Due to the fact that the Su-27 differed considerably from the rest of Soviet fighters both in performances and weapons control system, navigation suite and other equipment, its twin-seat combat trainer version designated Su-27UB had to be designed. It was supposed that retaining the full avionics and weapons suites inherited from the combat version would enable the twin-seater to be used as an effective combat aircraft. Moreover, considering the Su-27's range and fuel endurance, the crew of two was supposed at times to be a better option.

The Sukhoi team had begun designing the Su-27UB (developer's designation T-10U) in the late seventies even before the testing of first new-configuration aircraft commenced. The developer was tasked with providing a high degree of unification of the single-seater and twin-seater's avionics design with the fighter retaining its superior performances. In 1980, the predesign work was completed. The Su-27UB's aerodynamic configuration mainly matched that of the single-seater. The primary differences lay in the design of the airframe fore part which incorporated a two-seat tandem cockpit, as well as in the design of the tails whose area had to be increased to preserve directional stability. To provide good visibility from the back seat, the rear part of the cockpit was elevated a little in relation to the forward part with both cockpit parts having been equipped with a single up-and-backwards opening hinged part of the canopy to ensure the ease of operation and ejection safety. Placing the backseat crew member higher than the pilot required changing the fuselage spine fairing shape above the forward fuel tank and centrewing with the aircraft acquiring a hunchback appearance. However, this allowed the plane to retain the design and configuration of the nose wheel and its well with two lateral avionics bays having been placed under the rear cockpit. This also prevented the reducing of the fuel capacity and extending the fuselage length. Each tail of the twin-seater comprised a single-seater tail panel and a 420 mm spacer that increased the area by 1.55 sq.m.

In 1984, Sukhoi prepared the Su-27UB intended for static tests followed the next year by the assembly of the first flying prototype (T10U-1). On 7 March, 1985, test pilot N.F.Sadovnikov took it to the sky for its maiden flight. The prototype was followed by the second and third prototypes (T10U-2 and T10U-3 respectively). The prototype twin-seaters were built in Komsomolsk-on-Amur with the final assembly being performed at Sukhoi's Moscow-based facilities and the large-scale manufacture established at the Irkutsk Aircraft Plant. The fly-out of the first series-made Su-27UB (T10U-4) built in Irkutsk was performed by the plant's test pilots G.E.Bulanov and N.N.Ivanov on 10 September, 1986. Soon, combat trainers began being fielded with Air Force and Air Defence Forces' line units slated to be equipped with Su-27s.

Late in 1986, the press reported new rate of climb records established by the new Soviet jet - P-42. On 27 October, 1986, pilot V.G.Pugachov climbed to an altitude of 3,000 m in 25.4 seconds while on 15 November he climbed to 6,000, 9,000 and 12,000 m in 37.1, 47.0 and 58.1 sec respectively, thus exceeding by over 2 sec the decade-long records set by American pilot R.Smith flying the F-15. The records were set in two classes simultaneously - both in the class of jet aircraft and in that of 12-16 t service aircraft. The latter raised quite a few brows among experts who realised fast enough that the P-42 designation was applied to the new fighter - the Su-27. The point is the 20 t fighter could hardly fit the 16 t aircraft class, and, as was revealed later, the IAF protocols had the P-42's take-off weight at 14,100 kg, which is as much as two tonnes lighter than an empty Su-27. This was simple enough: to set records, Sukhoi's leadership took a decision to use one of the first series-built Su-27s - the T10-15 that by then had completed its testing programme. According to Designer General M.P.Simonov, the aircraft's unusual designation was used due to the following reason: "it was designated P-42 to commemorate the turning point of the Stalingrad battle in November 1942. Then, the Soviet aviation's role in defending that stronghold was crucial." The plane was stripped of the weapons control system and radar to make it fit the weight constraints. Besides, it had the central tail boom shortened, tail area reduced, drag chute and fences removed, wing high lift devices keyed, radar nosecone was replaced with a lighter metal one. Some other measures to reduce weight were taken too. To perform the record flights, the aircraft had only a limited amount of fuel in the tanks, that was enough only for take-off, achieving the goals and landing. The designers managed to soup up the engines with each having its thrust increased by over 1,000 kg/f (the R-32 engines and the thrust of 13,600 kg/f were included into IAF protocols ). The measures taken allowed the designers to achieve a unique take-off thrust-to-weight ration totalling nearly 2. Due to this, the P-42 was capable of accelerating and even crossing the sonic barrier during the vertical climb.

