A team of developers from the Kulon Machine-Building Plant headed by Pavel Osipovich Sukhoi initiated in 1969 the development of an advanced new-generation fighter for the Soviet Air Force and Air Defence Forces' aviation. By the mid-seventies, a general concept of the new fighter was formulated in co-operation with a number of science and research institutions. This concept provided for a high-manoeuvrability long-range fighter with a powerful weapons system and a perfect sighting/navigation system that would allow the pilot to participate efficiently in both long-range missile exchange and dogfight. The basic performances of the new fighter were to be of top class globally while a number of characteristics was to exceed those of its American F-15 counterpart the American leadership was reasonably vesting its high hopes in. The design bureau was planning to introduce a number of major innovations and solutions into the design of the new fighter designated T-10.

In 1970, the Sukhoi design bureau developed the first version of the fighter airframe incorporating key features of the integrated aerodynamic configuration. The aircraft was to have a lifting body featuring a smooth coupling of the wing and fuselage, two turbojet engines located in isolated belly-mounted engine nacelles and two vertical stabilisers. Such an integrated design allowed a significant improvement of the fighter's aerodynamics and provided for a more spacious interior to accommodate fuel tanks and various equipment. For the fighter to achieve intended flight performances within a wide range of altitudes and speeds, the new fighter's wing had ogyval shape and a leading edge extension. According to the developers, the leading edge extension was supposed to ensure required high-lift characteristics during the aerodynamic focus shifting at supersonic speeds and generate vortice increasing wing, tail unit and control surface efficiency. At the same time, Sukhoi was engaged in developing a traditional non-integrated version of the same fighter powered by two adjacent engines, which were located in the fuselage's aft part, and fitted with side-mounted air intakes and two vertical stabilisers. In 1972, both versions were submitted to the Air Force board for examination and consideration. The board was to assess proposals submitted by three aircraft design bureaux (Sukhoi, Mikoyan and Yakovlev) regarding development of an advanced tactical fighter slated for entering the service with the Soviet Air Force in the early eighties.

As a result of thorough evaluation and assessment of all versions submitted for consideration, the proposals presented by Sukhoi and Mikoyan were approved by the board for further development. The Su-27 (T-10) programme was to be developed into a heavy multirole advanced tactical fighter, while the MiG-29 programme was to produce a series-built light advanced tactical fighter. Among the major combat objectives the both fighters were to meet, there was the dogfight and medium-range aerial combat capability, interception of aerial targets in the front and rear hemispheres both against the sky and ground as well as the secondary task of ground target destruction. The Su-27, which had a better fuel endurance, more weapons load and more sophisticated navigation, self-defence and communication systems, was intended to operate independently within an air group deep behind the enemy lines at an operational-tactical range of up to 250-300 km, while a lighter and cheaper MiG-29 would operate only at ranges of up to 100-150 km into the enemy-held territory. The Su-27's weapons control system was expected to ensure successful countering the F-15 fighter, the most capable fighter of that time available to the potential opponent, as well as successful engagement when outnumbered by less capable but numerous fighters (for instance, YF-17, YF-16 and J-6). Besides, the Su-27 fighter was intended for entering the service with the Soviet Air Defence Forces after its appropriate reequipping and rearmament.

Proceeding from the updated and more specific requirements the client set for the tactical fighter of the eighties, Sukhoi started in 1972 developing a preliminary design of the T-10 fighter subsequently followed by the conceptual design stage. During 1970-75, over 15 options of the fighter configuration were given consideration. Those configurations varied not only in general approaches to the problem (integrated or classic configuration) but differed in solutions suggested for individual problems (engine and air intake positioning, landing gear design, type of the control system, etc.). Finally, preferences were given to the aircraft design with an integrated statically unstable configuration. The Su-27 was to become the first Soviet unstable fighter with longitudinal in-flight balancing to be provided by a fly-by-wire control system. Accepting the longitudinal static instability concept (in other words, "electronic stability") promised a lot of advantages: for the aircraft to balance at a high angle of attack, the leading edge slat's upward deflection was required with its lift being added to the wing lift, which allowed significant improvement of the fighter lift with a minor increase in its drag. Due to the use of the integrated statically unstable configuration, the Su-27 was to acquire outstanding manoeuvrability allowing the fighter altitude changes unavailable to typical configuration fighters and an increase in its range of up to 4,000 km without external fuel tanks. There were no other series-produced fighter in the world boasting similar performances.

