From 1933 to 1990, Reimar Horten, assisted by his brother, Walter, designed and built a series of swept-wing aircraft without fuselages or tails and they did not use any other surfaces for control or stability that did not also contribute lift to the wing. The National Air and Space Museum owns five of these aircraft: a Horten II L, Horten IIIf, Horten IIIh, Horten VI V2, and the Horten IX V3 turbojet interceptor.
Horten began to consider his sixth major design when construction began on the first Horten IV in December 1940. The H IV became the most successful all-wing aircraft that Horten developed and it encouraged him to explore further the potential of high aspect ratio (AR) wing design. He completed the first Horten VI four years later but this sailplane was not intended for series production and Reimar designated it 'vee-number-one' to indicate its prototype status.
One of Reimar's primary design goals was to build and test a sailplane with a very large AR so he increased the new wing's span to 24.25 m (79 ft 7 in) but lessened its chord to produce a 32.4:1 AR. By comparison, the Horten IV had spanned 20.3 m (66 ft 7 in) and the wing had a 21.8:1 AR. Although all-wing research was his immediate focus, Horten was very keen to duplicate the AR and better the performance of the Darmstadt D-30, at that time without peer in the exclusively world club of high-performance sailplanes. Students at the Darmstadt Akaflieg (a small group of aeronautical engineering students led by a professor who worked at several major universities) designed and built this experimental aircraft in 1938 with an AR of 33.6:1 and a Lift-to-Drag ratio of 37.6:1. For twenty years, no other sailplane could surpass this level of performance. The unswept D-30 wing spanned 20.1 m (65 ft 11 in) and it sat atop a conventional fuselage and tail. The wing was so long and narrow that it required an aluminum alloy box-spar for strength and rigidity.
Reimar Horten's workshop at Bonn began building the Horten VI V1 ('six-vee-one') in 1943 and the sailplane was finished on May 24, 1944. The second prototype was designated the Horten VI V2 and finished some months later. Reimar planned to continue to develop the design but the war ended before he could continue the work. Without revealing actual figures, Horten claimed these gliders were expensive to build but he hinted at the costs when he reported that building the Horten VI required about 8,000 man-hours. A structural tour of the Horten VI reveals why it required so much time to build. Within each wing half, 120 nose ribs covered with thin plywood formed the critical airfoil-shape of the leading edge. Every nose rib was slightly different in length and curvature but all resembled the letter 'D' and every rib was hand-made from fifteen pieces of plywood so fragile that jigs were needed to support the rib during assembly.
Horten actually stepped beyond what was known and practical and strode boldly into a purely experimental and largely unknown realm of sailplane construction. A main wing panel spanned 8.4 m (27 ft 7 in) but the root chord measured just 1.25 m (4 ft 2 in). Even a massive wooden spar could not keep such a long and narrow wing section rigid enough for safe flight but the aluminum plug-in wingtips, each spanning 2.9 m (9 ft 7 in) and containing both an aileron and a spoiler, also contributed to weakening the wing. Photographs of the Horten VI V2 taken after the war reveal pronounced flex along almost the full length of the wing but the flexibility is greatest in the joint between wing and wingtip.
Horten trimmed every unused cubic centimeter from the center section and pared it down to barely 1.6 m (5 ft 3 in) wide. He built the basic structure from large and small-diameter steel tubing welded into an interlocking network of trusses. It had to be stout to support nearly 25 m (80 ft) of wing. Horten covered the frame with plywood and curved sheets of aluminum. There was no door to enter the cockpit - Horten transformed most of the top half of the center section into a removable hatch and he designed a handy tray inside the hatch to take the weight of the pilot's low-slung parachute. During a bailout, pins to secure the hatch would have retracted automatically if the pilot released his seat belt harness as the air stream blew the cover away from the airplane.
When he fitted out the cockpit, Horten had in mind serious, high-altitude flying. He added a complete oxygen system: a blinker-type regulator, plumbing, and a pressurized tank packed neatly inside the ventral fin. Horten installed batteries and wiring to power heated flying gloves and he mounted all but two instruments inside the leading edge of the wing near small mirrors that reflected information from these gauges to the pilot. No other space was available to mount the instruments so reading backwards became a prerequisite for flying the Horten VI. Safe flight was impossible, particularly in cloud, unless the pilot could make out these vital dials quickly and accurately.
