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Tail-Sitters

By Robert Guttman
10/24/2017 • Aviation History Magazine

What goes up must come down, and therein lay the problem with these vertical takeoff aircraft.

Early in World War II, when the Allies didn’t have enough aircraft carriers to provide air cover for convoys, Axis long-range maritime reconnaissance planes could  shadow cargo ships with impunity, reporting their positions to subs and bombers. Among the measures the British used to alleviate that threat was to launch fighters from catapults mounted on merchant vessels. These Hawker “Hurricats,” as they were unofficially called, proved surprisingly effective even though they could only be used once—after which the pilot had to ditch or parachute into the sea, in hopes of being rescued by one of the ships he was protecting.

It was undoubtedly with those desperate wartime expediencies in mind that the postwar U.S. Navy initiated Project Hummingbird. The program called for the development of fighter aircraft that could operate from the decks of warships independent of carrier task groups, or even from merchant vessels. To that end the Navy issued a requirement for an entirely new type of high-performance fighter designed to operate from a very small space. In response, both Convair and Lockheed submitted tail-sitting vertical takeoff and landing (VTOL) fighter proposals. Although neither achieved production, they were among the most interesting and unusual flying machines ever to leave the ground.

As was the case with many postwar aerospace innovations, the idea for the tail-sitting fighter actually came from prewar Nazi Germany. In 1938 Siemens engineer Otto Muck patented a design for an aircraft without any wings, in which lift and thrust were provided by controllable-pitch rotors spinning around the axis of the fuselage. Nothing was seriously done about it until September 1944, when Focke-Wulf submitted a design to the Luftwaffe for a fighter based on Muck’s peculiar aerodynamic patents.

Designed by Heinz von Halem, the Focke-Wulf Triebflügeljäger (powered-wing fighter) would have looked something like a cross between a helicopter and a V-2 rocket. On the ground it would have stood erect, supported by four small wheels that retracted into the tips of its cruciform tail surfaces. The pilot was seated in a fairly conventional cockpit near the nose, reached by means of a long ladder. The three non-tapered rotors that provided lift and thrust were mounted about one third of the way back from the nose. The rotors were attached to a ring that rotated freely around the fuselage, and powered by ramjet engines installed at their tips. Unlike turbojets, ramjets are not self-starting and cannot begin to operate until air is forced through them; consequently, the rotor tips were also fitted with small rocket engines, to get them rotating fast enough to start the ramjets.

The Triebflügeljäger was to be armed with two 20mm and two 30mm cannons mounted beneath the cockpit. Its designers calculated it would assume a slightly nose-up attitude in level flight, so the cannons were to be mounted to fire at a slight downward angle from the centerline.

Focke-Wulf intended the Triebflügeljäger to be a point-defense interceptor, able to operate from any site that could accommodate its 38-footdiameter rotor. Powered by three 2,000-poundthrust Pabst ramjets and three 3,300-poundthrust Walther rocket-assisted takeoff (RATO) units, it was expected to attain a speed of 621 mph.

Landing was seen as a potential problem. Not only would the pilot have to control the aircraft in vertical descent, his rearward visibility would be obstructed by the three whirling rotors behind him. Of course, all that remains conjecture, since the Triebflügel concept never progressed beyond preliminary testing of wind-tunnel models before the war’s end.

There’s little doubt American engineers were aware of the Triebflügel by the time the Navy issued its VTOL fighter requirement in 1947. But their solutions to the problem were less radical than Focke-Wulf’s. The design submitted by Convair incorporated yet another aerodynamic feature developed by the Germans during the war, the delta wing. Although it lacked the Triebflügel’s ramjet-powered rotor, Convair’s XFY-1 still represented a radical aerodynamic departure. Its huge delta wings were matched by an equally large dorsal and ventral fin. On the ground it stood on tiny wheels mounted on the trailing edges of its wings and fins. The cockpit was accessible only from a tall scaffold. The pilot’s seat could be adjusted 45 degrees from vertical so that he sat in a more upright attitude during takeoff and landing. Power was provided by a 5,500-hp Allison T-40 turboprop engine driving a pair of huge three-bladed, contrarotating propellers. The propeller hub was covered with an enormous spinner that was also intended to house the fighter’s airborne intercept radar.

The odd-looking XFY-1, known as the Pogo, could take off and land vertically, and hover briefly as it transitioned from normal flight back to a vertical attitude. To land, the pilot had to look over his shoulder while descending vertically, slowing his rate of descent with engine power while controlling the aircraft with the rudder and elevons.

