Why a Biplane with Tension Wires? by Hans Appel

Why a biplane with tension wires?

Hans Appel

1 / 8 © Hans Appel, 09-05-18 Why a biplane with tension wires?

Preface A biplane is an airplane with two main wings one above the other. The first, motor-driven, aircraft that was heavier than air was the Wright Flyer. This aircraft had a double-wing construction, just like many other aircraft in the first years of aviation. Although a biplane wing configuration has a structural advantage over a monoplane, namely the stiffness of the wings, a biplane produces more resistance than a cantilevered monoplane wing. Improved construction techniques, better materials and more speed ensured that the biplane configuration was outdated by the end of the 1930s.

Engine power vs stall In the first years of aviation, the engines only delivered a low power output. The low power meant that planes could only fly slowly. This lack of power, and therefore the low flight speed, was the cause of the low stall speed of those first aircraft. The slower the air flow over the wings, the less lift those wings generate. Increasing the wing area produces more lift and the angle of attack at which the stall takes place is thereby favorably influenced. However, a large wing area means a greater wing weight and weight hinders lift. So a large wing area with a low weight had to be combined.

A double wing configuration with a specific wingspan and cord has twice the surface area of a comparable wing with a single wing. As a result, such a biplane can fly slower without increasing the angle of attack and thus produce more lift than a monoplane with the same wing surface.

Biplanes offer more advantages over cantilevered monplane designs: they have lighter wing constructions, a lower wing load and a shorter wingspan for a given wing surface. However, the interference between the airflow between each wing increases the aerodynamic resistance considerably and, moreover, biplanes require extensive reinforcement of the wings (struts), which in turn causes additional resistance.

Aerodynamic interference Biplanes therefore suffer from aerodynamic interference between the two wings. This means that in practice a biplane will not produce twice the lift of a monoplane with the same wing area. The farther apart the wings are, the less interference is produced, but the struts between the two wings must therefore be longer. Due to the low speed of the early aircraft, the resistance of the wires and struts and the mutual interference of air currents were relatively small and acceptable factors. When the engine power increased after the First World War, the heavy thick-walled cantilevered wing with great stiffness became possible for the monoplane (cantilever monoplane). And with its higher power and faster speed, from 1918 the monoplane began replacing the biplane with many new designs.

2 / 8 © Hans Appel, 09-05-18 With a biplane, two wings are placed one above the other. Each wing provides part of the total lift, but as noted earlier, they do not produce twice as much lift as a single wing of comparable size and shape. The cause of this is that the upper and lower wings work in the same part of the atmosphere and therefore interfere with each other's behavior. For example, in a wing with a slenderness of 6 (slenderness = wingspan / chord) and a separation distance between the wings with the length of the associated chord, a biplane configuration will produce only about 20 percent more lift than a single wing with the same shape and surface.

To achieve maximum stability of the biplane, the upper and lower wings will be constructed 50% offset from each other. Furthermore an angle of 4˚ is given to the upper wing (figure 1).

4˚

Figure 1

In a biplane configuration, the lower wing is usually attached to the body. The upper wing is placed above the body with a construction of struts (cabane struts) and tension wires. One or both of the main wings may be provided with ailerons, while wing flaps are usually placed on the lower wing. Between the upper and lower wings, extra support is often given by means of wires and / or slender struts which are placed symmetrically on either side of the hull.

Tension wires When designing the first motor-driven aircraft, heavier than air, we always see many tension wires applied. These wires are needed to achieve the greatest possible rigidity and robust construction when using relatively light materials. Basically it means that we want to use a frame (light weight, flexible) as a beam (heavy, stiff, robust).

The cross-section of the tubular structure, for example the hull of an old-fashioned biplane, looks like Figure 2A. The hull is not massive, because then it would be way too heavy. The hull is therefore a tubular construction, covered with woven fabric. We therefore come across a tube “window” in a number of places in the hull as shown in 2A. Now this construction is far from stable. In Figure 2B we see that when a force is applied to the top left corner the "window" will twist. The construction of a diagonal pipe, however, results in a construction that will retain its stiffness with an exerted force from left or right, figure 2C.

