Sydney Camm

“ The choice of a suitable wood for aircraft construction is a matter of some difficulty, engendered by the variety of considerations of which at least some observance is essential. The fundamental principle of aircraft construction, that of obtaining the maximum strength for a minimum of weight, affords one standpoint from which a particular wood may be regarded, but this does not constitute in itself a sufficient reason for its choice. ”

“ PLANE CONSTRUCTION. Of the various components which comprise the complete machine, the wings, aerofoils, or planes, as these items are variously designated, may be said to contribute the greater part of the ultimate success of the complete machine. The aerodynamical properties of a wing are now fairly well determined, and have been the subject of a great number of experiments, resulting in the clearing away of many hazy ideas and notions, so that the actual design of the wing section for machines of given purpose is almost standardized. ”

“ One cannot assert that any very decided indication exists of a trend in modern design towards metal construction, and it is quite possible that this will not arrive until it is rendered imperative by reason of the scarcity of timber. The precise composition of the metal is rather a controversial matter, some authorities favouring steel, and others some alloy of aluminium, such as, for instance, “duralumin.” The production of a suitable alloy constitutes a real problem and one upon which the Advisory Committee for Aeronautics have already made investigations and experiments. ”

“ If we take the case of a machine rolling over bumpy ground, all that is required of the suspension is that the wheel movement over the inequalities shall not be transmitted to the whole machine. So far both steel springs and rubber cord satisfy these conditions, but in the operation of alighting the machine not infrequently strikes the ground with some force, sometimes the result of gusts or pancaking. With steel springs, and to a lesser degree those of rubber, the energy of landing is not absorbed, but is stored up, being given out again in the form of a rebound. ”

“ A detail which does not always receive sufficient attention is the provision of adequate bracing against the thrust of the engine. Where the construction is such that the engine-bearers form an integral part of the fuselage structure, there is generally little fault to find, but with some sheet steel mountings, particularly those employed for the rotary type of engine, the only bracing in a fore-and-aft direction is that provided by the flanged edges of the plate, which are usually much too narrow to be of real use. ”

“ As the arrangement of planes in a biplane forms the extremely simple yet enormously efficient box-girder, it is generally considered superior in strength to weight requirements, although for monoplanes of small span it is doubtful if this is so, which affords some indication of the possibilities of the small monoplane as the sporting machine of the days to come. ”

“ There seems no valid reason why many of the fittings for all machines within certain dimensions should not be of standard design, and a brief review of the various details which could be standardized without detracting in the least from aerodynamical efficiency will indicate the extent to which the conserving of labour could be carried. In the construction of the fuselage, the clips fastening the longerons and cross struts could easily be of one design, whatever the make of the machine. ”

“ A great deal depends on the method of making the joint, and an operation such as gluing a laminated wing spar is usually carried out in a special room of certain temperature. Such spars are generally additionally fixed by rivets, bolts, or screws through the flanges. The material should always be dry, and as straight and close-grained as can be procured. The straightness and closeness of grain affect the strength to a remarkable degree; and here it may be remarked that the use of the best material is a most important factor for ensuring sound construction, and one that in the end pays. ”

“ In view of the dwindling supplies of suitable timber, it certainly seems more than probable that some form of metal construction will one day constitute the structure of the aeroplane. The manufacture of the various components in wood does not necessitate an extensive plant, the labour necessary is comparatively cheap and easily available, and moreover the transitory nature of the whole business, and the ease with which essential changes in type can be made without the wholesale scrapping of the expensive jigs associated with the use of steel, all strengthen the case in favour of wood. ”

“ Numerous suggestions, ideas, and patents exist, having as their object the housing of the undercarriage in the fuselage during flight, with a resultant reduction in resistance; and excellent as the principle is, its practical application is difficult of achievement—at least, for machines of the present. In flight the undercarriage is a useless encumbrance, adding weight and head resistance, so that an arrangement whereby this component could be folded into the main structure would apparently effect a saving in resistance. ”

“ Brazing and Welding.
It is somewhat amazing that brazing as an essential operation in the making of a joint should still be employed, as it is difficult to imagine anything less suited to the conditions under which aircraft operate. The advantages of a uniform high-grade steel possessing a high ultimate tensile strength are dissipated by the intense heat necessary for the action of brazing, resulting in the strength of the finished joint becoming an extremely problematic quantity, indeed this is rendered the more so by the individuality of the workmen. ”

“ Rapidity of production is a most cogent argument in favour of metal construction, for once the necessary machines are set up, and the jigs and dies made, and given a constant supply of material, output is only limited by the speed of the machine. In addition, there are the very exacting demands of interchangeability. Now, it is infinitely more easy to obtain exactitude in metal than in wood, and, moreover, assuming46 that it is possible to produce woodwork to the nearest ·01 of an inch, what preventive is there against shrinkage, which occurs even when using the dryest of timber. ”

“ Take as a hypothesis the case of, say, ten makes of scouting biplanes in use, each with approximately the same arrangement of longerons and struts, and with a similar overall size of fuselage. Each of these machines will incorporate a different fuselage clip, which means that somewhere highly skilled labour is being unnecessarily expended in the making of jigs and press tools, whereas a suitably standardized clip for all scout machines of certain dimensions would involve the making of one set of press and bending tools only for the machines of the one type built. ”

“ The almost universal arrangement for the small single-seater scout is the single bay, and from this method the progress of design has inclined towards the elimination of as many struts and wires as possible, which has its culminant in the type of truss embodying one strut and one pair of wires, lift and counter-lift, each side of the body. Quite a number of machines have incorporated the single strut assembly, the earliest perhaps being the Brequet, and one also remembers a small Avro scout, the strut in this case being built up with spars and stringers, covered with fabric. ”

