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AEROPLANE
CONSTRUCTION

A Handbook on the various Methods and
Details of Construction employed in
the Building of Aeroplanes

BY
SYDNEY CAMM
ASSOCIATE FELLOW ROYAL AERONAUTICAL SOCIETY

LONDON
CROSBY LOCKWOOD AND SON
7, STATIONERS’ HALL COURT, LUDGATE HILL

1919

PRINTED BY
WILLIAM CLOWES AND SONS, LIMITED,
LONDON AND BECCLES.

PREFACE

The articles embodied with other matter in this book, were intended as a broad survey of the principles and details of modern aeroplane construction, concerning which there is a noticeable deficit amongst existing aeronautical literature.

They were written at a time when specific references to modern British aircraft were forbidden, and although from a comparative point of view this is to be regretted, the details and methods dealt with are, in the author’s opinion, representative of those most generally used in machines of present-day design. It is hoped that the book will appeal not only to those engaged on the manufacture, but also to those concerned with the uses of aircraft.

S. C.

TABLE OF CONTENTS

AEROPLANE CONSTRUCTION

CHAPTER I.
INTRODUCTION.

The purpose of this book is to give some indication of the principles and methods of construction of modern aeroplanes, as distinct from those considerations pertaining purely to design, although occasional references to various elementary principles of aerodynamics have been found necessary to illustrate the why and wherefore of certain constructional details.

To many the aeroplane is a structure of appalling flimsiness, yet the principle which it exemplifies, that of obtaining the maximum strength for a minimum of weight, constitutes a problem of which the solving is not only an unceasing labour, but one demanding the observance of the best engineering procedure. The whole future of aviation, commercially or otherwise, may be said to be indissolubly bound up with the development of efficiency; and whether this is to be attained in improvements in aerodynamical qualities, by the discovery of a material giving a greatly enhanced strength to weight ratio, or by progress in the arrangement of the various members of the complete structure of the aeroplane, is a matter upon which some diversity of opinion exists. However, it is certain that the very great developments of the last few years are due more to refinements in design rather than construction; and it is questionable whether the constructional work of the modern aeroplane has developed equally with design, so that, even taking for granted the oft-repeated, but very doubtful, statement that we are approaching the limitations of design, there is certainly plenty of scope for experiment and improvement in the constructional principles of the modern aeroplane.

Standardization of Details.

Whatever may be said for the standardization of aeroplane types, a scheme which should effect a considerable saving in labour and material, and which offers chances of success, would consist in the standardization of metal fittings and wood components generally, for in this direction there is certainly great need for improvement. Taking as a hypothesis the various makes of scouting machines, we find hardly any two details the same. This means that if in this country there are six firms producing machines of their own design (these figures, of course, being purely suppositionary), there will be six sets of detail drawings, six sets of jigs, templates, and press tools, and sundry special machine tools. 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. At present we find some clips are bent up from a stamping and attached to the longeron without the drilling of the latter; some built up from various parts, such as washer-plates, duralumin pressings, and bolted through the longeron; while some combine advantages and others the disadvantages of both. In some cases the longerons of spruce are spindled out for lightening; in others no spindling occurs; while in a few instances hickory or ash, with or without channelling, is used. There are the interplane strut attachments, stern-post fittings, control-surface hinges, and undercarriage attachments, all showing great variations, and in all of which the design could be brought within reasonable limits. As indicating how unnecessary a good deal of the variation is, one may instance the fact that for the swaged streamline, or R.A.F. wires, there are at least three different terminals in use. Although more difficult of achievement, there is scope for improvement in the different arrangements for the fixed gun mounting, while a standard instrument board would benefit the pilot.

Methods of Manufacture.

It is fairly well known that the output of some firms is considerably better than others, although the machines are of the same design. Although a good many factors may contribute to this result, it seems fairly certain that in some cases the methods of manufacture must be superior, which calls for some system of standardizing the broad principles pertaining to manufacturing procedure. Under this arrangement a much better estimate of probable output could be made. It is also necessary by the fact that some firms have been developed through the exigencies of war, and not as a result of any great manufacturing ability, whereas in peace time the spur of competition would force the adoption of the most rapid methods of production. The creation of a central or universal office for the design of the various jigs used in the manufacture of aircraft, with power to decide the process of manufacture, although a somewhat far-reaching reform, would certainly eliminate a number of useless experiments made by the individual constructors, and would also greatly improve the interchangeability of the various components. In addition, fresh firms to the aviation industry would be at once acquainted with the general methods of manufacture, which should be of considerable assistance in expediting initial output. Of course, this system would tend rather to destroy individual initiative, in that much that is now left to the skill and experience of the workman would be predetermined, although this would be more than compensated for by the increased benefits accruing to the State. Jigs designed to produce the same work in different works often differ in detail considerably, and this, of course, often influences the rate of production. As an instance, in some works elaborate benches are considered necessary for the erection of fuselages, while in others a pair of trestles suffices. With this system of unified manufacturing procedure extreme regard would have to be paid, in the design of various jigs and fixtures, to adaptability for modifications in design. Otherwise the various alterations which are bound to occur would result in an unnecessary expenditure on fresh jigs. It is somewhat unfortunate that in the general design of an aeroplane, in numerous cases, far too little regard is paid to considerations of ease of manufacture, and this is frequently responsible for the many changes in design after a contract has been started. Under an ideal system of standardization, the requirements of manufacture would necessitate consideration in the design of the constructional details.

Metal Construction.

