M.Simons The World's Vintage Sailplanes 1908-45
THE D-28 WINDSPIEL
In 1932 opinion was divided about the most suitable type of sailplane for thermal flying. Aerodynamic considerations alone favored a large wingspan with high aspect ratio, but if. as many theorists suspected, most thermal
currents were rather small, the best machine would be very small, very light, very manoeuvrable and highly refined aerodynamically.
The Akaflieg Darmstadt, combining engineering skill with considerable imagination, produced in 1933 a little sailplane to test these theories. It was called the Windspiel (literally, wind-sport). With a span of 12 metres the D-28 was as perfect as the knowledge and materials available permitted. It weighed even less than its pilot, yet hardly anything was conceded aerodynamically. The wing had the Goettingen 535 profile, which wind tunnel tests indicated was the best for low drag at slow airspeeds. In the interests of still less drag, the section was thinned by ten percent. The fuselage was of very small cross-section and nearly circular except for the narrow pylon which carried the wing clear of any disturbed flow. The fully enclosed cockpit, close to the centre of gravity, was under the leading edge, and was designed to take any pilot to a height of 186 cm. There were no struts and external protruberances were reduced to the minimum.
To achieve such a low structural weight great care was taken. Superior materials were selected for every part, surplus glue was scrupulously cleaned off joints, and all measurements were kept within 0.1 mm of their designed figures. Every wing rib, fuselage frame and all other similar components were spindled out to 'U' cross-section, light alloys being used for all fittings and for the aileron spars. 7000 man-hours went into the work of completing the aircraft.
The single-spar wing was designed to a 'g' factor of only 4, and was stressed for airspeeds up to 180 km/h, the twisting loads being taken partly by the 1 mm thick plywood ‘D’-nose covering and partly by the 0.5 mm plywood which formed the aileron gap seal and held the rear ends of the wing ribs. The aileron hinges were carried by three specially stiffened ribs in each wing. There was no aileron spar. To aid manoeuvrability the ailerons ran from root to tip, and they could be raised and lowered together to act as camber flaps.
To hold the wing root fittings to the spars, light hollow rivets instead of steel bolts were used. For further lightness the wing was constructed in one piece from tip to tip. The fuselage skin was 1 mm plywood, the rear being a true monocoque with laminated wooden rings to hold the shape but with no longitudinal members. Light box spars carried the landing and pilot’s weight loads at the front, but the streamlined form was retained by a built up 1 mm shell of plywood strips scarfed together.
Unlike most contemporary gliders there was a fixed tailplane which provided a degree of pitch stability ‘stick free'. When the rudder was deflected, the fin also moved through an angle half that of the rudder itself, so the combined effect was greater than rudder alone, permitting the vertical surface area to be reduced accordingly, saving both weight and drag. Rudder pedals and ailerons were also linked so that operation of either control aided the other.
The fabric covering of the rear section of the wing and the control surfaces was pure silk, again for extreme lightness.
In flight the Windspiel was able to circle with shallow bank of 25 degrees, in ten seconds, at an airspeed of 47 km/h, corresponding to circling radius of 80 metres. This was what the students had been hoping for; if thermals were less than 100 metres radius the little D-28 would be able to circle within them.
After competing with moderate success in the 1933 Rhoen contest, Hans Fischer in March 1934 broke the world distance record, flying the Windspiel from Darmstadt into France, 240 km. The following year Fischer completed the world’s first record goal flight, 140 km from Darmstadt to Saarbrucken, though goal flying was not yet recognised as a world record class. Had it not been for the fact that Wolf Hirth broke Fischer’s record with his big, 20 metre Moazagotl, which represented the antithesis of the D-28, the Windspiel's successes might have led to a general acceptance of the 'small sailplane’ policy. But in the long run the success was limited. Unfortunately the D-28 was found too delicate for general use, requiring expert handling on the ground and in the air; it was also structurally weak and would not have withstood flying in really rough conditions.
The original D-28 was smashed to pieces in 1935, before that year’s Rhoen contest, when a powered aeroplane landed on top of it on the aerodrome at Darmstadt. Fischer was in the cockpit at the time but struggled out with four broken ribs. The Windspiel was rebuilt and flew again in 1936, this time stronger and heavier and now re-named the D-28B. With it, in 1937 at the Salzburg meeting, Osann made a spectacular trans-Alpine flight of 180 km. It was later taken on a tour to the Sahara to investigate thermals over the desert and fly into dust devils. Its small size and agility were doubtless very useful but the thermals there turned out to be bigger than expected. In any case the trend was now toward large machines with their higher wing loadings, for they had shown themselves capable of thermalling with ease.
D-28: Span. 12.00 m. Wing area, 11.4 sq m. Aspect ratio, 12.63. Empty weight. 55.5 kg. Flying weight, 136 kg. Wing loading, 11.9 kg/ sq m. Aerofoil at root. Goettingen 535 reduced 10%, tip, Goettingen 535 reduced to 8% thick.
D-28B: As for D-28 but empty weight 72 kg, flying weight 152 kg and wing loading 16.65 kg/sq m.