Early vertical take-off and landing fighters from southern California, part 1: the turboprop-powered tail-sitters

The German invasion of the Soviet Union and the Japanese attack on Pearl Harbor in 1941 became stark reminders of the dangers of airfields being mauled by enemy airstrikes if they were not fully prepared for an enemy attack. Although helicopters were America's premier aircraft optimized for vertical take-off and landing (VTOL) beginning in World War II, the biggest practical limitation of the helicopter is the fact that a helicopter itself has a built-in speed limitation caused by the motion of the rotor blades, meaning that the average helicopter can attain a speed of only 160 miles per hour (260 km/h). As a helicopter flies through the air, one rotor blade is moving away from the other blades, and if a helicopter flies too fast, then a raft of onrushing air will overtake the retreating blade, causing that rotor blade to lose lift and the helicopter to stall from the air, a phenomenon called retreating blade stall. Aware of the constraints on the top speeds of existing helicopters due to the motion of the rotor blades, beginning in the late 1940s the US armed forces decided to look at the idea of combat aircraft designed to combine the vertical lift of helicopters with the combat performance of a fighter plane. The aviation industry in southern California would take the lead in envisaging designs for early in vertical take-off fighter planes, and as the 1950s progressed there would a be a diversity of ideas regarding the best means of vertical takeoff for a fighter. Therefore, I've dedicated the first post in a three-part series on VTOL fighter designs from southern California to tail-sitter designs. 

Top: Sideview drawing of the Northrop N-63 proposal
Bottom: Desktop models of the Lockheed XFV-1 (left) and Northrop N-63 (right) in a model store of the Planes of the Fame Museum (photographed by me)

In 1948, the US Navy announced a requirement for an aircraft capable of vertical takeoff and landing aboard platforms on the afterdecks of conventional ships. By 1950, this requirement had evolved into the OS-122 specification for a research aircraft that could be developed into a ship-based VTOL convoy escort fighter. Convair, Goodyear, Grumman, Lockheed, Martin, and Northrop submitted designs for the OS-122 requirement, all of them powered by a single Allison T40 turboprop engine driving counter-rotating propellers and standing to their tail empennages for VTOL. The Convair Model 5 featured the delta-wing layout of the earlier XF-92 and XF-92A as well as the future F-102 Delta Dagger and F-106 Delta Dart, and it had two large tail fins for support in an upright position. By contrast, the Lockheed  L-200-1 had the straight wings of the P-80/F-80 Shooting Star, T-33 trainer, and F-94 Starfire, and it was designed to stand on four tail fins in upright position. (A number of L-200 designs with backswept wing and canards were also envisaged, and a horizontal take-off and landing version was proposed as the L-210-2, but these were eventually discarded.) The Northrop N-63 had a straight wing with dihedral and a very large ventral T-tail, with provisions for four 20-mm cannons mounted in large pods on the wing tips. The tail-sitter concept, however, was not new in these designs. In the final months of the Third Reich, German aircraft manufacturers Focke-Wulf and Heinkel came up with designs for tail-sitting VTOL fighters, the Focke-Wulf Triebflügel with a three-blade rotor around the center of the aircraft behind the cockpit, powered by three ramjets at the tips of the blades, and Heinkel Lerche and Wespe with ducted propellers. None of these proposals left the design phase before Nazi Germany's surrender, but some design attributes of the wartime German tail-sitter projects, namely the cruciform tail empennage, would eventually find their way into the VTOL fighter designs for the OS-122 requirement.

Models of the Convair XFY (left) and Lockheed XFV (right) at the Planes of Fame Museum

The Navy selected the Convair and Lockheed proposals for full-scale development and on March 31, 1951, a contract for three Convair Model 5 prototypes (BuNos 138648/138650) was issued, and the Convair design became XFY-1, whereas the Lockheed L-200-1 was designated XFO-1 (later changed to XFV-1), with a contract for two prototypes (BuNos 138657/138658) plus a static test article being signed on April 19. The XFY-1 was 34 feet 9.3 in (10.6 meters), with a wingspan of 27 feet 7.9 in (8.43 meters), a wing area of 355 square feet (33.0 square meters), and a horizontal height of 22 feet 1 in (6.73 meters). Compared to the XFY-1, the XFV-1 was a bit larger, measuring 36 feet 8.94 in (11.2 meters) long with a wingspan of 30 feet 10 in (9.4 meters) and a wing area of 246 square feet (22.8 square meters). A full developed production version of the Convair design would be armed with either 20-mm cannons or two separate packs of 2.75 in (70 mm) folding-fin unguided rockets in wingtip pods. A planned production version of the XFV-1, the FV-2, would carry the same type of armament as the XFY-1 but would have the T40 replaced by an Allison T54 turboprop. Because the mode of vertical take-off and landing for which the Lockheed and Convair machines were designed resembled the action of a pogo stick, the XFV-1 and XFY-1 would be collectively nicknamed the "Pogo".

