Triplane

A flyable reproduction of the Fokker Dr.I, the best known triplane aircraft of World War I.

A triplane is a fixed-wing aircraft equipped with three vertically-stacked wing planes. Tailplanes and canard foreplanes are not normally included in this count, although they may occasionally be.

Design principles

The triplane arrangement may be compared with the biplane in a number of ways.

A triplane arrangement has a narrower wing chord than a biplane of similar span and area. This gives each wing plane a slender appearance with higher aspect ratio, making it more efficient and giving increased lift. This potentially offers a faster rate of climb and tighter turning radius, both of which are important in a fighter. The Sopwith Triplane was a successful example, having the same wing span as the equivalent biplane, the Sopwith Pup.

Alternatively, a triplane has reduced span compared to a biplane of given wing area and aspect ratio, leading to a more compact and lightweight structure. This potentially offers better maneuvrability for a fighter, and higher load capacity with more practical ground handling for a large aircraft type.

The famous Fokker Dr.I triplane was a balance between the two approaches, having moderately shorter span and moderately higher aspect ratio than the equivalent biplane, the Fokker D.VI.

Yet a third comparison may be made between a biplane and triplane having the same wing plan - the triplane's third wing provides increased wing area, giving much increased lift. The extra weight is partially offset by the increased depth of the overall structure, allowing a more efficient construction. The Caproni Ca.4 series had some success with this approach.

These advantages are offset, to a greater or lesser extent in any given design, by the extra weight and drag of the structural bracing, and the aerodynamic inefficiency inherent in the stacked wing layout. As biplane design advanced, it became clear that the disadvantages of the triplane outweighed the advantages.

Typically the lower set of wings are approximately level with the underside of the aircraft's fuselage, the middle set level with the top of the fuselage, and the top set supported above the fuselage on cabane struts.

History

Pioneer years

Illustration of a triplane

The Bousson-Borgnis canard triplane was built ca. 1908. The first triplane known to have flown was the Goupy No.1, designed in 1908 by Ambroise Goupy, built by Voisin, and flown with a 37 kW (50 hp) Renault engine. A few weeks later Hans Grade's triplane became the first German-built aeroplane to fly. In the same year Farman modified his original Voisin machine to triplane configuration, and Dorand constructed his military triplane.

In 1909 Bokor constructed his own canard triplane. Through 1909 and 1910 the British aviation pioneer A.V. Roe built a series of four experimental triplanes—types I, II, III and IV—before abandoning the design. And in 1911 the Russian Rodjestveisky also constructed a triplane.

The fighting triplanes

The Sopwith Triplane, the first triplane to see service in World War I.

During World War I, some aircraft manufacturers turned to the triplane configuration for fighter aircraft. In practice these triplanes generally offered inferior performance to biplanes and only two types were successful enough to be produced, although in relatively small numbers.

Nieuport built a series of triplane prototypes between 1915 and 1917, featuring a top wing heavily staggered backwards to improve the pilot's view and a characteristic triangular strut arrangement bracing the three wings. The design resulted in poor handling and was dropped.

Sopwith developed three different designs in 1916. One, known simply as the Sopwith Triplane, went into production and became the first military triplane to see operational service. It had equal-span wings of high aspect ratio, mounted on a fuselage very similar to that of the preceding Pup biplane, and braced by one sturdy strut on each side with minimal wire bracing. The type was ordered by both the RFC and RNAS, but in the event the RFC traded theirs for another type and the Sopwith saw service only with the RNAS, where it served with success.

The Sopwith type's performance advantage and early successes over the Albatros D.III spurred military interest in the design, especially in Germany and Austro-Hungary. A flurry of prototypes were produced through 1917 and 1918, sometimes reluctantly under pressure from the military. Examples were produced by Albatros, Aviatik, Brandenburg, DFW, Euler, Friedrichshafen, LFG Roland, Lohner, Naglo, Offag, Pfalz, Sablating, Schütte-Lanz, Siemens-Schuckert, W.K.F and in ther USA by Curtiss. None of these went into production. Fokker's V.4 prototype of 1917 (identified by some as the V.3) had unusual cantilevered wings without bracing, the uppermost wing being attached only by cabane struts to the fuselage. The wings vibrated excessively in flight, and the next prototype, the V.5 featured a single interplane strut on each side similar to the Sopwith, but with no wires. This became the prototype of the famous Fokker Dr.I triplane of 1917 which became immortalised as the aircraft most closely identified with Manfred von Richthofen, the "Red Baron". Although it had a good rate of climb and was highly manoeuvrable it was not particularly fast. Following the break-up of two examples in the air the type was withdrawn from service for strengthening, and by the time it was re-introduced it was no longer at the forefront of performance.

The performance of the fighting triplanes was soon overtaken by improved biplane fighters. However, as late as 1919 three prototype Sopwith Snarks were flown.

