With the Ilyushin Il-86 under way, the Soviets apparently were looking to purchase L1011s with GE CF-6 turbofans. But, then, again, this was the height of the Cold War; the TriStar and its Rolls-Royce high-bypass engines and automated ailerons were high-tech goodies that would never see the other side of the Iron Curtain. So, Antonov NKB decided to come up with their own trijet, though no engines existed in the USSR to power such an aircraft. An-318 is more like the An-218 with a third motor in the tail.
During the 1960s the US aircraft industry and the airlines were planning the introduction of "jumbo' subsonic jets as a follow-on for highly productive long-range transports. The 747 was announced in 1966, first flew in 1969, and entered service in 1970. The second wide-body concept was the McDonnell-Douglas DC-10, a trijet swept wing design that was slightly smaller than the 747 and was intended for carrying a large load of passengers from smaller fields and over short-to-medium ranges. Lockheed, having been involved with the C-5 military transport program, later entered the wide-body civil transport field with the trijet L.1011, the first civil transport built by Lockheed since the Electra turboprop [and the last].
In transport aircraft propelled by jet engines the arrangement of the engines is variable in particular depending on the number of engines and in practice also on the size of these engines, size introducing important constraints.
When the number of jet engines is even, generally two or four jet engines, exceptionally six or eight, the jet engines are most often fixed under the wing of the aircraft, according to a symmetrical arrangement with respect to the axis of the aircraft. This arrangement is very common at present, on civil transport jet aircraft, because of its good performance in terms of the aerodynamic integration of the aircraft and despite the constraints it imposes on the undercarriages in order to maintain a sufficient ground clearance of the jet engines.
Another arrangement of the jet engines consists in fixing the jet engines on the rear part of the fuselage, symmetrically on each side. This arrangement gives rise to difficulties with regard to certification when two jet engines must be fixed on each side of the fuselage of an aircraft having four jet engines and generates difficulties in the management of the position of the center of gravity of the aircraft, in particular with large-sized jet engines like the modern jet engines with high bypass ratios. For these reasons, only aircraft of modest size having two jet engines using relatively small jet engines are at present designed with this system of mounting jet engines on the sides of the fuselage at the rear of the aircraft. When the number of jet engines is odd, the extra engine with respect to the even number is generally placed in the axis of the aircraft, in the rear zone of the fuselage, and the even number of jet engines are arranged symmetrically as described before.
The examples of transport aircraft having three jet engines are rather limited and there can be mentioned in particular the DC10, comprising two jet engines fixed under the wing and a third jet engine fixed on the vertical stabilizer at the rear of the fuselage, the L1011 also comprising two jet engines fixed under the wing and a third jet engine fixed in the rear part of the fuselage, the B727 which comprises two jet engines fixed on the sides of the fuselage at the rear and a third jet engine fixed in the rear part in the axis of the fuselage.
These configurations with three jet engines, and in particular the arrangement of the third jet engine in the axis of the aircraft, present problems which explain why they are seldom used despite the advantage that the use of three engines can provide rather than two or four engines.
A first problem is related to the aerodynamic integration of such jet engines with the aircraft. In fact, the proximity with respect to the fuselage of these air intakes induces losses of aerodynamic loads and distortions of the flow speeds field that are prejudicial to good efficiency of the jet engine thus placed.
A second problem, also very critical for companies operating the aircraft, relates to the maintenance of these jet engines. Placed higher and less accessible than under-wing jet engines, they necessitate special means for the daily or periodic checking operations and their installation and removal operations are much more problematic than for jet engines suspended under a wing, which proves to be penalizing in operation. Moreover, their specific installation necessitates a special design of the nacelles, different for example from the jet engines fixed under the wings, and therefore different spare parts for the nacelles, different maintenance manuals and different training for maintenance personnel.
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