The Pogo took off for first time, albeit inside a hanger and tethered for safety, on April 19, 1954. In that configuration, test pilot Lt. Col. James F. “Skeets” Coleman accumulated 60 hours handling the XFY-1 before performing an unrestrained outdoor vertical takeoff and landing on August 1. He successfully transitioned from vertical to horizontal flight and back again on November 2. Coleman was the only pilot to fly the Pogo before the program was terminated in August 1955.

While somewhat similar in configuration to the XFY-1, Lockheed’s competing design, the XFV-1, was longer and narrower and had more conventional flying surfaces. Like the Pogo, the XFV-1 was powered by a T-40 turboprop engine driving contrarotating propellers, and also incorporated a huge propeller spinner that was meant to house the radar. It acquired its nickname, Salmon, not because of any resemblance to a fish but in honor of test pilot Herman R. “Fish” Salmon, the only pilot ever to fly it.

Unlike the delta-wing Pogo, the XFV-1 was fitted with a pair of thin, straight wings that looked as though they had been borrowed from an F-104 Starfighter. Its backward-swept cruciform tail, set at a 45-degree angle to the wings to prevent control blanketing, culminated in a tiny set of castoring wheels.

Lockheed’s prototype first flew on December 23, 1953, seven months earlier than the Pogo, but it took off and landed on a jury rigged conventional landing gear. Although Fish Salmon eventually managed to transition the aircraft successfully between horizontal and vertical flight, he never did perform a vertical takeoff or landing.

One major drawback to the Navy’s VTOL fighter program was the engine it selected. Essentially two gas turbine engines mounted side by side and coupled together via a set of reduction gears driving a single prop shaft, the notoriously unreliable Allison T-40 proved to be a serious handicap to every aircraft in which it was installed. The only T-40- powered plane to achieve production was Convair’s ill-starred R3Y Tradewind flying boat. Just 13 were built, and they were withdrawn from Navy service after two years due to chronic engine problems.

Perhaps the biggest problem with the Navy’s tail-sitter program was that the basic concept simply wasn’t practical. Although Skeets Coleman demonstrated that the Pogo could indeed be flown, it clearly required exceptional skill. And hard as the tail-sitters were to land on an airfield, they’d have been a hundred times harder to land on the helipad of a ship that was rolling and pitching at sea. Moreover, aviation had advanced since the two tail-sitters had been conceived. By the mid-1950s, the estimated top speeds of the Pogo and Salmon, 610 mph and 580 mph respectively, were no longer sufficient to make them competitive fighters.

Despite the failure of the U.S. Navy’s program, tail-sitting VTOL aircraft became quite a fad in the 1950s. Perhaps science-fiction films depicting rocket ships taking off and landing vertically inspired the aircraft engineers of the day to find out whether it was actually feasible.

In any event, the U.S. Air Force developed a tail-sitter of its own, the Ryan Vertijet, ominously designated the X-13. The Air Force took a different approach to the VTOL problem than the Navy. Apparently its planners visualized a scenario in which the Soviets could lay waste to all the conventional air bases in Western Europe. Given that concept, the Air Force believed it might still be able to deploy a force of VTOL interceptors from specially built truck trailers, in much the same way tactical ballistic missiles were launched.

Ryan Aircraft began investigating the possibilities in 1947. After extensive trials of a couple of jet-powered test rigs, Ryan received an Air Force contract in 1953 to build the X-13 flying prototype. Only 23 feet 5 inches long, the X-13 had a vertical tail fin almost as large as its shoulder-mounted delta wing. The Vertijet was powered by a 10,000- pound-thrust Rolls-Royce Avon turbojet. Unlike with the Navy’s tailsitters, the thrust from its engine could be vectored to control pitch and yaw while the aircraft was vertical.“Puffer jets”mounted in the endplates attached to the tips of the delta wings provided lateral control.

The Vertijet was transported on a flatbed truck trailer, which was raised to a vertical position for takeoff. As in the Pogo, the pilot took off with his seat tilted forward at a 45-degree angle. To land, he returned the X-13 to a vertical attitude, then backed slowly down onto the trailer, engaging a hook under the nose of his aircraft with a horizontal bar at the top of the trailer. Since the pilot couldn’t see the bar from his cockpit, a long, graduated pole was mounted on top of the trailer to help him gauge his position during landing.

