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An advanced oval-shaped VTOL ducted fan concept produced by Edmond Doak in the late 1950s. This was the anticipated next stage on from the Model 16. US Patent Office weight of the aircraft progressively increased during development, reaching 3,1801b (1,443kg). Maximum speed was never a major issue with this prototype, but the aircraft was capable of reaching about 230mph (370km/h) in level flight. Propulsion took the form of a Lycoming T-53-L-1 turboshaft engine rated at 840hp (626kW), which was located in the fuselage and coupled to the ducts by shafts. The VZ-4DA made its first test flight on 25th February 1958 at Torrance Airport, with Jim Reichert at the controls. This initial test pro- gramme was completed by June 1958 and the aircraft was then moved to Edwards AFB during October 1958. After a further fifty hours of flight-testing, the US Army formally accepted the aircraft and it was moved to NASA-Langley for further evaluation in 1960. The VZ-4DA performed reasonably well, although it exhibited a fairly severe nose-up tendency during transition from hover to for- ward flight and it was generally accepted that the T-53-L-1 engine provided insufficient power and need to be replaced with some- thing offering better performance. Throughout the 1950s Doak developed advanced concepts that used the same engi- neering principles as the VZ-4DA and he envisaged the successor to this proof-of-con- cept demonstrator as a wingless elliptical craft, with four separate fans. The ducted unit would be shaft-driven by a pair of gas tur- bines, which were capable of propelling the aircraft at relatively high speed in level flight. By coincidence, in 1959 Bell Aircraft pro- duced two somewhat similar designs known as the D-2005 and D-2022. Both were intended to be assault transporters capable of carrying up to thirty soldiers. However, Doak Aviation closed for busi- ness during 1961 and the rights to the VZ-4DA and all the R&D material were sold to Douglas Aviation in Long Beach, who did not exploit the concept further. The VZ-4DA remained at NASA-Langley until August 1972 when it was finally presented to the US Army Transporta- tion Command Museum at Fort Eustis, where it is currently on display. Norman Nelson joined Lockheed’s highly secretive Skunk Works, becoming involved with the A-12/ SR-71 project; later he was appointed as pro- ject manager for the F-117A stealth interdictor. Jim Reichert also joined Lockheed, working on radar-absorbent materials for the F-117A. Davis Flying Saucer Throughout the 1950s, most of the major defence contractors were secretly undertak- ing or commissioning independent consul- tants to carry out design studies of circular-winged aircraft. Propulsion using an enclosed rotor system gained considerable favour and Texas-based designer John W Davis produced an interesting concept, which was reviewed by several contractors, but there were no takers for further development. Nevertheless, Davis applied for a patent in 1957 and the design of his Silver Bug-sized air- craft does have several interesting features. Intended for a military role, the aircraft had a centrally mounted single-seat cockpit with a blister canopy. The lenticular-shaped fuse- lage contained two rotor blade assemblies, each with four arms on a common central bearing. These were contra-rotating to elimi- 91 An unusual flying saucer design produced in 1957 by John Davis, a Texas- based aerodynamicist. via Bill Rose Postwar Discplane Development