In the years following World War Two, the idea of military aircraft capable of vertical take-ff and landing (VTOL) began to interest designers and military strategists around the world. It wasn’t long before one of the most radical designs to emerge from these studies was the Convair XFY-1 Pogo.
With the performance of land-based jet aircraft increasing, it meant that longer runways were needed. Longer runways were vulnerable to attack: a single conventional bomb placed on a runway could render an airbase and its aircraft unusable.
For naval aviation, aircraft carriers were also vulnerable to attack. No matter how many combat aircraft a carrier could operate, if the vessel was damaged, all its air power was negated. Aircraft capable of VTOL could potentially avoid this problem.
They could be launched and recovered from small, temporary forward air bases or even from ships considerably smaller than aircraft carriers. As early as 1947, the USAF and US Navy were actively looking at the possibilities for VTOL combat aircraft.
In 1947, the USAF and US Navy jointly sponsored Project Hummingbird, a design study looking at the practical application of VTOL technology.
This project was able to use data captured from Nazi Germany at the end of World War Two and in particular, design work on a projected German VTOL aircraft, the Focke-Wulf Triebflügeljäger (thrust-wing fighter).
The Triebflügeljäger was a radical design concept created in 1944 in response to the increased Allied bombing of German industrial targets. This aircraft was intended to be a tail sitter VTOL fighter that would provide point-defense for factories or other installations where no adjacent airfield was available.
Thrust was to be provided by three ramjet engines, each mounted at the tip of a large three-blade propellor mounted in the centre of the fuselage.
The Triebflügeljäger design never went further than the creation of wind-tunnel models, but the concept of a tail sitter VTOL aircraft captured the interest of the US Navy.
They were interested in a fighter that could be launched from any warship and even from transport ships or tankers. If each US Navy ship could be provided with its own point-defense interceptor, the need for vulnerable aircraft carriers would be greatly reduced.
In 1948, the US Navy began a more detailed study based on the results of Project Hummingbird and in 1949 asked the National Advisory Committee for Aeronautics (NACA) to examine the feasibility of producing a tail sitter VTOL interceptor.
NACA built a model powered by a 5hp engine to test the concept. In some ways, this was similar to the Triebflügeljäger, but it used a conventional fuselage with an engine driving contra-rotating propellors in the nose.
The model flew successfully and demonstrated the capability of taking off and landing vertically and making the transition to conventional forward flight. In May 1951, the US Navy awarded contracts to both Convair and Lockheed to each produce two prototypes of a VTOL tail sitter fighter.
The XFY-1 Pogo
Both the Convair aircraft (which received the US Navy designation XFY-1) and the Lockheed example (XFV-1) were to be provided with an extremely powerful engine then under development by Allison.
The T40 engine was based on a pair of T38 Allison turboprop engines mounted side by side and providing power to a common reduction gearbox.
Output was to be provided to a pair of 16ft diameter Curtiss-Wright turbo-electric contra-rotating propellors.
The power of the engine was estimated to be over 5,000 kW: in comparison, the Pratt and Whitney radial engine fitted to the Vought F4U-4 Corsair then in service with the US Navy provided around 1,700 kW.
In the Convair design, this powerplant was to be mounted in a stubby, streamlined fuselage provided with short delta wings and large ventral and dorsal tail surfaces.
Four castor wheels were fitted, with one on each wingtip and one on each of the upper and lower tail surfaces. Each wheel was provided with a long shock-absorber that compressed on landing. This led to the unofficial name given to this aircraft, the Pogo.
The pilot’s seat was able to rotate through 45˚ to allow it to be used comfortably in both vertical and horizontal flight. No armament was ever fitted to the XFY-1, but this was expected to take the form of either four 20mm cannon fitted in the wings outboard of the massive propellors or a pack containing 48 folding-fin unguided aerial rockets.
The first XFY-1 prototype was completed in early 1954 and tested in a fixed vertical stand at Lindbergh Field in San Diego. It performed satisfactorily and it was decided to proceed with the next stage of development, tethered and untethered flight testing.
The second prototype was taken to NAS Moffett Field near Sunnyvale, California where a tethered test rig had been created in the gigantic Hanger Number One, originally built in the 1930s to provide accommodation for the US Navy Airship USS Macron.
The rig incorporated a motorised reel attached by cable to the XFY-1 propellor hub. In an emergency, this could be reeled in to rapidly bring the aircraft back to an upright position.
Four additional tethers were attached to each wingtip and the dorsal and ventral tail surfaces.
The test pilot chosen for the challenging task of finding out whether the XFY-1 would actually fly was a US Marine Corps Reserve pilot who was also working as an engineering test pilot for Convair, lieutenant colonel James F. “Skeets” Coleman.
He was to be assisted in tethered flights by Convair flight test engineer Bob McGreary. McGreary would control the tether mechanism, ready to pull the XFY-1 upright if Coleman lost control.
Rarely can any test pilot have been faced with the testing of such a radically different and potentially dangerous aircraft. No one really knew how the XFY-1 would perform when tethered flight testing began in April 1954.
