S-22 / S-23 / S-25
A major shortcoming of the entire Su-7 aircraft family was poor takeoff and landing characteristics (TOLC), which were entirely due to the aerodynamic configuration selected, featuring as it did a relatively slender sharply-swept wing. This resulted in high takeoff and landing speeds, requiring therefore a long runway. With a view to improving the Su-7B's TOLC, the Design Bureau investigated a variety of solutions. Thus, in the period 1960-61, the Su-7 was used as a platform to develop and build an experimental aeroplane, the S-25, which featured a blown flap based on a blowing-type boundary layer control (BTBLC) system with air tapped from behind the engine compressor. The plane underwent manufacturer's tests in the period 1961-62, yet the BTBLCS failed to bring about a sufficient reduction in the takeoff/landing speed so it was not recommended for production.
Improvement of the baseline aircraft TOLC was achieved using another solution, validated on the experimental S22-4. Between 1960 and 1963, this aircraft was used to try out a suspension powder rocket booster to provide faster acceleration during takeoff, and innovative larger braking parachutes to cut down the landing run. The solutions were recommended for production.
Another innovation introduced by the Design Bureau was ski-equipped landing gear designed to improve the takeoff/landing performance of frontline aircraft deployed from unpaved airstrips with soft surfaces. The Design Bureau had been doing experimental research in this area since 1956. With the Su-7 and Su-7B used as the platform, several prototypes were built with ski undercarriage, viz. S-23 and S-26, which were then tried out on a variety of unpaved and snow-covered aerodromes between 1959 and 1966. The skis installed on the Su-7B's main struts proved highly effective on unpaved runways and were recommended for production and operational use, but it was another version, the so-called wheel-ski undercarriage, tried out on S22-4, that was finally put into production.
This way, S22-4 became the prototype of the last mass-produced variant of the baseline aircraft, which received the designation Su-7BKL (short for "wheel-ski"). The Su-7BKL replaced the Su-7BM on the production line in Komsomolsk-on-Amur in 1965, the later variant having been produced till 1972. A flight of Su-7BKLs was exported to Czechoslovakia and Poland.
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