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Boeing 7X7

In 1972 Boeing began a development study, code-named 7X7, for a new wide-body aircraft intended to replace the 707 and other narrow-body jets. The aircraft would provide twin-aisle seating, but in a smaller fuselage than the Boeing 747, McDonnell Douglas DC-10, and Lockheed L-1011 TriStar wide-bodies. A trijet is an aircraft powered by three jet engines. Early twin-jet designs were limited by the FAAs 60-minute rule, whereby the flight path of twin-engine jetliners was restricted to within 60 minutes' flying time from a suitable airport, in case of engine failure. By 1976, a twinjet layout, similar to Airbus A300, became the baseline configuration of what became the Boeing 767.

Airplanes are built to meet the specific needs of their users. One U.S. domestic airlines requirement was for a large-capacity medium-range aircraft. Originally it was desired that the aircraft carrying a full load transcontinental with one fuel stop also have a good airfield performance in terms of noise, maneuverability, takeoff and landing distance requirements, climb and landing slopes, etc. Large capacity aircraft with twin engines and a 300-seat capacity were first studied. However, a later survey revealed the need for an aircraft with coast-to-coast non-stop capabilities without sacrifice of shorter-range economics, and it was felt that the only satisfactory way such a range could be achieved was by the use of three engines.

A new generation of high bypass turbofan engines with improved power-weight ratio and a significant reduction in fuel consumption has provided airframe designers with the challenge of designing a single airframe adaptable to carry a full load off 6,000 ft. runways for short ranges and the same load off 10,000 ft. runways for distances in excess of 3,000 miles. One basic type of aircraft with this versatility can do a very large share of the world's air transportation.

Two engines on one side and one on the other, either on the wings or on the fuselage, lacked symmetry. Three engines on the tail was the configuration of an existing commercially acceptable aircraft. However, for proper balance, the wings must be placed more rearwardly than if two of the engines were on the wings. Also, the tail surfaces must be made larger to compensate for the shorter tail distance to the center of balance of the aircraft. The larger tail size adds weight and drag, making operational costs higher. A saving of over 4 tons of weight can be achieved simply by mounting two engines on the wing and one in the tail for better total mass distribution, instead of having all three engines far aft on the fuselage.

Such was the case in particular with tri-jet aircraft such as the Lockheed L1011, and the McDonnell Douglas DC-10, or MD-11. In fact, while two of the engines are respectively carried by the wings, the third engine was structurally and geometrically located between the horizontal tail unit with its two symmetrical stabilizers and the vertical tail unit with its vertical stabilizer disposed on the nacelle or the fairing of this third engine. Other aircraft integrate the tail engine at the end of the fuselage, directly in its structure.

Whatever the type of aircraft equipped with such an engine at the tail of the fuselage may be, particular maintenance problems arise due to the location of said engine compared with that of a very accessible and easily removable wing engine. In fact, when it was desired to remove the engine for an important overhaul, or even to replace it, it was necessary to previously carry out the removal of numerous surrounding components and devices that are independent of the engine in order to gain access to the latter, which results in particularly long, tedious and costly interventions.

Furthermore, it was necessary to provide complex and heavy lifting and/or handling installations (considering the height of the intervention) in order to be able to carry out in complete safety the various movements (horizontal and vertical) for taking out and moving the engine from the rear tail units or the fuselage to a receiving stand placed on the ground.

Engine-in-the-tail configurations include the aft pod inlet, bifurcated inlet, engine forward of vertical stabilizer, S-duct inlet and now the straight inlet. Several contemporary commercial aircraft have the S-duct inlet with the inlet above the fuselage curving down to join the engine mounted rearwardly on the fuselage. Weight, drag, inlet loss and flow distortion, inlet-engine compatibility, susceptibility to engine-disk failure, engine growth, commonality with the wing-mounted engines, and reverser effectiveness were considered for each configuration. The straight inlet through the vertical stabilizer concept proved to be the best.

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Page last modified: 01-07-2021 17:55:18 ZULU