The situation was made even more hazardous because the massive backwash from the propellors bounced back from the hanger walls causing unpredictable turbulence. More than once during over 60 hours of tethered flight, Coleman was forced to call “catch me!,” causing McGreary to operate the tether to pull the aircraft safely upright.
Despite these problems, in August 1954, testing moved outdoors to begin untethered flights. Coleman began with more than 70 simple hops, taking off, hovering and then bringing the XFY-1 back to land safely.
In November 1954, he made the first transition to horizontal flight, staying in conventional forward flight for over 20 minutes before returning to make a safe landing.
It soon became clear that the combination of massive power and a tiny, streamlined airframe made the XFY-1 very fast indeed.
With the throttles at their minimum setting, the little aircraft would cruise at over 300mph and in level flight it often outpaced the conventional chase aircraft sent to monitor its progress. As it wasn’t provided with speed brakes or spoilers, high speeds made landing very challenging.
Coleman developed a technique where he would approach the landing field at around 300mph and, when close to the landing point, he would pull the stick all the way back, causing the XFY-1 into a vertical climb.
As speed decayed, he would carefully add power until the aircraft was in a hover at an altitude of around 1,000 feet.
He would then gently reduce power while looking over his shoulder at the ground below as he descended. The XFY-1 was not stable in the hover, and during descent it required constant corrections on the controls to maintain attitude.
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This was particularly difficult as the aircraft neared the ground when it would encounter additional turbulence from its own reflected prop wash.
One of the most challenging aspects of landing was judging whether the aircraft’s rate of descent was within safe limits: the pilot had to do this while looking at the ground over his shoulder and providing constant input to the flying controls to keep the XFY-1 stable.
As an aid, Convair mounted a small radar altimeter pod in the left wingtip. This was connected to three lights in the cockpit: green meant that the aircraft was in a stable hover, amber meant that it was descending within safe limits and red meant that the rate of descent was too fast.
Even with this device fitted, the XFY-1 was a fiendishly difficult aircraft to land. Coleman was clearly an exceptional pilot and when another US Navy pilot was allowed to fly the XFY-1 for the first time in May 1955, he was lucky to survive the experience.
Coleman made his last flight in this aircraft in June 1955 but soon after, routine maintenance revealed that the main gearbox needed urgent refurbishment.
The XFY-1 never flew again and the project was formally closed in August 1956. Lockheed’s XFV-1 tail sitter was abandoned without ever having achieved vertical take-off or landing.
In strictly technical terms, the XFY-1 was a success. It made many flights where it demonstrated the ability to fly conventionally and to land and take off vertically. In this limited sense, this was the first successful military VTOL aircraft. However, as a potential operational aircraft, it was doomed by two separate issues.
First of all, it was very, very difficult to land safely. Without any form of automated flight stabilisation in the hover, it required constant correction from the pilot who was also required to closely and constantly monitor the rate of descent, position and attitude.
The workload was extremely high and any loss of control in the landing phase was likely to lead to disaster. Now, try to imagine doing that while approaching a small landing pad on a pitching, rolling ship at sea in marginal weather…
Perhaps there are a few truly exceptional pilots who could manage that reliably, but any successful military aircraft must be useable by pilots of average ability. That was simply never going to apply to the challenging XFY-1.
The second issue that led to the cancellation of this project was performance. By the mid-1950s, jet interceptors were being produced that were capable of exceeding Mach 1. Within a few years, operational aircraft would be capable of Mach 2.
Despite the awesome power of its turboprop engine, the little XFY-1 would never be capable of reaching even Mach 1. As a combat aircraft, that would have put it at a distinct and unsurmountable disadvantage.
Even while the XFY-1 project was nearing its end, in Britain a new VTOL concept was being developed. In April 1957, the Short SC.1 made its first flight. This used four Rolls-Royce turbojet engines to lift it into the hover in a horizontal attitude and a single turbojet to allow it to transition to forward flight.
This approach rather than the tail sitter concept would prove to be the solution to the development of practical military VTOL aircraft. But the whacky XFY-1, looking as if it had escaped from an episode of the television show Thunderbirds, remains an icon representing a post-war willingness to explore the outer limits of aircraft design.
- Crew: 1
- Length: 32 ft 3 in (9.83 m)
- Wingspan: 27 ft 8 in (8.43 m)
- Empty weight: 11,139 lb (5,053 kg)
- Max takeoff weight: 16,250 lb (7,371 kg)
- Powerplant: 1 × Allison YT40 -A-6 Turboprop engine, 5,100 shp (3,800 kW)
- Maximum speed: 610 mph (980 km/h, 530 kn) at 15,000 ft (4,572 m)
- Range: 500 mi (800 km, 430 nmi) ~
- Service ceiling: 37,500 ft (11,400 m)
- Rate of climb: 9,980 ft/min (50.7 m/s) to 20,000 ft (6,096 m)
- Guns: 4 × 20mm (0.79 in) cannons
- Rockets: 48 × 2.75 in (70 mm) Mk 4 Folding-Fin Aerial Rockets