IS-1 Sęp
The IS-1 Sęp (Instytucie Szybownictwa – gliding institute) was a single-seat high-performance glider designed and built in Poland from 1947.[1]
Development
The IS-1 Sęp (Vulture) was designed as part of the effort to re-invigorate Polish gliding after World War II, which included the IS-2 Mucha for training and medium performance, IS-A Salamander (resurrected by copying the sole survivor, with improvements) transition solo trainer and IS-3 ABC primary trainer.[1]
The first flight, piloted by Piotr Mynarski, took place on 2 June 1947, but nearly ended in disaster as the ailerons were cross-connected. Mynarski abandoned the launch and landed safely.[2] The Sęp was cleared for cloud flying, high speeds and basic aerobatics with comparable performance with foreign contemporaries such as the DFS Weihe and its derivative the Slingsby T.34 Sky. Tests were satisfactory and the prototype was taken to the international glider meet held at Samedan, Switerland in July 1947, by Adam Zientek, achieving eighth place, winning a speed task on one day.[1]
Results of flight tests and reports from pilots prompted modifications to the airbrakes, increased dihedral and increased aileron range of movement as well as a more compact cockpit. Three production IS-1 Sęp bis aircraft, and an IS-2 Mucha, were entered in the 1948 International Gliding Championship at Samedan in 1948, but were withdrawn when the Polish team was withdrawn for political reasons.[2] Competition success came in the 1948 Polish National Gliding championships, taking the first three places, and many Polish national records were broken in Sęps, including the International Feminine Record for speed round a 100 km triangle set by Irena Kempowna on 10 June 1948; 1.04.48, A. Zientek duration 18 hours 23 min; I Kepówna A. Zientek and R. Matz two Sęps fly 270 km Vienna to the Caloris Basin; 9.05.49 Justin Kępówna a record 3720 m height gain; A. Zientek 100 km triangle at 28.7 km/h; 10.06.48 I. Kępównę flew a 100 km triamgle at 50 km/h for a Polish natioanal and world record. 23.07.50 T.Gora achieved 5,737m altitude; 22.07.50 A.Zientek flew out and return 232 km for a Polish record; 11.05.50 T.Gora flew a 100 km triangle at 52.63 km/h; 23.04.53 J. Popiel flew a 100 km triangle at 68.52 km/h.[1]
The IS-1 Sęp was constructed of wood throughout with steel for high-stress areas and fittings. The fuselage was a plywood-covered semi-monocoque structure with smooth lines. Incorporated in the fuselage was the cockpit, which was covered by a built-up plexi-glas canopy, and an integral fin which supported the plywood stressed-skin tailplane just above the fuselage top decking level. The undercarriage consisted of a long pneumatically-sprung ash skid under the nose and a rubber-sprung tail skid. For take-off a jettisonable two-wheeled dolly was attached to the front skid. As with all drop-off dollies it was important to release the dolly at the right moment to avoid the dolly bouncing into the structure, causing damage.[2] An aero-tow hook was provided in the extreme nose, as well as a bungee hook and twin winch-launch hooks either side of the fuselage.[2]
The cantilever gulled wings followed normal practice with a plywood-covered torsion box leading edge, mainspar, ribs and false rear spar to support the full-span flaps and ailerons. DFS-style airbrakes were fitted at the rear of the mainspar with the upper plates hinging forwards and the lower plates hinging rearwards. A comprehensive instrument panel was fitted in the cockpit which included:-A large plaque on the instrument panel; AirSpeed Indicator (8), Vertical Speed Indicator to 5 m / sec, Vertical Speed Indicator to 15 m / sec, an artificial horizon with slip ball, altimeter, longitudinal clinometer, stopwatch, compass and oxygen pressure gauge (oxygen equipment was housed behind the pilots seat).[1]
Six IS-1 Sęps were built, with several continuing to fly into the 1960s with one surviving at the Krakow Aviation Museum.[2]
Variants
- IS-1 Sęp – The sole prototype SP-443, (Fabr no. : 006).
- IS-1 Sęp bis – Five production models with new airbrakes, increased dihedral and greater aileron range of movement. Registration numbers: SP-549 - SP-553.
Specifications (IS-1 Sęp bis)
Data from http://www.piotrp.de/SZYBOWCE/pis1.htm
General characteristics
- Crew: 1
- Length: 7.5 m (24 ft 7 in)
- Wingspan: 17.5 m (57 ft 5 in)
- Height: 1.25 m (4 ft 1 in)
- Wing area: 17 m2 (183 ft2)
- Aspect ratio: 18 :1
- Wing profile: Göttingen 549 > M12
- Empty weight: 276 kg (608.5 lb)
- Gross weight: 353 kg (778 lb) each
Performance
- Maximum speed: 225 km/h (134 mph)
- Stall speed: (40km/h / 25mph / 21kts with flaps) 50 km/h (31 mph)
- Maximum glide ratio: 27:1 @ 72 km/h (39 kts / 45 mph)
- Rate of sink: 0.68 m/s (134 ft/min)
See also
- Aircraft of comparable role, configuration and era
- Slingsby Type 34 Sky
- DFS Weihe
References
- Taylor, J. H. (ed) (1989) Jane's Encyclopedia of Aviation. Studio Editions: London. p. 29
- Simons, Martin. Sailplanes 1945-1965 2nd revised edition. EQIP Werbung und Verlag G.m.b.H.. Königswinter. 2006. ISBN 3-9807977-4-0
- "Wings and Motor" No. 30-31 (162-163), August 1949, p. 339
External links
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