T10-15 ( P-42 ) record aircraft

However, the aircraft's great thrust-to-weight ratio evoked a very peculiar problem: the P-42's brakes were unable to hold it at the starting position when the engines went in afterburner. So, a rather ingenious solution was made: the plane was linked up with a towrope and a special lock to a powerful tractor protected from the exhausts with an armoured plate. The tractor would go to the runway and keep the aircraft from the premature run by its impressive weight. At a predetermined moment, the lock would separate the towrope from the plane, the cameras and stopwatches would be switched on and the P-42 would rush to set new world records. The work on the P-42 record preparation was headed by leading engineer (later - Chief Designer) R.G.Martirosov. On 10 March, 1987, the P-42 flown by N.F.Sadovnikov exceeded its own 9 and 12 km climb records by another three seconds - 44.2 sec and 55.5 sec respectively. On the next day, the aircraft was used in the STOL class. The rate of climb to an altitude of 3, 12 and 15 km was 25.4, 57.4 and 75.7 sec respectively. On 10 June, 1987, the P-42 flying as a STOL-class aircraft set a record for the level flight altitude that amounted to 19,335 m. The last of the official records set by V.G.Pugachov while flying the P-42 was a climb to 15,000 m in 81.7 sec with a weight of 1 tonne. All in all, the aircraft had established 27 world records during 1986-1988 with world record-holder diplomas having been issued to V.G.Pugachov, N.F.Sadovnikov, O.G.Tsoy and Ye.I.Frolov.

The Soviet press was first to penetrate the wall of secrecy shrouding the Su-27 fighter with its reports on the P-42 records. Truth be told, as early as in summer 1985, the Central TV run a documentary on Designer General P.O.Sukhoi to commemorate his 90th birthday. A 10-second-long fragment about the new fighter was included with a few frames showing the take-off of the T10-1 prototype. In the same year, the first aircraft was given to the Air Force museum in the town of Monino (Moscow region). Two years later, first pictures of series-built Su-27s began cropping up in the Soviet aviation and military press. In autumn 1987, the first detailed photo report featuring close-up pictures of a missile-carrying Su-27 appeared in the western press. The pictures were taken under rather unusual circumstances. On 13 September, 1987, a Norwegian Air Force Lockheed P-3B Orion was monitoring a group of Soviet warships in the Barents Sea neutral waters. The pilot of a Su-27 on station nearby was tasked with a mock interception to prevent Orion from deploying buoys. The Orion crew closed up on the Su-27 to oust it out of the patrol area, let the Su-27 under its belly and cut the speed. The fighter pilot did the same. The Orion crew lost track of it since they had underestimated Sukhoi's manoeuvrability. As a result of dangerous manoeuvring, the aircraft collided: the fighter's tail tip was hit by one of Orion's propellers with the latter disintegrating. The fragments pierced through the fuselage of the recon plane and depressurised it. Thanks God, no casualties were suffered - Orion went shorewards while the Su-27 RTB'd safely.