“ Additional important features are the provision for rapid assembly and detachment, ease of manufacture, and the absence of brazing, welding and soldering as mediums for forming connections, at least for those parts subject to any stress.
The qualities of strong construction and good design are paramount considerations in the manufacture of these fittings, as the purpose of an interstrut joint is not merely to form a connection between the upper and lower planes, but also to distribute the intricate stresses encountered in flight. ”

“ Cross-country flying, which more often than not means “getting off” in a restricted space, requires that the machine shall attain flying speed in the shortest time, and conversely in alighting the machine should come to rest in the quickest time. Innumerable smashes have been caused after a perfectly good landing by failure to pull up before a hedge, fence, or ditch. These are the main principles involved, and at least they indicate how and why the undercarriage is necessarily a compromise. ”

“ For the main spars of the planes spruce is almost universally used, as here great strength for the least weight is of extreme importance, while a consideration almost as important is the necessity of a good average length, straight grained and free from defects. It is also used for the webs and flanges of the wing ribs, the leading and trailing edges and wing structure generally. The longerons or rails of the fuselage of many machines are spruce, although in this instance ash and hickory are used to a moderate extent. ”

“ The writer has often contended that, where duralumin is used, it should be with a real desire to reduce weight. Too often one sees a fitting of such lavish dimensions as to entirely nullify the advantage of the lighter metal. ”

“ Machines are now designed for air performance pure and simple, so that an undercarriage of the simple Vee type is all that is permissible; but in the post-war machine general utility will be the desideratum sought for by designers. At one time the majority of the undercarriage arrangements incorporated one or more skids. The material most suited for this purpose is hickory, although some designers prefer ash, steamed to the desired curve, and generally channelled out between the points of intersection of the struts, fittings, etc., in a similar manner to longerons and wing spars. ”

“ Fabric and its attachment is a matter requiring considerable attention, with the great pressure to which modern wings are subjected. In the old days any fabric which was light with a moderate degree of strength was utilized. Nowadays, it is required to stand a certain strain in warp and weft, and rightly so, since the bursting of fabric in flight can only have one result. It is interesting to note that the fabric used on the Deperdussin hydro-monoplane was specially woven with threads running at right angles, forming innumerable squares. ”

“ For production in quantity a box jig, between which a dozen ribs might be clamped and shaped, is preferable. Templates of wood are of doubtful accuracy, for not only do corners wear, but gradual shrinkage soon renders them useless. The incorrect shaping of the most insignificant piece of wood may have far-reaching effects when assembled, and any extra trouble taken in the preparation of parts is more than repaid by the subsequent ease and precision of erection. ”

“ Now, the strength of a beam varies as the square of the depth of the beam, and it is obvious that in the case of a circular steel tube the material is evenly distributed about the neutral axis, and therefore its strength in both horizontal and vertical directions is equal ”

“ A rather unusual feature may be noticed in the attachment of the bracing wires, which are not finished off with the orthodox wire ferrule, but are arranged as a loop, the turnbuckle85 forming the anchorage for the two ends. The trend of design in this country seems to incline towards the clip stamped out from sheet steel and bent up. This class of fitting can be produced accurately and quickly, and, in the writer’s opinion, is by far the best manufacturing proposition. Aluminium castings are quite obsolete, and the built-up fitting, involving welding or brazing, does not seem greatly in vogue. ”

“ With rubber, elongation and its consequent depreciation of ultimate tensile strength prevents any energy of moment being returned to the aeroplane, which is why, for light machines of modern design, say, up to 2500 lbs. total weight, rubber is the better material. Steel springs being deficient in the power to damp out shocks, it becomes necessary to use these in conjunction with some other medium possessing this quality, and one of the most suitable arrangements extant is that known as the oleo-pneumatic gear, consisting of a combination of helical coil spring and oil plunger. ”

“ The advantages of metal as a material considered briefly, are that it permits of design to close limits without the allowance of so-called factors of safety, which are now necessary through5 the great variation in the strengths of wood, manufacturing procedure would be expedited, while one can reasonably expect a greater degree of precision in the finished machine, due largely to the increased facilities for accurate manufacture of components which metal affords. It is quite possible, of course, given a uniform grade of steel, to design to extremely close limits without fear of collapse ”

“ A wide wheel base therefore means an undesirable increase in weight and resistance. To make up for the deficiencies of the almost unavoidable narrow wheel base, it is usual to make use of the wing tips by fitting skids of malacca cane or laminated ash, which are brought into action when the machine is excessively canted over sideways. At one time the wing tips were almost invariably used to assist the undercarriage, the wing tips of the Nieuport monoplane being specially constructed for the purpose, and no skids were fitted. ”

“ With regard to the flying-boat type of machine, a favourite practice is to mount the motor on the four struts supporting the centre section of the upper plane, which is braced by struts and wires from different points on the hull or body.
Although the modern aircraft engine is of greatly increased power, compared with the engine of the period 1912–1914, one does not find any great difference in the structural features of the mounting employed, and in view of the very diverse arrangements for mounting the same type of engine which now exist, there is need for greater uniformity. ”

“ This element of inherent stability is obtained by suitably grading the camber and incidence of the wings, until at the wing tips the chord of the plane section forms a negative angle to the line of flight. Although this arrangement is undoubtedly of value, especially for the touring machine of moderate power, its chief fault lies in the relatively slow righting movements, which, although of no great consequence at a reasonable altitude, becomes a source of danger when alighting, and certainly entail the use of ailerons, or warp, to counteract it. ”