Sunday, July 26, 2015

Yakovlev Yak 15

Russia’s first Jet fighter
The closing stages of the Second I World War saw as radical a change in fighter development as that brought about a decade earlier by the appearance of the first fighter monoplanes with retractable undercarriages; the turbojet began to supplant the piston engine for fighter propulsion. Both the Luftwaffe and the RAF had introduced the jet fighter operationally, and the USAAF was preparing to do so, but the Soviet Union had failed to evolve an acceptable turbojet and, with the Third Reich rapidly crumbling in front of the victorious Russian forces, it had become a matter of the utmost importance that the Soviet Union compete, at least ostensibly, in all fields of technical development. The Soviet air forces had to have a jet fighter for prestige purposes!

Copied from Germans
Fortunately for the Russians, their forces had acquired a number of German BMW 003A and Junkers Jumo 004A turbojets by the beginning of 1945, and these were hurriedly shipped back to Russian experimental establishments for examination. A crash programme was immediately initiated to expedite the development of jet fighters, and prepara- tions were made to mass produce copies of the German engines, a task in which the Russians were much assisted by Czech technicians who succeeded in passing valuable data on production techniques to Russian agents. Work on adapting the turbojets to Soviet manufacturing standards had reached an advanced stage some time before Germany's final collapse, and the subsequent capture of factories building the BMW and Jumo engines expedited the Russian programme. Captured German technicians were hastily transported to the Russian plants where production of the turbojets had started, the BMW 003 under the designation RD-20 and the Jumo 004 under the designation RD-I0 " (the prefix “RD " signifying Reaktivnyi Dvigatel or Reaction Motor), and Russian designers were already at work evolving suitable airframes.

One of the design teams allocated one or two of the precious captured Jumo 004 turbojets was that of Alexander S. Yakovlev, whose piston-engined fighters had been responsible, perhaps more than those of any other individual designer, for turning the tide of the air war over the Soviet Union. No fewer than 30,000 of Yakovlev's piston-engined fighters had been manufactured by the Russian aircraft industry during the war years, and in order to expedite the development of an interim jet fighter, he decided to use major components from his last wartime fighter design to see widespread service, the Yak-3. Although designed in parallel with the better-known Yak-9, the Yak-3 had not been introduced until 1944. Intended specifically for low-altitude combat and army co-operation, it had a smaller wing span than that of the Yak-9, and pilots who flew both the earlier versions of the Spitfire and the Yak-3 claimed the Russian fighter to be lighter on the ailerons, smoother to fly, and superior in speed and initial climb rate.

The new jet fighter, which received the designation Yak-15, employed the wings, undercarriage and tail assembly of the Yak-3, these components being married to anew, all-metal fuselage. The Jumo 0048 turbojet, rated at 1,980 lb.s.t., was mounted below the wing main spar, being fed via a small, circular intake in the nose and exhausting beneath the rear fuselage, which was protected from the jet stream 'by a convex steel " bath." Although rather crude and displaying ample evidence of its hasty design, the Yak-15's small size and light construction compensated to some extent for the low output of the turbojet, resulting in a performance that compared reasonably well with those of contemporary West- ern jet fighters. The first prototype Yak-15 was flown for the first time on April 24, 1946, and was presumably powered by one of the captured German engines, but production machines which were being delivered to units of the IA-PVO early in 1947, employed the Russian adaptation of the Jumo engine, the RD-10, and on August 3, 1947, during the annual air display at Tushino, the fighter was revealed to the public for the first time, a trio of Yak-15s giving an aerobatic display.

The Yak-15 was evidently considered as little more than an interim type by the Russians, suited only to providing the Air Forces with some jet experience and useful in building up a nucleus of trained jet pilots while more advanced designs were being investigated. The Yak-15 was reputedly extremely manoeuvrable, but the tail wheel resulted in a rather lengthy take-off run and the pilot's view from the aft-positioned cockpit was extremely limited, particularly during take-off and landing. Armament comprised two 23-mm. Nudelman-Suranov NS-23 cannon installed in the upper decking of the forward fuselage, and performance included an approximate maximum speed of 495 m.p.h. at 9,840 ft., a cruising speed of 404 m.p.h., and a range of the order of 460 miles. Overall dimensions included a span and length of 31 ft. 6 in. and 28 ft. respectively.

The impracticability of the tailwheel undercarriage resulted in the design of a nosewheel undercarriage for the fighter at an early stage in its development, the variant embodying this modification receiving the designation Yak-17. A comparatively small number of Yak-15 fighters had been delivered to the Soviet Air Forces when, early in 1948, it was supplanted on the production line by the Yak-17. The main undercarriage member attachment points were moved from the front to the rear spar, which was suitably reinforced, and a semi-retractable nosewheel was introduced. When retracted, this nosewheel was only partly enclosed, the air intake duct leaving insufficient space for a housing large enough to accommodate the whole wheel, but it served as a convenient bumper in the event of a" wheels-up " landing, and drag was reduced by a small fairing attached to the nosewheel leg. Simultaneously with the introduction of the nosewheel undercarriage, Alexander Yakovlev took the opportunity to install the improved RD-10A turbojet, rated at 2,200 Ib.s.t., and to redesign and enlarge the vertical tail surfaces, the curved fin-and-rudder assembly which, until that time, had characterised all Yakovlev fighters.

The performance of the Yak-17 was slightly higher than that of its predecessor, maximum speed being 510 m.p.h. at 9,840 ft., and a tandem two-seat version, the Yak-17UTI, was evolved as Russia's first two-seat jet conversion trainer. A second cockpit for the pupil was mounted ahead of the standard cockpit in place of the forward fuel tank, and the Yak-17UTI remained a standard conversion trainer with the Soviet Air Forces until the appearance of the two-seat MiG-15UTI. One Yak-17UTI was supplied to the Polish Aero Club and, bearing the civil registration SP-GLM, was used for several years to provide Polish reserve pilots with jet experience.

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