Design Trijet

the hawker siddeley trident featured s-duct.



the lockheed tristar , dc-10 share similar configuration, biggest difference mounting of tail engine. tristar s tail-mounted engine, 727, located in rear fuselage , fed through s-duct.



the mcdonnell douglas dc-10 not use s-duct design, instead tail engine mounted above fuselage in straight layout.

one issue trijets positioning central engine. on trijets placed @ tail along centerline, producing technical difficulties. common configuration having central engine located in rear fuselage , supplied air s-shaped duct – used on hawker siddeley trident, boeing 727, tupolev tu-154, lockheed tristar and, more recently, dassault falcon 7x. s-duct design has advantage of reduced drag, additionally since engine outlet located lower down, aircraft easier handle in event of engine failure. however, s-ducts complicated , costly designs. furthermore, central engine bay require structural changes in order accommodate future larger-diameter engines in case of boeing 727 , lockheed tristar. boeing 727 s central bay wide enough fit low-bypass turbofan , not newer high-bypass turbofans had greater output being quiet enough meet new noise regulations of 1980s, such redesign prohibitively expensive , boeing ended production of 727 instead of pursuing further development. lockheed tristar s s-duct cross section small fit existing double spool engine designed accommodate new triple spool rb211 engine, , delays in rb211 development in turn pushed tristar s entry service affected sales. dc-10 , related md-11 use alternative straight layout, allows easier engine installation, modification, , access. however, has inferior aerodynamic properties when compared s-duct designs. also, engine outlet located higher wing-mounted engines, engine failure produce greater moment aircraft s lateral axis, making more difficult control.

placement of remaining 2 engines varies. smaller aircraft, hawker siddeley trident, boeing 727 , tupolev tu-154 have 2 side-mount engine pylons in t-tail configuration. larger widebody lockheed tristar , dc-10/md-11 mount engine underneath each wing. preliminary studies done on tristar reuse fuselage , wing twinjet design though these never materialized due lockheed s lack of funds, while in late 1990s boeing had taken on mcdonnell douglas had considered removing tail engine md-11 make twinjet ending instead cancelled md-11 production altogether.

trijets more efficient , cheaper quadjets (four-engine aircraft, engines under wing), engines expensive part of plane , having more engines consumes more fuel, particularly if quadjets , trijets share engines of similar power, making trijet configuration more suited mid-range medium size airliner compared long-range large quadjets. however, difficulty , complexity of mounting center engine through tail in trijet design negate advantage. main disadvantage trijets fuel efficiency, trijet design consume more fuel comparable twinjet design, resulting in higher operating costs. worth trade off during 1970-1990s when trijets , twinjets shared engines of similar output, such when dc-10/md-11 trijets , airbus a300/a330 twinjets powered general electric cf6, additional power third engine gave dc-10/md-11 advantages in longer range and/or heavier payload on a300/a330 twinjet. since 1990s, further advancements in high-bypass turbofan technology, large twinjets have been equipped purpose-designed engines boeing 777 s general electric ge90 , 2 engines match or exceed output of trijets , quadjets.