Plywood in the Construction of Aeroplanes (1942)

* I AHAT man has aspired to fly since the dawn of history is evident from the early legends of Greek mythology. The story of the escape of Daedalus and Icarus, his son, from the displeasure of Minos, King of Crete, is familiar to most of us. The tale tells of the wings built by Daedalus, for himself and his son by the use of which they were able to escape from the island. It tells of the disastrous results to Icarus when he flew so high that the sun melted the wax . by which his wings were held together, and how as a result he fell into the ocean and was lost. The philosophers of Ancient Greece doubtless discussed the problem of flight and one of the dominant characteristics of early religion was the attributing to the disembodied spirit of wings and the power of flight. From a practical standpoint, however, little in the way of experiment was done. The Chinese flew their kites and learned something of the power of moving air to lift inclined planes ; the Dutch built their windmills to harness the strong winds of the Low Countries, but these activities scarcely advanced the knowledge of the science of flight. Bird flight was studied by many medieval philosophers, of whom Leonardo da Vinci is perhaps the best known. The observations he recorded for posterity, and hints of his own aspirations to build a flying harness with which to conquer the realms of the higher air are still preserved in these records. Unfortunately, da Vinci had neither the machinery nor the materials with which to produce such an apparatus. Had he been able to carry out practical experiments he would, without doubt, have done much through his genius to advance the science of flight. He did recognize and record, however, one of the principles of such a flying mechanism, the principle that it must be of great strength and of light construction. Later investigators in the field of aeronautics probed more deeply into the secrets of wing structure and methods of propulsion, and much information as to the action of wings in flight was accumulated. Various mechanisms were invented, dependent for their lift and motion upon the flapping wing principle ; none were successful, but the studies made eventually gave the answer to the riddle of soaring flight and laid the foundation of the art of fixed wing flying. This development in the science of flight brought demands by the investigators for materials which had the combination of qualities, which da Vinci had many centuries ago discovered to be essentiallightness combined with strength. Early models of successful aircraft were designed with thin wing sections of linen or cotton, stretched on frames, and varnished to increase tautness and reduce resistance to air-flow. The inherent structural strength of the covering was not great, and it added little to the strength of the wind-frame to which it was attached. Wing loads at that time were very much lower than those of to-day, and the fabric covering proved reasonably satisfactory ; but, with the increase in engine power and the demand for greater speed, cleaner lines and heavier wing loadings were required. These could not be obtained efficiently from fabric-covered wings and fuselages, so resort to other construction materials became inevitable. Materials which combine within themselves all the desirable qualities of the perfect skin are not readily found. These qualities are strength, lightness, resistance to puncture, ease of manipulation, stability of form, elasticity, procurability and cheapness. One material, however, meets all these requirements very fully and that material is wood. Thin sheets of wood have in abundance all the desired qualities but one. That quality is strength. Wood is not strong when stress is applied in a direction at right angles to the grain. It is, however, very strong in