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Evolution of aircraft structures

 

The Flyer I, designed and built by the Wright brothers made its maiden flight in 1903. That flight is considered as the first in aviation history because it was an aeroplane equipped with an engine and flight controls. The device was made of steel, wood and impregnated fabric, a true feat taking into account the era’s knowledge and engines.

Some people want to go much further by questioning the basic principles of aircraft structures as we have known them since the Wright brothers first took to the skies.

The structures of the very first aircraft built were based on wood and impregnated fabric and their flight controls were made of steel wires. Enough rigidity to lift the aircraft in flight was sought with the materials used. Hence, a design was needed where the distribution of force vectors was divided in such a way that the stress on the wood would not reach a point that could cause excessive torsion or breakages. In order to achieve this, the design was comprised of a large number of struts and ribs of a certain size that added to the aircraft’s overall weight. The problem of the materials’ instability with regard to flight conditions, manoeuvrability and weather conditions was added to all this. Aside from the conditioning factors caused by the aircraft’s design, calculations of in-flight forces were as yet unknown, since it was literally impossible to measure these forces with real values using suitable equipment. Despite the engineering calculations made on aerofoils and on the structure as a whole, there were undetermined and uncontrolled factors that meant that flight capabilities were rather limited.

Once the Second World War broke out, the need arose for heavier means of transport capable of carrying troops and weapons with better aircraft than the ones then in existence, along with greater speeds and more manoeuvrability, especially in the case of patrol and fighter aircraft. It was then that the development of new control techniques commenced and, more importantly, a more detailed study of the structure in search of the best possible performance with the capacity- speed-consumption ratio. Metal structures with aluminium as the main element began to be used.

The new aircraft’s transport capacity required higher traction than the ones which had been available up to that time. New means of propulsion and propeller manufacturing techniques were studied. The new engines which began to appear from that moment on provided the necessary force to drag the useful load foreseen. They, however, required greater consumption and increased the aircraft’s total weight due to both the engines used and the amount of fuel. The very first jet engine for human transport and rocket engine to lift unmanned vehicles appeared at this point in history. Apart from the new propulsion systems, it was observed that aerofoils and aircraft controls needed fairly high handling forces. In fact, a pilot using steel wire systems cannot handle certain control surfaces (ailerons and flaps). The conclusion was therefore reached of the need for indirect control systems using hydraulic servos. The power-assisted steering we presently use in cars was thus invented.

Added to all this were the weights and forces exerted on aircraft structures. Designs were therefore modified in order to achieve a much more complex distribution of vectors. The most immediate consequence of all this was that the aircraft’s weight was increased even more in an effort to eliminate direct action by using ribs and struts. The study of aeronautical design began to be viewed as a fairly important field in engineering.

After the war, civil aviation took advantage of all the advances of that period and the study of new air transport capabilities became widespread. The measurement of in-flight tensions and forces commenced, which gave rise to structural design changes and the use of new materials which weighed less and provided greater flexibility and resistance to forces. Composite materials were thus invented.

Thanks to composite materials like fibres (carbon, polyester, etc. fibres), metal sandwich systems and other similar techniques, progress is being made in increasing the aforementioned ratio and better performance with minimal consumption is being achieved. Today it is assumed that the useful load of a commercial aircraft is around 30% of its total weight, most of which is accounted for by the fuel needed for take-off, the engines and the rest of the equipment.

However, the most important thing about today’s advances does not reside in the philosophy of searching for a lighter or more robust structure, but rather in achieving means of generating thrust that do not require a great deal of fuel and in doing away with equipment by manufacturing light electronic systems, using fibre optics instead of traditional wiring or even using robust wireless systems so that the aircraft’s useful load is higher than its empty weight. Obviously, the study of aircraft structures is headed towards more resistant materials which are as light as possible. Studies today are focusing on the search for lighter materials and a new material known as “metal foam” has been invented.

There are three ways to improve a material’s strength-weight ratio: try to improve strength, reduce density or both. In order to attain a strength-weight ratio that manages to reduce density while improving strength, a foam material has been designed where the existence of internal cells is controlled and corrected so that they have a certain direction, separation and number to subsequently make the structure more rigid with a thin layer of aluminium oxide. This results in a hardness similar to that of steel, but with a density equivalent to that of water (PNAS – Proceedings of the National Academy of Science of the United States of America). The problem with this new material resides its cost, which until now is excessively high.

There are presently a number of approaches which aim to achieve all that has been mentioned above. Some, however, want to go much further by questioning the basic principles of aircraft structures as we have known them since the Wright brothers first took to the skies. In a few years’ time, all these concepts may become obsolete, but there is no doubt at all that as of today the constant evolution of aircraft structures will carry on unabated.

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