SZD-56-1 Diana was designed in late 1980'th in Polish sailplane factory SZD-Bielsko-Biala. Chief of the design team was Bogumil Beres. Its first flight was in 1990. The sailplane was quite unusual. It had the thinnest airfoil (13%), the highest aspect ratio wing - 27.6, the smallest wetted surface of the fuselage, the lowest empty mass and the best ratio empty mass to water ballast mass between all 15m-class sailplanes. The empty mass of the Diana is 175 kg/385 lb, while empty mass of other sailplanes in Diana's class are about 235 kg/515 lb. Such design characteristics were possible thanks to the very inventive structure, especially in case of wing. Unlike other sailplanes, Diana's wing was made as a spar-less, monocoque pipe, where skin takes all: torsional load, shear forces and bending moment. This allows really to take full advantage of the modern construction materials: epoxy resin, kevlar and carbon fiber. Each wing panel weighs only 46 kg / 102 pounds. The spar-less design leaves extra room for ballast: the '56's wet wings hold 160 liters / 42.2 gallons.
Aerodynamics of the sailplane was based on Wortmann type, free transition airfoil NN27-13.
With today's aerodynamics, computers and software, it was possible to make the sailplane really modern.
Estimated possible reduction of profile drag of the actually used Diana's airfoil can achieve about 20%.
Taking into consideration the possible effect from lowering the adverse aerodynamic interference, the reduction of parasite drag by making wing slightly bigger and installing winglets to reduce induced drag, the increase of max. L/D about 10% and farther reduction sink rate at higher speeds can be expected. Aerodynamic design of the new wing was performed by Krzysztof Kubrynski (Warsaw University of Technology). The new wing planform is totally curved. This solution lets reduce both induced drag and wing profile drag, and allows to expect proper wing stall progression. Wing profiles are changing along the span and meet aerodynamic requirements in all sections in the best manner. Diana-2's airfoil sections are even thinner then before, ranging from 12.8% at the root to 12.2% at the tip (and much less at the winglets). Additionally they are modified at the wing/fuselage junction. The flaperon chord is 17% and they can be deflected -2 to +28 deg. Aerodynamic philosophy of new Diana's airfoils is slightly different then that of other sailplanes and aerodynamic characteristics are expected to be slightly more profitable, especially at lower speeds.
At the same time, even the profiles are thinner than original Diana's airfoil, the stiffness and strength characteristics are much better, and a cross section area is higher. This is important for wing weight and volume of water ballast. Pneumatic turbulators are used to enforce turbulent transition on the lower flap surface.
Maximum L/D is expected to be exceed 50:1. Calculated speed polars are presented below.
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