The question of the aircraft materials of the future is not so much a problem as a matter of gradual evolution. 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. The conclusion of hostilities would introduce another state of affairs, and it is conceivable that the various types will then be standardized for different purposes, which may necessitate the greater use of steel. Certainly the advantage of steel would be better realized under some system of standardized design, but this unfortunately is not possible while present conditions obtain. 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 through 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; but the human factor in the shape of fitter, welder, or operator introduces the unknown element, and one for which some allowance must always be made. 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. A disadvantage with steel is that, although it is quite possible to produce, say, a fuselage entirely of this material to withstand easily the greatest stress encountered in flying, such a structure, owing to the thin nature of the various components, would suffer damage through shocks induced by rolling over rough ground, and also by handling. In addition the effects of crystallization would require some considerable study. These and other reasons indicate that an alloy of aluminium, which for a given weight would be considerably more rigid than steel, offers possibilities as a material. It might prove advantageous to combine both metals, using steel for the more highly stressed parts, such as, for instance, wing spar attachments, interplane bracing lugs, and indeed any part where the load to be carried is one induced by tension.

The foregoing is indicative of some of the more important directions in which improvement and development are possible, and certainly ample scope yet exists for the attention of the student, or indeed any one interested in the future of the aviation industry.

CHAPTER II.
MATERIALS.

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.

Fig. 1.—Heart shake.

Fig. 2.—Star shake.

Fig. 3.—Cup shake.

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

Fig. 4.—Twisted grain.

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.

Fig. 5.—Shrinkage of boards
due to position in log.

Fig. 6.—Effect of drying
on a squared-up section.

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.

CHAPTER III.
SPARS AND STRUTS.

Having thus considered generally the chief materials of aircraft construction, we will proceed to examine the various types of spars and struts in present use. The main spars of the wings are by far the most important items of the complete structure, and very great care is always taken to ensure that only the best of materials and workmanship are concerned with their manufacture. Looking back at the days one usually associates with the aero shows at Olympia, multitudinous methods of building wing spars can be recalled. Some composed of three-ply and ash; others, less common, of channel steel; and a few of steel tubing, either plain or wood filled. Various reasons and causes have combined to eliminate these methods of construction. For instance, the spar of channel steel proved much too flexible, although this characteristic was no great disadvantage in those machines employing wing-warping for lateral control, for with this arrangement a certain amount of flexibility in the wing structure is essential. While steel tubing is excellent for many details it can hardly be said to be really suitable for wing spars, which are stressed essentially as beams. 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; although employed as a strut, this feature becomes of real value. One, however, still encounters its use on modern machines; indeed, it must not be supposed that the progress made in construction generally since 1914 has tended greatly towards a reduction in the number of different methods employed, and this will be realized from a consideration of the accompanying spar sections which are in use to-day on one make of machine or another.

Spar Sections.

Fig. 7.—Solid spar.

The I section form of wing spar, shown by [Fig. 7], is in general use, being spindled from the solid. It is comparatively easy to produce, which in a measure explains its popularity, and it also disposes the material in probably the best manner for the stresses involved. The laminated spar, [Fig. 8], is an improvement on the solid channelled spar; it is stronger, will withstand distortion to a greater degree without injury, and the strength is also more uniform than with the solid spar. An additional point in its favour is that it is much easier to procure three pieces of small section timber free from defects than one large piece, which, in view of the increasing scarcity of perfect timber, is an important consideration. In order to minimize the risk of the glue between

Fig. 8.—Laminated spar.

the laminations failing, the usual practice is to copper rivet or bolt the flange portion, while both spars are left solid at the point of attachment of the interplane strut fittings and wire anchorages. The spar shown by [Fig. 9] is of the hollow box variety, chiefly used for machines of large wing surface, where weight reduction is an important factor. The two halves of channel section are spindled from the solid and glued together. The joint is strengthened by the provision of small

Fig. 9.—Hollow box-spar.

fillets or tongues of hard wood, and in some instances the complete spar is bound with glued fabric. Comparing the hollow spar with the solid, and neglecting the cost factor, the writer contends that the advantage is indisputably with the former. The tendency of the I-section spar to buckle laterally is of much lesser moment in a hollow spar of the type shown by [Fig. 9], while for a given weight it shows an increase in strength, and for equal strength it is much lighter. A different version of the hollow spar system is that indicated by [Fig. 10], consisting of two channelled sections, tongued together at the joint, the sides being stiffened with three-ply. The disposition of the joint in a vertical plane is a distinct improvement on the hollow spar previously considered, mainly in that better resistance to a shearing stress is afforded.

Fig. 10.—Hollow spar with
stiffened sides.

Fig. 11.—Hollow spar with
multi-ply sides.

The principle underlying the construction of the spar shown by [Fig. 11], is that in its manufacture the lengths of wood necessary are of small section. The sides of this spar are built up with a centre of spruce about ⅛ in. thick, to each side of which is glued thin three-ply, these being glued, screwed, and bradded to the flanges. The wing spar shown in section by [Fig. 12] is unique in that it really constitutes two spars placed closed together, the connection being formed by the top and bottom flanges of three-ply. This spar was used in a machine with planes of small chord, but of very deep section, and in which no interplane wiring occurred, the wings functioning as cantilevers. Its chief advantage is great rigidity for a low weight, but such a spar necessitates a deep wing section, and is not in general use.

Fig. 12.—Twin box spar.

Hollow Spar Construction.

The advantages of the hollow type of spar summarized are (1) greater strength for a given weight; (2) it can be produced from wood of small section, and is therefore a better manufacturing proposition. On the other hand, the strength of a hollow spar is greatly and almost entirely dependent on the glue used. Now, however well the joint may be made, the glue is susceptible to a damp atmosphere, and if so affected is of greatly reduced strength, while possible depreciation in the glue due to age renders the life of the spar a problematic quantity. Where the various fittings occur it is also necessary to place blocks before the spar is glued up, which is rather an unmechanical job. The practice of forming vertical sides of a hollow spar from three-ply is not to be commended, by reason of the doubtful character of the glue used in its manufacture. However, in spite of these disabilities, there is a future for hollow spar construction in the manufacture of the big commercial machines of the future, for with these the question of maximum strength for minimum weight, to permit the carrying of the greatest possible useful load, will be a primary consideration. This, of course, assuming that the era of the all-steel machine has not arrived.

