How Light Members are Assembled to Produce Strong Airframes
FRAMEWORK OF A SINGLE-SEATER FIGHTER with the fabric removed. It is a Czechoslovak all-metal machine, and is a typical example of the older type of biplane construction now largely superseded by the method by which wings and fuselage are metal-covered. This metal covering takes much of the load from the framework.
THE building of an aeroplane and the method and materials used are intimately connected with the aerodynamics and layout of the machine, and also with the number of machines to be built. It is illogical to say, because a certain type of structure has proved suitable in one set of conditions, that it is therefore a completely satisfactory type to be used universally. A small civil aeroplane designed for the “private owner” market in England may well be built of wood, as for this type wood is both cheap and light. If, however, such an aeroplane is taken out to tropical or Arctic regions, signs of trouble may quickly show themselves, which would never have been suspected in a temperate climate. On the other hand, to have built the type entirely in metal would have handicapped its sales amongst the greater number of possible customers at home because of the extra weight and expense.
The modern all-metal “monocoque” or “lobster claw” construction justifies itself for large aeroplanes of a standard type produced in considerable quantities. If the designer is experimenting with a new aeroplane of which only one will be built, and that one liable to have extensive alterations, then, he will find it much cheaper to use wood.
In the early days of aviation, when the constructors scarcely knew if their machines would even leave the ground, and when the problems of aerodynamics were still the greatest that they had to solve, they used the simplest and cheapest material, wood. Now, when their problem is to get the greatest number of a standard model into the air in the shortest time, they are studying the problems of punching out steel metal to the finished shape in large hydraulic presses.
The advantages of metal construction may be stated briefly. By its use it is possible to build large aeroplanes which would not be possible in wood. It would scarcely be possible, for example, to contain within the thickness of the wing sufficient material to withstand the heavy loads coming on it unless that material were a strong metal capable of carrying high stresses. Although this argument does not hold for small machines, the medium and large-sized aircraft can be built with a lower weight in metal than in wood. The dividing line occurs where a weight of about 5,000 lb is reached.
EXAMPLES OF MODERN CONSTRUCTION METHODS. On the left is a representative type of fuselage joint, part of a Czechoslovak military biplane. The joint is made up of plates and machined fittings connected together with pins, bolts and tubular rivets. On the right is a section of an Armstrong Whitworth high-tensile steel spar. The corrugations stiffen the thin metal and prevent buckling. The spar is about eight inches across and the metal is approximately one-hundredth of an inch thick. These spars are always built into the wing upright.
Metal is more reliable than wood, as its manufacture ensures a much greater consistency; moreover, there is not the same necessity to make allowances for hidden defects. Metal is not affected by weather and climate to the same extent as wood. Timber shrinks in a hot dry climate and swells in a humid climate. Further, the glue with which it is joined together deteriorates rapidly, and is subject to fungus growth and the attacks of bacteria. This does not mean that it is hazardous to use a well-maintained wooden aeroplane in adverse conditions, but that more constant inspection and care are required. Wooden parts are joined together by glue, but the corresponding metal details may be attached one to the other by bolts or rivets. Not only are such joints more durable, but also their strengths can be forecast much more accurately by the designer. They can therefore be made more efficient, as it is not necessary to make the same allowance for defects and deterioration.
In the earlier days of flying the risk of fire was great. This risk will never be completely eliminated as long as petrol and oil are needed in the engine. Although metal will burn at high temperatures there is much less danger of the main structure catching fire if timber is not used.
An important feature of metallic materials is the ease with which they may be worked into a variety of shapes and sizes. Whereas only two or three kinds of timber are suitable for aircraft construction, there is an almost infinite range of suitable metals, from magnesium to steel, each with its range of alloys, and the characteristics may be varied for particular needs.
