Aeroplane construction

Seeing that wood constitutes the material for the greater part of the structure of the aeroplane, that is with very few exceptions, some notes on the characteristics and qualities of those woods most commonly used may prove of interest. 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. Of almost equal importance are such considerations as the length and size of the balks obtainable from the log, the total stock available, the relative straightness of grain and freedom from knots as well as the durability of the wood.

Variable Qualities of Wood.

The choice is additionally complicated by the very great variation found in the strength and characteristics of trees of exactly the same species, and also of different portions cut from the same tree. The nature of the site upon which a tree is grown exercises a marked influence upon its properties, while as a general rule, it may be taken that the greater number of annual growth rings per inch, the greater the strength. It is also a general rule that up to certain diameters, the timber contained in that part of the tree the greatest distance from the pith, or centre, is the stronger.

The wood obtained from the base of a tree is heavier than that at the top, and one finds the influence of this in the necessity for balancing and alternating the different laminæ of air-screws before gluing.

Shrinkage.

Another point, and one which is intimately concerned with the proper seasoning of timber, is the amount of moisture contained in a specimen, and this latter point is of some considerable importance, as not only is a large amount of moisture detrimental to the strength values of the timber, but it also renders useless any attempt at precision of workmanship. It is this very point of shrinkage, which constitutes the greatest bar to the achievement of a measure of component standardization, and it is also one of the most serious disabilities of wood as a material for aircraft construction. It is now necessary in the production of finished parts to make some allowance for resultant shrinkage, which is a matter of guesswork, and only practicable where some time will elapse between the finishing of the part and its erection in the complete machine. Under present conditions, more often than not the parts are assembled almost immediately they are made, which means that no allowance over the actual size is possible, this being due to the various fittings which in the majority of machines are of set dimensions and clip or surround the material.

As a natural sequence shrinkage occurs subsequent to the attachment of the fitting, followed by looseness and loss of alignment in the structure. Until the proper period for seasoning can elapse, between the cutting of the tree and its conversion into aeroplane parts, it is difficult to see how this disability can be obviated, although latterly some considerable advances have been made with artificial methods of seasoning. The prejudice against kiln drying is founded on the belief that the strength of the timber is reduced, and that extraneous defects are induced. A method which is a distinct improvement on those systems, using superheated steam and hot air, is now being used with apparently good results. In this system, steam under very low compression is constantly circulated through the timber, drying being effected by a gradual reduction in the humidity of the atmosphere.

Unreliability of Tabulated Tests.

The various tables which exist indicating the strength, weight, and characteristics of various woods are of very doubtful utility, in some cases fallacious, and in nearly all cases far too specific. The foregoing enumeration of some of the variations existing with wood will indicate the enormous difficulty of obtaining with any exactitude a result representative of the species of wood tested, and which could be regarded as reliable data for the calculation of stresses, or for general design. The moisture content of timber, an extremely variable quantity, greatly affects the figures relating to the strength and weight of timber, so that tables indicating the properties of woods should include the percentage of moisture contained in the examples tested. Again, certain woods possessing relatively high strength values, are frequently short-grained and brittle, and therefore not so suitable as other woods of lower strength values, but of greater elasticity and resiliency.

Woods in Use.

Silver Spruce.

The wood most extensively used for the main items of construction is silver spruce, or Sitka spruce, found in great quantities in British Columbia. Experience has proved this wood pre-eminently suitable for aeroplane construction, its strength-weight ratio is particularly good, it can be (at least until recently) obtained in long lengths up to 30 ft., and, moreover, is particularly straight grained and free from knots and other defects. There are other woods possessing higher strength qualities, but in most cases their value is greatly diminished by reason of the greater weight, and that only a limited portion straight of grain and free from knots is obtainable. The weight of Sitka spruce varies from 26 to 33 lbs. per cubic foot, and although it is difficult to give a precise figure, a good average specimen fairly dry would weigh about 28 lbs. per cubic foot. Some impression of the extent to which it enters into the construction of the aeroplane will be gathered if the components usually of spruce are detailed. 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 growing practice is to make the front portion of the fuselage of ash, as this is subject to the greater stress, while the tail portion is of spruce; but in a number of cases the latter material is used throughout. The cross struts of the fuselage are invariably of spruce, as well as such items as interplane and undercarriage struts and streamline fairings.