T-10-1
The first Su-27 prototype

The fighter's aerodynamic configuration featured a regular scheme with the empennage being situated behind the wings on the load-carrying beams on the outer sides of the separated engine nacelles. The aircraft had twin tails mounted on the engine nacelles. The plane was to be controlled by the all-moving horizontal stabiliser whose panels could deflect in different directions, as well as by ailerons and rudders. The wing high lift devices included flaps. To enable the powerplant to produce the optimal performances at any speed and altitude, the air intakes mounted under the centrewing were made controllable by the horizontal ramp. The fore part of the fuselage housed an integral radar covered by a radio transparent nosecone as well as the cockpit featuring a canopy that provided good visibility in all directions.

The landing gear featured a classical tricycle design with the nose strut having been moved far forward and the gear well placed under the cockpit. The main gear struts were made forward-retractable into centrewing gear wells with the wheels being turned in the process and the wells' forward doors serving as air brakes.

The Su-27 fighter powerplant included two powerful and economical AL-31F bypass turbofan engines producing thrust of 12,500 kg each. They were developed by the Saturn Mechanical Plant headed by Arkhip Mikhailovich Lyulka and supposed to ensure take-off thrust-to-weight ratio exceeding 1. Low specific fuel consumption along with about eight tonnes of fuel stored in internal tanks filling most of the airframe inner space was to ensure required operational range of the fighter.

T-10-1 The first Su-27 prototype

The Su-27 fighter weaponry which included 30 mm high-rate cannon, K-27 medium-range air-to-air missiles and K-73 or K-14 short-range missiles was to be standardised with that of the MiG-29 light tactical fighter. The only difference in armament was the warload carried: while the MiG-29 could house only six missiles, including two K-27 missiles, the Su-27 was capable of carrying eight, including four K-27s, as well as the K-27E improved longer-range radar-homing and heat-seeking missiles.

An integrated weaponry control system of both fighters was substantially standardised and, for the first time in the history of aviation, included two complementing channels - a radar sighting system and an optoelectronic sighting system, as well as a helmet-mounted sight. The sighting system initially designed for the Su-27 fighter had better characteristics. The N001 radar was built by the Phazotron Research and Development Association under the guidance of Victor Grishin. The OLS-27 optics-based detection and ranging system combining an acquisition/tracking IR locator and a laser rangefinder was developed by the Moscow-based Geophysics Central Design Bureau headed by Chief Designer D.Khorol.

Vladimir Ilyushin test pilot

The Su-27 fighter conceptual and detailed design stages were completed in 1975-1976, and after issuance of required design documentation and drawings, the manufacturing of first prototypes was initiated at the Kulon plant. Unfortunately, Pavel Sukhoi did not live long enough to see the fighter - he died in 1975 to be succeeded by Yevgeny Ivanov. From 1976 on, the Su-27 programme has been under immedeate guidance of Chief Designer Mikhail Simonov. The first prototype designated T10-1 was completed in early 1977. Due to the lack of the Al-31F turbofan bypass engines, the prototype was equipped with two AL-21F-3AI engines - a modified version of the series-production AL-21F-3A engine mounted on other Sukhoi-developed fighters (Su-17, Su-24). After the completion of required ground checks and run-ups, everything was ready for test flights and on 20 May, 1977 Vladimir Ilyushin, Sukhoi's chief pilot, took the T10-1 off the ground for its maiden flight. The first prototype was used to evaluate its key flight performances as well as stability and controllability.