The advantages of metal have been set forth above, but it would be possible to give at considerable length certain rival claims of timber. Our purpose is not to decide which, if any, is the ideal material, but rather to give reasons for the present popularity of metal construction. It is possible, however, that the industry may see the introduction of a completely new kind of structural materials, the synthetic resins or “plastics” having characteristics belonging neither to timber nor to metal. Since metal construction of aircraft first became general in Great Britain in 1925-26 there have been two generations or types of structure bearing little resemblance to each other. The first type consisted of a braced framework of tube or strip members covered with fabric. It was widely used for the biplanes which were popular at that time. In the early thirties the monoplane came into favour and, with it, the “lobster claw” structure. This was not entirely new, for Short Bros., Ltd., of Rochester, Kent, and other firms, had experimented with it years before. As, however, it was more suitable for the monoplane than for the biplane, it had to wait for aerodynamic developments before it became popular.
The principle of the “lobster claw” or “stressed skin” construction is that as much of the strength as possible is carried in a metal sheathing which forms the outside envelope of the structure. The skin is riveted on to a light framework of transverse and longitudinal members. Fuselages built in this way are spoken of as “monocoques”.
UNCOVERED WING TIP of the top plane of a Supermarine Stranraer flying boat. The light braced girder ribs which hold the fabric covering to the correct aerofoil section, and some of the cross-bracing wires are clearly shown. The ribs carry the load on the wing to two longitudinal spars placed at about one-quarter and two-thirds of the width of the plane from its leading edge.
Another distinction between the older fabric-covered braced structure and the stressed skin structure lies in the material used. For the braced structure it would be possible to build with equal efficiency either in high tensile steel or in duralumin - an alloy of aluminium with small quantities of copper and other metals. Because steel was the major product of one of the largest industries in Great Britain, whereas aluminium was imported, steel was the more generally favoured.
Steel is too strong to be used for a monocoque structure, or - putting it another way - if its full strength were to be used the metal skin would be only a few thousandths of an inch thick. This “tinfoil” thickness is not practicable. To use a greater thickness suggests the need of a lighter material. Duralumin is almost one-third of the weight of steel, and its tensile strength of about 28 tons per square inch allows the use of thicknesses from fifteen up to fifty thousandths of an inch in the appropriate parts of the skin. Duralumin, therefore, or one of the other hard aluminium alloys, is now generally used for the modern stressed skin monoplane.
Having now outlined the reasons for arriving at the types of structure in use today, we may turn to some examples of metal construction and examine them in detail. Taking first the main planes of the older kind of biplane, we find that they follow the conventional practice established in the early days of wooden construction, metal members being substituted for the timber members.
Principle of Corrugation
In the Czechoslovak single-seater fighter illustrated above, the air load is taken on a fabric covering, which transmits it to the ribs. These conform to the aerofoil section of the wing and are of braced girder construction.
In the Supermarine Stranraer flying boat (see above) the ribs carry the load to two spars placed at about one-quarter and two-thirds of the chord back from the leading edge. The spars act as beams and are supported at one or two points by the interplane struts running from top to bottom planes. The panels so formed are cross-braced by wires.
The design of the spars is perhaps the most difficult part of the whole structure. The material must be very thin to save weight, particularly if it is of steel. But thin strip steel is liable to buckle, and therefore means must be found of stiffening it. A piece of corrugated iron is much stiffer than the corresponding piece of flat sheet iron. The same principle was applied to spar design.
The Armstrong Whitworth spar, a section of which is seen above, is made of high tensile steel 1/100 to 1/80-in thick, and measures eight inches deep. It will develop a stress of over 60 tons per square inch in the booms. It weighs less than three-quarters of a pound per foot length. The corrugations give the spar great stiffness and allow it to develop so high a stress. Without them the spar would be flimsy and would buckle up at a low stress, perhaps at only one or two tons per square inch.
PORTION OF A MONOCOQUE FUSELAGE of American design being built on a rotating jig. This type of jig increases the accessibility of the parts on which work has to be done The sheet-metal covering is first bolted in position, as seen at the top of the picture, before being riveted. A completely riveted section is visible at the left-hand end of the portion of the fuselage shown.