The X-13 flew for the first time on December 10, 1955, taking off in a normal attitude from a temporary fixed tricycle landing gear. In that guise, the prototype was extensively test flown in level flight, as well as in transition between horizontal and vertical flight modes. Finally, on April 11, 1957, Ryan test pilot Peter F. Girard successfully took off and landed vertically from the trailer in the second X-13 prototype. In spite of a spectacular flying demonstration conducted at the Pentagon on July 30, however, the Air Force was insufficiently impressed to continue development, and the Vertijet program was shelved.

The French got into the tail-sitting act with what may have been the most peculiar-looking VTOL prototype of all. Known as the Coléoptère (Beetle), the SNECMA C-450 was unusual not only by virtue of being a tail-sitting VTOL jet, but in having a unique annular, or cylindrical, wing.

Like the Americans, the French got their idea from a former Nazi scientist—in this case Helmut von Zborowski, an expert on liquid-fueled rocket motors who helped develop the engine used in the Messerchmitt Me-163 rocket fighter. He’d also been the director of a BMW factory that produced the first jet engines for the Luftwaffe. In addition, Zborowski developed the principle of an annular airfoil, a closed wing that encircled the fuselage. After the war he went to France where, in 1950, he established the Bureau Technique Zborowski. In 1952 he sold his annular wing design to a French jet engine manufacturer, la Société Nationale d’Etude et Construction de Moteurs d’Aviation.

Utilizing Zborowski’s patents, SNECMA hoped to develop a VTOL jet fighter capable of Mach 2. Since SNECMA was essentially an engine manufacturer, Nord Aviation constructed the C-450 prototype’s airframe, with SNECMA providing its 6,400-pound-thrust Atar-D turbojet engine. As completed, the Coléoptère was 22 feet long. Its surrounding wing had a diameter of 10 feet 6 inches and a chord of 9 feet 10 inches. Control in normal flight was provided by four small movable triangular surfaces attached to the outside edge of the annular wing, while control for hovering was effected by movable vanes in the jet nozzle.

Like his American counterparts, the Coléoptère’s pilot could adjust his seat to a 45-degree angle during vertical flight. The C-450 was also designed to operate from a truck trailer, some what like that used by the X-13. But as with the U.S. Navy’s VTOL turboprops, the French aircraft rested on four tail-mounted wheels.

SNECMA began developing its VTOL aircraft in 1954, progressing through a number of experimental jet-powered test stands in 1955- 57. The Coléoptère began tethered flight testing in April 1959, and made its first free flight on May 3. During the aircraft’s ninth flight, on July 25, the pilot lost control while trying to transition from horizontal flight into a hover. Although he ejected safely, the crash of the only prototype ended one of the most unusual aircraft projects of the 1950s.

In hindsight, none of the 1950s tail-sitters were really practical. They tended to be topheavy, so if they didn’t land squarely they might topple over. Without today’s computerized stabilization systems, transitioning into and out of hover mode and landing required great skill. And all were designed with severe weight restrictions, which meant they lacked suitable flaps or air brakes to effectively retard forward speed before transitioning into hover. Another problem, evident in a cursory survey of all four prototypes, is that the pilot’s view must have been poor during landing despite tilting seats. Not one had a bubble canopy, to let the pilot see directly behind him, and the Coléoptère’s annular wing would have severely restricted visibility to the rear in any case.

A practical VTOL fighter design finally emerged in the 1960s, though it wasn’t a tailsitter. Using rotating jet nozzles, Hawker-Siddeley produced the Harrier, which could take off and land vertically in a normal flying attitude.

Although the sole Coléoptère prototype was destroyed, examples of all three American tailsitters still exist. One of the two Ryan X-13 Vertijet prototypes can be seen at the National Museum of the U.S. Air Force in Dayton, Ohio, and the other at the San Diego Air and Space Museum. The only complete Lockheed XFV-1 Salmon prototype survives at the Sun ’n Fun Air Museum in Lakeland, Fla. The Convair XFY-1 Pogo has long been in storage at the National Air and Space Museum’s Paul E. Garber facility in Suitland, Md., and currently awaits transfer to the new NASM restoration shop under construction at the Udvar-Hazy Center at Dulles International Airport.

 

Robert Guttman writes frequently for Aviation History about unusual and offbeat aircraft. For further reading, try The World’s Worst Aircraft, by Jim Winchester.

Originally published in the May 2011 issue of Aviation History. To subscribe, click here

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