Virginia Spruce.

This is of a lower weight per cubic foot than Sitka spruce, but does not possess such a good strength value, cannot be obtained in such large pieces, and is generally subject to small knots, which limit the straight-grained lengths procurable.

It is distinguishable from Sitka spruce by its whiteness of colour and general closeness of grain.

Norwegian Spruce.

This wood is also known as spruce fir and white deal, and is grown principally in North Europe. Selected balks can be obtained to weigh no more than 30 lbs. per cubic foot, which compares very favourably with silver spruce. It can be obtained in average lengths, but it is subject to the presence of small hard knots and streaks of resin, although the writer has seen consignments with very few knots. A material known as Baltic yellow deal and Northern pine is procured from the same source, and is more durable than Norwegian spruce. It is inclined to brittleness when dry, and is heavier than white deal, weighing about 36 lbs. per cubic foot. The recent shortage of silver spruce has led to the employment of Norwegian spruce for items such as fuselage struts, hollow fairings to tubular struts, the webs and flanges of the plane ribs, and generally for those components for which long straight-grained lengths are not absolutely essential.

For fuselage struts, where the chief consideration is stiffness, to resist the bending strain produced by inequalities of wiring, fittings, etc., it may actually give better results, being slightly more rigid than silver spruce—at least that is the writer’s experience of it. In addition, very little increase in weight would result, as this wood can be obtained of almost the same weight per cubic foot as silver spruce. The defect usually met with in this wood, of knots occurring at intervals, would be of no great detriment, the lengths needed for the fuselage struts being approximately 3 feet and less, and it would therefore be easily possible to procure wood of this length free from knots. The other items enumerated are of varying lengths, which, with care in selection and conversion, could be arranged for. The practical application of this would be the increased amount of silver spruce available for such highly stressed items as wing spars, interplane struts, and longerons.

Ash.

This wood is one of the most valuable of those employed, being extremely tough and resilient. There are two varieties in use, English and American, the former being considered the better material. It is used mainly for longerons, undercarriage struts, and for all kinds of bent work. It possesses the quality of being readily steamed to comparatively sharp curves, and will retain the bend for a considerable period. The strength and characteristics of ash vary greatly with the climate under which it is grown, and it is also much heavier than spruce, the weight per cubic foot ranging between 40 and 50 lbs. Difficulty is also experienced in obtaining lengths greater than 20 ft., and even in lengths up to that figure, continuity of grain is somewhat rare. It is notable that on various German machines, ash in conjunction with a species of mahogany is used for the laminæ of the air-screw.

Hickory.

Hickory, a species of walnut, is imported from New Zealand and America, and possesses characteristics similar to those of ash. It is obtainable in about the same lengths as ash, but in the writer’s experience is of greater weight. Its chief property is extreme resiliency, which makes it especially suitable for skids, and it has also been used to a limited extent for longerons. It is subject to excessive warping in drying, is not so durable as ash, and the great difficulty experienced in obtaining straight-grained lengths is responsible for its waning popularity.

Walnut.

This wood is almost entirely devoted to the making of air-screws, although the dwindling supplies and the very short lengths obtainable has practically enforced the employment of other woods for this purpose.

Mahogany.

The term “mahogany” covers an infinite variety of woods, possessing widely different characteristics, many of the species being quite unsuitable for the requirements of aircraft work. That known as Honduras mahogany possesses the best strength values, is of medium weight, about 35 lbs. per cubic foot, and is in general use for airs-crews and seaplane floats. It has been used on some German machines for such parts as rib webs, but is not really suitable for parts of comparatively small section, such as longerons, as it is inclined to brittleness. It is of particular value for seaplane floats and the hulls of the flying-boat type of machine, as it is not affected by water. A defect peculiar to Honduras mahogany is the occurrence of irregular fractures across the grain known as thunder shakes. Although other so-called mahoganies are similar in appearance to the Honduras variety, a species quite distinct in appearance is that known as Cuban or Spanish mahogany, which is of darker colour, and much heavier in weight, averaging about 50 lbs. per cubic foot, which latter factor almost precludes its use for aeroplane construction.

