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

THE D-30 CIRRUS

  Preliminary design work on the Darmstadt Akaflieg’s D-30 began in 1933. The D-30 surrendered all to high aspect ratio and minimum drag. It was vastly expensive but had a performance which was not surpassed until 20 years later.
  The ДальшеMore>>> machine’s phenomenal aspect ratio of 33.6 was a record. The wing section was 14% of the chord thick at the root, tapering to 12% at the tip, so with the span at 20 metres the spar depth at its maximum was only 13.5 cm. At the half-span position, due to the wing taper, the thickness was less than 8 cm. At this point the wing was hinged to allow the dihedral angle to be changed in flight. The hinge and control mechanism were contained entirely within the wing. This variable dihedral feature was intended to establish the best configuration and to investigate the effects on controllability of raising and lowering the tips. The tests established that small dihedral angles, either positive or negative, had little effect either way upon handling but as the angles increased, controls became both heavier to operate and slower in response. The wing of the D-30 was quite flexible so that with the hinge in the central position some dihedral was created by upward bending. With the outer panels drooped, the combined effect was like a ’gull’ wing.
  The D-30 also had camber flaps which combined with the ailerons to extend across the entire span to allow efficient flight at all speeds. This device was not generally adopted on sailplanes until long after the D-30 flew.
  The sailplane was stressed to normal strength requirements but due to the thin wing it was quite impossible to use timber for the spars. Instead an elaborate box spar was built up from light alloys. There were three vertical shear webs, with pressed ribs running chordwise, and upper and lower skins of heavy gauge duralumin, curved to conform to the aerofoil section. Corrugated sheet stiffening was flush-riveted to these skins inside the box between the vertical webs, the corrugations running lengthwise. To complete the wing profile, plywood leading and trailing edge portions were built up with wooden ribs in the usual way and riveted to the spar. Air brakes were fitted and the controls were operated by pushrods. To reduce drag the total area of surface or ‘wetted area’ of the fuselage was reduced by a ‘pod and boom' form. The pilot sat in a tiny plywood capsule, streamlined as perfectly as possible and tapered aft to a knife edge. The tail, of minimum size, was carried on a light boom of electron alloy. The undercarriage comprised a small wooden skid but drop-off dolly wheels were used. The tail unit was of wooden construction with plywood and fabric covering. The tailplane moved with the elevator but only half as much.
  Until the advent of low-drag, laminar flow aerofoils and plastic structures, the D-30, named Cirrus, represented the technical peak of sailplane design. It was not a very practical club or competition aircraft but soon after its first flight, Bernhard Flinsch on 7th July 1938 set up a new world record out-and-return flight, 305.6 km from Bremen to Luebeck and back. At the Rhoen contest that year the aircraft was damaged on its first take-off and did not compete, nor did it appear at the 1939 Wasserkuppe meeting. Flinsch flew it to win the Student World Games competition held at Vienna in August 1939, just prior to the outbreak of war.
  In 1945 the D-30 was apparently captured intact by the American troops but was deliberately destroyed under the same orders that led to the loss of so many other sailplanes.

  Technical data:
  D-30 Cirrus: Span, 20.10 m. Wing area, 12.00 sqm. Aspect ratio, 33.6. Empty weight, 203 kg. Flying weight, 288 kg. Wing loading, 24 kg/sq m. Aerofoils: root, NACA 2414\ tip, NACA 4412 with wash-in (positive twist) over inner wing panels and washout over tip panels, to preserve elliptical lift distribution. Maximum permitted speed, 300 km/h. Best glide ratio (measured), 1 : 37.6, at 77 km/h. Minimum sinking speed, 0.55 m/sec at 72 km/h.