The new SZD-21-2 retained the overall shape of the original project, but was constructed with wood throughout, using multiple spars for the wings.

Flight testing of the SZD-21 Kobuz began on 3 June 1961 with Stanisław Skrzydlewski at the controls,[2] but the results were disappointing prompting a re-build as the SZD-21-2A, with a completely revised fuselage, landing gear, canopy and differential ailerons.

[1] The final version, the SZD-21-2B Kobuz 3, which was of all wood construction with very stiff wings to resist bending and flutter, was produced in small numbers in Wrocław.

Unfortunately, the Kobuz 3 was not immune to accidents and one high-profile fatal accident, at the world glider aerobatic championships at Hockenheim in Germany, highlighted problems with condensation soaking highly stressed wooden parts causing structural failure.

The pilot, Krzysztof Wyskiel, was carrying out his competition routine with high accuracy until the penultimate manoeuvre when the wings failed, clapping hands above the fuselage which shot straight down to the ground like an arrow, killing Krzysztof Wyskiel instantly.