Victor Pugachev test pilot

In autumn 1988, the policy of glasnost declared in the USSR became effective as to the combat aircraft. At the prominent international airshow in Farnborough (UK) the Soviets unveiled two warplanes - the MiG-29 fighter and the MiG-29UB twin-seat combat trainer. The unprecedented demonstration of that advanced Soviet fighter left the world public and western businessmen quite impressed with real prospects of arms export contracts looming. Satisfied with the success at Farnborough, the Soviet leadership took a decision to display several combat aircraft at once at the upcoming Le Bourget airshow. There were two Su-27 fighters - a single-seater (tail number 388) and a combat trainer twin-seater (tail number 389). Early in July 1989, the fighters flew to Paris. Authoritative western experts called the supersonic Su-27 fighter the star of that airshow. The public was immensely impressed with the aerobatics performed by Hero of the Soviet Union test pilot V.G.Pugachov. The zest of the performance comprising the sequence of advanced and aerobatics manoeuvres was a unique, inaccessible to any other aircraft in the whole world manoeuvre - the so-called dynamic deceleration later called Pugachov's Cobra after the first performer. The gist is the following - the aircraft accelerates, pitches up its nose without going upwards and carries on moving forward with the angle of attack reaching 120 degrees. In fact, the plane flies tail-first. In a few seconds, it decelerates down to 150 km/h, drops its nose and returns to the level flight. It's a unique manoeuvre unmatched by any other combat aircraft. According to experts, dynamic deceleration could be used in aerial combat while engaging the target from a disadvantageous position, for example, while launching missiles into the rear hemisphere.

Flight testing of dynamic deceleration with taking the plane to extreme AoAs had been held by V.G.Pugachov since February till May 1989. He was flying the T10U-1 prototype twin-seater fitted with an counterspin chute and counterspin rockets to enhance safety. On 28 April, 1989, test pilot Pugachov showed the famous "cobra" to experts at the LII Flight research Institute. At an altitude of 500-1,000 m the pilot performed about 10 such manoeuvres in three passes. All in all, during the testing, dynamic deceleration had been performed about 1,000 times, which allowed it to be included into aerobatics.

The skies over France witnessed great success of Soviet aircraft. Here is what Reuters reported in June 1989: " Looks like the competition between Soviet and American fighters in the skies of Le Bourget has been won by the Soviet Union. The Russians succeeded thanks to their snake-like aircraft whose promising design and ease of control shocked the experts". In August 1989, the Su-27 aerobatics programme was shown to the Moscovites during the aircraft displays dedicated to the Soviet Aviation Day in the towns of Tushino and Zhukovsky. The Su-27 and Su-27UB fighters were demonstrated during the aircraft exhibition held at the Khodynka Central Airfield. Since 1989, the Su-27 has been a permanent participant in all important airshows, such as ones held in Zhukovsky, Le Bourget, Farnborough, Dubai, Singapore, etc.

While the Su-27 was establishing world records and winning the applause of airshow visitors, the Sukhoi design bureau headed by Designer General M.P.Simonov carried on with the hard work on refining the fighter's design. One of the variants to advance the fighter's configuration provided for fitting it with foreplanes. This idea cropped up for the first time as early as in 1977, but the wind tunnel testing revealed some problems with longitudinal control in certain flight modes. The problems were resolved when a new version of foreplanes was developed in 1982. That variant was yet to be tested in flight. One of the first series-made Su-27s - T10-24 - was used as a flying lab. After it had been fitted with foreplanes, in May 1985, N.F.Sadovnikov commenced its test-flying.

As is known, the Su-27 can fly at a very high angle of attack. However, there is a critical AoA, at which the empennage finds itself in the wing-generated vortex sheet with its effectiveness slumping sharply. In this case, deflecting the stabiliser at a maximum angle with its forward edge deflected upwards may be insufficient to create a required negative pitching moment that would return the aircraft in a normal position. It is this case when the foreplanes could come in handy. The T10-24 had them deflecting automatically in proportion to the AoA. The foreplanes increased the fighter 's instability, which provided a greater g-load while required no increase in the strength of the wings and empennage and eliminated problems with balance during manoeuvring. The foreplane-equipped T10-24 testing displayed a considerable increase in maximum lift, caused by favourable interference of the foreplanes and lifting body due to their configuration. Thanks to those advantages, it was decided to incorporate the foreplanes into the aerodynamic configuration of new Su-27 versions, first of all, the navalised Su-27K and upgraded Su-27M.

