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Aviation History
1939
1939 - 0078.PDF
b FLIGHT. JANUARY 12, 1939 material is next put in a trough about 30ft. long by 6in. wide. This trough is then led through an 800-ton hydraulic press (Fig. 5) specially constructed for this pur pose. The trough is electrically heated and is so arranged that the temperature is high during the operation of mould ing, and remains at a high temperature after the trough has passed through the press. The press has two rams, one at the top and the other at the side. The ram at the side is to give lateral support to the trough ; the actual moulding is done by the top ram. With this press we can also produce finished spars of I-section of unlimited length, but normally we produce a plank 6in. wide by 30ft. long and of any required thickness. Some may feel disappointment at not seeing pictures of the production of complete moulded wings and fuselages. I do not think, myself, that that is the way in which development will take place in the immediate future ; I believe that the contribution which plastic materials can make to aircraft construction at the present day is in the form of standard materials which can be fabricated by any aircraft manufacturer into structures of any size or shape, using only the simplest possible tools. It would be unwise to dogmatise, but it does seem difficult to visualise anyone (except, possibly, a communistic or totalitarian government) being prepared to lock up thou sands of pounds of capital simply to produce one particular size and shape of wing or fuselage. At any rate, at the moment I do not think the moulding of really large com ponents is a profitable line of development to pursue. But, having made our long planks (which, of course, can be longer than 30ft. if required, as there is no limit to their length), the next step is to build them up into an aeroplane. For this purpose let us suppose that we have to make the front spar of a cantilever monoplane of about 12,000 pounds all-up weight. The density of the material is sufficiently low to make unnecessary the use of elaborate extruded sections, so that we can use our material in plarik form. The chief problem is that of making connections. The shear strength of the material is only one-eighth that of its tensile strength ; the ratio for wood is, of course, far worse, but wood is a material which can be glued, and we are not yet satisfied with joints made by glueing our material. Clearly, if we are to use mechanical joints we cannot use the common but crude expedient of the hole and pin, and therefore we have developed a system of interlocking dowels which has proved under numerous tests to be efficient. Joint Construction Fig.- 6 shows the construction of a typical spar joint. In one flange we make a groove into which we insert a ring which locks into a similar groove in the adjacent web members. This method transfers the load straight from one surface to another far more efficiently than would the use of bolted connections with their attendant concentra tions of stress. The joint always gives central loading on compression members. The use of a circular dowel is advantageous not only because of the ease with which one can make circular grooves, but also because it gives the greatest stability to the ring under the particular loads imposed, which produce Fig. 4. The machine used for drawing out the fibres in a uniform parallel arrangement, and then impregnating them. couples in the same sense at opposite ends of a diameter. We take a plank and we drill it with 2 B.A. holes at each joint position ; then, by the use of a trepanning tool, we cut grooves around each 2 B.A. hole. The load on the joint at the root-end exceeds 40 tons, which, as Fig. 7 illustrates, is quite a considerable load if thought of in zoological concepts. The next view shows this joint in some detail; it consists simply of dural plates sandwiched between the synthetic material. One's first reaction is to say that there is far too much metal, but when it is remembered that the synthetic and the metallic materials have both about the same ultimate strength, it will be seen that the joint is not so ill-proportioned as might at first appear. On test it has stood up to the required load. The construction of such a spar is a simple process ; it is held together with 2 B.A. nuts and bolts. I think that I have said enough to give an idea of the position we have now reached in the development of plastic materials for aircraft structures. Briefly, we can produce a material of good strength-to-weight characteristics in almost unlimited lengths and which can be built up easily into structures capable of withstanding heavy loads. Development, of course, is in no way finished, and we are working now at the production of materials with char acteristics specially suited for lightly loaded structures. The table of comparative strength figures already given is only a fair comparison for tension members, and few components in a structure are subjected to tensile forces only. If we try to make a more general comparison cover- Fig. 6. Method of making a typical spar joint by a system of interlocking dowels.
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