Twin-boom aircraft
A twin-boom aircraft is characterised by two booms. The booms may contain ancillary items such as fuel tanks and/or provide a supporting structure for other items. Typically, twin tailbooms support the tail surfaces, although on some types such as the Rutan Model 72 Grizzly the booms run forward of the wing. The twin-boom configuration is distinct from twin-fuselage designs in that it retains a central fuselage.
Design
The twin-boom configuration is distinct from the twin fuselage type in having a separate, short fuselage housing the pilot and payload. It has been adopted to resolve various design problems with the conventional empennage for aircraft in different roles.Engine mounting
For a single engine with a propeller in the pusher configuration or a jet engine, a conventional tail requires the propeller or exhaust to be moved far aft, requiring either a very long driveshaft or jet pipe and thus reducing propulsive efficiency. The twin-boom configuration allows a much shorter and more efficient installation. The Saab 21 was originally built as a pusher type and was later adapted to jet power as the 21R.In these designs, the tailplane is typically high-mounted on twin tail fins to keep it clear of the engine wake. The Scaled Composites SpaceShipOne and SpaceShipTwo sub-orbital spaceplanes adopted twin booms with outboard tails or outboard horizontal stabilizers to keep the airframe clear of the more widely-spreading rocket engine exhaust.
Twin booms have also been adopted for twin-engined designs where the engine system includes bulky additional items such as turbochargers and heat exchangers, taking up a large volume of space. Examples include the Lockheed P-38 Lightning.
Field of view
For a rear observation or gunnery position to have an unobstructed field of view, placing it at the rear of a conventional tail moves it so far aft that problems arise with the centre of mass and balancing the aircraft. Getting rid of the conventional empennage allows the rear position to be located more forward, resolving the balance problem. An example is provided by the Focke-Wulf Fw 189.However the twin booms and bridging tailplane still obstruct the field of view to some extent and guns in this position are especially restricted in firing to the side.
Transport access
Loading and unloading large freight or cargo items such as vehicles and containers requires large access doors. In conventional designs these doors must be located at the nose or side of the fuselage, necessitating heavy reinforcement of the main structure. Side doors limit the length of an item to the width of the door and access may also be obstructed by engines or undercarriage. The twin-boom configuration allows a large door to be placed at the rear of the fuselage, free from obstruction by the tail assembly, as on the Armstrong Whitworth AW.660 Argosy.However access to the rear door remains limited, especially for trucks backing up to it, and a high-mounted conventional rear fuselage is often preferred.
Efficiency
Twin booms typically offer greater drag than a conventional arrangement. They are also typically shallower than the fuselage and thus inherently less stiff, requiring additional reinforcement to maintain a rigid tail position in pitch. On the other hand tip effects on the tailplane are avoided and it is supported at both ends, allowing it to be made smaller and lighter. Moreover, span loading along the wing can reduce the structural forces between the booms and thus overall weight.Some modern high-efficiency designs have twin booms which distribute the load along the wing span and/or stiffen the overall structure. Capable of flying non-stop round the world, the Rutan Voyager was a canard design with tractor propeller, in which the twin booms extended forwards to brace the foreplane as well as aft to support twin fins. The later Virgin Atlantic GlobalFlyer was jet propelled but with a similar range, still with large twin booms to accommodate the jet fuel in a lightweight span-loaded structure, but with a small conventional tail on each boom.
History
Twin boom designs can trace their history back to the lattices of booms used on many early boxkite aircraft. With the recognition of the tremendous drag these imposed, more compact structures covered in fabric were developed during the World War One. Prime examples include the Caproni series of trimotor bombers.Around the same time, the first wooden monocoque fuselages appeared, and it wasn't long before this technique was applied to provide twin booms. Possibly the first of these was the pre-war Nieuport pusher, which used paper impregnated with Bakelite however the most successful were the AGO C.I and C.II which used a more conventional wooden shell, built up from strips of wood glued over a form.
With the development of aluminium stressed skin monocoques later in World War One, the same technique was extended to twin boom designs, beginning in the 1920s.
Most of the early designs used twin booms to clear a rear mounted propeller, however even in World War One, several larger aircraft used them to provide a gunner with the ability to cover the underside of the tail without having to have the weight at the very extreme end of the aircraft where it posed balance and control problems.
Only in World War Two, with the increasing prevalence of transporting bulky items and vehicles by air was the utility of a rear door, in line with the cabin to ease loading realized, and with it, the utility of moving the rear fuselage structure to the sides to avoid excessive height in the rear fuselage as on the Gotha Go 242 glider.
With the beginning of the jet age, the need for clearance for the propeller was replaced with the need to provide a clear path for hot exhaust gases. Jet engine efficiency was hampered by long intake and exhaust trunks, as were used on many early designs, and one solution was to use twin booms to shorten the exhaust trunking to the minimum, such as de Havilland used on their successful Vampire and Venom jet fighters.
A small number of designs used twin booms for other reasons, most notable being the Lockheed P-38 Lightning, whose booms contained the overly lengthy engine turbo-superchargers, which would have made for an unusually long nacelle. The final use for a twin boom to be developed was in tying together very high aspect ratio wings and canards as on the Rutan Voyager, to reduce flexing, and the weight needed to otherwise constrain it. Also, by having the mass from most of the fuel mid-span, it reduces the forces on the wings considerably, much in the same manner mounting the engines mid-span on most jet transports does.
Despite these anticipated benefits, twin booms remain unusual. For most cases, the booms are less efficient structurally in providing pitch stiffness, and produce more drag. In the case of those using twin booms to improve the field of fire downwards, it severely reduces it laterally, and often directly astern. For transports, the booms may facilitate access to the fuselage, but trucks then have to be extremely careful to not hit parts of the aircraft that they are then getting closer to. As a result, the C-119 remained an anomaly, and most successful post-war transports, such as the C-130 Hercules, reverted to a single rear fuselage.
Even worse, at transonic speeds attained in long power on terminal dives, the location of the tail on the P-38 caused it to be blanked by the wing and fuselage, which caused Mach tuck, when the nose would pitch down violently and dangerously, although it was otherwise extremely forgiving. It is also unclear that the twin booms really offered any advantages over a conventional fuselage and extended nacelles, which worked well in several other designs.