Birch.

One finds very few instances of the use of this wood for aeroplane details, although it is used fairly extensively in America for air-screw construction, for which it is only moderately suited. It possesses a high value of compressive strength across the grain, but is much affected by climatic changes, and does not take glue well. It is useful for bent work, and might conceivably be used instead of ash for small bent work details. Its weight is about 44 lbs. per cubic foot.

Poplar.

Under this name is included such woods as American whitewood, cotton wood, bass wood, etc. The wood sold under one or other of these names is generally very soft and brittle, and although of a light nature, weighing about 30 lbs. per cubic foot and less, it is of very little utility for the work under discussion. It has been used for minor parts such as rib webs, and fairings to tubular struts.

Oregon Pine.

The scarcity of silver spruce has led to the adoption of the wood known as Oregon pine for most of the components for which the former wood has hitherto been used. The term “Oregon pine” is applied to the Douglas fir, one of the largest of the fir species, a length of 200 ft. being an average. It is altogether heavier than silver spruce, weighing about 34 lbs. per cubic foot, and also differs greatly in appearance, possessing a reddish-brown grain, with very distinct annual rings. Its strength to weight ratios are practically equal to those of silver spruce, although in the writer’s experience it has a tendency towards brittleness, and is not so suitable as Sitka spruce for components of small scantling. With some specimens of this wood it is noticeable that the effect of drying on freshly sawn lengths for longerons, etc., is the appearance of “shakes” or cracks, not previously discernible. Its appearance generally is reminiscent of pitch pine, for which wood it is sometimes substituted in connection with building.

Other Woods.

The foregoing constitute woods which are in fairly general use for one purpose or another, there being, of course, very many other varieties, some of which may be called into use with the progress of the industry. Of the conifer species, silver spruce is easily the most suitable timber for aeroplane construction, and one realizes this more as the various substitutes are tried. As an instance, cypress is straight of grain with no very great increase over the weight of spruce, being also well up the table of strengths. It is, however, much too brittle for the various members of small section of which an aeroplane is composed, and does not seem to have any extensive future for aircraft work. Another, at one time much-advertised wood, is Parang, a species of mahogany. It has been reputed to bend well, but it certainly does not enter into the construction of modern aeroplanes. A consignment handled by the writer some years ago and intended for bending, was found to be exceedingly brittle, and although standing a good load, fractured almost square across the grain, in a manner known colloquially in the workshop as “carrot-like.” The latter term is indicative of a characteristic which precludes the use of many woods possessing other physical properties especially suitable for aircraft work.

Multi-ply Wood.

This term is applied to the sheets of wood composed of a number of thin layers glued together with the grain reversed. As the layers are obtained by rotating the tree against cutters in such a manner that a continuous cut is taken from the outside almost to the centre, it is possible to get very great widths, which makes it particularly suitable for aircraft work. It is made in varying widths up to 4 ft., and in thickness from 1/20 in. up to ½ in., consisting of three, five, and seven layers, although the three-ply variety in thicknesses up to 3/16 in. is more commonly used. It is made up in nearly all woods, but those mostly utilized in the aeroplane industry are birch, ash, poplar, and satin-walnut, birch being superior by reason of its closeness of grain. Ash ply-wood in some instances tends towards brittleness, while poplar, although exceptionally light, is very soft and only used for minor parts. Satin-walnut is very even in quality but is apt to warp.

Defects in Timber.

