Fourth Gen­er­a­tion Fighter Trainer

De­vel­op­ing the L-39MS and L-59 Su­per Al­ba­tros

Aviation Classics - - CONTENTS -

The ad­vent of the fourth gen­er­a­tion of fight­ers with their glass cock­pit tech­nol­ogy, fly by wire con­trol sys­tems and dig­i­tal en­gine man­age­ment sys­tems re­quired a trainer that could ready the stu­dent pi­lot for the ad­vances he would find when he pro­gressed to such air­craft as the MIG-29, Su-27 and F-16.

By the mid­dle of the 1970s, pro­to­types of fourth gen­er­a­tion fighter air­craft were fly­ing in coun­tries all over the world. The end of the decade would see a num­ber in ser­vice, most no­tably the F-15 and F-16 in the US, fol­lowed only a few years later by their equiv­a­lents in the Soviet Union, the Su-27 and MIG-29. All these air­craft were ex­tremely ag­ile, had ad­vanced man­age­ment sys­tems and avionics, the in­for­ma­tion from which was con­veyed to the pi­lot on Head Up Dis­plays (HUDS) and Multi Func­tion Dis­plays (MFDS) in the cock­pit. The age of the ‘glass cock­pit’ had ar­rived and pro­vided pilots with all the per­for­mance, tac­ti­cal, mis­sion and threat in­for­ma­tion they needed in a mod­ern elec­tronic war­fare en­vi­ron­ment. Cou­pled to this, most of the new gen­er­a­tion of fight­ers had fly by wire con­trol sys­tems and ad­vanced aero­dy­nam­ics which meant fly­ing them was a very dif­fer­ent ex­pe­ri­ence to fly­ing those air­craft they re­placed. Aero was quick to recog­nise that the gap be­tween the ex­ist­ing trainer air­craft and the front line they were sup­posed to be pre­par­ing pilots for was grow­ing again, so the L-39MS pro­gramme was be­gun in or­der curb this trend and pro­duce a trainer that would al­low stu­dents to make the tran­si­tion to the new com­bat air­craft with rel­a­tive ease. The Soviet Air Force was also in­ter­ested in an im­proved trainer, cit­ing in­creased power and per­for­mance along with such fea­tures as a com­put­erised in­te­grated weapons de­liv­ery and nav­i­ga­tion sys­tem among its prime re­quire­ments for such an air­craft. The need for a more pow­er­ful en­gine was ad­dressed in a bi-lat­eral agree­ment be­tween the Soviet Union and Cze­choslo­vakia to de­velop and pro­duce a new tur­bo­fan of su­pe­rior per­for­mance to the ex­ist­ing AI-25 de­sign.