In 1978, the second prototype designated T10-2 was completed though its life was brief. On 7 July, 1978, the fighter suffered a mid-air accident resulting in death of test pilot Yevgeny Solovyov. The crash was triggered by the plane's oscillation in the longitudinal plane at a supersonic speed, due to which it suffered an extremely high g-load and disintegrated in midair. The black box recorder readings showed that the fighter got into an unexplored resonant mode, which caused the mid-air disintegration of the aircraft. This happened so fast that Yevgeny Solovyov, distinguished test pilot and Hero of the Soviet Union, had no chance to eject. The after-action review established the actual reasons for the tragedy and allowed the developer to introduce necessary changes into the fighter's design. During 1978, preparations for preliminary production of the Su-27 were made at the Gagarin machine-building plant situated in the Far Eastern city of Komsomolsk-on-Amur. At the same time, the Sukhoi mechanical plant commenced assembling another two Su-27 prototypes. Unlike the first two prototypes, those two fighters were to be powered by AL-31F engines. The new powerplant was 500 kg lighter, its thrust was 12 percent higher than that of the predecessor with fuel consumption being lower. In comparison with the AL-21F-3s, the AL-31F engines had smaller diameter and length while their nozzles were accommodated with a secondary (bypass) duct control system. On 23 August, 1979, Vladimir Ilyushin took the T10-3 off for its maiden flight. Two months later, the T10-4 joined the flight testing too. At first, both aircraft were used for in-flight engine testing. Then the T10-3 was remodeled to be further tested on the Nitka training facility in support of the development of the Su-27 ship-borne version while the T10-4 was used for the further refining of the weapons control system.

By the early 1980, as many as three prototypes (T10-1, T10-3 and T10-4) had been taking part in the Su-27 testing programme with first pre-series aircraft expected to join soon. It seemed that everything was in line with the schedule and the new fighter would have been fielded in a couple of years. However, there were staunch objections against commencing series production of the aircraft in the then airframe configuration, made by the Siberian aviation research institute's (SibNIA) aerodynamic experts and... Chief Designer Mikhail Simonov himself. According to the SibNIA experts who had done the bulk of aerodynamic research under the Su-27 programme, several mistakes were made during the fighter development stage. The combination of the accepted wing planform and leading-edge extension configuration caused premature vortex flow separation: the non-stationary airflow around the wing would begin as early as at the 8-to-10 angle of attack (AoA), which would cause deterioration of the airframe's lifting capability, buffeting and decrease in lateral stability. The tail unit configuration designed for the T10 would fail to provide required effectiveness of the longitudinal control surfaces, lateral and directional stability devices. The SibNIA-held T-10 wind tunnel tests performed in 1975-76 indicated that there was a slim chance of developing a highly manoeuvrable fighter without dealing first with the above problems.

The necessity of the Su-27 programme's radical reconsideration was looming ahead. Such fundamental elements of the fighter design as the form and area of the wing, leading-edge extension configuration, horizontal and vertical control surface arrangement were have to be redesigned. Mikhail Simonov was a staunch supporter of such an approach but the Aviation Industry Ministry's brass had a different opinion. Designer Ye.A.Ivanov was not too keen on taking risk by radically revamping the design. As a result, first Su-27s were tested in their initial configuration. The flight tests of the T10-1 and T10-3 corroborated to the Siberian aerodynamics experts' doubts. To make up for the vertical stabiliser effectiveness decrease resulting in a decrease in the fighter's directional stability at a high AoA, first planes were fitted on the TsAGI advice with top-of-the-wing-mounted fences. However, the fences reduced the airframe's lifting capacity and negated the advantage of using the leading edge extension roots.

