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Source: SZD-19 Zefir

The SZD-19 Zefir (Szybowcowy Zakład Doświadczalny - Glider Experimental Works) is a single-seat glider aircraft that was designed and built in Poland from 1957.
Although they all bore the same name, the different versions of Zefir were actually quite different gliders in terms of materials used, design and performance. What unified them most was the person of the lead designer - Bugumił Szuba and the use of large Fowler flaps to get superior (for the era) performance both at low and high-speed flight. The Zefir series (in particular Zefir-3, with its L/D of 42+) is perhaps the ultimate design of the wooden glider construction era.
Intended to replace the SZD-8 Jaskółka as the Polish team mount at the World Gliding Championships in 1958 in Leszno, the SZD-19 Zefir was a high-wing glider with a wooden and glass-fibre fuselage, retractable undercarriage and all-metal wings incorporating hydraulically actuated flaps. The first prototype SZD-19X Zefir flew on 31 December 1958, missing the 1958 championships, but the flight tests revealed problems with control and stability as well as operation of the flaps and complexity of the hydraulic system.
The SZD-19 was totally re-designed with all-wooden wings, revised fuselage and tail section, and towing hook relocated to the retractable undercarriage chassis, designated as the SZD-19-2 Zefir 2, first flying on 11 March 1960, showing a marked improvement over the Zefir. Two prototypes of the Zefir 2 were built, flying in the 1960 World Championships at Köln in Germany to gain second, piloted by Edward Makula, and third, piloted by Jerzy Popiel, places in the Open class.
With significant success in the World Championships, the SZD-19-2 was refined with new stabilisers and landing gear to become the SZD-19-2 Zefir 2A, flying for the first time in 1962, in time for the 1963 World Championships, held in Argentina, where Makula and Popiel took first and second places. As a result of these successes, demand for the SZD-19-2 was significant with 14 units built, most being sold to other countries, (according to some sources, 20 SZD-19-2A units were constructed.). From the beginning until (and including) Zefir 3 these gliders did not have spoilers (airbrakes) - a tail chute was used. The chute drag could be controlled by the pilot (in versions 2B and 3).
Several records were established by the Zefir 2A/B's, including; a flight distance of 714 km (444 mi) and a 100 km (62 mi) closed triangle speed record of 102 km/h (63.4 mph).
To improve the performance, further development with wings extended to 19m was carried out as the SZD-29 Zefir 3, of which two prototypes were built, and the Open-class 19m SZD-31 Zefir 4 of which three were built. Two SZD-31 Zefir 4's competed in the 1968 World Gliding Championships at Leszno with mediocre (mainly due to the unrecognized issue of an air leak through airbrake housings) results - they took 14th and 28th place.
Because the Zefir series pushed the limits of a wooden wing construction, all construction elements were carefully designed with unusually narrow tolerance margins. Jerzy Śmielkiewicz (of the design team) describes a catastrophic in-flight wing failure
of one of Zefir's caused by
a careless minor change of the design of a few rivet placements - the whole original design was accounting for forces resulting from and/or involving individual rivets.