DFS Reiher

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

THE REIHER

  The Reiher represented the peak of achievement of the classical designers. In its day it was the best production sailplane in the world. The DFS team under Hans Jacobs undertook a lengthy series of tests and made numerous calculations to establish the best shape and structure. The most suitable compromise between aerodynamic efficiency and structural simplicity and lightness was considered to be achieved with a 19 metre span. The wing would weigh well over half the total empty weight of the aircraft and the mainspar would account for more than half the wing. Spars in Germany were usually fabricated from Baltic pine. Spruce of aircraft quality had to be imported from Canada. For the Reiher the research team considered using steel, light alloys, ash or impregnated beech as alternatives to these more orthodox materials for the spar. Specimens of all types were tested, but in the end the team settled for spruce. The timber for the Reiher was specially selected. A higher stress value than usual was used in the calculations, knowing that the wood actually employed would be much stronger than the average specimen. The spar, when built, was a laminated box type, with the ‘gull’ dihedral form which was still believed to be helpful for stability in turns.
  The best aerofoil for gliders was the Goettingen 549, comparative wind tunnel tests proving this point to everyone’s satisfaction. In view of this profile’s rather sharp stalling characteristics, the section was changed progressively from the semi-span position to Goettingen 676 at the tip, with several degrees of washout. Large airbrakes were fitted and there were camber-changing flaps, the ailerons drooping or rising with the flaps.
  The fuselage was a simple, streamlined form with fully contoured cockpit canopy. No landing wheel was fitted, touchdown being, as usual, on the skid sprung with tennis balls. A drop-off wheeled dolly was used. The wing root was faired to the fuselage as neatly as possible but there was no attempt to construct a ‘lifting’ fuselage as had been tried on the Fafnir 2.
  In flight tests of the prototype several problems appeared. The ailerons had a habit of suddenly snatching the stick out of the pilot’s hand in rough weather. This was cured by mass balancing the ailerons. The air brakes also behaved oddly. On opening, there was an appreciable delay before any effect was felt. This could be alarming, but worse, on closing the brakes their effect remained for some moments so that it was easy to slam the glider on to the ground accidentally even though the brakes were fully closed. The cause this time was the ‘gull’ kink in the wing, and the trouble cleared up when the upper surface brake paddle was shortened. The fuselage also needed lengthening to increase longitudinal stability.
  The wing was flexible although Jacobs knew it was safe. At the maximum allowed bending load the Reiher prototype wing flexed upward 181 cm, which was thought to be too much by pilots accustomed to the comparatively thick, stiff wings of the older sailplanes. The taper ratio of the wingtip to the root chord was changed so that the root could be made broader and thicker. The result of the increased taper was a stiffer wing without increase of weight, and the tip deflection under load now came down to 91 cm. These changes produced the Reiher 2. On this model also the camber flaps were not incorporated since they were considered too ineffective to justify the cost and complication involved in building them. Some further modifications, including an enlargement of the rudder and incorporation of the longer fuselage, were carried out. Six further Reihers built after this were known as the Reiher 3.
  Internally the Reiher had control pushrods instead of cables and the rigging system was one of the neatest devised to that date. A single, vertical cylindrical steel pin was used to join the mainspars together inside the fuselage, on the centre line. Two horizontal pins, front and rear, held the wing to the fuselage on each side. These two pins at the side were linked by a toggle bar and handle, so they could be inserted simultaneously, an idea that Jacobs developed further on the Weihe, his later design. All controls were automatically coupled as the wings were assembled.
  The prototype was finished just in time for the International Soaring Competitions at the Wasserkuppe in 1937. It was flown by Hanna Reitsch and in spite of control difficulties she achieved a 351 km distance flight on the first day and finished sixth. She established a ladies' world record and went on with the improved Reiher, setting German feminine records for goal flight and out-and-return distance records which remained unbroken for more than thirty years.
  In 1938 Wolfgang Spaete won the Rhoen Competitions with the prototype. The wing had been stiffened with a resultant weight increase of 90 kg. The increased wing loading proved an asset to Spaete, who had recently developed an early form of the 'speed to fly’ theory which later, perhaps wrongly, was named after Paul MacCready. On one flight, Spaete went 420 km to land across the border in Holland.
  At the last Rhoen gathering before the outbreak of war, the contest was won by a Reiher 3. The pilot was Erwin Kraft.
  Only one Reiher survived the war and was taken to England, but was allowed to stand in the open in bad weather and finally had to be scrapped after the glue had deteriorated.
  The performance figures and weights below are those reported after flight tests by the DFS.

  Technical data:
  Reiher: Span, 19.00 m. Wing area. 19.36 sq m. Aspect ratio. 18.64. Empty weight. 234.5 kg. Flying weight. 329.5 kg. Wing loading, 17.0 kg/sq m. Aerofoils, Goettingen 549 at root and to mid-span, tapering to Goettingen 676 at the tip. Best glide, measured in flight. 1 : 33 at 72 km/h. Minimum sinking speed, 0.50 m/sec, at 56 km/h.