The T-10 testing also revealed the failure of the fighter's certain characteristics to match those of the performance specifications. First of all, that was true for the range: the mismatch between the required and actual ranges exceeded 20 percent. The Designer General reported to the ministry that there were two main reasons for non-compliance with certain requirements set in the performance specifications. Firstly, avionics developers failed to meet the weight limits set out in the avionics performance specifications. The summary avionics excessive weight comprised a few hundred kilograms, which, naturally, led to the aircraft's overall weight increase, hampered its manoeuvrability and reduced its range. Secondly, the engine's specific fuel consumption ordered in the performance specifications had not been achieved by the developer either. Truth be told, the issue was settled later when the requirements to the engine's specific fuel consumption were found to be a tall order which could not be met then. In spite of the T-10's considerable deficiencies revealed during the research and flight tests, Yevgeny Ivanov hoped, nonetheless, for possibility to gradually hone that configuration through minor design modifications, fuel capacity increase, etc. Otherwise, Mikhail Simonov was pushing hard for radical aircraft reworking, since as early as in 1976-77 a team of his subordinates in cooperation with the SibNIA scientists developed on their own and later tested in the wind tunnel a new configuration of the airframe devoid of the deficiencies which the previous configuration had in abundance. Justice should be done to M.P.Simonov (who went to work for the Aviation Industry Ministry in 1979 to return to Sukhoi as Designer General in 1983) who managed to persuade the leadership to risk radical change in the configuration of the fighter that had been already undergoing tests by then. With the passage of time, this decision proved to be right and led no matter what to building the aircraft that still - almost two decades later - has been regarded as one of the best warplanes in the world. Having commenced production of the Su-27 in its final configuration, Sukhoi confirmed its reputation of a world leader of aviation industry in line with its old tradition of never fielding mediocre aircraft.

The fighter's version featuring the new configuration was designated T-10S by the Sukhoi design bureau with its full-scale design work starting in 1979. Preliminary attempts to deal with the first T-10 version's glitches and meet the performance specifications' requirements, made by the design bureau and SibNIA (in the SibNIA the work was headed by Stanislav Kashafutdinov, candidate of technical sciences) resulted in setting the guidelines for modifying the initial configuration. With the developing of those guidelines, differences in design between the T-10S and T-10 became even more obvious. In the end, it became clear that the designers would have to develop an utterly different aircraft. According to Mikhail Simonov, the only things the T-10S inherited from its T-10 predecessor were the main landing gear's wheels and ejection seat. Only general principles set up for the Su-27 by P.O.Sukhoi himself, such as the integrated airframe configuration, statically unstable design with the aft centre of gravity, fly-by-wire control system, mounting the engines in isolated engine nacelles with belly-mounted air intakes, etc., were retained.

The T-10S had a new wing featuring a straight leading edge, an aerodynamic twist and a reconfigured sharp leading edge extension. Ogyval wing tips lost the ground to traditional permanent leading edge sweepback angle ones featuring pylons for air-to-air missiles, which resulted, firstly, in discarding flutter-preventing weights carried by the T-10 and, secondly, in beefing up the number of missiles from eight to 10. The wing area grew from 59.4 sq.m to 62 sq.m with the wing high-lift devices being changed drastically. Ailerons and flaps were replaced with unified control devices - flaperons, wing leading edges were fitted with slats (the T-10 had no leading edge high-lift devices), the automatic adaptive deflection mode for the flaperons and leading edge slats was ensured providing for the fighter's "polar curve envelope" flight concept. The airframe's lifting characteristics were enhanced through the use of the new wings and leading-edge extensions while maintaining the negative pitching moment at positive angles of attack and extending significantly the range of operational angles of attack, which ensured satisfactory lateral stability and prevents buffeting.

