KB-2 Udarnik
KB-2 Udarnik design came second at the 1947 competition mentioned on KB-1 Triglav page. The plane design was very modern and the plane was to be used for long cross country flights. To achieve this goal the flight polar of the plane should be as flat
as possible, glide ratio should decrease only slightly with speed and
sinking speed at minimum flight speed should be minimal. To help achieve
all this NACA 2b flaps were installed. After long consideration NACA
23014 was used for wing section tapering to NACA 23008 at wing tip. For
good stability in turns gull wing was chosen with 15 m (49 ft 2 1/2
in)wing span to assure mobility. The fuselage length of 7.6 m (24
ft 11 1/4 in) was adequate to provide sufficient longitudinal
stability. |
It took two years for the plans to be finished and the plane was built in 'Letov' aircraft factory. First flight took place at the Ljubljana airfield on October 29, 1949 with Milan Borisek as test pilot. First flights were successful but later it showed that the plane did not meet all expectations. Although the chosen wing section was very good in wind tunnel testing in practice the wing could not be built accurately enough to be able to use all the potentials of the wing section. The plane had an all wood shoulder-set wing covered with plywood. Flaps moved together with ailerons, inbuilt Huetter airbrakes were very efficient. Tail surfaces were built from plywood and fabric, the elevator was mass balanced by the means of a lead counter weight. Trim tab was on the right side of the elevator. |
For reasons stated above only the prototype was built. It was entered in some competitions, the 1950 National championships in Ruma being one of them, and had some good results, it's longest cross country flight measuring 290 km (162 mi). On these competitions the plane's main deficiency showed up - it's sinking speed in steep turns while thermalling was too high because of excessive wing drag. As laminar wing section data were beginning to show up at that time further development of the plane was abandoned. |
| Data | Prototype |
|---|---|
| Wing span | 15.00 m (49 ft 2 1/2 in) |
| Length | 7.60 m (24ft 11 1/4 in) |
| Wing area | 15.0 m2 (161.2 sq ft) |
| Aspect ratio | 15 |
| M-dihedral | 10o |
| Wing section | NACA 23014/23008 |
| Empty weight | 200 kg (441 lbs) |
| Max weight | 300 kg (661 lbs) |
| Wing loading | 20 kg/m2 (4.09 lb/sq ft) |
| Best glide ratio | 25 at 79 km/h (43 kt) |
| Min sinking speed | 0.57 m/sec at 63 km/h (34 kt) |
