill-fated J40 proved to be a turning point for Westinghouse, who were forced to pull out of jet engine manufacturing completely. The re-engined F4D-1 (later re-designated F-6A) now showed itself to be a more capa- ble aircraft than initially envisaged, with a top speed approaching Mach 1 and a service ceil- ing in excess of 50,000ft (15,240m). Maximum loaded weight was 27,000Ib (12,247kg) and with drop tanks this allowed a range of about 87 A postwar design by Alexander Lippisch for a heel- shaped ducted fan aircraft. US Patent Office Postwar Discplane Development Oo