To minimize drag, Horten adopted a semi-prone position for the pilot that was identical to the layout that he had used on the Horten IV. The NASM Wright 1903 Flyer and the Horten IIIf accommodate a pilot lying flat on his stomach without bending either his legs or his waist but the Horten VI V2 is the only aircraft in the NASM collections piloted from a semi-prone, 'praying mantis' position. As a pilot flew the 'VI V2 in level flight, she rested her torso on a special cushion that inclined her spine about 30° degrees to the horizon. She bent her waist about 70°-80° and placed her knees in padded aluminum "knee cups" that were adjustable for height and secured to the floor of the ventral fin. Reimar described it as a configuration "well-suited to the flying wing, since only a small plastic bubble is necessary above the pilot's head to give the required view. The knees are in a "keel" below the wing, which also provides wing-to-ground clearance. [This position] feels more natural, bird-like, and is safer than [other] positions." Horten's claim that most pilots were enthusiastic about the semi-prone position may be true. "Several pilots," he said, "flew the Ho IV up to 10 hours at a time in thermals and on instruments during extended flights in clouds, and compared their experience with conventional sailplanes. All found the semi-prone position less tiresome."
The origins of the semi-prone layout are elusive but the idea may have sprung from Reimar's long association with motorcycles that began in 1933 when he contracted with the Imperia Motorrad Werke (Imperia Motorcycle Factory) to manufacture metal fittings for his Horten I sailplane. He had begun experimenting with seating position in 1935 and after designing and flying the first glider and propeller-driven airplanes with supine seating, he learned about the pioneering work done by the DFS glider research group in Stuttgart. These experiments and the disappointing results of his supine configurations led Reimar to try prone cockpits in several Horten II gliders and successful trials encouraged him to develop the definitive semi-prone configuration that he used in all Horten IV and Horten VI sailplanes. In 1937, the year Reimar and Walter flew the NASM Horten II equipped with a prone cockpit, Ernst Henne rode a BMW motorcycle faster than any man alive and set a new world land-speed record not broken until 1951. Technicians at BMW tightly cloaked Henne and his motorcycle in a flowing aluminum cocoon that left only his head and the two wheels exposed. A pilot flying an H IV or H VI sailplane crouched almost exactly the same way as Henne on his record-setting motorcycle.
Horten cited several advantages to the semi-prone layout. "Visibility," said Reimar, "in this position is unsurpassed, not only forward, but also straight down, which improves navigation and helps locate turnpoints. The unrestricted view over the top of the aircraft provides good collision avoidance visibility when thermaling with other aircraft." He admitted instruments restricted the pilot's view forward but Reimar revised the layout in the H VI to minimize this problem. Some of the instruments were equipped with reversed labels and mounted out of the field of view. The pilot checked these indicators with a small mirror conveniently mounted in the tiny cockpit.
To make up for the control lost by deleting a conventional fuselage and tail, Reimar Horten designed a flight control system for his Horten VI that consisted of 6 elevons, 4 spoilers, and 4 drag rudders - a total of 14 control surfaces! This control arrangement is one of the most complex ever used on a high-performance sailplane. Three pairs of elevons formed the back edge of the wing panels and a pilot moved these devices using the 'rams horn' control wheel according to a complex schedule which was similar to the operation of the elevons on the Horten IV (please refer to page 10 in K. G. Wilkinson, "The Horten Tailless Aircraft," R. A. E. Report No. F. A. 259/1, Technical Note No. Aero 1703, Royal Aircraft Establishment, Franborough Hants, October 1945, or see the document at this web address, http://www.twitt.org/Farnborough_03.html#top). To flex these flight controls, the pilot reached beneath his chin and gripped the control wheel in both hands.