The fuselage construction generally considered to be appropriate to a biplane consisted of a braced framework built of round tubes, and covered with fabric. The tubes might be either of steel or of duralumin, steel being more usual. When steel was used, a popular method of joining the tubes at the joints was to weld them with an oxy-acetylene blow lamp. An example of this, given further down this page, shows the cockpit portion of the fuselage of a Fokker C 5 fighter. The advantages claimed for this method of construction were that it was cheap, light, and simple to repair. It was cheap because only low carbon steel can be welded successfully. The more expensive high tensile and stainless alloys are not easy to weld. Further, but little workshop equipment was needed, and if only a few machines of any particular type were to be built, they could be turned out quickly. There was a saving in weight because there were no bulky fittings at the joints of the tubes.
BUILT-IN WING STUBS are often used to join the main plane to the fuselage. In this picture a German Junkers 160 monoplane is seen under construction. The outer lengths of the wings are built separately and are joined to the stubs by strong bolted fittings.
Equally strong claims were made for the fuselage built up of tubes connected together by bolted joints. An example of such a joint is illustrated above. This, taken from the Avia fighter, built in Czechoslovakia, is similar to the system used on the Hawker aircraft in England. This method perhaps lends itself to a larger scale of production than the method of welding where production is directly proportional to the number of skilled welders employed. The various pieces which go to make the joint may be stamped out of sheet metal or turned on an automatic lathe. They may therefore be made quickly and cheaply and the rate of their production depends more on an ingenious design of tools than on the number of workmen.
The tubes of the fuselage framework are all cut to length in jigs and the final work of assembling the tubes and fittings may be done by semi-skilled labour. As with many other engineering operations, the less skilled the labour required to carry it out, the more skilled must be the designers, the tool makers and those who plan the order of the work. A few highly skilled men may between them build a few aeroplanes at a slow rate. But if the rate of production is to be high, then large numbers of skilled men are needed to control the work of even larger numbers of unskilled workers.
This point is further exemplified by comparing the construction of welded steel tube fuselages with that of the modern monocoques which are described later in this chapter. With welded steel fuselages the workshops can start soon after the designer has conceived the design and completed his calculations. The work will then go forward steadily and the aeroplanes will be completed in direct proportion to the number of “man-hours” which are put in on them.
To achieve rapid production of the metal sheathed monocoques, the process is dissimilar. The drawing office work is much more elaborate and the details must be thought out with much more care before the designer will risk sending a single drawing to the works.
A WELDED-JOINT FUSELAGE, of which the cockpit section is seen here, is used on the Fokker C 5 military biplane. Round tubes of low-carbon steel are joined together by oxy-acetylene welding. The advantages claimed for this method of construction include cheapness, lightness and simplicity in the making of the necessary repairs.
When the drawings have been received there they are handed over to another set of draughtsmen under the control of the works manager. Their job is to design the tools and jigs which are necessary to make the thousands of small parts which build up into the whole. This will occupy some time. One complete set of everything is generally made to begin with, and this is assembled for checking purposes.
Adjustments and corrections are then made and the main production can begin. If, for example, five hundred of the type are to be made then five hundred of each part are made straight off. As each set is completed it is put into the “finished part” stores until assembly can begin. The erection shop will remain empty until sufficient parts are made to justify starting the final assembly. Thus months may pass before any visible progress is apparent. Suddenly the point is reached when the assembly lines can start up, but weeks will pass before the first complete aeroplane comes off and is wheeled on to the aerodrome. Once that has happened the rest will follow at a great rate and it will then be possible to equip new squadrons every week.
This brief and somewhat idealized account of the production of modern metal aircraft gives but an inadequate idea of the many thousands of processes involved and of the thorough coordination which is necessary between all the departments of the factory.
Analogy of Shipbuilding
The structure of a modern monocoque or stressed skin fuselage is similar in many ways to the structure of a ship. The internal framework is first built up of transverse bulkheads and frames, crossed at right angles by the fore-and-aft “stringers”. The channel, angle and Z sections used for these may be bent up out of strip, or “extruded” by pumping a billet of duralumin through a die of the required section. The lighter frames may be made by bending hoops of angle or channel section to the transverse shape at the appropriate point. The stronger bulkheads are made by building a similar hoop and then plating it across with flat sheets of duralumin, riveting the parts together with duralumin rivets. Notches are generally cut in the frames and bulkheads to allow the main stringers to run through from front to back of the fuselage. This framework is then covered with thin duralumin sheeting, riveted on in panels. The appropriate holes are cut for doors, windows and ventilators, and the edges of these holes are stiffened up with doubling plates or bent angle sections.