Strut Sections.

In the construction of the interplane and undercarriage struts, one does not find a very decided preference for any one particular method, although the interplane strut spindled from the solid to a streamline section is common to many types of modern aircraft. The strut shown in section by [Fig. 13] is in use for both interplane and undercarriage struts. This consists of ordinary round section steel tubing, to which is attached a tail piece or fairing of wood, this being bound to

Fig. 13.—Steel tube strut
with fairing bound on.

Figs. 14, 15.—Interplane struts
spindled from the solid.

the tube by linen tape or fabric, doped and varnished. This strut is of practically equal strength in both lateral and longitudinal directions, and from this point of view is superior to the solid spindled strut, which is usually of great strength in the fore and aft direction, but always possesses a tendency to buckle laterally. [Fig. 14] indicates a hollow plane strut, in which the sides of spruce are spindled from the solid, and glued to a central stiffening piece of ash; while [Fig. 15] is arranged so that a stiffening web is formed in the spindling process. Owing to the rather extensive nature of the latter operation, one does not find many instances of its use. Where the hollow wood struts used are not completely bound with tape or fabric, they should at least be bound at intervals with tape or fine twine, as there is always the possibility of the glued joint failing under the combined attentions of rain and heat.

A type of strut which is now being widely used is that of streamline section steel tubing, drawn or rolled from the round section. It is employed for both the interplane and undercarriage struts, but for the latter has not given entirely satisfactory results, owing to the tendency to buckle under extra heavy landing shocks. This would be more pronounced with a tube of fine section than with one possessing a bluff contour; but in any case, a strut of parallel section, whatever the material, is not well suited to withstand sudden shocks. This point is referred to later. Seeing that progress is being made with the production of a seamless streamline tapered strut this defect should soon disappear.

Fig. 16.—Interplane support from body.

Fig. 17.—Section of built-up strut.

In some machines the top plane is supported from the fuselage by struts which are formed integrally with a horizontal compression member, as in [Fig. 16]; the section of the vertical struts being shown by [Fig. 17]. The ply-wood is cut to the shape of the complete component, and forms a tie for the spruce layers, which are jointed at the junction of the vertical and horizontal members.

Strut Materials.

Referring again to the material generally employed for struts, i.e. silver spruce, it is perhaps necessary to explain further the reasons for its predominance over ash, as on a strength-for-weight ratio the latter wood is slightly the better material. The points already detailed, indicate that an interplane strut is stressed essentially in compression, and therefore the chief characteristic of ash, great tensile strength, is of but secondary importance. There is also the fact that, for the same weight, spruce would be thicker, and correspondingly more able to resist collapse. However, in machines of the flying-boat class, where the engine is invariably mounted between the four central plane struts, and consequently subjected to an amount of vibration varying with the type of engine used, ash forms the material.

Tapering of Interplane Struts.

The correct shaping of struts longitudinally, particularly those for interplane use, is apparently a rather controversial subject. Taking the case of an untapered strut, it is evident that the greatest stress will be located at or near the centre, so that if at this point the section is strong enough, clearly there must be an amount of superfluous material at the ends. By suitably reducing or tapering the strut from the centre one can obtain the same degree of strength for less weight. Conversely, for the same weight a much stronger strut is possible. So it has always appeared to the writer. It is, however, admittedly possible that unless carefully done, the operation of tapering a strut may actually diminish the strength. One method of tapering, that of making the maximum cross-section at the centre, and from this point diminishing in a straight line to the ends, is undoubtedly open to criticism, and a way more nearly approximating to the correct method of shaping is to reduce the cross-section at various points so that the finished contour is curvilinear, as in [Fig. 18]. In this connection it is pertinent to emphasize the importance of ensuring that all strut ends are cut to the correct bevels, and this is particularly applicable to those struts which seat directly in a socket. The slightest irregularity will cause considerable distortion when assembled under the tension of the bracing wires, and frequently the writer has seen an ostensibly perfect strut assume the most hopeless lines directly the operation of truing up is commenced.

Fig. 18.—Tapering of interplane struts.

Design of Strut Sections.

Although, strictly speaking, the design of strut shapes is outside the scope of this book, a few remarks anent the development of streamline may emphasize the advances made, and also the need for careful construction. The resistance of a body is generally considered to increase as the square of the speed, i.e. double the speed and head resistance is doubled, and while this is true for a moderate range of speeds, experiment has proved that for high speeds, exceeding say 100 miles per hour, resistance increases at rather less than as the square of the speed. However, it is certain that the correct shaping or otherwise of the struts and other exposed members, affects generally the performance in flight of the aeroplane. The accepted feature of all streamline forms is an easy curve, having a fairly bluff entrance and gradually tapering to a fine edge. The ratio of length to diameter, called the fineness ratio, varies in modern machines, being in some instances 3 to 1 and in others 5 to 1, a good average being 4 to 1. Considering only the point of head resistance, it would be better to choose a section of high fineness ratio, but constructionally such a strut would buckle sideways under a moderate load, and therefore the cross section must be sufficient to resist this. The strut section used on the earliest aeroplanes, such as the Wright biplane, shown by [Fig. 19], is

Fig. 19. Fig. 20.

Fig. 21. Fig. 22.

Figs. 19–22.—Strut sections.

nothing more than a rectangle with the corners rounded off. [Fig. 20] shows a development of [Fig. 19] consisting of a semi-circular head with a cone-shaped tail, which by gradual evolution has resulted in the section [Fig. 21]. Some experiments carried out a considerable time ago by Lieut.-Col. Alec Ogilvy, revealed the rather interesting point that a strut shaped as in [Fig. 22] gave the same results as a similar strut taken to a fine edge. The reasons for the non-suitability of a sharp-pointed section are apparent from a consideration of [Fig. 23], showing the action of a side wind with the resultant dead air region.

