Jet engine location
The location of the jet engines on jet aeroplanes varies considerably and fashions have evolved as requirements and technologies have matured. Besides the number of engines, many other considerations are taken into account.
History
The optimum location for jet engines varies according to operational requirements and the technologies of the day. Designers have for the most part followed broad historical trends, although many individual types have differed for one reason or another.
Early jets
The first prototype jet aeroplanes such as the Heinkel 178 and Gloster E.28/39 "Whittle" had a single engine buried in the fuselage with long through-ducting.
But burying the engine made access difficult and the long intake and exhaust ducts wasted a lot of power, so many designers turned to short pods or nacelles as more practical solutions. At the same time, practical warplanes needed two or more of these early engines to provide sufficient power. Nobody quite knew the best place to put them; the Arado Ar 234 and Messerschmitt Me 262 housed them in underwing nacelles, the Junkers Ju 287 in various wing and fuselage locations on different prototypes, and the Gloster Meteor in mid-mounted wing nacelles.
The advent of more powerful and reliable engines such as the de Havilland Goblin, coupled with a split intake to eliminate the long nose duct, allowed a return to the single-engined layout for smaller jets such as the de Havilland Vampire and Lockheed F-80 Shooting Star.
Large aircraft
While several of the first American jet bombers followed the Arado pattern, the Boeing B-47 placed its engines in underwing pods. Flying in 1947, it set the standard pattern for large aircraft.
When the passenger jet airliner followed, the cabin noise from their turbojet engines proved significant. In a bid to reduce the noise, several 1970s designs such as the Sud Aviation Caravelle moved the pods back to the rear fuselage. This made possible the trijet configuration, which was seen on some medium-sized airliners. However rear mounting was aerodynamically and structurally less efficient and the advent of turbofans with increasingly high bypass ratios and improved sound-reduction technologies soon led to a return to underwing pods.
Large jet aircraft originally had four or more engines. These were needed to provide sufficient power, as well as redundancy in the case of engine failure. From around the millennium, engines have gained sufficient power and reliability for large twin-engine jets to become practicable, taking advantage of their greater simplicity and fuel efficiency.[1] By 2020, only the very largest commercial airliners were still four-engined.
Supersonic jets
At supersonic speeds engine pods cause significant drag. Although early multi-engine types such as the Convair B-58 Hustler had podded engines, most were buried in the rear fuselage as with their subsonic predecessors, or grouped in nacelles under a delta wing.
The 1980s trend towards heavy twin-engined air-superiority fighters led to moving the engines a little outboard such as the McDonnell Douglas F-15 Eagle and dropping them below the wing to create a central tunnel in which the waverider effect could provide increased lift without increasing drag. When stealth became paramount in the 21st century, the trend was reversed.
Private jets
The first practical small, light jets were developed in the 1950s to meet USAF requirements. They created a new market for business jets. Most are twinjets with rear-mounted engines although other variations exist, such as the four-engined Lockheed JetStar.
Stealth
Jet engines have spinning radar reflectors at the front and a strong infrared source at the back. The advent of low-observables has led to a return to buried engines. Typically both intake and exhaust are located on the upper surface of the wing, to hide them from below.
Layouts
Jet engines do not have a propeller and so the thrust line can be located closer to the ground. However they require direct axial front entry and rear exit of the airflow and this constrains their location in various other ways.
Engine maintenance, especially removal and replacement, requires large removable panels which compromise structural integrity. This is one of the reasons why pods and nacelles are often preferred over buried locations. Buried engines are often inserted and withdrawn through the rear opening for the exhaust duct but access to the forward mounting, attachment and routine maintenance points is still necessary, making the procedure fiddly and time-consuming.
Safety considerations also affect the location of jet engines. The two main scenarios are in-flight failure and crash survivability.
- Primary structure, crew and passengers must be protected from in-flight fires and from debris ejected by mechanical failure of the rotating components. For a multi-engine type, engine failure on one side of the craft must be trimmable to maintain a straight heading with some yaw control authority still remaining. For this reason, only a few experimental types have ever had their engines mounted on the wing tips.
- In the event of a survivable crash on takeoff or landing, podded engines of a civil airliner must be located so that if they come loose they will not be thrown onto the passengers, crew or fuel tankage. The fuel tanks and pipework should also not spill directly onto or under the accommodation.
Buried engines
Also known as integral engines.[2] The advantage of burying the engine integrally with the main airframe is to reduce frontal area and wetted area, and hence drag. It can also reduce structural weight, provided the requirements for a strong structure can be reconciled with the large access panels needed for ease of maintenance and replacement.
Common techniques to avoid long intake ducts include S-ducts as on the General Dynamics F-16 or split intakes as on the de Havilland Vampire. However these also suffer some losses.
S- or split ducts are less common for exhaust ducts due to the high temperature of the exhaust gases but have occasionally been used, as on the Hawker Sea Hawk. Another technique for single-engine types, first used on the de Havilland Vampire, is to adopt a twin-tailboom layout with a high-mounted tailplane. The tailless swept wing naturally allows a short fuselage and exhaust duct, as on the de Havilland DH.108.
Engines may also be buried in the wings if they are thick enough, as on the Horten Ho 229 flying wing, although they typically create bulges at the rear.
