Sukhoi Su-27 T-10S

When it became clear in 1979 that the original T-10 could neither meet the specified performance nor match the F-15, the Sukhoi bureau decided not to put an inferior aircraft into production and set about creating a new configuration, designated T-10S in-house. The preliminary studies on how to overcome the shortcomings of the "first-edition" T-10 were carried out at the bureau and at the Siberian institute SibNIA, where they were led by chief aerodynamicist Stanislav Kashafutdinov. As the work progressed the T-10S moved further and further away from the original, until it was obvious that an almost entirely new aircraft was being designed. In the words attributed to Mikhail Simonov, all that survived from the T-10 to the T-10S were the main-gear tyres and the ejection seat. What was never questioned were the underlying principles: the integral lifting body, the statically unstable layout, fly-by-wire control and engines in isolated nacelles with intakes beneath the lifting body. The design was completed in 1980.

The wing became fully trapezoidal, with a straight leading edge and leading-edge flaps, sharp-angled root extensions and flaperons instead of separate flaps and ailerons. The ogival wingtips, which had not lived up to expectations, gave way to conventional tips carrying air-to-air missile launch rails, which doubled as anti-flutter masses and raised the number of hardpoints from 8 to 10. The fins were moved off the engines and spread wide apart on booms on either side of the nacelles, as on the F-15, at the optimum point within the vortex system generated by the wing root extensions; directional stability at high angles of attack and sideslip improved markedly, and ventral fins were added under the booms to improve spin behaviour. The stabilator changed planform and its pivot axes were shifted, which made the anti-flutter masses unnecessary.

The forward fuselage was reshaped, with a new canopy hinged to open up and back, as on the F-15, instead of sliding. The airbrake, which on the T-10 was under the fuselage and caused buffeting when extended, moved to the spine behind the cockpit. The nose gear was moved some 3 metres aft so that spray from the wheel would not enter the intakes, and the intakes received retractable screens against foreign-object ingestion and auxiliary suction doors. The central tail boom was extended with a cylindrical fairing that enlarged the rear fuel tank, and total internal fuel rose to 9.4 tonnes. The engine became the version of the AL-31F turbofan with the accessory gearbox on top, which allowed slimmer, lighter nacelles and put the accessories within easy reach from above.

The first prototype, T-10-7 (also T-10S-1, serial number 04-03), was completed at Sukhoi's experimental plant at the end of 1980, moved to the Zhukovsky flight station in March 1981 and took off on 20 April 1981 with Vladimir Ilyushin, the same pilot who had flown the first T-10 four years earlier. The same year saw the completion of the static airframe T-10-8 (T-10S-0) and the second flying aircraft, T-10-12 (T-10S-2), built at Komsomolsk-on-Amur in March 1981 for fire-control testing. T-10-7 had no radar or mission avionics and was used for handling, manoeuvring and AL-31F trials. Neither aircraft lived long: on 3 September 1981, during a maximum-endurance test near the LII, T-10-7 unexpectedly ran out of fuel and Ilyushin had to eject; the project chief was replaced and the flight engineer dismissed. On 23 December 1981, T-10-12 broke up in flight at 2300 km/h in a critical regime when the nose section failed, killing test pilot Alexander Komarov; the general designer was dismissed after the crash. In 1981 Alexei Knyshev, who had headed the bureau's branch at Komsomolsk, was appointed chief designer of the Su-27, and in 1983 Simonov became the company's general designer.

KnAAPO built further batches in 1982 (T-10-15 and T-10-17 to T-10-22) and in 1983 (T-10-23 to T-10-27). T-10-17 was the first aircraft built to full production standard and first flew on 26 May 1982. On 16 July 1983, near Akhtubinsk, during structural-strength testing at 1000 m and more than 1000 km/h indicated airspeed, the leading edge and part of one wing broke away; pilot Nikolai Sadovnikov, later Hero of the Soviet Union and world record holder, managed to land the aircraft without much of its wing and with a truncated fin, holding it at 350 km/h, nearly a hundred more than the normal landing speed. In similar circumstances T-10-21 was lost at the LII on 25 May 1984 after a structural failure of a leading-edge flap; the pilot ejected. The cause was excessive hinge moment on the slat: the wing and airframe were strengthened and the slat area reduced, and the aircraft kept receiving modifications even once in series production.

Flight-test data showed that a fighter had been created that yielded nothing to the F-15 and exceeded it in some respects. The T-10S is the direct precursor of the production Su-27; its third prototype, converted into the P-42, took the time-to-height records of the F-15 Streak Eagle between 1986 and 1988, and the whole Flanker family descends from its configuration.