In the midst of the Yak-26 testing process, on March 28, 1956, a decree of the Central Committee of the CPSU and the Council of Ministers of the USSR No. 424-261 (MAP order No. 194 of April 6) was issued, instructing OKB-115 to begin the development and construction of a new light high-altitude supersonic front-line bomber. According to this decree, the aircraft with a crew of two was to be equipped with two R-11-300 engines designed by OKB S. K. Tumansky with a thrust of 3900 kgf at maximum mode and 5300-5500 kgf at afterburner. The following basic requirements were imposed on the car: takeoff weight - 12000-13000 kg; maximum speed at an altitude of 10,000 m with afterburner - 1500-1600 km / h (without afterburner - 1200-1300 km / h); time to climb 10,000 m with afterburner - 3-3, 5 minutes; practical ceiling - 16000-17000 m; takeoff run - 1000 m, run - 1100 m; flight range at an altitude of 10,000 m with a bomb load of 1,200 kg (special item) - 2,200-2400 km; normal bomb load - 1200 kg, overload - 3000 kg. To reduce the likelihood of being hit from the rear hemisphere, the aircraft needed to be equipped with a stern gun mount with a 23-mm cannon and 50 rounds of ammunition.
On August 15, 1956, the following resolution of the Council of Ministers No. 1115-578 (MAP order No. 453 of August 21) was issued, according to which OKB-115 was also instructed to develop a version of the aircraft for two very powerful VK-11 engines with a maximum thrust of 6100 kgf and 9000 kgf on afterburner. At the same time, the requirements for the flight characteristics of the bomber have significantly increased. So, the maximum speed with the afterburner was supposed to increase to 2500 km / h, the practical ceiling - up to 20,000-21000 m, the flight range at an altitude of 14000-15000 m at a speed of 1000 km / h - up to 2500 km, and when flying at an altitude of 19000 - 20,000 m - up to 2,000 km (while 500-600 km - at a speed of 2000 km / h and 1400-1500 km - at a speed of 1000 km / h). The first of the two prototypes was to be presented for factory tests in the first quarter of 1958, and in the fourth quarter - for state tests. However, the engines designed by V. Ya. It was not possible to bring Klimov to the series. It's a pity. With a thrust of 5000 kgf in the nominal mode and 9000 kgf in the afterburner, such engines would probably allow the Yakovlev OKB-115 to create a very balanced and useful aircraft for the Air Force.
Frustrated by the failure of the Yak-26, General Designer A. S. Yakovlev was openly skeptical about the new assignment, believing that it would not be possible to build a successful bomber on the basis of the "twenty-sixth". However, a number of OKB employees, in particular E. G. Adler, nevertheless, believed that there was nothing impracticable in the customer's requirements. After analyzing the list of military comments on the Yak-26, they came to the conclusion that the design of the base aircraft should be basically preserved. However, it is necessary to increase the rigidity of the wing at the root section; shift the ailerons to the engine nacelles and thus eliminate the reverse of the ailerons, or at least move it to the area of higher speeds; to increase the wing area by changing its shape in plan in the area between the engine nacelles - to straighten the trailing edge, and to "mow" the front edge at an even greater angle so that, without changing the relative thickness of the profile, to increase the construction height of the load-bearing elements; Install Fowler-type flaps on the straight section of the trailing edge; raise the wing, which will allow installing more powerful and at the same time larger engines, increasing the distance between the air intakes and the ground,increase the height of the bomb bay, which will make it possible to place in it not only bombs of all calibers, but also aircraft torpedoes; revise the design of engine nacelles to ensure the operation of engines at high supersonic speeds; make the navigator's seat a fixed ejection, and mount the sight on a folding platform and equip it with an elongated eyepiece (when ejection, the sight must shoot along the guide rails).
To speed up the construction of the first experimental Yak-129, at the request of Adler, one of the serial Yak-26s available at the OKB flight station was allocated. The alteration was completed at the beginning of 1958. And although outwardly the aircraft seemed very similar to the Yak-26-1 modified in 1957, it was a completely new type of bomber. The high position of the wing reduced the pitching moment from the thrust of the engines, which increased the stability of the machine on takeoff and when flying at high angles of attack. This was also facilitated by an increase in the chords of the wing root, which reduced the effect of the flow slope on the horizontal tail and increased its efficiency. The new flaps gave a big boost in lift. The displacement of the ailerons significantly increased the efficiency of lateral control at high speeds due to the decrease in elastic deformations of the wing. The use of a braking parachute made it possible to shorten the length of the run. To increase the angle of attack during takeoff, the rear main landing gear was equipped with an automatic "drawdown" system.