To increase the fighter's range, the work on providing the Su-27 with in-flight refuelling capability was initiated in the mid-80s. The first experimental in-flight refuelling probe was mounted on the T10U-2 combat trainer prototype. The refuelling was supposed to be performed both from same-type aircraft fitted with the standardised UPAZ refuelling unit and UPAZ-equipped Su-24M bombers as well as series-built Ilyushin Il-78 Midas tanker planes. In June 1987, Sukhoi's test pilots N.F.Sadovnikov and I.V.Votintsev made two long-range non-stop sorties flying the T10U-2 from Moscow to Komsomolsk-on-Amur (16 June) and back (19 June) with the 13,440 km-long non-stop flight from Moscow to Komsomolsk and back following on 23 June, 1987. During that flight lasting 15 hours 42 minutes, the pilots made four refuellings (vic. the cities of Novosibirsk and Chita). Soon, the T10U-2 escorted by Air Defence Forces' line unit Su-27s flew to the northernmost Soviet airfield - Graham Bell situated on the Land of the Franz-Josef. The in-flight refuelling system tested on the T10U-2 was later mounted on later versions of the Su-27.

T-10-26 flying laboratory

Su-27s were widely used as flying labs for testing other new aircraft systems. Thus, they were employed to develop the thrust vector control (TVC) system and test the controlling of the aircraft via the side potentiometer-type throttle. The flying lab derived from the T10-26 was used to test the asymmetric swivelling nozzle. In 1990, the T10U-5 combat trainer was converted into a flying lab whose left engine was fitted with a flat variable-geometry nozzle. The aircraft control via the side throttle had been tested since 1989 on the LMK-2405 flying lab retrofitted from a series-produced Su-27. The results of those and other tests have been used in designing new versions of the fighter and prospective combat aircraft. However, the thrust vector control and the side throttle have been incorporated into the Su-37 fighter.

In autumn 1989, the sea trials of the first Soviet heavy through-the-deck cruiser (TAVKR) Tbilisi (currently Admiral Kuznetsov) commenced. Kuznetsov's main armament was to be composed of fourth-generation fighters. Development of such aircraft designated Su-27K and MiG-27K was being performed by the two leading Soviet design bureaux in line with the governmental resolution of 1984. However, the idea of navalising the prospective Su-27 frontline fighter first cropped up as early back as in 1973 when the consideration was underway of building a multipurpose large-displacement nuclear aircraft carrier fitted with a catapult for launching various aircraft. The change in the plans for further development of the Soviet Navy resulted in the removal of the full-size carrier issue from the agenda. Instead, in 1977, the decision was made to carry on with the Project 1143 aircraft-carrying cruisers with the fifth ship of this type provided with a Su-27K unit as well as some other planes. The catapult-assisted take-off was retained. By 1978, Sukhoi had prepared its proposals for the development of a ship-borne version of the Su-27 (then still in its initial configuration). The navalised Su-27 fighter powered by two AL-31F engines was supposed to have a regular take-off weight of 22.8 tonnes, which was 26.6 tonnes when the warload was mounted. The maximum warload was two K-73 short-range air-to-air missiles and six K-27E medium-range air-to-air missiles as well as 150 rounds for the integral automatic cannon. Topped up to the full extent (7.7 t), the Su-27K had a range of 1,150-1,270 km with a CAP endurance of about two hours at a distance of 250 km from the carrier. 18 Su-27K and 28 MiG-29K fighters as well as 14 Ka-252 helicopters were supposed to be deployed with the carrier-borne air task force.

Su-27K carrier-borne fighter

In 1981, the carrier fleet development programme was corrected once more. That time it was decided that only VTOL aircraft were to stay. So, the take-off catapult was cancelled too. However, considering the backlog of experience in developing the fourth-generation ship-borne fighters featuring high thrust-to-weight ratio, Sukhoi and Mikoyan's brass supported by LII and Air Force's representatives suggested the Su-27K and MiG-29K fighters be not removed from the carrier and promised to provide them with catapultless take-off via a take-off ramp to be installed at the nose of the carrier. The Su-27 and MiG-29's catapult-assisted take-off capability was to be confirmed by ground tests at the ground-based Nitka training facility which had been built by then in the Crimea.

