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The Lockheed XFV-1 was an experimental VTOL aircraft that never made a vertical take off or landing, but that did fly with a temporary conventional undercarriage.
Both the US Air Force and the US Navy became interested in a VTOL fighter in the aftermath of the Second World War. In 1947 the two services jointly sponsored Project Hummingbird, a study of VTOL technology. In 1948 the Navy initiated a formal study based on the results of Hummingbird. The aim was to produce a fighter that could be carried on most naval warships, including destroyers, transports, tankers, cruisers and battleships, in order to provide every significant ship in a fleet its own air defence. In 1949 the Navy asked NACA to help with the project, and they were able to produce a working model of a tail landing VTOL aircraft. This design had a standard aircraft fuselage, contra-rotating propellers powered by a 5hp engine, and a cruciform tail which also acted as the landing gear. The aircraft would launch and land in the vertical position, and would shift into horizontal flight while in the air.
This test model showed promise, and so on 31 May 1951 the Navy awarded Convair and Lockheed contracts to develop similar VTOL aircraft, to be powered by the Allison YT-40 turboprop engine. At this early point the alarm bells should have been ringing. The YT-40 was an experimental engine that was still under development. It was a coupled engine, consisting of two turboprop engines that drove a single gearbox, which would then be used to power contra-rotating propellers. The same basic engine layout had been used in the failed Avro Manchester and the much delayed Heinkel He 177, both of which were badly let down by their engines. Allison was able to produce lower powered versions of the engine, but the more powerful version needed for any potential production aircraft never appeared.
Of the two projects the Convair XFY-1 Pogo was probably the more successful, as it was the only one of the two to actually take off and land vertically and then transition into horizontal flight.
Lockheed's design was similar to the NACA test model. It had a standard aircraft fuselage, with stubby tapered wings. All of the control surfaces and the landing gear were built into the cruciform tail, which was mounted in a 'X' configuration. The T-40 engine was used to power specially designed counter-rotating six bladed Curtiss propellers that were designed to be especially effective when hovering.
In order to test the new design a ¼ scale wind tunnel model was constructed, powered by two 38hp electric motors. The model performed fairly well. It was easy to control when hovering, but there were some problems during the transition from vertical to horizontal and back that got worse at higher speeds. Landing in still air appeared to be straightforward, but in windy weather would get tricky.
A number of test cockpits were built, in order to try out various control methods. They were also used to test the rotating reclining seat that would move between positions during the transition between vertical and horizontal flight. Some time also had to be spent developing a system that would allow the pilot to tell which way the aircraft was moving when hovering, and eventually a 'weather vane' system was chosen, mounted on the wing tips.
The first XFV-1 prototype went to Edwards Air Force Base in October 1943. At this stage the aircraft had been given its temporary conventional landing gear, a very ungainly fixed undercarriage looking like it had been built out of scaffolding poles. There was also a fairly simple vertical erection rig, used to move the aircraft from the horizontal to the vertical while on the ground, and a boarding platform to allow the pilot to reach the cockpit when the aircraft was in the vertical position, both features that indicated some of the problems that would have had to be overcome in service.
At this point the aircraft was using the prototype XT-40 engine, as the first of the purpose built YT-40 engines designed for vertical flight had gone to Convair, whose aircraft couldn't use a temporary undercarriage. The XFV-1 made an inadvertent maiden flight on 23 December 1953 when it accidently took off during a taxing test and flew for a mile and a half.
The first official test flight was made on 16 June 1954, and was a 35 minute flight that reached 10,000ft. A key element of the early test flights was to test out the aircraft's stall speed and low speed characteristics, as these were key to the transition between vertical and horizontal and back when landing and taking off, which had to be carried out at low speeds.
Once these tests were over, the programme moved onto vertical flight. The transition was carried out at a safe altitude. The test pilot reported that it was easy to switch into vertical flight when going at speed, and the aircraft was very easy to control in vertical flight. The biggest problems came when descending, where it became difficult to maintain a safe speed. However the powerful XT-40 made it easy to recover if something did go wrong in the descent.
The test programme included a number of practise landing approaches. These proved to be more difficult that the switch into vertical flight at higher speeds, as the aircraft became difficult to control between 30 degrees and 80 degrees at lower speeds. However once it was vertical control was restored.
By 1954, when the aircraft made its test flight, it was being left behind by standard jet aircraft, which were approaching speeds of Mach 2, twice the best speed available with the turboprop VTOL aircraft. The 7,100 shp Allison VT40-A-14 engine had also failed to appear. Although both VTOL aircraft had worked, they were both quite complex aircraft and required a skilled pilot. This meant that they weren't really suited to be scattered around the fleet. The XFV-1 made thirty two test flights, and logged 23 hours before the project was cancelled on 16 June 1955. The Convair project was cancelled six weeks later, on 1 August 1955.
The first XFV-1 went to Hiller, where work was still continuing on VTOL aircraft, but it was destroyed during tests. A second prototype had also been ordered, but was never completed, and ended up being put on display outside the recruiting building at NAS Los Alamitos.
Estimated performance figures
Engines: Allison VT40-A-14
Power: 7,100shp
Crew: 1
Wing span: 27.39ft
Loaded weight: 12,823lb
Maximum speed: 667mph at 15,000ft
Rate of climb: 10,820ft/ min
Service ceiling: 43,300ft