Fig. 23.—Showing inefficiency of pointed section in a side wind.

Fuselage Struts.

Fig. 24.—Channel-section fuselage strut.

Fig. 25.—T section fuselage strut.

In the general features of those struts associated with the construction of the fuselage and nacelle, there is very little diversity of practice, the majority of constructors favouring a square spruce strut, [Fig. 24], channelled out for lightness. A defect with this type of strut is the tendency, engendered by irregularities in the fittings and wiring, to buckle laterally, although this can be obviated by the provision of a strut of larger section at the centre and diminishing in width to the ends. A strut not nearly so popular but nevertheless in use is that indicated by [Fig. 25], consisting of spruce spindled to a T section the web being of considerable width at the centre. It would seem that the piece of wood necessary to obtain such a strut is out of proportion to its actual finished dimensions, and from the standpoint of economy in both labour and material is not justified. The circular turned and tapered strut noticeable on a number of machines disposes the material in probably the best manner for the conditions applicable to this component, although it necessitates the provision of tubular ferrules in the fuselage clip. On one modern machine the fuselage struts are circular, but of hollow section, built up of two pieces glued together. An obsolescent method is that in which the strut is shaped to something approaching a streamline section, as the fact that all aeroplane bodies are now fabric covered renders it unnecessary.

CHAPTER IV.
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. From this it might be deduced that the methods of construction were equally well determined, and although absolute uniformity of practice does not exist, the wing construction of most machines is similar, as far as the main assembly is concerned.

Effects of Standardization.

Incidentally, one may point out the detrimental effects of undue standardization as applied to an industry in its preliminary stages. These effects are well exemplified by certain machines, in which standardization has been studied to an almost meticulous extent, with the logical result that their performance is considerably inferior to that of other machines of contemporary design, but in which desirable improvements are incorporated as they occur. Although at present one cannot give actual figures, the average performance of modern British aircraft in range of speeds, rate and extent of climb is superior to the products of any other country, and one certainly cannot cite the construction of the average British machine as an example of standardization. Seeing that, as a typical instance, wing sections are frequently altered in minor detail, the impracticability of standardization is apparent, for this would entail, to a firm wishing to keep pace with developments, a considerable loss, through scrapping of jigs, etc., consequent upon the new design. When the principles of aeroplane design are as well defined as those pertaining to internal combustion engines, one may expect the various manufacturers to produce one type of machine per year, and the various improvements adduced from the year’s experience would be incorporated in the type of the succeeding year.

Fig. 26.—Plan view of wing assembly.

However, leaving the realms of vaticination for the more prosaic subject of wing construction, it will be realized that the process of producing the full-sized wing, accurately conforming to the measurements, etc., deduced from experiment, and so constructed that the chief characteristic of the section will permanently remain, is of importance. As one or two of the spar sections in use were dealt with in the first chapter, it will be unnecessary again to consider them in detail.

[Fig. 26] shows diagrammatically the plan view of a wing assembly typical of modern practice, so far as the disposition of the various components is concerned.

Shaping of Main Spars.

Fig. 27.—Shaping of main spars.

Taking in greater detail the different parts, it is apparent that the spars form the nucleus of the general arrangement. There are two methods of shaping the spar longitudinally, and, as shown by [Fig. 27], the one consists of leaving it parallel for the greater part of its length, while the end forming the tip of the wing is gradually tapered to a comparatively fine edge. This may be said to constitute prevailing practice. The other method which is illustrative of monoplane practice is not used to anything like the same extent, and differs in that it is constantly tapering from root to tip. The advantage of this spar construction is the improved distribution of the material for the stresses involved, and also that a wing built with this spar may possibly possess a greater degree of lateral stability owing to the weight of the complete wing being located nearer the centre of gravity. Against this one must balance the fact that each rib must necessarily be different in contour, entailing a greater number of jigs, an increase in the time taken in building, with a consequent increase in cost. In addition, all strut fittings would differ in size, so that, taking all things into consideration, this construction is hardly justified. It will be noted that at the point of attachment of the interplane strut fittings, or, in the case of the monoplane wing, the anchorage for the wires, the spar is left solid. It is possible to channel the spar right through, from root to tip, and to glue blocks where fittings occur; and although there is a possible saving of labour thereby, it hardly conforms to the standards of modern workshop practice.

Defects of Glue in Wing Spars.

Although gluing is a most necessary operation in modern wing construction, it is not what one would call an engineering proposition. It has a tendency to deteriorate with time, especially if exposed to a humid atmosphere. 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. If a spar should happen to be cut from a wet log, it may in the interval between its finishing as a part and subsequent assembly in the wing cast or warp, which may cause trouble in assembling, and is more likely to result in eventually being sawn up as scrap. The resultant section of any wing is really dependent upon the spar being of correct section, and should the spar be out of “truth,” the section will vary at different points. This may not be eradicated even in the erection of the machine, so that finally the actual flying properties of the machine will be affected—another illustration of the importance of thorough construction in ensuring a good and lasting performance. To secure uniformity and interchangeability the wing spars are set out for the wing positions, and the necessary holes for the fittings drilled to jig, before being handed over to the wing erectors.

Arrangement of Planes.

The usual arrangement on machines of the scout type is for the lower plane to butt against the lower members of the fuselage, and the top planes being the same span, the width of the body is made up by a centre plane. Another method is to make the top plane in two portions only, thus obviating the centre plane; and occasionally the spars of the top plane run through, from wing-tip to wing-tip, although this is only possible in machines of small span. Apart from the fact that such a wing requires extra room, it is difficult to procure timber of length exceeding 20 ft. sufficiently straight in the grain; and a minor detail would be the difficulty of repair, as a damaged wing-tip would practically entail a new spar, as splicing, although permissible in some parts of the machine, should not be tolerated as a means of repairing wing spars.

