View source for SZD-21 Kobuz

{{Short description|Polish single-seat aerobatic glider, 1961}}
<!-- This article is a part of [[Wikipedia:WikiProject Aircraft]]. Please see [[Wikipedia:WikiProject Aircraft/page content]] for recommended layout. -->
{|{{Infobox aircraft begin
 | name=SZD-21 Kobuz
 | image=SZD21-2B Kobuz3 p120829.jpg
 | caption=SZD-21-2B Kobuz 3 in the [[Polish Aviation Museum]]
}}{{Infobox aircraft type
 | type= [[Glider (sailplane)|Glider]]
 | national origin=[[Poland]]
 | manufacturer= Glider Works in [[Wrocław]]
 | designer=Jerzy Trzeciak, Marian Gracz
 | first flight= 3 June 1961<ref name=bab>{{in lang|pl}} Babiejczuk, Janusz and Grzegorzewski, Jerzy. ''Polski przemysł lotniczy 1945–1973'' (Polish aerospace industry 1945-197). Warsaw: Wydawnictwo MON, 1974, p. 126</ref>
 | introduced=
 | retired=
 | status=
 | primary user=
 | number built= 30<ref name=bab/>
 | developed from=
 | variants with their own articles=
}}
|}

The '''SZD-21 Kobuz'''  was a single-seat aerobatic [[Glider (sailplane)|glider]] designed and built in [[Poland]] at [[Szybowcowy Zakład Doświadczalny]] (Glider Experimental Works) in [[Bielsko-Biała]] from 1958.

== Development ==
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.

Flight testing of the SZD-21 Kobuz began on 3 June 1961 with [[Stanisław Skrzydlewski]] at the controls,<ref name=glass>Glass, A. (1965), p. 124-126</ref> 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. Flight tests resumed on in May 1962, but serious problems with the pitch and roll stability at high speed as well as flutter in a certain speed range prompted a further round of modifications. The resulting SZD-21-2AZ (still registered as SP-1990) continued the test program until 20 April 1963, when the aircraft broke up in flight during flutter testing, killing the pilot Sławomir Makaruk.<ref name=bab/>

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]]. It first flew on 10 December 1964.<ref name=glass/> It competed at the highest levels into the late 1980s. 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. The three remaining Kobuz 3's in the competition were grounded and remained so until the aircraft were cleared for limited manoeuvres only, curtailing their aerobatic competition careers.

In 1991 the design of a new (composite build) aerobatic glider, [[S-1 Swift]] had been finished, based on the Kobuz 3 design (the wings of the first production series of the Swift glider had been manufactured using the wings of Kobus 3 as molds).<ref>[http://www.sagach.ch/Deutsch/Flugzeuge/Swift.htm Segel-Kunstflugzeuge Swift S-1]</ref>

== Variants ==
* '''SZD-21 Sokół''' – The original designation of the project as begun in 1958.
* '''SZD-21-2 Kobuz''' – The initial prototype version, (reg'n SP-1990), which proved unsatisfactory, modified to Kobuz 2A.
* '''SZD-21-2A Kobuz 2A''' – The second version of the prototype, (reg'n SP-1990), again disappointing, modified to SZD-21-2AZ
* '''SZD-21-2AZ''' – The final modification state of the SZD-21 prototype, (reg'n SP-1990), which crashed on 20 April 1963
* '''SZD-21-2B Kobuz 3''' – The final version of the Kobuz, first flight 10 December 1964,<ref name=bab/> which proved acceptable and a small production batch were completed.

==Specifications (SZD-21 Kobuz 3)==
{{Aircraft specs
|ref=http://www.piotrp.de/SZYBOWCE/dszd212b.htm
|prime units? = met
|crew= 1
|length m= 7.25
|length ft=23
|length in=9
|span m= 14
|span ft= 45
|span in=11
|height m= 1.9
|height ft=6
|height in=2.8
|wing area sqm= 13.5
|wing area sqft= 145.3
|aspect ratio=14.5:1
|airfoil =NACA 64<sub>1</sub> 412
|empty weight kg= 311
|empty weight lb= 685.6
|gross weight kg= 401
|gross weight lb= 884
|max speed kmh=250
|max speed mph= 154.3
|max speed kts= 189
|stall speed kmh=<!-- aerobatic and STOL aircraft -->73
|g limits=<!-- aerobatic aircraft -->+7 / −5 as built – (+4 / −1.5 post 1989)
|glide ratio=30:1 (19:1 inverted) at 100 km/h (54 kts / 62 mph)
|sink rate ms=0.9
|sink rate ftmin=177
|sink rate note=at {{convert|100|km/h|kn|abbr=on}}
}}

==See also==
{{aircontent
<!-- include as many lines are appropriate. additional lines/entries with carriage return. -->
|see also=
|related=<!-- related developments -->
|similar aircraft=<!-- similar or comparable aircraft -->
* [[Vogt Lo-100]]
* [[Swift S-1]]
* [[MDM MDM-1 Fox|MDM-1 Fox]]
* [[PZL Bielsko SZD-59]]
|lists=<!-- related lists -->
}}

==References==
{{reflist|30em}}
*  Taylor, J. H. (ed) (1989) ''Jane's Encyclopedia of Aviation''. Studio Editions: London. p.&nbsp;29
* Glass, Andrzej (ed) (1965) ''Konstrukcje lotnicze Polski Ludowej ''[Aviation constructions of People's Poland], WKiŁ, Warsaw, p.&nbsp;124–126 (Polish)
{{refbegin}}
{{refend}}

==External links==
{{commons category|SZD-21 Kobuz}}
* http://www.abpic.co.uk/search.php?q=PZL-Bielsko%20SZD-21%20Kobuz%203&u=type
* https://web.archive.org/web/20090916142935/http://www.szdjezow.com.pl/ofirmie_eng.html
* [https://archive.today/20070519153309/http://luftfahrtmuseum.org/htmi/itf/szd21.htm]
* http://www.piotrp.de/SZYBOWCE/dszd212b.htm
* http://www.piotrp.de/SZYBOWCE/dszd21.htm
* http://www.sagach.ch/Deutsch/Flugzeuge/Kobuz.htm
* http://www.flightglobal.com/pdfarchive/view/1962/1962%20-%200016.html
* http://www.flightglobal.com/pdfarchive/view/1962/1962%20-%200017.html
* https://www.youtube.com/watch?v=DjwNOoqf0WM

{{Polish gliders}}

[[Category:PZL aircraft]]
[[Category:1960s Polish sailplanes]]
[[Category:SZD aircraft]]
[[Category:Aircraft first flown in 1961]]