The PIK-5 Cumulus is a Finnish wooden single-seat training glider developed by the Polyteknikkojen Ilmailukerho (Flying Club of Polytechnic Students, PIK) in the mid-1940s.
In August 1945 Kaarlo J. Temmes began envisaging a replacement for the relatively heavy German Schneider Grunau Babys. He chose the Polish WWS.1 Salamandra as the basis, as it was well liked by the PIK. The Salamandra's wings were carried over to the PIK-5 with very little changes, but the fuselage and the tail were substantially redesigned. The first prototype, OH-PAA, was built with funds collected from industrial companies and made its first flight on 8 September 1946. The flying characteristics were considered to be pleasant with little to criticize.
In 1947 the Finnish Aeronautical Association built six PIK-5 kits in Jämijärvi. These kits were equipped with air brakes, and designated the PIK-5a. OH-PAA was also retrofitted with air brakes later. PIK attempted to market the PIK-5a in Sweden, but without success.
OH-PAA was severely damaged on 18 August 1948. During the repairs Ilkka Lounamaa modified the original detachable forward fuselage and made it fixed, while a wheel was added to supplement the landing ski. This variant became the PIK-5b. 16 were built in Finland, and one in Sweden. Some PIK-5as were modifed into PIK-5bs.
OH-PAA crashed again on 4 July 1957, and was again subjected to modifications during the repairs, now by PIK chairman Keijo Tiusanen. Almost everything was refined, but most importantly the fuselage was made more streamlined, the wing structure was redesigned and the air brakes were enlarged. All in all, the aerodynamic shape of the wings was the only thing that had not changed. This definitive version was known as the PIK-5c.
By the time the last PIK-5s were completed in 1963, it had become the most common aircraft in Finland. The PIK-5s, also known as Koukku (Hook) and Vitonen (Number Five) were a central part of civilian pilot training.
However, the aircraft was known for its erratic, highly sensitive controls. When the old primary gliders like the Harakka were retired in late 1950s, many rookies made their first flight in the tricky PIK-5, as two-seat gliders were rare for some time. This inevitably led to many accidents, although only two of them were fatal.
Most PIK-5s were retired in the late 1960s and early 1970s. Four examples remain airworthy.
- PIK-5c OH-139 is flying.
- PIK-5b OH-151 is airworthy and on display at the Karhulan ilmailukerho Aviation Museum in Kotka, Finland.
- PIK-5c OH-188 is flying.
- PIK-5c OH-237 is flying.
- PIK-5b OH-PAR is on display at the Finnish Aviation Museum in Vantaa, Finland.
- PIK-5b OH-157 and PIK-5c OH-PBA are on storage at the Finnish Air Force Museum in Tikkakoski, Finland.
- PIK-5b SE-STZ is on display at the Jämtlands Flyghistoriska Museum in Östersund, Sweden.
- PIK-5 – The prototype.
- PIK-5a – Air brakes. Six kits manufactured, five completed.
- PIk-5b – New fuselage and landing wheel. 17 built.
- PIK-5c – Thoroughly reshaped version. 11 built.
- Crew: 1
- Length: 6.4 m (21 ft 0 in)
- Wingspan: 12.4 m (40 ft 8 in)
- Wing area: 14.7 m2 (158 sq ft)
- Aspect ratio: 10.4
- Airfoil: Göttingen 533
- Empty weight: 120 kg (265 lb)
- Max takeoff weight: 210 kg (463 lb)
- Stall speed: 45 km/h (28 mph, 24 kn)
- Never exceed speed: 190 km/h (120 mph, 100 kn)
- Rough air speed max: 120 km/h (74.6 mph; 64.8 kn)
- Aerotow speed: 20 km/h (12.4 mph; 10.8 kn)
- Winch launch speed: 90 km/h (55.9 mph; 48.6 kn)
- Terminal velocity: with full air-brakes at max all-up weight 180 km/h (112 mph; 97 kn)
- g limits: +4 -2
- Maximum glide ratio: 18 at 60 km/h (37.3 mph; 32.4 kn)
- Rate of sink: 0.85 m/s (167 ft/min) at 52 km/h (32.3 mph; 28.1 kn)
- Wing loading: 14.3 kg/m2 (2.9 lb/sq ft)