Central wing mounting
Underwing mounting
Overwing mounting
For a seaplane, mounting the engines over the wing helps to avoid ingestion of spray when taking off and landing. Types include:
- Beriev A-40 Albatros: Soviet/Russian ASW amphibian
- Beriev Be-200 Altai: Russian utility amphibian
- Martin P6M Seamaster: American pre-production strategic bomber flying boat
For a low-wing landplane, engines under the wing require long and heavy undercarriage to provide adequate ground clearance. Placing the engines over the wing resolves the problem and also allows the fuselage entry and exit doors to be closer to the ground. Types with such short undercarriage include:
- Beechcraft PD 290: prototype for a jet-powered version of the Beechcraft Super King Air
- Honda MH01 and MH02: Japanese business jet prototypes
- Honda HA-420 HondaJet: Japanese/American business jet
- VFW-Fokker 614: German regional airliner
Forward-mounted overwing jets can be used to blow air over large flaps on the trailing edge, employing the Coandă effect to give short takeoff and landing (STOL) performance. Types using this technique include:
- Antonov An-72: Soviet/Ukranian transport aircraft, also its variants Antonov An-74 and An-71
- Boeing YC-14: American transport aircraft prototype
Some designs featured four engines mounted in pairs with one above the wing and one under. Types include:
- Messerschmitt Me 262 Lorin: Projected variant with additional ramjet engines mounted above the main engines, which were in underwing nacelles.
- Short Sperrin: British jet bomber prototype with two engines in each deep mid-mounted wing nacelle.
- Sukhoi Su-10: Early postwar Soviet jet bomber project.
Wingtip mounting
Wingtip pods improve the aerodynamic efficiency of the wing by reducing the tip vortices in its wake. They also offer a form of span loading allowing a lighter structure, as with any wing-mounted items, while avoiding the drag problems with either nacelle junctions or pod support pylons. A further advantage for jet engines is that they keep the exhaust stream clear of the tail structure throughout the flight envelope.
However maintaining heading and adequate yaw control in the event of a single engine failure poses an extreme challenge and the configuration is seldom used. The Sud-Ouest SO 9050 Trident II interceptor is a rare example.
Rear fuselage mounting
Single engine
Twinjets
Trijets
Some trijets have two underwing engines. Should they go in a different section or, just address the central engine here, or what?
The most common trijet configuration places the central engine in the rear fuselage and supplies air via an S-shaped duct; this is used on the Hawker Siddeley Trident, Boeing 727, Tupolev Tu-154, Lockheed L-1011 TriStar, and, more recently, the Dassault Falcon 7X. The S-duct has low drag. However, S-duct designs are extremely complex and costly. Furthermore, the central engine bay would require structural changes in the event of a major re-engining. For example, the 727's central bay was only wide enough to fit a low-bypass turbofan and not the newer high-bypass turbofans which were quieter and more powerful, while the Lockheed Tristar's tail section was too short to fit an existing two-spool engine as it was designed only to accommodate the new three-spool Rolls-Royce RB211 engine and delays in the RB211's development pushed back the TriStar's entry into service.[3]
The McDonnell Douglas DC-10 and related MD-11 use a straight-through layout with the engine part way up the fin, which allows easier installation, modification and access. It is also easier to re-engine. However it causes more drag than an S-duct. Also, as the engine is located much higher up than the other engines, engine failure will produce a greater pitching moment, making it more difficult to control.
The placement of the remaining two engines varies. Most smaller aircraft, like the Hawker Siddeley Trident, the Boeing 727 and the Tupolev Tu-154 have two rear-mounted pods in a T-tail configuration. The larger widebody Lockheed TriStar and DC-10/MD-11 mount an engine underneath each wing.
Trijets include:
- Boeing 727
- Boeing X-48
- Dassault Falcon 50
- Dassault Falcon 900
- Dassault Falcon 7X
- Dassault Falcon 8X
- Hawker Siddeley Trident
- Lockheed L-1011 TriStar
- Martin XB-51
- McDonnell Douglas DC-10
- McDonnell Douglas MD-11
- Tupolev Tu-154
- Yakovlev Yak-40
- Yakovlev Yak-42
Notable design projects include:
- Boeing 747-300 Trijet – downsized 747 to compete with the DC-10 and L-1011, changed to four engines
- Blended Wing Body Trijet – proposed design based on the Boeing X-48
- McDonnell Douglas MD-XX – stretched derivative of the DC-10, project shelved
- North American NR-349 – proposed interceptor derivative of the A-5 Vigilante, cancelled
- Airbus twin-tail trijet,[4] – status unknown
- Dassault Supersonic Business Jet – suspended
- Aerion AS2[5]
- Sukhoi-Gulfstream S-21
- Boom Technology Overture
- Boeing 777 – Originally envisioned as a trijet 767 in the 1970s to compete with the DC-10 and the L-1011; later became a new twin-engine design.
Four-engined jets
Other positions
See also
References
- ^ Mutzabaugh, Ben (2017-10-09). "For airlines, two engines are better than four". USA TODAY. Retrieved 2018-11-08.
- ^ William Green; The Observer's World Aircraft Directory, Warne, 1961. See for example pp. 189-199.
- ^ "Requiem For a Trijet Masterpiece – The Lockheed L-1011". September 30, 2015.
- ^ Airbus files patent for new trijet design, FlightGlobal.com, Retrieved 2008-12-11.
- ^ "Archived copy". Archived from the original on 2014-07-28. Retrieved 2014-07-20.
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