intended to use the Yakovlev-designed version of the afterburning RD-10F engine. However, wind tunnel testing of the Su-9 in September revealed that drag could be reduced if the engine nacelles were mounted in the wing rather than underneath it, and the wing tips were redesigned to use a different airfoil that significantly reduced Mach tuck. These changes required that the entire wing be redesigned; the wing spar was bent into an inverted U to accommodate the engines and slotted flaps replaced the simple flaps previously used. The tailplane was given a 5° dihedral to move it out of the engine exhaust. Before the prototype was finished, Sukhoi was ordered to use the Lyulka TR-1 turbojets originally intended for the Su-9. This aircraft was designated the Su-11 and was called Samolyet KL by the OKB. Each TR-1 developed only 12.7 kN (2,865 lbf) thrust each, rather than the 15 kN (3300 lbf) required. The aircraft made its first flight on 28 May 1947 and also participated in the flypast at Tushino in August. It had a maximum speed of at sea level, but flight testing revealed that it lacked longitudinal stability at high speeds. Modifications of the wing/nacelle fillets and lengthening the engine nacelles failed to cure these problems. Coupled with the unavailability of mature TR-1 engines, these problems caused the program to be cancelled. Su-13 The Su-13, (Samolyet KT), was the final attempt to further increase performance of the basic Su-9 design, using wing sections reduced from 11% to 9% thickness/chord ratio and swept tailplanes. The aircraft was also fitted with a pair of Klimov RD-500 (unlicensed copies of the Rolls-Royce Derwent) engines with 15.6 kN (3,500 lbf) thrust each. The armament was changed to three 37 mm Nudelman N-37 cannon, while drop tanks could be fitted underneath the wingtips. A night fighter version with a Torii radar was also proposed, but this required major structural changes to accommodate the radar. Neither version made it off the drawing board. Operators Soviet Air Force Variants Su-9UT – Planned two-seat trainer version, never built. Su-11 (Samolyet KL) – Modified Su-9 with new wings and Lyulka TR-1 engines. One prototype built. Su-13 (Samolyet KT) – Su-9 with Klimov RD-500 engines. Night fighter version also proposed. Neither version was built. Specifications (Su-9) See also Notes References Gordon, Yefim. Early Soviet Jet Fighters. Hinckley, Leicestershire, UK: Midland, 2002. . Gunston, Bill. The Osprey Encyclopedia of