The difficulty of obtaining timber will necessitate the wings of large machines being made in sections; and there are several instances where this form of construction has been adopted, in one case the sections being only five feet in length. This construction seems eminently suited to the post-war sporting machine, as chance damage would be confined to a smaller area, transport simplified, and, providing the joints are well made, no appreciable loss in efficiency should ensue.

Types of Wing Ribs in Use.

From a survey of the plane diagram, [Fig. 26], it will be noticed that the chief components, in addition to the main spars, are the ribs, box-ribs, stringers, and leading and trailing edges.

Fig. 28.—Construction of ribs.

The ribs, which is the term applied to the very light framework built over the spars to maintain the correct curvature, are variously constructed; one of the most popular methods in vogue is that shown by [Fig. 28]. The central portion, or web, which includes the nose and trailing edge formers, may be cut from either spruce, whitewood, cotton wood, which can be bent to a surprising degree without fracture, and three-ply. Three-ply, while excellent for some items, is hardly suited for this purpose, as the laminations have a tendency to come apart, especially in the lower grades, which is aggravated by the screws or brads necessary for the attachment of the flange. A rib, fretted out as in [Fig. 28], with the web of cotton wood and a spruce flange, can be made extremely light. A rib for a chord of from 4 ft. 6 in. to 5 ft. would weigh about 5½ oz. As it is very necessary that every rib should correspond, these parts should be made to a metal jig, which is about the only way to ensure exactitude. This should be made from mild sheet steel, about 16 B.W.G., and need only be shaped to the outer curve, as the lightening holes are of but secondary importance, these being usually marked out in the saw mill, and cut to the line with a fine jig saw. 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.

While the method of rib building previously described constitutes general practice, there are, of course, other arrangements in vogue. [Fig. 29] illustrates a system in which the front spar forms the leading edge, a procedure which is somewhat rare now, owing to the features of modern wing sections, but at one time quite common. In this case the web is of three-ply lightened with a series of graduated holes, according to the width of the web, and the flanges of spruce.

The rib assembly, [Fig. 30], is extremely simple and light, as in this case the web proper is superseded by thin strips of three-ply, glued and bradded each side of the spruce flange. The amount of woodwork between the spars is reduced to a minimum, although one can hardly imagine such a system answering for a chord over five feet. Even then the wing curvature would require to be fairly simple, as a pronounced curve would flatten out. As a point of fact, this assembly is rarely used for chords exceeding 4 ft. 6 in. In another arrangement as shown in [Fig. 31], the connection between the top and bottom flanges is formed by blocks, a method which is certainly economical of material.

Fig. 29.

Fig. 30.

Fig. 31.

AFig. 32.

Figs. 29–32.—Construction of ribs.

An interesting form of rib design is that shown by [Fig. 32], and in this instance the fretting is specially designed to prevent any flattening out of the camber. The rib section is shown at A, [Fig. 32], and it will be noticed that the flange of chamfered section is grooved to take the three-ply web. The vertical parts of the web are stiffened by small semi-circular fillets.

Ribs under Compression.

For those ribs contiguous to the interstrut joints, a different construction is necessary to withstand the tension of the cross-bracing of the planes and, to a lesser degree, the internal plane wiring, so that at this point the rib performs two functions, that of maintaining the wing curve, and also taking the strains due to compression. Where such provision is not made, the tension of the wiring will result in either or possibly both of the following: (1) the rib will buckle laterally; (2) the camber will increase to an extent varying with the pressure on the wires, both results being extremely detrimental to efficiency. In this respect the old box-kites of varying origin used to offer some interesting studies in variable camber, and when it is remembered that the wing ribs were commonly composed of a single ash lath, steamed to shape, and the fabric attached on the top side only, the wonder is that extended flying was possible at all. For all that, some comparatively classic cross-country flights were accomplished. One popular system is to incorporate a box-rib at these points, sometimes made by placing two ordinary ribs close together and connecting them with three-ply or thin spruce, so that, although the overall width of the finished box-rib would be approximately 2 in., it is exceptionally rigid and withal light.

Fig. 33.—Compression rib.

Another solution is to use a solid web, lightly channelled out, as in [Fig. 33].

In some wing structures the ribs are uniform throughout, a strut of either steel tube or wood being inserted and to which the internal wiring is attached. This latter method is possibly more desirable, that is, if the joint between the compression strut and spar can be combined with the interstrut fitting. This may necessitate a little extra work in the latter, but this is preferable to the use of a separate fitting, involving additional piercing of the spar.

Importance of Even Contour.

Whilst on the subject of rib building, one cannot over emphasize the desirability of even contour, and the template, illustrated by [Fig. 34], serves as an admirable check. It is cut from very dry material to the outside curve of the section, and if this is tried on as each rib is fixed, one may be sure of comparative uniformity. The root rib is generally of stouter construction, and usually follows the same lines as the compression ribs. At this point the pull of the fabric has to be contended with, which is not infrequently a considerable strain. The same conditions prevail at the wing tip, which is one reason against excessive reduction of material at this point. Instances occur where the tension of the fabric after doping has considerably deformed the tip curve, which is at least unsightly, and may entail reconstruction.

SHAPED TO UNDER SURFACE

Fig. 34.—Template for testing rib contours.

Wing Tip Details.

The actual shape of the wing tip varies with the make of machine, and forms one of the distinctive features of the complete assembly. There is a general tendency to rake the ends, making the back spar longer than the front, on the score that increased efficiency due to reduction of end losses is attained. While this is somewhat problematic, seeing that several notable machines have square tips, and some actually constructed with the longest edge leading, it undoubtedly imparts a pleasing and distinctive appearance.

