Ilyushin Il-62

Development

The Ilyushin OKB presented a proposal for a four-engined long-range jet airliner in February 1960, receiving the go-ahead from the Soviet Council of Ministers on 18 June 1960, with the Kuznetsov Design Bureau being instructed at the same time to develop the NK-8 turbofan to power the new airliner. The official specification required that the airliner, designated Il-62, must carry 165 economy-class passengers over a distance of 4,500 km (2,800 mi) or 100 first-class passengers over 6,700 km (4,200 mi).

The Il-62 replaced the fast turboprop Tu-114 on long-range routes. As the Tu-114 was just entering service when the Il-62 was on the drawing board, Ilyushin had time for an unhurried design, test, and development programme. This was useful, since the Il-62 did call for significant development.

The Il-62 and the British Vickers VC10 are the only commercial airliners with four engines fitted in twinned/paired nacelles by the sides of, and beneath, a T-shaped empennage (T-tail). In the case of Ilyushin, the configuration was dictated by TsAGI, the Soviet Union's aerospace agency, since Ilyushin's design bureau lacked the resources to engage in configuration studies. This layout allowed the wing design to be optimised for aerodynamic efficiency, without being cluttered by having to carry engines. In addition, the rear-mounted engines reduced engine noise in the cabin and allowed smaller vertical tail surfaces (as the yawing moment in the event of an engine failure was reduced compared to wing-mounted engines). These advantages are balanced by a number of drawbacks. The wing structure, lacking the weight of forward engines to pull down against the wing torsional moment, needed to be heavier, as did the rear fuselage structure, which had to carry the engines. In addition, aerodynamic wash (shadow) from the wing blankets the tail when the nose is pitched up (high angle of attack) leading the aircraft into a condition known as deep stall. This called for complex and (in the 1960s) unreliable automatic stall warning systems such as stick shakers and stick pushers to prevent the aircraft from stalling, although the Il-62's wing was designed to prevent deep stall.

Early aircraft (prototypes, pre-production and initial production aircraft) display an evolution from thin or thick kinked leading inboard edges to the ultimate thick and straight 1966 shape. The characteristic "dog-tooth" also moved until fixed before production began. The engine installation also evolved, with the engines' longitudinal axes canted by 3 degrees from the horizontal, addition of thrust reversers to the outer engines, and slimming down of the entire installation as production began.

The prototype was grossly underpowered, as its intended NK-8 engines were not ready, and small Lyulka AL-7 turbojet engines had to be installed temporarily. The prototype with AL-7PB engines (registered СССР-06156) first flew on 3 January 1963, but crashed after clipping a perimeter fence during a maximum-weight testing flight of the development program. The production Il-62 was powered by the originally intended rear-mounted Kuznetsov NK-8-4 engines. The first Il-62 powered with NK-8 engines (registered СССР-06153) first flew in 1964.

The Il-62M variant (first flight in 1971, introduced in 1973) has more powerful, more efficient, and quieter Soloviev D-30KU engines and a fin fuel tank. Beneath the skin, the Il-62M has simpler and lighter single-slotted flaps and incremental aerodynamic improvements. Most important of these was the addition of spoilerons (spoilers or wing-mounted lift-disruptors which can act in the same capacity as ailerons by differential deployment in cruising flight, or in concert to kill lift on landing), and the ability to use idle reverse thrust in flight during the final approach so as to shorten the landing run. All but one example in service today are Il-62M. In 1978, the Il-62MK was further developed to seat up to 198 passengers and carry about 2 tonnes (4,400 lb) more payload or fuel than the Il-62M.

Other versions were also planned, including a stretched version to seat up to 250 passengers and a shrunk version to suit smaller airfields.

The Il-62 has tricycle landing gear with an additional lightweight gear strut at the rear of the fuselage which extends when the aircraft reaches its parking position. Aircraft with rear-mounted engines are usually tail-heavy when sitting empty on the ground, and to prevent the aircraft from tipping up on its tail, various devices are used for supporting the tail – from simple "pogo stick" fixed struts on small aircraft to light-weight extendable struts (Il-62). Aircraft like DC-9 or Boeing 727 use an airstair door under their tail, which serves the dual purpose of a tail support and an extra door for passenger loading.

The Il-62 is the only airliner in its class with unpowered, all-manual flight controls, using steel cables, rods, pulleys, aerodynamic and weight balances, and aerodynamic trim tabs. The Il-62 also has a forward-mounted tank serving as a water ballast. This may be used when the aircraft flies empty or lightly loaded.