In 1982, Sukhoi prepared the T10-3 prototype fighter for the Nitka facility testing. On 28 August, 1982, test pilot N.F.Sadovnikov performed the first take-off from the T-1 experimental jump-off ramp. The 18.2-tonne aircraft run made up 230 m with a lift-off speed of 230 km/h. Later on, the run was reduced down to 140 m with a lift-off speed of 180 km/h, max take-off speed of the T10-3 having been made 22 tonnes during the ramp take-off. In August 1983, Sadovnikov commenced testing the T10-3 at the ground-based arrestor facility. In summer 1984, this was joined by the first series-built new-configuration aircraft - T10-25 - fitted with an arrestor. On 30 August, 1984, V.G.Pugachov flew the T10-25 to perform the first arrestor-assisted landing followed on 25 September, 1984 by N.F.Sadovnikov who was first to take that aircraft off the new T-2 jump-off ramp whose design matched that of the carrier being built then. Unfortunately, on 11 November, 1984, the T10-25 crashed due to the hydraulic system fatigue failure (Sadovnikov managed to punch out). In 1986, the Nitka facility testing was joined by the T10-24 prototype outfitted with foreplanes, as well as a two-seat T10U-2 outfitted with an arrestor.

By then, the work was in full swing at Sukhoi on assembling the first navalised fighter prototype designated T10K by the developer. The ship-borne fighter differed from the series-produced counterpart in certain design features. It incorporated the foreplanes tested on the T10-24. The wing was redesigned with its panels having been made foldable; the single-section flaperon lost the ground to two individual control devices - flap and aileron. The nose strut and main struts were reinforced with the nose strut becoming telescopic and two-wheeled, while a hook was mounted under the reduced tail beam. Due to this, the airframe as a whole was reinforced too. The nose part of the fuselage was fitted with an extendable in-flight refuelling probe installed in front of the cockpit with the IR sensor having been shifted to the right of the centreline. To diminish the size of the fighter for it to occupy less space in below-deck hangars, beside making the wing planes foldable, the fighter's design provided for folding the empennage panels; the tail beam casing and nosecone could fold upwards too (Sukhoi prepared one of the first series-made Su-27s - T10-20 - as the T10-KTM design mock-up for the ship-borne headroom testing). The navalised version's avionics was bolstered with systems providing the flying over the water and landing on the carrier. The Su-27K's weapons control system was borrowed from the Su-27 without any significant modifications. The ship-borne fighter's weapons suite matched that of the baseline version, though the number of hardpoints totalled 12 with the maximum warload weight growing up to 6,500 kg. A further installation of anti-ship missiles at the Su-27 was planned. The Su-27 ship-borne version development was headed by Sukhoi's Chief Designer K.Kh.Marbashev.

Test pilot V.G.Pugachov was first to take the T10K-1 ship-borne fighter prototype to its maiden flight on 17 August, 1987. At first, the plane featured the standard wings the Air Force Su-27s were equipped with. It was fitted with the set of folding wings in August 1988. However, on 28 September, 1988, it crashed during the imitation of the failure of a hydraulic system with pilot N.F.Sadovnikov ejecting safely. The testing was carried on through the use of the second prototype (T10K-2) that made its maiden flight on 22 December, 1987. It is that aircraft that Pugachov landed on the deck of the vessel for the first time in the history of the Soviet aviation and Navy. Soon, Pugachov was followed first by other test pilots and then by military pilots. In 1989, the series production of the Su-27K commenced at the Komsomolsk-based KNAAPO aircraft production association plant with the Sukhoi design bureau designating the new aircraft as Su-33. The first series-built aircraft (T10K-3) was taken to the skies for the first time on 17 February, 1990 by test pilot I.V.Votintsev. In 1991, the operational trials of the Su-27K on the carrier commenced to be end up in adopting the fighter into the inventory three years later. By late 1994, all 24 Su-27K were redeployed with the Northern Fleet the Admiral Kuznetsov carrier was organic to and became part of Kuznetsov's fighter regiment. Since the late 1980s, Sukhoi has been developing the Su-27KUB two-seat combat trainer. Unlike the Air Force's twin-seater, it is going to have the crew seated side by side, which is expected to make it easier to train naval pilots to land on the deck. The Su-27KUB prototype's flight testing began in spring 1999. The maiden flight was performed on 29 April, 1999 by test pilots V.G.Pugachov and S.N.Melnikov.