Zeppelin killers

A few British designers pursued the triplane configuration in the anti-Zeppelin role. From 1915, Armstrong Whitworth developed the F.K.5 and F.K.6 prototypes. These were large three-seat types with twin engines and the middle wing of noticeably longer span than the others. Then in 1917 Blackburn produced their single-seat triplane. It was something of a throwback, featuring a pusher propeller and boom-mounted empennage in the manner of an earlier era. The arrangement was intended to allow fitting of an upwards-firing 2-pounder recoilless gun in the forward fuselage. Neither type progressed beyond the prototype stage.

Bombers and transports

A Caproni Ca.48 airliner.

The Caproni Ca.4 of 1917 entered service with the Italian air force as a heavy bomber in 1918. It was a successful design for its day and many variants were produced. Later on, after the war, Caproni re-numbered many of these variants as new types, including the Ca.48 airliner variant; a Ca.48 crashed while flying over Verona, Italy, on August 2, 1919, killing everyone on board (at least 14 and perhaps as many as 17 people) in Italy's first commercial aviation disaster and one of history's earliest – and, at the time, deadliest – airliner accidents.

The unsuccessful Caproni Ca.60 prototype transatlantic seaplane had three sets of triplane wings taken from the Ca.4, making nine wings in all, and is generally classified as a multiplane.

From 1918, Bristol developed a series of heavy triplanes which, like the Caproni design, appeared in different variants aimed at different roles. The first was the Bristol Braemar bomber, flying in 1918 with the Mk II version in 1919. The Bristol Pullman 14-seat transport variant flew in 1920. This was followed by two examples of a new, larger design for a military freighter - the Bristol Tramp.

The Tarrant Tabor, another and much larger British bomber, was built with three wings to carry the 6 engines required. It crashed on its maiden flight in 1919. Its designer Walter Barling went on to create the similar-sized American Witteman-Lewis XNBL-1, known as the "Barling Bomber", which first flew in 1923.

Racing triplane

In 1921 the "Cactus Kitten" racing triplane, formerly the monoplane "Texas Wildcat 1 (there was a biplane Texas Wildcat 2"), was created. It is the only design in history having gone from a monoplane to biplane to triplane configuration. Referred to as the Curtiss-Cox racer, designed by Cox, a Texan who sponsored it, the Cactus Kitten placed 2nd in the 1922 Pulitzer race behind a Curtiss biplane. It was powered by a 435 hp Curtiss C-12 engine with a 20' wingspan. In its triplane configuration it surpassed it's monoplane and biplane antecedents in handling and speed, and for a brief moment, the triplane was once again being noticed, the Kitten being touted as being the world's fastest plane in 1922, being capable of surpassing 200 miles per hour. The same year it was donated to the Navy and used as a trainer for the 1922 Pulitzer race, fame having proven very fleeting.

Tandem triplanes

The Caproni Ca.60 Noviplano in 1921

.A tandem triplane has two sets of triplane wings, fore and aft. Few have been made.

Dufaux produced Switzerland's first native aircraft design in 1908, as a tandem triplane with a smaller biplane horizontal stabiliser.

The 1909 Roe I Triplane has also been described as a tandem triplane due to its relatively large triplane aft plane.

The Fokker V.8 of 1917 was another tandem design although not a true tandem triplane, having a triplane fore wing, biplane rear wing and monoplane tail stabiliser.

In 1921, the Italian Gianni Caproni mated three stacks of triplane wings from his Ca.4 series to a single fuselage in a tandem triple triplane arrangement, to create the Caproni Ca.60 Noviplano prototype transatlantic airliner. It proved unstable and crashed on its first flight.

A further example was under construction in Kansas City, USA as late as 1922.

Recently, the term "tandem triplane" has been used for some new monoplane types that have active canard surfaces in addition to conventional wings and horizontal tailplane. A configuration having three comparable lifting surfaces in tandem is more correctly referred to as tandem triple or tandem triplet, and is not a triplane as such. These modern types may also be compared to the pioneer Voisin-Farman I and Curtiss No. 1 which also had a large main wing with smaller fore and aft planes; the smaller planes were not regarded as part of the main wing arrangement, and they were not described as tandem types.

See also

• Multiplane (aeronautics)

References

Bibliography

• Angelucci, E. and Matricardi, P.; World Aircraft - Origins-World War 1, Sampson Low, 1977.
• Green, W. and Swanborough, G.; The complete book of fighters, Salamander, 1994.
• Guttman, Jon, "Crazy Capronis", Aviation History, July 2008.
• Jane, F.T.; All the world's aircraft 1913, Sampson Low, 1913, facsimilie reprint David & Charles, 1969.
• Lamberton, W.M. and Cheeseman, E.F.; Fighter aircraft of the 1914-1918 War, Harleyford, 1960.
• Sollinger, G.K.; Villehad Forssman: Constructing German Bombers 1914-1918. Rusavia, 2009.