The Ilyushin Il-22, USAF/DOD designation Type 10,[1] was the first Sovietjet-enginedbomber to fly. It used four Lyulka TR-1turbojets carried on short horizontal pylons ahead and below the wing. The engines did not meet their designed thrust ratings and their fuel consumption was higher than planned. These problems meant that the aircraft could not reach its required performance and it was cancelled on 22 September 1947.[2]
Design and development
The Council of Ministers ordered the Ilyushin design bureau on 12 February 1946 to begin work on a bomber that would use four of the new TR-1 jet engines. Experiences with the first generation of jet fighters had revealed unsuspected problems involved with high-speed flight and Ilyushin devoted much effort to mitigate them. The long and thin unswept wing was conventional in appearance, but it was shaped to improve lateral stability at high angles of attack and to prevent the onset of tip stall.[3]
Another problem discovered by the jet fighters was unexpectedly dropping a wing at high speeds and high altitudes. This was traced to manufacturing defects in the wings that made no difference at low speeds and altitudes, but meant that each wing had a slightly different airfoil and, hence, a different amount of lift. To counter this Sergey Ilyushin and his team developed a new manufacturing technique that reversed the traditional practice where the internal supporting members were affixed to the assembly jig and the aircraft's skin panels were then attached. This new method meant that the skin panels were placed in the jigs where the correct curvature and shape could be guaranteed and the internal structure was then fastened to them. This required that manufacturing joints be used along the chord lines of the wings and tail surfaces, which split the spars and ribs in half. Similarly, the fuselage was built the same way, although it was split vertically along the centerline. This new technique did impose a small weight penalty but had the unexpected advantage of greatly accelerating the assembly process, as the internal equipment could be installed before the halves were joined together. This allowed several teams to work on a single sub-assembly before they were mated.[4]
Most of the other multi-engined jet aircraft in existence, when the Il-22 was being designed, either had the engines in a nacelle (singly or in pairs) directly attached to the underside of the wing or were buried in the wing itself. Clustering them in a nacelle offered several advantages over individual nacelles, as it reduced overall drag and minimized interference drag, but had the major operational disadvantage that an uncontained fire in one engine could disable its neighbor as well. Early jet engines were not reliable, so this was a significant risk. Ilyushin chose to put the TR-1 engines ahead and below the wing leading edge on short horizontal pylons. This gave them the beneficial effect of acting as anti-flutter weights and proved to be more efficient aerodynamically than underwing nacelles. This also facilitated engine changes and maintenance by making them more accessible to the ground crews.[5]
Neither the thin wing nor the engine nacelles offered any place to store the main landing gear so the fuselage was designed as a flattened oval to give them as wide a track as possible. This also provided plenty of room for the 9,300 kg (20,500 lb) of fuel stored in three bags, one each ahead, above and behind the bomb bay. This could carry up to 3,000 kg (6,600 lb) of bombs. The stepless fuselage nose was largely glazed and came to a rounded point (similar to the noses of the Boeing B-29 Superfortress, Heinkel He 111 and Arado Ar 234) to reduce drag. The Il-22 had a crew of five, two pilots in the nose, the bombardier-navigator in front of them in the tip of the nose, the dorsal gunner/radio operator immediately behind the pilots and the rear turret gunner behind the tail.[6]
A 23-millimetre (0.91 in) Nudelman-Suranov NS-23autocannon with 150 rounds was fixed on the lower starboard side of the nose; it was fired by the pilot who had a primitive ring sight to use for aiming. The dorsal turret mounted two 20-millimetre (0.79 in) Berezin B-20E guns with 400 rounds per gun and was capable of 360° of traverse, with special microswitches preventing the gunner from firing into the bomber's tail. The turret was remotely controlled by the radio operator and was powered by electric motors for both traverse and elevation. The gunner and his gunsight used a small observation blister at the rear of the main crew compartment to lay the guns on their target. The sight automatically compensated for parallax between the gunner and the turret as well as the required amount of target lead and the shell's ballistics. The remote-control system offered several advantages including a smaller turret that had less drag, the guns could be fixed more rigidly to their mounts, the sight was not exposed to vibrations from firing and could track targets more smoothly and the gunner's comfort did not have to be sacrificed to optimize the performance of the turret. The major disadvantage, of course, was that the analog computer remote control system was exceedingly complicated for the period and prone to breaking down, just like the even more complex systems in use on the B-29 Superfortress. The rear gunner was placed at the very tail of the Il-22 to optimize his field of fire in an electro-hydraulically powered Il-KU3 turret that mounted another NS-23 cannon. The turret could traverse a total of 140°, elevate 35° and depress 30°.[7]
Testing and evaluation
The prototype Il-22 was rapidly assembled and made its first flight on 24 July 1947. It proved to have docile handling characteristics, but was severely underpowered as the TR-1 engines produced only 80% of the required thrust. During the latter part of the manufacturer's flight tests the Il-22 made the first-ever Soviet jet-assisted (rocket-assisted, RATO) takeoff on 7 February 1948 with a pair of SR-2 boosters. As the thrust of the engines could not be increased in a timely manner Ilyushin made the decision not to submit the bomber for state acceptance trials as its performance did not meet the requirements laid down for it in 1946.[8]
Gordon, Yefim and Dmitriy and Sergey Komissarov. OKB Ilyushin: A History of the Design Bureau and its Aircraft. London: Ian Allan, 2004. ISBN1-85780-187-3.
Němeček, Václav. The History of Soviet Aircraft from 1918. London: Willow Books, 1986. ISBN0-00-218033-2.