The Ivchenko-progress De­sign Bureau and En­gine Man­u­fac­tur­ing Plant in the Ukraine was to de­sign and de­velop the en­gine, which would then be pro­duced by the new Slovak en­gine man­u­fac­turer that had re­cently been built in Povázská Bys­trica, Po­vazské Stro­járne Letecké Mo­tory (PSLM). Un­usu­ally for a mil­i­tary en­gine, the new de­sign had a sin­gle stage fan, more typ­i­cal in civil ap­pli­ca­tions. Known as the DV-2, the new en­gine pro­duced one third more thrust than the ear­lier AI-25, de­vel­op­ing 4850lb (2200kg) as op­posed to 3800lb (1724kg) in the orig­i­nal en­gine. The first en­gine was run in static tests dur­ing 1984, and later dur­ing the en­gine de­vel­op­ment a mod­i­fied L-39 fuse­lage with the DV-2 fit­ted to it was mounted in the bomb bay of a Tupolev Tu-16 twin en­gined bomber fly­ing test­bed in or­der to per­form flight tests. The greater power would off­set the in­creased weight of the ad­di­tional sys­tems the new ver­sion of the trainer would re­quire and would have the ad­di­tional ben­e­fit of giv­ing the new ver­sion im­proved per­for­mance and han­dling closer to that of the new gen­er­a­tion of fight­ers. The pro­ject be­came known as the L-39MS. Aside from the en­gine and its mounts, there were many other ex­ten­sive changes to the air­frame which, although the ini­tial plan had been to main­tain a high de­gree of com­mon­al­ity with the L-39, es­sen­tially made the new ver­sion of the air­craft a new de­sign in many re­spects. The cock­pit canopy changed to a sin­gle piece de­sign hinged at the rear and hy­drauli­cally op­er­ated, which, in the event of an emer­gency, was jet­ti­soned us­ing rock­ets. In­side the cock­pit, the ejec­tion seats were changed as well, the VS-2 ‘zero-zero’ seat be­ing fit­ted in pace of the ear­lier VS-1. The avionics changed sig­nif­i­cantly with MFDS in the in­stru­ment panel and a head up dis­play to more closely re­sem­ble the new gen­er­a­tion of fight­ers, and in­cluded a new in­te­grated nav­i­ga­tion and com­mu­ni­ca­tions suite, all de­vel­oped in Prague’s Výzkum­ný a zkuśební letecký ús­tav, VZLÚ or Aero­nau­ti­cal Re­search and Test In­sti­tute. The wing was strength­ened to cope with the in­creased air­craft weight and to en­able greater loads to be car­ried on the un­der­wing py­lons to ex­pand the new ver­sion’s abil­i­ties as a weapons trainer and light at­tack air­craft. To mon­i­tor the health of the air­frame and its sys­tems, a di­ag­nos­tic sys­tem was de­vel­oped to track a wide va­ri­ety of pa­ram­e­ters and re­port their sta­tus, en­abling engi­neers to main­tain aware­ness of the over­all con­di­tion of the air­craft in ser­vice. For the first time in the de­vel­op­ment of the L-39, the flight con­trol sys­tem was changed from the sim­ple di­rect link­age me­chan­i­cal sys­tem the air­craft had been fit­ted with since the be­gin­ning. The ailerons and el­e­va­tor were now hy­drauli­cally op­er­ated by ir­re­versible ac­tu­a­tors, partly due to the more ad­vanced flight char­ac­ter­is­tics the in­crease in en­gine power im­bued in the air­craft, and partly to give the new ver­sion sim­i­lar han­dling char­ac­ter­is­tics to the new gen­er­a­tion of fight­ers. This would ease the tran­si­tion be­tween types for the stu­dents as they would al­ready be used to fly­ing with pow­ered con­trols. The first L-39MS was the X-21 pro­to­type, which had all of the air­frame, avionics and sys­tem changes but was still pow­ered by the AI-25TL tur­bo­fan. This made its first flight in 1983 be­fore the DV-2 en­gine was ready to al­low the flight test pro­gramme of the many up­grades to be­gin. The first of the L-39MS pro­to­types to fly fit­ted with the DV-2 en­gine was the X-22, which got air­borne on Septem­ber 30, 1986. This was fol­lowed by two more pro­to­types, the X-24 on June 26 1987 and the X-25 on Oc­to­ber 6 the same year. Con­sid­er­able flight test­ing was re­quired, the pro­to­types of­ten fly­ing with their rear cock­pits full of teleme­try and record­ing equip­ment to mon­i­tor the per­for­mance of the air­craft and its new sys­tems. By 1989, the first pre-pro­duc­tion air­craft was com­pleted, mak­ing its first flight on Oc­to­ber 1 and be­gin­ning a se­ries of mil­i­tary flight tri­als for the Cze­choslo­vak Air Force with the iden­ti­fy­ing num­ber 0001. In 1992, five L-39MS train­ers were built for the

BE­LOW: As part of the flight tests for the Ivchenko-progress DV-2, an L-39MS fuse­lage fit­ted with the en­gine was sus­pended in the bomb bay of a Tupolev Tu-16 fly­ing lab­o­ra­tory from the Rus­sian LII. Pavel Kucˇera

ABOVE: The in­creased power of the L-39MS with the DV-2 en­gine is ob­vi­ous in this shot of 0006 of the Cze­choslo­vak Air Force loaded with the un­der nose cannon, rocket pods and air-to-air mis­siles. Pavel Kucˇera

ABOVE: A Pslm-built Ivchenko-progress DV-2 tur­bo­fan on dis­play in the Mu­seum of Avi­a­tion, Košice, Slo­vakia. www.leteck­e­mo­tory.cz ABOVE RIGHT: Still pow­ered by the AI-25TL tur­bo­fan, the X-21 pro­to­type of the L-39MS was the first to fly in 1983. Note the sin­gle piece cock­pit canopy hinged at the rear. Pavel Kucˇera RIGHT: The third L-39MS pro­to­type, the X-24, first flew on June 26, 1987. Note the rear cock­pit is full of record­ing and teleme­try equip­ment. Pavel Kucˇera

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