M.Simons The World's Vintage Sailplanes 1908-45
THE PWS 102, REKIN
Some technically superb sailplanes have passed almost forgotten. The PWS 102 was probably at least equal to the German Reiher in respect of aerodynamic and structural excellence and it had a number of other interesting
features which have since been re-introduced, or perhaps copied, on some of the most advanced modern sailplanes. War came to Poland before the testing of the prototype was completed and the second aircraft off the production line was still in the factory when the building was captured by advancing Soviet troops. Three of the PWS 102 sailplanes, in various stages of completion, were taken to Russia at this time, and as far as the rest of the world knows, that was the last of them. If they ever flew in the USSR, no news of it reached the west.
The design was by Waclaw Czerwinski, and followed naturally from his very successful PWS 101. The 102, Rekin (Shark), was a refinement of the earlier type. Like its predecessor it had a 19 metre wingspan with ‘gull’ dihedral. The wing profiles were different, only 13% thick and with less camber. The current emphasis was then shifting to faster flight between thermals. Such a thin wing required a very strong mainspar, and Czerwinski assumed a maximum permitted airspeed of 300 km/h. At such a speed the wing would be under very severe twisting loads, so the entire surface was covered with plywood to give torsional stiffness, with the grain of the wood running diagonally.
The wing root fittings were cleverly designed to save weight. A special alloy was used, and the upper and lower attachment points, where one spar joined the other, were well spaced out to give an effective deepening of the spar just at the point where bending loads would be greatest. The PWS 102 had ailerons of comparatively small span but very wide chord, and to achieve adequate control with light stick forces, they were slotted and aerodynamically balanced. This entailed some quite complex hingeing, on brackets fastened to the wing's auxiliary spar. The ailerons were also mass-balanced to prevent flutter. The drive mechanism was entirely hidden within the wing. The system used has since been copied, with various minor refinements, on many more modern sailplanes. A cable and chain drive turned a sprocket wheel on which was mounted a rod with a bend at its further end. The angled end of the rod slid inside a pivoted sleeve which swivelled on its bracket inside the aileron. Rotation of the sprocket wheel and its attached, angled rod, moved the aileron up or down according to the direction of the wheel’s turning.
In flight, these ailerons gave a rate of roll for the aircraft of between 5 and 7 seconds from 45 degrees bank in one direction to 45 degrees bank in the opposite direction. Inboard of the short-span ailerons, flaps occupied the rest of the wing trailing edge. When lowered 12 to 15 degrees, which was found best for circling, they reduced the stalling speed by about 10 km/h, and with 5 degrees deflection the speed for minimum sink came down from 58 to 51 km/h. There was, wrote Czerwinski, an increase of sinking speed with the flaps fully down, but this was compensated for by the corresponding reduction in turning radius, enabling the pilot to work into the core of small thermals.
All the controls connected automatically when the wings were attached. Three bellcranks, for ailerons, flaps and spoilers, were pivoted on brackets extending from a shaft between main spar and rear spar. On the fuselage, similar bellcranks, with pads on each end, interdigitated with the wing pivots, the pads coming into firm contact. Such a linkage, though simple in concept, contains some subtleties of design since the mating sets of bellcranks must all rotate on a common axis; the system will not work otherwise.
In the cockpit, pockets were provided for food and maps, the rudder pedals were adjustable, and the instrument panel could be moved back or forward to suit different pilots' preferences. The flap lever and canopy opening knobs were on the left with the trim wheel on the right. The seat itself could be moved back or forward in flight; in every respect the cockpit was very comfortable and convenient, a feature by no means usual on sailplanes of the time.
The tail unit was large for the sake of stability. Slight dihedral was built into the tailplane to keep its tips clear of the ground. The trim tab, as on the PWS 101, acted as an antibalance tab. The elevator controls connected automatically, two spur-like levers sliding into pivoted sleeves so that no horns or cables projected into the airflow. Cables were used for rudder and elevator drives, and in the wing, but rods were employed for that part of the aileron and flap control mechanism within the fuselage. Most of the metal fittings were special alloys rather than steel, all cranks, brackets and levers being made of tempered magnesium alloy castings.
Test flights indicated that all the design objectives had been achieved but the PWS 102 never had a chance to prove itself in service.
Czerwinski designed the PWS 103 as an aerobatic sailplane and the prototype flew just before he had to escape from Poland.
PWS 102: Span; 18.99 m. Wing area. 19.3 sq m. Aspect ratio. 18.68. Empty weight. 260 kg. Flying weight, 350 kg. Wing loading. 18.1 kg/sq m. Maximum permitted speed. 300 km/h. Best glide ratio. 1 : 31 measured in flight. Minimum sinking speed, 0.65 m/sec at 58 km/h. Sink at 100 km/h, 1.2 m/sec. Sink at 120 km/h. 2.25 m/sec. Stall, flaps down. 42 km/h. Stall, flaps up. 51 km/h. Aerofoils, special.