Triplanes an introduction

by

Hans Appel

1 / 7 © Hans Appel 17-08-19 The "Triplane" introduction In 1843, Sir William Samuel Henson, together with John Stringfellow, already published an article in the Mechanic's Magazine in which they described an aircraft heavier than air, equipped with wings that had a wingspan of no less than 45 meters. The aircraft was powered by a steam engine. In the same magazine was also an article by Sir George Cayley that indicated that a shorter wingspan with three more compact wing pairs placed below each other would be wiser.

This was probably the first time that a triplane was mentioned. John Stringfellow worked out the idea of Sir George Cayley but had no success with it. It took until 1908 until a motor-driven triplane was built.

Ambroise Goupy constructed his Goupy 1 and flew it about 150 meters.

Farman constructed a three-decker and flew with it over a distance of 8 kilometers on January 6, 1909.

2 / 7 © Hans Appel 17-08-19 Hans Grade constructed a small three-decker in Germany and used it to fly about 400 meters.

In England, Alliot Verdon-Roe constructed some three-deckers around 1910 with which he experimented and set the A.V. Roe Company.

First World War During the First World War, the company Sopwith on Thames set up a whole new project: the LC1TTr, which we now know better as the "Sopwith Triplane". This aircraft was successfully used against the German Fliegertruppe. In the professional hands of pilots from the Royal Naval Air Service (RNAS), this aircraft was a fearsome and successful weapon.

The Inspektion der Fliegertruppe (Idflieg) was now convinced that the English had a powerful weapon in their hands with the Sopwith three-decker and ordered Hauptmann Mühlig- Hoffmann to write a treatise on the phenomenon of triplanes on 27 July 1917.

The Germans had already captured a Sopwith Triplane in Aldershof (the German test center) and Idflieg invited the German industry to come and inspect the aircraft and then design and build such an aircraft itself.

It seems that many of the companies that made use of the possibility to investigate the plane were not really interested in constructing a triplane. Many of the invited companies took a successful biplane and provided it with a third wing set. This resulted in aircraft that performed poorly. Eventually, until the end of 1918, the different manufacturers would construct around 34 types of triplanes.

Remarkably, only two manufacturers were able to build a suitable prototype: the Pfalz Flugzeugwerke GmbH in Speyer am Rhein and the Fokker Flugzeugwerke GmbH in Schwerin. Both companies had made a completely new design that was not based on existing aircraft.

The Pfalz Dr.I In October 1917 the test phase for the Pfalz dr.I in Adlershof was completed. During the tests at Adlershof it was found that the Pfalz Dr.I equipped with a 160 hp Siemens-Halske rotary

3 / 7 © Hans Appel 17-08-19 engine delivered better performance than the Fokker especially when it came to the speed of ascent.

The first 10 aircraft were sent to the front but the performance was very disappointing. In addition to structural shortcomings, a major problem was due to the unreliability of the engine. In no time, 9 out of 10 machines delivered were on the ground. There was some experimentation with other engines, but that did not help much. That is why the Pfalz Dr.I was never taken into production.

Fokker Anthony Fokker was the only manufacturer who came to the front to consult with the pilots. So it happened that he was at the front in April 1917 when Manfred Freiherr von Richthofen had a nerve-racking fight with a Sopwith Triplane behind him on 20 April 1917, flying himself in an Albatros D.III. Von Richthofen explained to Fokker that the urgency to have such a type of aircraft available was very high. Fokker probably saw the captured Sopwith Triplane before it became available in Adlershof.

Already on 3 June 1917, Fokker commissioned its design agency to redesign a Fokker D.VI biplane with work number 1661 and to convert it into a triplane. The lower, middle and upper wings had a gradient spanwith. The upper wing was equipped with so-called horn-balanced ailerons (ailerons, also called elephant ears). The wings were self-supporting, so no tension wires and supports (struts) were needed.

Leutnant Werner Voss was the first to have the opportunity to fly the 1661 and he thought the maneuverability and ascent speed of the aircraft were excellent. He still had some technical remarks including his concern about the flutter effect of the wings. He therefore proposed to put struts between the wings.

4 / 7 © Hans Appel 17-08-19 On July 5, 1917, Fokker commissioned his design agency to design a second D.VI under work number 1697 (later 101/17). Idflieg approved the design and wanted to conduct formal tests with the aircraft type as quickly as possible. That meant three planes. One for material testing under tension and two for combat testing. It was started on July 11, 1917 under work numbers 1729 and 1730. On July 14, 1917, an order for 20 Fokker triplanes followed.

Triplanes Why did people ever come up with the idea of constructing triplanes? Well, we have seen that the first ideas arose around 1843.

It was therefore known in that time, around 1910, when this type of aircraft appeared that: the more wing area was present, the more lift (upward force) was produced by those wings at the same speed.

