M.Simons The World's Vintage Sailplanes 1908-45

THE HORTEN FLYING WINGS

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  With the Horten 4, the brothers for the first time used a high aspect ratio. The span was the same as the '3 but the area was halved, nearly doubling the wing loading. The best glide ratio was expected to be better than 37:1. The wing root now was too thin for the pilot to be housed entirely inside it in the sitting position. Instead of spoiling the line by adding a nacelle, he lay or half knelt, face down, on a foam rubber padded couch. His legs and the rudder pedals, with stirrups, were housed in a small underwing fin which also carried the rear landing skid. At the front end of the couch, his hands and arms hung over the edge to grasp the controls, and his chin rested on a rubber pad. Without this, the pilot’s neck muscles would have become too tired to support the weight of his head, especially in tight turns. The leading edge of the wing in the centre bulged up slightly to give room for the shoulders and head, with transparent mouldings to give a view upward, to the sides, and straight down.
  A retractable skid and drop-off wheel was provided at the front, to complement the fixed skid under the fin at the rear. The control system was similar to the well-tried arrangement of the earlier designs, but large airbrakes were fitted, above and below the wing. The prototype flew in the summer of 1941. Three further examples were built with metal wingtip sections since the small dimensions in this region made it very difficult to obtain adequate strength with wooden structures. The spars were reinforced with a plastic laminate material, ‘Lignafol’.
  The DFS carried out a series of careful performance measurements by flying the Horten 4 against the D-30, and established that the best glide ratio was 1 : 34. The Hortens, whilst still carrying on with various military powered aircraft projects, persevered with the sailplane. The Horten 4B had a laminar flow aerofoil of 14% thickness based on the NACA profile used on the American Mustang fighter. The mainspar was stiffened with dural in place of the Lignafol and the plywood covering of the leading edge was internally supported by another light plastic material. This, called ‘Tronal’, was intended to preserve a very accurate profile shape. The Hortens thus anticipated by many years both aerodynamic and constructional trends that later became established everywhere. The prototype crashed near Goeppingen.
  Comparison tests of the Horten 4A against the D-30 suggested that if the aspect ratio of the flying wing equalled that of the famous Darmstadt aircraft, a true proof of the superiority of the tailless layout would appear. The Horten 6 was the result. This was an astonishing achievement. To attain the desired aspect ratio of 33, the span was increased to 24.2 metres, a size surpassed only by the old RRG Obs and Kronfeld’s ill-fated Austria 1. The outer wing panels were made from light alloy as were those of the Horten 4. The cockpit arrangements were the same but since it was proposed to use the sailplane for high altitude wave flights, oxygen gear was incorporated. It was calculated that the glide ratio would equal 1:43.
  Two Horten 6s were built. In March 1945, with the American armies 40 km away, one was test flown against the Horten 4. It was established that the performance in straight flight was as much better than the '4 as the '4 was better than the orthodox types. The work had to stop as the Americans soon over-ran Goettingen. One of the new aircraft was destroyed on the ground by the conquerors, as were most German sailplanes at this time. The other Horten 6 and parts of several other flying wings were taken to the USA to be studied at the Northrop factory. Two Horten 4s survived the war in flyable condition. One was flown by the British Air Force of Occupation gliding clubs until it was badly broken in a landing accident. The other was taken to England, where it was flown at Farnborough but badly damaged. After repairs it was sold to Hollis Button in the USA.
  Although capable of good flights in expert hands, the Horten 4 seemed to fall short of expectations. In 1959 the sailplane was the subject of a thorough study by the Aerophysics Department of Mississippi State University under August Raspet. The results were reported to the OSTIV Congress in 1960. The researchers found that the flying wing had a number of serious defects. The Hortens, when they heard of these results, were convinced that the controls of the aircraft tested had been incorrectly rigged. If so, this would account for the poor results Raspet's team obtained.
  The Horten 4 in the USA was still extant in 1980. The aircraft including the Horten 6, sent to Northrops, were re-discovered in crated condition in the Smithsonian Institution’s store in 1977, and will probably be preserved.
  Reimar Horten, in post war years, emigrated to Argentina where he produced several further flying wing aircraft, including the Horten 15, which competed in the 1952 World Championships, but without success. Walter Horten remained in Germany and also built more tailless aircraft, particularly the Horten 33, a two-seat powered version of the Horten 3. His advice was sought during the preparation of this description.

  Technical data:
   Horten 4, 1941: Span. 20.00 m. Wing area, 19.1 sq m. Aspect ratio, 21. Flying weight, 350 kg. Wing loading, 18.3 kg/sq m.
   Horten 6, 1945: Span, 24.20 m. Wing area. 17.75 sq m. Aspect ratio, 33. Flying weight. 425 kg. Wing loading, 23.9 kg/sq m.