To reduce drag, the fore part of the airframe was reworked: the airframe cross section in front of and around the cockpit was decreased, canopy's midsection was reduced while its rearward movement was enhanced, the airframe's fore part cross section in the area of first fuel tanks was increased. Besides, airframe and spine fairing couplings were introduced along its whole length, spine fairing's lateral cross section in the airframe's midsection was reduced, central tail boom arrangement was altered through addition of a cylindrical tip which was an extension of the rear integral tank. At the same time, the total internal fuel capacity increased making up 9.4 tonnes. Making the shape of the engine nacelles more graceful while reducing their weight was achieved through fitting the T-10 aircraft with AL-31F engines featuring top-mounted aircraft and engine accessory gearbox (the T10-3 and T-10-4 were powered by the AL-31F engines featuring bottom-mounted accessory gearboxes). While preserving the general arrangement of air intakes, the new aircraft had a new foreign-object damage (FOD) system introduced for preventing FOD during taxiing, run and take-off through the use of extendable screens in the air intakes' ducts. Supplementary air inlet ramps were added to the lower surface of the air intakes.

To enhance directional and lateral stability through raising efficiency of the corresponding control surfaces, the empennage underwent substantial redesigning. The two vertical stabilisers were set wide apart at the reinforced booms on both sides of the engine nacelles with an optimum position for the stabilisers chosen in the vortex system generated by the leading edge extensions and wing panels. This resulted in considerable increase in directional stability and controllability of the fighter at high angles of attack and slipping. Also, the T-10S was fitted with ventral fins increasing its directional stability and anti-spin performances. Mounting the tails on the tail beams provided an increase in the tails' and horizontal stabiliser area, as well as to place the stabiliser's actuator fairings behind the tails. The planeform of the empennage was altered too, while the change in the position of stabiliser panel rotation semiaxes enhanced their flutter characteristics and allowed the designers to discard the anti-flutter loads mounted on the T-10. The air brakes - the main landing gear doors featured by the earlier version and discarded due to horizontal control surface flutter caused by their extending - were ousted by the large air brake mounted behind the cockpit.

The landing gear was reworked too: main struts were provided with "slant" main pivot, which made it possible to retract the gear into the wing centre section, thus requiring no folding struts to be installed. This also allowed to reduce the lifting body's cross section in the area of the landing gear wells. The nose gear strut was reinforced and moved aft, which enabled better taxiing and decreased a chance of foreign object damage during taxiing, run and take-off. In general, the fighter's configuration modification provided the lifting body midsection reduction by 15 percent, which led to reduction of drag by 18-20 percent in the subsonic and supersonic speed range. This, combined with an increase in the airframe's lifting characteristics as well as lateral and directional stability and controllability in all three planes in every flight mode, provided the aircraft with superior manoeuvrability with special emphasis put on high angles of attack and met the requirements set for the fighter's range.

In 1980, when the new-version prototype assembly was in full swing at Sukhoi, the pre-production batch assembly was nearing the end at the series production plant in Komsomolsk-on-Amur. As far as their design was concerned, they were the deadringers for the T10-1 and T-10-2 prototypes with only difference being their canted tails resembling those of the T10-3 prototype. Their powerplant still retained the AL-21F-3AI engines. Despite the fact that the planes had little in common with the future series-made Su-27, it was decided against cancellation of the pre-production batch. The decision was made to use them for honing the weapons control system and other equipment as long as first T-10S were being manufactured and put through initial stages of the testing programme. This was intended to make up for inevitable dragging behind the schedule due to the need of reequipping the production lines for manufacturing the new-configuration aircraft. The first aircraft of the pre-production batch designated T10-5 was finished in July 1980 and followed in the same year by the T10-6 and T10-9 (numbers 7 and 8 were assigned to first T-10Ss). In 1981, the Komsomolsk-based plant produced another two fighters - the T10-10 and T10-11, thus making five flying prototypes of the pre-series batch aircraft designated Su-27, Type T10-5 to discern them from future series-built aircraft. By 1982, there have been produced nine initial-configuration aircraft and one for static tests including those assembled at the Sukhoi plant.

