The Power to Train
From the Ivchenko AI-25 to the Williams FJ44-4M
The vast majority of L-39s built used the first engine to power the trainer, the Ivchenko AI-25. As greater performance was required in the later models of the aircraft, this was replaced by other engines from both the Ukraine and the US.
THE IVCHENKO-PROGRESS AI-25 The development of one of the first small turbofans to be produced in the Soviet Union began in 1965 and was intended to produce a suitable powerplant for a new class of rugged regional jet airliners. Aeroflot required a new type of transport to replace its ageing fleet of piston-engined airliners on local routes, aircraft such as the Ilyushin Il-12 and Lisunov Li-2. The new airliner had to be rugged to operate from the many grass airfields on the internal routes in the Soviet Union, and powered by a simple, reliable and easy to maintain jet engine. This also had to be able to operate from basic airstrips with no external support equipment, so an auxiliary power unit to start the engines was also required. The Yakovlev design bureau settled on a 32 seat, three crew airliner with a straight wing for short field performance and a three engined layout for redundancy. To power the new design, given the designation Yak-40, a lightweight engine that produced around 3300lb (1497kg) thrust was needed. The Ivchenko-progress Design Bureau and Engine Manufacturing Plant at Zaporozhye in the Ukraine were already working on the AI-25, which was initially designed with the airliner in mind. Aero required a very similar engine for the new L-39 trainer, particularly in terms of reliability, simplicity and ease of maintenance. However, a number of modifications were required to the civil engine, for example, to enable it to withstand the high G forces required in military training. The former
Walter engine factory at Jiinonice in Czechoslovakia, now knnown as Motorlet, was given the task of producing the AI-25W, a modified version of the engine to power the prototype L-39s and allow flight testing to begin. The AI-25 was the firstt turbofan to be considered forf a military trainer, and consissted of three fan or low pressure sttages followed by nine high presssure compressor stages then a ringr of annular combustion chambbers. The last element of the engine was the turbine that consisted of twwo high and one low pressure stages. Fitted witth the AI-25W, the second prototype L-39, X-02,2 was theh fifirst to flyfl on November 4, 1968, when test pilot Rudolf Duchon took off from the Aero factory airfield at Vodochody. The prototype and pre-production versions of the L-39 were all powered by the AI-25W, but problems with the engine were to delay the delivery of the first 15 production standard aircraft. The test programme had already proved that the engine was otherwise ideal for the trainer, so Ivchenko-progress, who had been kept informed throughout the project, had developed a version of the turbofan specifically tailored to the performance requirements of the trainer, the AI-25TL. The clearances between the turbines and engine casing were increased and a special lubrication system was fitted to enable normal operation under zero and negative G forces. The new version was fitted to the X-02 and X-07 prototypes to begin flight testing in 1971, passing Aero’s and the Czech Air Force’s trials in 1972 and the Soviet Air Force’s state verification trials in 1973. The AI-25TL became the standard engine for the L-39, and a smaller version, the AI25TLK was later produced for the Chinese K-8 trainer. Both versions produce 3792lb (1720kg) of thrust, achieving this increase over the earlier AI-25 through higher compressor pressure ratios and increased turbine inlet temperatures. After the dissolution of the Soviet Union, in the 1990s Ivchenko-progress and the engine manufacturer Motor Sich had developed an improved version of the engine, the AI-25TLSH. This was intended for upgraded versions of the L-39 developed in the Ukraine and flight tested in 2002. The new version could be produced by upgrading existing AI-25TL engines at the factory with a new compressor, high pressure turbine stage, fuel control and electrical systems among many other detail changes. The changes saw the thrust increase to 4078lb (1850kg), with a reduced throttle response time and improved fuel consumption. The companies are currently marketing the improved engine to L-39 operators around the world. Altogether over 9500 of all variants of this incredibly successful engine have been produced and are still in service today all over the globe. LOTAREV DV-2 The development of the L-39MS and L-59 variants of the Albatros gave rise to the need for a more powerful engine to provide the performance required of the improved trainer. Interest from the Soviet Air Force in the project saw the signing of a bi-lateral agreement betwween the Soviet U Czechoslovakianion and nc to develop and produce the new turbofan. Development would be the responsibility of the IvchenkoPprogress Design Buureau and Engine Manuufacturing Plant in the Ukrainee under their new design leader, Voloodymyr Lotarev, the engines he designed beeing known by his surname rather than thaat of the previous chief designer, Alekksandr Ivchenko. Pre-production and productioon of the new engine would take place at thhe new Slovak Ppovazskéké Strojárnesjá Lleteckéké Motory (PSLM) engine factory which had been built at Povázská Bystrica. Initially basing his concept on the AI-25, Lotarev decided on a new approach to the military engine by taking his design philosophy from civil turbofans of the type commonly produced for transport aircraft. Usually in military engines, a multistage fan had been the common practice, whereas in larger civil turbofans, a single stage fan sufficed to provide the necessary pressure ratio. Lotarev elected to fit a single stage fan as he desired a low specific thrust cycle for the new engine, so an efficient single stage fan that could produce a higher than normal pressure ratio would be sufficient, only with a higher airflow bypass ratio than was common in military engines at that time. The DV-2, as it was now known, also differed from the AI-25 in being a twin spool design consisting of a single stage fan, two stage low pressure compressor, seven stage high pressure compressor, an annular combustion chamber with 16 fuel injectors, then a single stage high pressure and two stage low pressure turbine. In this form, the DV-2 produced one third more thrust than the AI-25, developing 4850lb (2200kg) as opposed to the 3800lb (1724kg) of
ABOVE: The ancillary drives and systems were largely mounted below the AI-25TL for ease of access when mounted in the aircraft. Jakub Fojtík BELOW: The Ivchenko-progress AI-25TL was an incredibly successful engine with more than 9500 built. Jakub Fojtík
ABOVE: The Lotarev DV-2 powered both the Aero L-39MS and L-59. Jakub Fojtík RIGHT: The Lotarev DV-2 was based on the AI-25 but featured a new single stage fan among other changes. Jakub Fojtík BELOW RIGHT: A cutaway of the Garrett, now Honeywell, TFE 731, the engine that powered the sole L-139. Interestingly it was exactly the same diameter as the AI-25. Honeywell