The Yak-129 was equipped with R-11A-300 engines with a thrust at the afterburner of 4850 kgf. It was an experimental modification of the serial R-11-300 (used on the MiG-21), which differed from it in the location of the unit box, since intended for installation in a gondola under the wing. The nacelles of the new engines had a round inlet with a small central cone. The fuel system of the experimental vehicle included six fuselage tanks with a total capacity of 4025 liters.
March 5, 1958 test pilot V. M. Volkov and test navigator N. M. Shipovskiy made the first flight on an experimental vehicle. Factory tests were carried out under the guidance of leading engineer V. N. Pavlova. According to OKB-115 experts, the aircraft basically met the requirements for it, but the maximum flight range was 400 km less than the specified one. The lack of stability of the machine at high angles of attack was also noted.
The Air Force command demanded that the revealed deficiencies be eliminated, the tested ejection seats installed, the aircraft operated from unpaved airfields, the RBP-3 radar bomber sight, and others should be installed before the aircraft was transferred for state tests.
The refinement of the cars took a long time. In addition, the first prototype aircraft, as usual, was built without weapons at all, so in the summer of 1958 it had to be significantly modified. The aircraft "129" turned out to be almost the first domestic bomber from which they risked dropping a supersonic bomb from the internal bomb bay. In the fall of 1958, shortly before the October holidays, a FAB-1500 bomb was successfully dropped from the 129 aircraft while flying at a speed of 1400 km / h from an altitude of 12 km. As noted by the leading designer of the machine E. G. Adler, the bomb "fell near the target." Two problems remained - "itching", that is, shaking the nacelles together with the engines, and the reversal of the ailerons, however, at a higher speed (over 950 km / h near the ground).
In the late autumn of 1958, it was decided to install a forced R11AF-300 engine with a thrust of 5750 kgf on the afterburner. Motivating the use of a "new" power plant, A. S. Yakovlev achieved the release of a new government decree of January 5, 1957, according to which the deadline for presenting the car for state tests was postponed again - to March 1959. On the last day of March, the aircraft "129" was presented to military testers, but those, after checking the list of presented requirements, were convinced of the absence of a number of systems on the car (for example, an autopilot!) and refused to accept the bomber. Angry A. S. Yakovlev ordered to continue the factory tests, while carrying out fine-tuning at the same time.
During the tests, the aircraft reached a ceiling of 16.5 km and the number M = 1, 4 (about 1500 km / h) at an altitude of 11 … 12 km. There was not enough thrust from the engines to achieve the maximum speed set by the government decree! Adler decided to remove the side fins on the fuselage, shorten the tail section of the engine nacelles and improve the air intakes, making their edge sharp. On May 21, 1959, in the very first flight after the changes made, "129" accelerated to a speed corresponding to M = 1.56 (more than 1700 km / h). No shaking was observed in flight, but there was aileron reverse due to insufficient wing rigidity. It was decided to eliminate it on the second prototype of the Yak-28, which was planned to be built at the end of July 1959, after which this particular machine was transferred to state tests. However, for a number of reasons, the second prototype Yak-28 received a conventional, not reinforced wing, which did not prevent it from accelerating in August 1959 to a maximum speed corresponding to M = 1.74 at the tropopause altitude (11 … 12 km). The ceiling remained the same - 16.5 km, since the R11AF-300 engines refused to work normally above. The "itch" reappeared in some flight modes. Another attempt by A. S. Yakovleva to present the plane for state tests came across the tough position of the commander-in-chief of the Air Force K. A. Vershinin - until the reverse of the ailerons is eliminated, do not accept the car! It was necessary to urgently build a third prototype aircraft with a reinforced wing (its skin in the center section area was made of steel on the recommendation of TsAGI).
Only on September 14, 1959, state tests of an experienced Yak-28 bomber began (the commission was headed by L. V. Zholudev). Engineer S. I. Blatov, test pilot F. M. Sobolevsky and test navigator A. M. Khalyavin. Soon, despite the large number of military remarks, due to the lack of an alternative, the "so-called Yak-28" began to be introduced into serial production at the Irkutsk aviation enterprise. At the end of 1959, the plant built the first three serial Yak-28s ("no letter" - they only had an OPB-11 telescopic sight and were not all-weather). The following year, two more Yak-28s with the RSBN-2 short-range radio technical navigation system and 37 Yak-28B aircraft with a very imperfect RBP-3 radar sight were produced.
For the first time, Yak-28 bombers were publicly demonstrated at the air parade in Tushino in 1961. The nine bombers were headed by the test pilot of the Air Force Research Institute of the Air Force, p-k F. M. Sobolevsky. The aircraft passed extremely effectively - in a tight formation, in which each pilot held his place, guided by the leader's rivets, at low altitude at a speed of 900 km / h. The American newspaper "New York Times" admitted that the United States "has nothing … that can be compared with this plane."
Continuation of the story about the Yak-28 (its modifications, operation and general assessment of the project) follows.
