Yakovlev Yak-20
Yak-20 trainer and sports aircraft
In 1949 the Yakovlev OKB developed a small two-seater intended for use in air clubs as a pilot trainer and aerobatic sports aircraft. The concept behind it envisaged a cheaper and simpler aircraft than the Yak-18 that had established itself as the basic trainer in military and civil flying schools.
Designated Yakovlev Yak-20, the aircraft was a low-wing monoplane with a fixed tailwheel undercarriage; structurally it had much in common with the Yak-18. but featured a simplified design. The fuselage was based on a welded steel tube truss with fabric skinning supported by a light alloy secondary structure. The forward and rear fuselage had a duralumin skin.
The single-spar wings comprised two panels attached directly to the fuselage truss. They were covered with duralumin from the leading edge to the spar; with fabric skinning aft of it. The slotted flaps and ailerons were also fabric-covered. The tail surfaces had duralumin frames covered with fabric and were reinforced with bracing wires. The fin and rudder had angular contours. The pyramid-type main undercarriage units were made of steel tubes. Shock absorption was provided by rubber bungees. The steerable tailwheel was controlled by the rudder pedals.
The Yak-20 was powered by the newly developed 80-hp Ivchenko AI-10 five-cylinder radial engine driving a V-515 variable-pitch counterweight-type propeller. Its pitch was controlled automatically, assuming the correct setting under the influence of aerodynamic and centrifugal forces. The 80-hp output was considered sufficient for an aircraft with an AUW of 700 kg (1,540 lb).
The long-chord engine cowling was split into upper and lower clamshell halves and had a neat circular cross-section, as distinct from the helmeted cowling on the Yak-18. Two fuel tanks with a total capacity of 70 litres (15,4 Imp gal) were placed in the wing roots ahead of the spar. There was no oil cooler, the oil tank being cooled by the slipstream.
The Yakovlev Yak-20 differed from all other Soviet trainers and sports aircraft of that period in having side-by-side seating for the trainee and instructor instead of the usual tandem arrangement. This made the training more effective and helped reduce the airframe weight. A large blown canopy provided an excellent view for the crew. The aircraft had dual controls; all control surfaces were actuated by cable linkages. Both halves of the elevator were provided with trim tabs.
In the course of manufacturer’s flight tests conducted by Sergey N. Anokhin and Gheorgiy M. Shiyanov, the Yak-20 displayed exceptionally simple handling and good stability in all flight modes. The aircraft was fully aerobatic and entered a spin only at a very low speed, which obviated the need for slats. Spin recovery posed no problems. Manufacturer’s tests were conducted with the participation of several leading aerobatic sports pilots who were unanimous in their praise of the aircraft and urged its adoption as a standard primary trainer for air clubs. Indeed, the Yak-20 appeared to be eminently suitable for this role.
However, the specialists who evaluated the Yak-20 during the State acceptance trials took a different view of the requirements which this aircraft was expected to meet. Their suggestions and critical comments boiled down to raising the performance and capabilities of this trainer to the same level as those of the Yak-18. a more complicated and expensive machine (an approach some specialists later considered to have been erroneous).
The second prototype (sometimes referred to as the Yak-20-2) differed externally in having a smaller vertical tail with more rounded contours. It was built with due regard to the criticism; among other things, it was fitted with the necessary equipment for IFR flying. However, the overall result was disappointing. This machine having increased-area wings was considerably heavier than the first prototype; this resulted in a marked deterioration of performance and made the Yak-20 clearly inferior to the Yak-18, which was already in service, so there was no point in putting the new aircraft into production. Thus, the aircraft may be regarded as a victim of ill-advised ‘improvement’.
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