The actual construction is largely a matter for individual preference, as there are several ways of forming it. For instance, a single piece of ash may be bent to shape, or it may be cut out in sections from spruce boards and glued together with a long splice, while in another instance oval steel tube is the material. This small section steel tubing seems admirably suited for such items as wing tips, trailing edges, and the various components of the empennage, such as the fixed stabilizer, elevators, fin, and rudder.

Another method of construction used for the wing tips of some machines consists of a number of strips, about six for a wing tip 1 in. wide by ¼ in. thick, the joints between which are disposed vertically, forming a laminated wing tip. In manufacture, each piece is bent round bending jigs or blocks of the required shape, the edges of the strips having previously been glued. It is apparent that the smaller the section of strip used, the easier it can be bent, and with this arrangement quite sharp bends can be successfully formed in spruce. The alternative method of steaming a solid piece is often wasteful, apart from the fact that it enforces the use of ash.

CHAPTER V.
DETAILS OF PLANE CONSTRUCTION.

The tendency to lose lift, pronounced in some machines, hardly noticeable in others, may be directly traced and attributed to the manner in which the wings are built, which is largely dependent upon the design. In the preliminary stages of design it is usual to take as a basis the figures for lift and drift of a known tested section, that is if facilities are not available for testing an exact scale model of the section it is intended to use. Anyway, the whole design is dependent upon these figures, in respect of both the maximum and minimum speeds, and also the rate of climb, and the extent to which the actual performance of the machine complies with these calculations is determined solely by the exactitude and precision with which the full-size wing conforms to the scale model. By this means only is it possible to design with any degree of accuracy.

The Sagging of Fabric.

The sagging of the fabric between the ribs is one of the principal reasons for the failure of the finished machine to satisfy expectation and also of the tendency to lose lift. One or two causes contribute to this result. One is the spacing of the ribs, which in some cases is not nearly close enough. A rough average spacing is from 10 ins. to 1 ft., but in modern high-speed machines, loaded to anything from 5 lbs. to 8 lbs. per square foot, the spacing should be much closer. In addition, the ribs near the wing root should be closer than those at the tip, for at this point the stresses are greater, a certain amount of vibration from the engine having to be contended with, in addition to the effects of the slip-stream of the air-screw. Particularly noticeable is the tendency for the fabric to sag down on the top surface of the leading edge, a feature which imparts to the machine, especially when viewed from the front, a not unpleasing corrugated appearance. At this part of the section the curve is somewhat sharp, and naturally the fabric tends to conform to the definition of the shortest distance between two points, a straight line. This, of course, is aggravated in flight, when the planes are under load, and by far the greatest amount of pressure is located at the front portion, or leading edge, of the wing.

False Ribs.

FALSE RIBS

Fig. 35.—Arrangement of ribs at leading edge.

In some wing constructions the forces are minimized by the provision of subsidiary or false nose-ribs, [Fig. 35], which extend usually from the leading edge as far back as the front spar and occasionally to the longitudinal stringer. While this prevents, to a certain extent, the sagging in of the fabric, it does not entirely eradicate it. The only successful way in which the characteristics of the wing contour may be preserved is by covering the leading edge with thin veneer, spruce, or, still better, three-ply, as [Fig. 36]. Despite the great advantages attending this constructional feature, its use cannot be said to be really extended.

Fig. 36.—Three-ply covering for leading edge.

Pressure at Leading Edge.

The pressure at the leading edge produced by the enormous speed at which the modern machines fly (and the maximum diving speed of which, owing to the reduction of resistance, is correspondingly increased) must be abnormal, and calls for different methods of construction from those which at present obtain. There is at least one case on record where the fabric has burst at this point with fatal results. It is interesting to note that in the report of the N.P.L. for the year 1916–17 mention is made of the deformation of the wing form, due to the sagging of the fabric, which has been reproduced in model form, so that the allowances to be made and the resultant effects have been determined.

Effect of Lateral Control.

The system adopted for the lateral control is a decisive factor in deciding the general lines of construction. The arrangement of plane warping, whereby the wing was twisted or warped from root to tip, or the outer section only, has given place to the almost universal use of aileron control. With the old warping system the ribs, spars, and the whole wing collectively was subjected to a torsional strain, which could only have had a deleterious effect upon it. This fact was almost entirely responsible for the practice of using steel tube for wing spars, for by its use it was a fairly easy matter to arrange the ribs to slide or hinge upon the tube, which, at least, relieved some of the torsional stress.

Leading and Trailing Edges.

Fig. 37.—Leading edges.

The average practice concerning the formation of the leading and trailing edges is shown by [Figs. 37] and [38]. Where the section in use requires a bluff entry the spindled-out nose-piece is applicable, while for a sharp entry a fillet let into the nose-formers suffices. As previously mentioned, steel tubing makes a satisfactory trailing edge, although somewhat heavier than the spruce strip, while an extremely fine leading edge can be formed by steel wire. The edge, under pressure of the fabric, assumes a variegated shape, a distinctive feature of some types, but, nevertheless, a wire trailing edge is somewhat flabby and undulating, and as a method is obsolescent. Longitudinal stringers are employed to preserve the wing contour and also for a stiffening medium for the ribs in a lateral direction. About the only variation of the small spruce strip for the purpose is linen tape, crossed alternately.

Fig. 38.—Trailing edges.

Efficiency of the Raked Wing Tip.

In the previous chapter mention was made of the probable gain in efficiency resulting from the raked wing tip, and that this has some foundation in fact will be apparent from a consideration of [Fig. 39], which illustrates the flow of air across a plane, as generally accepted. Where the plane surface is continuous from wing tip to wing tip, the provision of the shaped tip would appear to compensate for any slight loss, but there are instances where the extent of the pilot’s range of view is of the utmost importance, and this may necessitate the cutting away of a portion of the centre section (which sometimes affords the only means of ingress and egress), or the root of the lower plane, as in [Fig. 40].