A typical monocoque of Czechoslovak design is shown on this page, along with an illustration of a short portion of such a fuselage being built up on a rotating jig in the Curtiss-Wright factory in America. This shows how the covering sheets are first bolted in position before being riveted.
FUSELAGE OF A TRANSPORT AEROPLANE typical of Czechoslovak monocoque design. The transverse bulkheads and frames, crossed at right angles by the fore-and-aft stringers, make the construction similar in many ways to the structure of a ship. The bulkheads are made by building a suitable hoop and then plating it across with flat sheets of duralumin, the sheets being joined with rivets of duralumin.
The similarity between such methods of fuselage construction and shipbuilding is noticeable. There is, however, one great difference between the two, and it lies in the different nature of the external loads applied to the structures. When a ship is afloat in still water the force of buoyancy is relatively even along the hull. Even in heavy seas the force varies gradually. When an aeroplane is flying there are heavy point loads applied to the fuselage at the attachments of the engine, main plane and tail unit. In between these the external leads are small.
A method of building in the wing is shown above, illustrating the fuselage of the Junkers 160. A short section of the wing is built into the fuselage on either side. Where the spars meet the fuselage there are strong frames, and these are robustly braced by panels of sheeting. The outer lengths of the wings are built separately and pick up the centre section on strong bolted fittings.
A monoplane wing under construction is also illustrated on this page. Such wings generally taper towards the tip, there being considerable aerodynamic advantages to be had from this practice. Many different types of structure have been evolved, but the one shown is perhaps the most usual. Two spars, or main girders, run from the tip towards the centre. These carry the main loads and must be stiff against the bending caused by the weight of the fuselage reacting against the “lift”. The ribs of such a wing are generally stamped out of flat sheets of duralumin. Every effort is made to save weight by punching large holes, the edges of
which are stiffened by lipping or flanging over the metal.
In some of the earlier stressed skin wings, notably those made by Junkers in Germany, the skin covering was corrugated to stiffen it. These corrugations ran fore and aft in the direction of flight, but even so they caused a great increase in the head resistance of the machine. Recent American practice has favoured the use of a double skin, the inner one of which is corrugated from tip to tip. In the example shown on this page there is only a single skin of fiat sheet. This is held up against buckling by channel section stringers parallel to the spars, riveted to the inner side of the skin.
Protection Against Corrosion
The distribution of the stresses in a stressed skin wing and fuselage is difficult to estimate mathematically. It is relatively easy to make such a structure adequately strong. This may be done, for example, by assuming that the whole of the bending stiffness is provided by the main lengthwise members, and the duty of the skin is to provide torsional stiffness, that is, to prevent twisting. On that assumption the difficulties of calculating the strength are removed, but the structure will be unnecessarily heavy, because
undoubtedly the skin does add to the bending strength. The designer must therefore carry out a mechanical test. A test section of the wing is built and loaded with weights until it collapses. Exact measurements of the deflection are made as the test proceeds, and from these data a complete analysis of the strength can be made.
Intensive inspection goes on throughout the work. From the raw material to the finished aeroplane, every stage is checked and cross-checked.
Two other important subjects in this connexion are the heat treatment of the metals and their protection against corrosion. Everyone is familiar with the importance of tempering a steel blade. Every metal and alloy has its appropriate treatment to bring out the best qualities. The aircraft factory is equipped with electrical or gas-fired furnaces, having accurate thermometers, in which to carry out this treatment. Similarly, the factory has tanks for chromium and cadmium plating of steel and for the anodic treatment of duralumin. Paint has a much greater importance than lies in its decorative properties. The right kind of paint, properly applied and well preserved, is of immense value as a protection against the effects of climate, salt water and other corroding influences.
TAPERED MAIN PLANE of a Fiat stressed skin monoplane under construction in Italy. It has two heavy girder spars that run out to the tip, ribs that are pressed out of flat duralumin, and sheet metal covering panels stiffened by light channel section stringers running parallel to the spars. These stringers are riveted to the inner surface of the metal skin.