3 / 8 © Hans Appel, 09-05-18 Figure 2A Figure 2B Figure 2C

Installing such an extra tube will now increase the weight of the structure and certainly if this is necessary in a number of places. And we want to avoid more weight. We solve this problem by using a tension cable instead of an extra tube (see Figure 2A), which of course is a considerably lighter construction. The problem here is that when a tensile force is exerted on the top right corner, the whole thing starts twisting again.

Figure 3A Figure 3B Figure 3C

Solution for this is found by using two tension wires. Figure 2C shows that. The construction is now stable, rigid and sturdy while the weight is much less than if one or two tubes were used.

Bays The space enclosed by a set of intermediate struts is called a "bay". In terms of force model, this space corresponds to the tubular "window" described above. A biplane (or triplane) with one set of such struts, which connect the wings on each side of the aircraft, is called a single-layer biplane or triplane. The bay provides sufficient strength for smaller aircraft. Examples are the Fokker D.VII fighter from the First World War, or the basic trainer from before the Second World War de Havilland Tiger Moth.

Large double-deckers for transport and bombardment flights often require several bays in order to gain sufficient strength and rigidity. These are called multi-bay biplanes.

Advantages and disadvantages of the double-decker The main advantage of the biplane over the monoplane is the combination of high rigidity with a relatively low weight. Stiffness requires a sturdy construction. And where complicated extra reinforcements had to be applied in early monoplane constructions, the biplane has a naturally stiff construction structure and it is therefore easier to make a biplane construction both light and strong. A wing of a monoplane must fully support itself, while the double wings help to reinforce each other. The biplane is therefore naturally stiffer than the monoplane.

4 / 8 © Hans Appel, 09-05-18 The structural forces in and on the wood of a biplane wing are also usually lower and can therefore be made much lighter.

A disadvantage of the biplanes can be that there is a need for placing additional struts to allow the spacing of longer wings.

Development In the period from 1914 to 1925, most of the new planes were double-deckers, although in 1918 the Germans experimented with a new generation of monoplanes such as the Junkers DI and Fokker D.VIII that canceled out the benefits of the biplanes. The French already employed the Morane-Saulnier AI "strut braced parasol" monoplane. Sesquiplane types, biplanes with abbreviated lower wings, such as the French Nieuport 17 and German Albatros D.III, had slightly less resistance than a conventional biplane while being stronger than a monoplane.

The available engine power and speed increased, while better materials could be used for wing construction. As a result, all kinds of reinforcements that negatively affect the performance of the aircraft were no longer needed. It was therefore possible to proceed to constructions with aerodynamically clean, cantilevered wings.

Triplanes Why did people ever come up with the idea of constructing triplanes? It was already known that around 1910, when this type of aircraft appeared, that the more wing surface was present, the more lift was produced at the same speed.

At the beginning of the 20th century people were not yet able to make self-supporting wing constructions with sufficiently light materials that could provide sufficient lift with a limited engine capacity. The biplane was a good solution for this problem. But, one might wonder, why not go further and make three-, four- or even multi-wing planes?

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

A balance has to be struck between increasing performance and increasing resistance, 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 this war, people experimented with triplanes. In the year 1917, even the largest number of such configurations were produced. Of the approximately 100 well-known three-deck designs in aviation history, three-quarters were designed between 1915 and 1918.

5 / 8 © Hans Appel, 09-05-18 The major advantage of the three-deck design is, as indicated earlier, the ratio of the size of the wing surface to the total wing width 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 with a shorter wingspan, more maneuverability and more 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 temsion 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 triplane are closer together.

A triplane gave no 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-deckers were less bothered by the fact that they were not adapted biplanes but were designed from the start as triplanes.

That, incidentally, is something that strikes the most about triplanes from the First World War: only a few were originally designed as such. Most were existing biplane designs, with the addition of an additional wing. Sometimes the upper and lower biplane wings were placed a little 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 add together the number of production units from Fokker (320), Sopwith (150) and Caproni, there are more built by them, compared to the other nearly 100 constructors / manufacturers.

6 / 8 © Hans Appel, 09-05-18 The decline of the triplane concept began in the spring of 1918 when the aerodynamic resistance (drag) of the tripane became a growing problem. The speed of (biplanes) fighter aircraft increased to such an extent that the use of larger and larger engines prevented an answer from the triplanes.

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).

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.

Graph of the climb speed of airplanes from the First World War

The Dr-I 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-I. 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.