Fig. 39.—Diagram showing flow of air across plane.

CENTRE SECTION CUT AWAY

SPAR ROOTS CUT AWAY

Fig. 40.

Wing Baffles.

An attempt to prevent air leakage caused by this is occasionally observed in the employment of vertical vanes, or wing baffles. In the case of a machine with the lower plane abutting against the side of the fuselage, these would not be necessary, the fuselage acting in the same manner. The baffles are usually of three-ply or spruce, and shaped to project above the top and bottom surfaces, this projection rarely exceeding six inches. A typical arrangement is illustrated by [Fig. 41], which also shows the exposed spars streamlined with a fairing of three-ply. It is typical of the varied opinions which still exist, that on some machines the wing roots are merely washed out somewhat abruptly. If this air leakage is of any moment, it is apparent that it must detrimentally affect the lift-drift ratio. As a proof of the existence of pressure at the openings in the wing, the writer remembers the case of a well-known seaplane, where the wing baffles on the centre section were made of somewhat thin three-ply. In flight it was noticed by the pilot that these were being forced away from the wing, and subsequently these were replaced by baffles of stouter construction.

Fig. 41.—Wing baffle.

Metal Wing Construction.

Of two machines, equal in air performance, the one which can be most easily produced has an obvious and, especially at the present time, a very important superiority. 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, assuming 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. By the more extensive use of metal there should be a considerably reduced proportion of scrapped parts, and erection would be accelerated. It is significant that the planes of some of the most recent German machines are constructed largely of steel tubing, which is at present the most practicable form in which steel can be used. Of course, steel tube spars are quite an old detail, although the more general English practice is to core them with spruce or ash, as in [Fig. 42]. One remembers a

Fig. 42.—Steel tube spar with wood filling.

monoplane, built some time before the war, in which the spars and ribs were of steel and the covering of thin aluminium sheet. In flight this machine was particularly fast, which may be accounted for by the reduction of skin friction, which a smooth surface such as aluminium would afford. In addition, the tendency of a fabric covering to sag was also obviated. Another example of metal construction is afforded by the Clement-Bayard monoplane, exhibited at Olympia in 1914. The plane construction of this machine, as shown by [Fig. 43], consisted of channel steel spars, steel leading and trailing edges, and thin steel strips replacing the usual wooden stringers. However, steel construction in modern English machines is restricted to the various organs of the empennage, and occasionally one finds ailerons so built. There seems no valid reason for the continued use of wood as the material for the construction of such items as the fin, rudder, and elevators, as a considerable saving of labour and time can be effected by using the various forms of steel tubing; moreover, the tendency which most controlling organs built of wood have to warp and twist with variations in temperature is prevented by the steel frame. One frequently sees such items as the ailerons and elevators distorted, which must result in excessive drift, if not erratic flying. At the present time it is difficult to obtain aluminium alloy in any large quantity, and this, in conjunction with the present high prices, precludes its extensive use. When this material is procurable in quantity, and when design is reasonably standardized, rolled or lattice spars and stamped ribs may come into vogue.

STEEL SPARS

Fig. 43.—Rib construction with metal spars.

Fabric Attachment.

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. The purpose of this was that, should a bullet or any object pierce any one of the squares, damage would be confined to that square, and thereby prevented from developing; but the writer cannot recall any instance of its use to-day.

In covering, the fabric should be tightly and evenly stretched from end to end of the wing, and only comparatively lightly pulled from leading to trailing edge. If too much strain is applied to the fabric crosswise it will result in undulations between each rib. The tendency of fabric to sag between the ribs is accentuated by this, and, of course, matters are not improved upon the application of the dope. It should be remembered that the efficiency of any machine is greatly dependent upon the tautness of the fabric. It should not be stretched too tightly, as the application of the specified coats of dope may result in the fibres or threads of the material being overstrained.

CANE STRIPS SCREWED TO RIBS

Fig. 44.—Attachment of fabric to ribs by cane strips.

With regard to the actual attachment of the covering to the wing framework modern practice is restricted to two methods. The older method is illustrated by [Fig. 44], and consists of strips of spruce, or more usually cane, tacked or screwed to the ribs. It is usual, and certainly preferable, to affix this beading to every rib of those sections of the planes adjacent to the fuselage, as the fabric on these portions is subjected to the slip stream of the propeller, which meets it in a succession of small blows. The fabric in the outer sections need only be affixed to alternate ribs. The alternate method is shown by [Fig. 45]. In this case the fabric is sewn to the

Fig. 45.—Fabric sewn to ribs.

ribs with twine or cord, the stitches occurring about every three inches. It will be noted that every loop or stitch is locked with a species of half-hitch knot. This stitching is then covered with bands of fabric, the edges being frayed to ensure perfect adhesion and doped to the main cover. It is largely a matter of opinion which system ensures the most even wing contour, although it would seem that the drift or resistance is slightly lessened by the sewing method. An obsolete method is that in which the fabric was tacked to the ribs with brass pins and taped with linen tape. All sewn joints in wing covers should be, and generally are, of the double lapped variety ([Fig. 46]), and arranged to run diagonally across the wing. A minor and somewhat insignificant detail of wing

Fig. 46.—Double-lapped joint in fabric.

covering is the provision of small eyelet holes in the under surface of the trailing edge, allowing water accumulated through condensation to drain away, and although not general practice, would appear to be necessary. A refinement which may be necessary on the post-war sporting machine is the attachment of small blocks, or “domes of silence,” to the leading edge, as a protection for the fabric against wear. When planes are dissembled more often than not they are stacked leading edge downwards on a concrete floor, and any movement or friction is likely to result in the rubbing away of the fabric, which, if unnoticed, may result in the bursting of the covering. Such fitments would hardly constitute an innovation, as the writer has distinct recollections of seeing such fittings on the D.F.W. biplane at Brooklands just prior to the outbreak of war. These consisted of brass balls, free to rotate in a socket, screwed to the leading edge. A narrow strip of aluminium screwed along the entering edge would be quite sufficient, and would not add appreciably to the weight.