Sukhoi completed the assembly of the first T-10S prototype designated T10-7 (a.k.a T10S-1) in early 1981 and in April 1981 it made its maiden flight piloted by test pilot V.S. Ilyushin. Also in 1981, the static version (T10-8, or T10S-0) and the second flying prototype featuring new configuration were built. From 1981 on, the Su-27 programme has been headed by Alexei Knyshev who has been the aircraft's Chief Designer ever since. The T10-7 and T10-12 were used to determine main performances of the new-configuration fighter, its stability and controllability, as well as to evaluate the new powerplant featuring top-mounted accessory gearboxes. However, both aircraft were not destined to fly long. On 3 September, 1981, the T-10-7 was lost due to a fuel system malfunction. Vladimir Ilyushin had to punch out while the fighter with its tanks nearly empty hit the ground in a fireball. On 23 December, 1981, the T10-12 crashed too due to entering uncontrolled spin while performing deceleration from the max speed. The fore part of the air frame was destroyed resulting in the aircraft hitting the ground. The test pilot, Aleksandr Komarov, died in the crash.

It proved to be impossible to determine all reasons for the crash. However, in 1983, Sukhoi's test pilot Nikolai Sadovnikov found himself in a similar situation while flying one of the first series-produced Su-27s - the T10-17. During the low-altitude high-speed level flight, Sadovnikov's fighter suffered destruction of a leading edge slat and part of the wing panel with the debris damaging vertical stabilisers. Thanks only to the high skills of the pilot (who was later awarded the title of Hero of the Soviet Union and set quite a few world records), the sortie did not end up in a crash. Nikolai Sadovnokov landed the damaged fighter with most of the wing panel missing and a tail clipped, thus providing the developers with precious information to rake their brains over. It was found out later that the reason was a mistake in the calculation of the hinge moment emerging due to the deflection of the adaptive leading edge flap in certain flight modes. Urgent measures were taken to rework the design, including the reinforcement of the airframe and wings as well as the development of lesser-area adaptive leading edge slats.

In 1982, the new fighter's testing programme was joined by first new-configuration aircraft manufactured in Komsomolsk-on-Amur, namely: the T10-15 (later converted into the P-42 record-maker), T10-16 and above-mentioned T10-17. The fly-out of the first series-built Su-27 took place on 2 June, 1982. The next year, Komsomolsk-based plant delivered another nine fighters, namely, T10-18, T10-20, T10-21, T10-22, T10-23, T10-24, T10-25, T10-26 and T10-27, most of which were used in the Su-27 joint official testing held in parallel with establishing series production of the new aircraft and its fielding with line units. The work on the T10-5 pre-series batch being in full swing too. The testing resulted in the substantial reworking of the fighter design. Thus, the fore part of the airframe and the wings were reinforced with the existing fighters being outfitted with additional external reinforcing plates and those under construction being given reinforced primary structural members and skin panels. The shape of the tail tips was altered too with previously-designed tail-mounted weights being discarded. The hinged part of the canopy was made to move up and rearwards (while earlier it was rear-sliding) with an additional behind-the-headrest metal casement being incorporated into it. To house the chaff dispenser, the length and height of the flipper - a rear-fuselage bay between the centre bean and engine nacelles - were increased. The wingtips were modified to carry ECM pods instead of air-to-air missile launching rails.

In 1984, first Su-27s were fielded with the Soviet Air Force followed by almost a hundred more fighters having been produced by the end of the next year as well as by Air Force and Air Defence Forces' units mass transition to the new fighter. The Su-27 joint official testing was completed in the mid-80s. Its results testified to the fact that a really outstanding aircraft had been made, which was second to none among the fighters as far as its manoeuvrability, range and combat effectiveness were concerned. However, certain avionics components required the additional testing held under special programmes after the joint official testing was over. With the avionics problems resolved, in 1990 the Su-27 was officially adopted in the inventory of the Soviet Air Force and Air Defence Forces aviation.