And at the beginning of the last century, when it was not yet possible to make self-supporting wing constructions with sufficiently light materials that could provide sufficient lift with a limited engine power, the biplane was a good solution for this problem. But, one might ask, why don't go further and make three-, four- or even more wings on a plane?

From a purely aerodynamic point of view, this is not a good idea, because constructing more wings not only increases the lift, but also increases the aerodynamic resistance. As a result, more power is required, so a larger engine that again uses more fuel.

A balance must be found between performance increase and resistance increase, with the advantage of a better slenderness (wing-length / width ratio) of the wings, so that lighter wings can be constructed.

Wars always give an extra boost to technological development and the "First World War" was no exception. During the "First World War" people experimented with triplanes. In the year 1917, even the largest number of such configurations were produced. Of the approximately 100 well-known triplane designs in aviation history, three-quarters were designed between 1915 and 1918.

The major advantage of the triplane design is, as indicated earlier, the ratio of the size of the wing area to the total wingspan of the wing. This meant that not only heavy bombers within, for that time, manageable construction limits could be designed, but gave to fighter aircraft, by loading the wings less and a shorter wingspan, more maneuverability and lift.

Now, triplanes also have intrinsic disadvantages, and moreover, the increased ratio between wing surface and wingspan is not just a gain. The third wing and the extra lines and struts that were needed to give the structure sufficient rigidity were a considerable disadvantage. Furthermore, useful effect was lost due to disruption of the aerodynamic air flow between the wings. This is due to the fact that the wings of a three-decker are closer together.

5 / 7 © Hans Appel 17-08-19 A triplane did not provide extra robustness, many triplanes were known to be less robust than their biplane colleagues. And with regard to maintenance: due to the complexity of rigging and adjusting the extra tension wires and struts, the complexity increased considerably. The Fokker and Sopwith single seat triplanes were less bothered because they were not adapted biplanes but were designed from the beginning as triplanes.

That, incidentally, is something that strikes the most about "First World War" triplanes: only a few were originally designed as such. As we have seen, most existing biplane designs were added with an additional wing. Sometimes the upper and lower biplane wings were set further apart and another wing was placed between them. For others, the wings were completely redesigned, which meant that substantial changes were needed to the hull and the rigging.

The reason for popularity around the triplane was the success of the English Sopwith triplane- fighter. After a brief successful combat test in June 1916, a number were ordered by the Royal Flying Corps (RFC). But for an inexplicable reason, these were delivered to the Royal Naval Air Service (RNAS) and made available to the operational squadrons in April 1917. In fact, the Sopwith triplane was a conversion of the Sopwith "Pup" which was replaced by an 80 hp standard engine and a 130 hp engine.

The German "Luftstreitkräfte / Idflieg" were so impressed with the performance of this English aircraft that they asked Fokker to develop a triplane. The advantage of the Fokker designers was that they could develop a triplane from the start. Which allowed them to assume the maximum aerodynamic and mechanical-structural benefits of the three-wing principle. The first Fokker triplanes appeared at the front in August 1917 and were immediately a success in the hands of "aces" such as Richthofen, Voss and Gontermann.

When we consider the number of production planes from Fokker (320), Sopwith (150) and Caproni (24) and add them together the coun is 494. That is more than there were ever built by the other nearly 100 constructors / manufacturers.

The decline of the triplane concept began in the spring of 1918 when the aerodynamic resistance (drag) of the triplanes became a growing problem. The speed of fighter (biplane) aircraft increased to such an extent that the use of larger and larger engines prevented an answer from the triplane.

Fokker Dr.-1 The Fokker Dr.-1 was a triplane of which all three wings were self-supporting (cantilever). That is, the wings were supported but that was done inside the wing. There were struts between the wings but that was because the pilots felt safer, but those struts were structurally unnecessary.

The wings of the Fokker Dr.-1 were very thick compared to the thin wings that were common at the time. It was apparently not understood that thick wings with more profile than their thin counterparts can also generate more lift. Research on the influence of wing thickness was done at the University of Göttingen, and Fokker applied it design for the so-called Göttingen 298 wing support for the Dr.-1. Furthermore, the wings were provided with "horns" (horn- balanced aileron) (elephant ears).

6 / 7 © Hans Appel 17-08-19 This was done to reduce the pivotal moments of the ailerons so that the pilot had to use less force on the stick to initiate the roller movement.

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The Dr.-1 had the lowest zero-lift drag coefficient (0.0323) of all fighter aircraft from the First World War. This value was mainly due to the relatively small wing area of the Dr.-1. In addition, the lack of struts and tension cables also had a major impact on this value. And finally there was the "thick" wing profile that was used.

The speed of the Dr.-1 was certainly not impressive (103mph) compared to other aircraft of that time, but the Dr.-1 had an excellent climbing ability and excellent maneuverability.

7 / 7 © Hans Appel 17-08-19