Trijets

A trijet is a jet aircraft powered by three jet engines. In general, passenger airline trijets are considered to be second-generation jet airliners, due to their innovative engine locations, in addition to the advancement of turbofan technology.

Other variations of three-engine designs are trimotors, which are aircraft with three piston engines.

One major advantage of the trijet design is that the wings can be located further aft on the fuselage, compared to twinjets and quad-jets with all wing-mounted engines, allowing main cabin exit and entry doors to be more centrally located for quicker boarding and deplaning, ensuring shorter turnaround times. The rear-mounted engine and wings shift the aircraft's center of gravity rearwards, improving fuel efficiency, although this will also make the plane slightly less stable and more difficult to handle during takeoff and landing. (The McDonnell Douglas DC-9 twinjet and its derivatives, whose engines are mounted on pylons near the rear empennage, have some advantages of the trijet design, such as the wings located further aft and a more rearward center of gravity.)

One issue with trijets is positioning the central engine. On most trijets this is placed at the tail along the centerline, producing some technical difficulties. The central engine is most commonly supplied with air by an S-shaped duct – this is used on the Hawker Siddeley Trident, Boeing 727, Tupolev Tu-154, Lockheed Tristar and, more recently, the Dassault_Falcon_7X. An S-duct has the advantage of reduced drag. Additionally, since the engine outlet is located lower down, the aircraft will be easier to handle in the event of an engine failure. However, S-ducts are complicated and costly designs. Furthermore, the central engine bay would require structural changes in order accommodate future larger-diameter engines; in the case of the Boeing 727 the central bay was only wide enough to fit a low-bypass turbofan and not the newer high-bypass turbofans which had greater output as well as being quiet enough to meet noise regulations, but such a redesign was prohibitively expensive and so Boeing ended production of the 727 instead of pursuing further development. The DC-10 and related MD-11 use an alternative "straight" layout, which allows for easier engine installation, modification, and access. However, this has inferior aerodynamic properties when compared to the S-duct designs. Since the engine outlet is located much higher up than the wing-mounted engines, an engine failure will produce a greater moment about the aircraft's lateral axis, making it more difficult to control.

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Wikipedia