Horten also employed spoilers set into the top and bottom of the main wing panel near the joint between the center section and the wing. At the full-open position, the spoilers projected about 15 centimeters (6 inches) above and below the wing. Each surface swung from a compact, steel-tube framework resembling a trapeze and the entire assembly fit compactly inside the wing. The pilot moved a spring-loaded handle to deploy these devices simultaneously to control airspeed, glide angle, and descent rate.
The drag rudders were actually two small spoilers set into the top and bottom of each wingtip. Their action supplemented the elevons for more precise turn control and they also minimized adverse yaw. The pilot controlled these devices with his feet by pushing two stirrups mounted at the back of the skid fairing. The pilot could also push both stirrups simultaneously and spoil the lift on both wingtips to control airspeed, glide path, and rate of descent.
Horten installed a fixed wheel and tire landing gear into the bottom of the aft fairing. At the nose, he used a retractable skid that rode atop a small two-wheel dolly that the pilot jettisoned immediately after takeoff.
Both prototypes were flying by summer 1944 but it was not possible to arrange any informal contests to compare the performance of Germany's best conventional sailplanes with the Horten VI. Even the D-30 was unavailable, however, the Horten test pilot, Heinz Schiedhauer, completed a half-hour flight aboard the 'VI V1. He found the wings very "soft" and prone to flutter at their own unique frequency, about 1.3 cycles per second at 128 km/h (80 mph). All four Horten IV sailplanes suffered flutter, too. The war ended before further flight-testing but we can deduce two reasons for the wing flutter. The Horten VI, like her H IV sister ships, had three control surfaces on each wing but none were balanced either statically or dynamically. The "elevons," wrote Karl Nickel, "are lagging behind the wing because of their mass moment of inertia [aft of the elevon hinge axis]. . . Hence the elevon deflections enforce the reversal forces, arouse the oscillations and thus the flutter of the flying wing." Horten's use of a bell lift distribution may have contributed to the flutter problem. The wing had a maximum speed of 200 km/h (124 mph) and a best glide speed of 84 km/h (52 mph).
Reimar Horten wrote in the postwar German aviation periodical "LUFTFAHRT International" that "In our view, of all Horten models of the pre-and post-war periods, the Horten IV and its successor, Horten VI, have come closest to the ideal of the "flying human being." They continued: "The pilot of these planes is enabled to fly by the wings which practically extend out from his shoulders with a minimum of aircraft technology" ("Horten VI and VI - High Performance Glider Aircraft," "LUFTFAHRT International," No. 12, November-December 1975, 1847-1870).
Reimar Horten reported that Allied ground forces burned the Horten VI V1 near Göttingen but the VI V2 was saved when the Allies picked up the sailplane in 1945. Allied technicians shipped the aircraft to the U.S., via England, where it was examined by aeronautical authorities, and then placed in storage at Park Ridge, Illinois. In 1947, Northrop Aeronautical Institute borrowed the Horten VI, 'IIIf and 'IIIh and studied them, and students and faculty published reports about these airplanes in 1948 and 1949. The Horten VI V2 was one of four Horten gliders shipped in 1993 from the National Air and Space Museum (NASM) to the Museum für Verkehr und Technik (renamed the Deutsches Technikmuseum) in Berlin, Germany, for restoration and preservation.
Wingspan 24.3 m (79 ft 7 in)
Center Section Length 2.5 m (8 ft 3 in)
Height 1 m (3.3 ft)
Weights, Empty 330 kg (726 lb)
Gross 410 kg (902 lb)
References and Suggested Reading:
Lee, Russell. "The National Air and Space Museum Horten Sailplane Collection: HortenII L, IIIF, IIIH, and VI-V2," "Bungee Cord," Vol. XXIII No. 4, Winter 1997.
Myhra, David. "The Horten Brothers and Their All-Wing Aircraft." Atglen, Penn.: Schiffer Publishing Ltd., 1998.
Nickel, Karl, and Wohlfahrt, Michael. "Tailless Aircraft in Theory and Practice." Reston, Va.: American Institute of Aeronautics and Astronautics, 1994.
Selinger, Peter F., and Horten, Reimar. "Nurflugel: Die Geschichte der Horten-Flugzeuge 1933-1960." Graz, Germany: H. Weishaupt Verlag, 1983.
Russ Lee, 9-2-04