Top: Lockheed XFV-1 in upright position (left) and forward flight (right)
Bottom: Convair XFY-1 during flight testing

The first XFV-1 prototype was completed in the spring of 1953. Due to the T40-A-6 engine offering insufficient power for a vertical takeoff, it was decided to fit the XFV-1 with a temporary fixed-position undercarriage (composed of two forward wheels supported by four struts and downward-facing wheels on the two lower vertical fins) and undertake horizontal takeoffs pending arrival of the more powerful T40-A-14 unit. A special ground-handling rig was also built to allow the aircraft to be moved easily in a horizontal position and tilted upright for take-off. Taxi trials and ground tests of the XFV-1 began in late November 1953, with test pilot Hermann "Fish" Salmon at the controls (hence the XFV-1 being known unofficially as the "Salmon"). During one high-speed taxi test on December 23, 1953, the XFV-1 made a short hop for a few seconds, but it would not be until June 16, 1954 that it made its first official flight at Edwards Air Force Base. A total of 22 flight were conducted until March 15, 1955, with Salmon himself carrying out transitions from horizontal to vertical flight and propeller vibration tests. With the T40-A-14 running into developmental problems, the XFV-1 would never make a vertical take-off or landing during flight testing. The second XFV-1 prototype was nearly completed but not flown, while the static test airframe was not completed. Meanwhile, in early 1954 the second of the three XFY-1 prototypes in order was completed, the first aircraft being partly assembled and relegated to use for engine tests and the third prototype being used for stress tests only. Tethered tests of the XFY-1 at the Moffett Field Naval Air Station near San Francisco began on April 29, 1954, piloted by Convair test pilot James "Skeets" Coleman. The first free flight of the aircraft took place on August 1 at Naval Auxiliary Air Station Brown Field in San Diego, where Coleman made the first successful vertical take-off, transition to forward flight, and vertical landing on November 2, making the XFY the first vertical take-off aircraft other than a rotorcraft to make a VTOL flight. Test flights of the XFY-1 continued until June 16, 1955, and by this time it was apparent that the tail-sitter idea was not going to an operationally practical concept because the pilot had difficulty landing the XFV or XFY, meaning that he had to look over his shoulder to determine the rate of descent and how close he was to the ground. Moreover, the T40 around which the Lockheed and Convair aircraft had been designed was plagued by teething troubles, also bedeviling the Douglas A2D Skyshark and North American A2J Super Savage attack aircraft, Convair P5Y and R3Y Tradewind flying boats, and Republic XF-84H turboprop fighter.

Left: Second Lockheed XFV-1 prototype (BuNo 138658) on display at the Florida Air Museum, Parkland, Florida.
Right: Convair XFY-1 (BuNo 138649) at the Garber Storage Facility of the National Air and Space Museum in Silver Hill, Maryland.

The US Navy cancelled the XFV and XFY programs in 1956 and the two XFV-1 prototypes as well as the only airworthy XFY-1 prototype miraculously survived the breaker's torch and found their way into museums. The first XFV-1 prototype was transported to Hiller Aviation in Palo Alto, California, where its engine was used for ground tests, before being tested to destruction, while the incomplete second XFV-1 was used as a gate guardian at Naval Air Station Los Alamitos for many years before being donated to the Florida Air Museum in Parkland, Florida. The only XFY-1 to fly was initially used as a gate guardian at Naval Air Station Norfolk in Virginia, before being donated to the National Air and Space Museum in 1973, and it currently resides in storage at the NASM's Garber Storage Facility in Silver Hill, Maryland. The US Navy must have made a practically common-sense decision to cancel XFV and XFY programs not just because of difficulties with landing a tail-sitter plane but also the fact that the Lockheed and Convair "Pogos" would never have kept up with the first generation of Soviet supersonic jet fighters due to the type of engine chosen for these aircraft.

References:

Buttler, T., 2007. American Secret Projects: Fighters and Interceptors 1945 to 1978. Hinckley, UK: Midland Publishing.

Chong, T., 2016. Flying Wings & Radical Things: Northrop's Secret Aerospace Projects & Concepts 1939-1994. Forest Lake, MN: Specialty Press.

Rose, B., 2013. Vertical Take-off Fighter Aircraft. Hersham, UK: Ian Allan Publishing.