Perhaps the most common and prolific defect encountered with the use of timber is the presence of cracks or shakes of different character, which are due to different causes. Fig. 1 indicates a very common form, known as a “heart shake,” dividing the timber at the centre; while Fig. 2, a “star shake,” is really a number of heart shakes diverging from the centre. The process of seasoning sometimes results in the separation of the annual rings, forming cup shakes, as shown in Fig. 3. It should be understood that the presence of shakes may render useless an otherwise perfect specimen of timber, as it frequently happens that in the conversion of timber so affected the usable portions do not permit of the sizes necessary for such items as wing spars and struts. The

defect of twisted grain (Fig. 4) is often found in ash, and is caused by the action of the wind when the tree is growing, and renders such wood of limited utility. Shrinkage affects all timber in varying degrees, and its effect on boards due to their position in the log is shown by Fig. 5, while Fig. 6 indicates the effect of drying on a squared-up section. Incidentally one may point out that the annual rings, viewed from the end of the section, should be as straight as possible, which would obviate to an extent the distortion due to drying in a component subsequent to its finishing. Another defect, and one somewhat difficult to detect, is the presence of a brownish speckled tint in the grain. Any evidence of this in a specimen indicates the beginning of decay, and is caused by insufficient seasoning and lengthy exposure in a stagnant situation.

Steel.

The greater proportion of the various fittings employed in the construction of the aeroplane are built up from sheet nickel steel, usually of a low tensile strength, to permit of working in a cold state, as, with a higher grade steel, the process of bending to template by hand, in many cases a none too careful procedure, would result in a considerable weakening of the material at the bend. In addition, the operation of welding, which now enters into the construction of a number of fittings, also necessitates a moderate grade of steel. A higher class of sheet steel, from 35 to 50 tons tensile, is used for parts subject to stress, such as interplane strut-fittings, wiring-lugs, etc. As a higher grade of steel is better from a strength-for-weight point of view, its employment for bent-up clips is desirable, although where such a steel is used it is almost necessary, if the original strength of the material is to be retained in the finished fitting, to effect the various bends in a machine, in conjunction with bending jigs. Careful heat-treatment after bending to shape is an important factor in removing the stresses set up by working, and in rendering the structure of the material more uniform.

Steel Tube.

Steel, in the form of tubing of various sections, enters largely into aeroplane construction, and may be said to contribute largely to the efficiency of the structure. It is now being used for the different items of the undercarriage, for struts in the fuselage, interplane struts, and in many cases control surfaces, such as the ailerons, elevators, and rudder, are being built of this material entirely. In the early days of aviation steel tubing attained some considerable popularity, many machines being built almost entirely of tubing; but difficulties in its manipulation, and the fact that very often the methods of attachment reduced its strength considerably, gradually led to the general employment of wood. The great advances lately made in the production of a high-grade nickel-chrome steel, with a high ultimate tensile stress, are responsible for its present increasing use.

Aluminium.

The present use of aluminium is restricted to the cowling of the engine, and occasionally as a body covering. Although it is light in weight, its extremely low strength values render it of very little use for other purposes. It attained some measure of popularity in the early days of aviation, particularly for the manufacture of different strut-sockets, which were cast from aluminium; but the general bulkiness of the fittings, in addition to the fact that it was generally necessary to incorporate a steel lug to form the wire anchorage, caused it to gradually fall into disuse. The tendency of aluminium to flake and corrode, which is intensified by the action of salt water, also limits its use for seaplane construction. Many attempts have been made through various alloys to impart greater strength to the material, and although progress has resulted, the characteristics of most of the products are unreliable.

Duralumin.

Of the different alloys, duralumin is probably the best, although one believes that its qualities are principally the result of special heat treatment. Its use is at present restricted to those parts not subjected to any great tensile strain. It is considerably less than half the weight of steel, bulk for bulk, and, properly used, may effect a considerable saving in weight. The fact that it has not achieved the popularity it deserves may be ascribed to the difficulties experienced in working it, especially for such parts as body clips, where several bends are necessary, and to the rather arbitrary methods in use. If properly annealed, no difficulty should occur in obtaining a reasonably sharp bend. The process recommended by the makers consists in heating the metal in a muffled furnace to a temperature of approximately 350° C., and the necessary work done as soon as possible after cooling. The importance of this is due to the fact that the process of annealing imparts to the metal a tendency to become brittle with time. 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.