Eurasian hobby
The SZD-21 was a successor to the IS-4 Jastrząb. Development began in January 1958 as the SZD-21 Sokół, but due to problems with the flying surfaces and a wood metal sandwich, a construction of the design was restarted as the SZD-21 Kobuz ("Eurasian hobby"), than SZD-21-2 Kobuz. 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. Design team was headed by Jerzy Trzeciak, later joined by M. Gracz in Kobuz 3 constructing.