CHAPTER VI.
INTERPLANE STRUT CONNECTIONS.

It may be taken as fairly conclusive that for war purposes the biplane has proved its superiority, and it appears also that for the commercial requirements of the future it is suited still better, and therefore, in view of the huge possibilities thus opened up, is likely to maintain this predominance.

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. Seeing that the principal difference between the biplane and monoplane consists essentially in the type of truss employed, the arrangement and attachment of the various members peculiar to the biplane truss becomes of interest, certainly of importance. It is intended to deal with the various trusses in a later chapter, confining the present remarks to the interplane strut fittings in use, and commencing by detailing the chief requirements and desirable features. The most desirable requirement is that the attachment of the fitting to the wing spars does not involve the drilling of the spar. In practice this is most difficult of accomplishment, for while no great trouble would be experienced in making a fitting fulfilling this requirement, it would be quite another matter to keep it in place under the tension of the bracing wires, and in the case of the outer strut fitting, to which any strain is ultimately transmitted, practically impossible. In spite of this, it must be remembered that the machine may occasionally, when landing or getting off, pitch over on to the wing-tip skid, and if severe, the shock transmitted to the spar may cause a fracture to develop which, starting at the hole due to the strut fitting, and owing to the fabric covering, would be difficult to detect. One or two similar mishaps, with a consequent increase in the extent of the fracture, give distinct possibilities of collapse in the air. Although one cannot give specific instances, it is a feasible contingency, and one that should be eliminated from the region of possibility.

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.

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.

Welding properly performed is less objectionable, indeed, its use may be said to be constantly increasing, although it is well to recognize its limitations. It should not be used for parts subject to any great tensile stress, such as the fittings forming the subject of this chapter. The efficiency of any welded joint is hard to determine, as apparent soundness on the surface is no indication of the internal nature of the weld. Regarded from the aphoristic “maximum strength for minimum weight” view point, and taking into account the advantages in this direction which can be obtained by the use of a high-grade steel, brazing and welding are not to be commended.

The operation of soft soldering, requiring only a moderate heat, does not weaken the material to any great extent, and for some items a properly pegged and soldered joint is superior to the two methods of jointing previously described.

Connections in Use.

Fig. 47.—Interplane strut attachment.

The illustrations given indicate the varying degrees of practice, taking as the standard for comparison the early Wright socket, [Fig. 47]. Although somewhat crude it was quite suitable for the purpose, especially as the wing warping system in the Wright machines necessitated a fair amount of flexibility in the joints. It serves also to illustrate that some advancement has been made in constructional work. The advantages of rapid erection and dismantling have been realized and provided for in most machines since the early days of the industry, and it is not surprising, therefore, that the salient characteristic of the joint ([Fig. 48]) used by S. F. Cody on his famous biplane was portability. The interstrut terminates in a kind of fork, which in turn is pinned to the head of a special bolt slotted to receive it. The fact that the wiring lugs were improvised from chain links is interesting.

Fig. 48.—Interplane strut attachment.

The method of packing the wings for transport consisted in detaching the two outer cellules from the central structure, when the removal of one set of wires enabled the planes to be folded one against the other. It is possibly of interest to record the fact that in the military trials of 1912 this machine was taken down and re-erected in 51 minutes, quite a good performance taking into account its large dimensions. Although this attribute is scarcely necessary at the present time, it will be undoubtedly required by the sporting owner of the future with limited storage facilities. The fitting shown

Fig. 49.—Interplane strut attachment.

by [Fig. 49] is only suitable for machines with light wing loading. The plate forming the anchorage for the wires is pressed out, the lugs bent to the different angles, and then attached to the spar by an eyebolt, to which is fixed the plane strut, the ends of the latter being capped with steel tube of streamline section. A similar arrangement is that shown by

Fig. 50.—Interplane strut attachment.

[Fig. 50], the lug plate being pressed out and bent, but in this example the strut terminates in a socket of oval steel tube welded to the plate. It is connected to the spar by a bolt passing through the centre of the socket, the strut end fitting over this.

Fig. 51.—Interplane strut attachment.

The practice of anchoring wires to eyebolts, as in [Fig. 51], forms the nucleus of many strut connections, but as a method cannot be recommended. Continual strain on the wire has a resultant in the bending over of the head of the eyebolt as in [Fig. 52]. As a point of fact the use of the eyebolt is distinctly elementary, and gives the impression of a makeshift. The fitting illustrated by [Fig. 53] constitutes an advance on the previous arrangements dealt with, and is also indicative of modern practice.

Fig. 52.—Interplane strut attachment.

Fig. 53.—Interplane strut attachment.

The main body of this clip is a stamping from heavy sheet-steel, bent up to the section of the spar, the bolts, it will be noticed, passing horizontally through it. The anchorage for the wires is formed by lugs, which have a direct pull on the bolts, and is so arranged that a slight clearance exists between lug and spar.

Fig. 54.—Plane strut attachment.

The plane-strut is shod with steel tubing, and connected to the fitting by a bolt, as shown. Of the strut connections described so far, hardly one can be said to conform to the leading principle of the ideal fitting, i.e. the secure attachment to the spar without piercing the latter for bolts. [Fig. 54] gives a fitting which is as good a solution of the problem as is constructionally possible. The basis of this connection is the lug-plate, to which is welded the strut-socket, the whole being fastened to the spar by four bolts, which are let in the flange of the spar just half their diameter, and tighten on a washer-plate on the opposite side. Lateral movement along the spars is thus adequately prevented, although the outer strut-socket might conveniently be bolted right through the spar, without materially reducing the strength thereof. This is made possible by the fact that the wing spars, disregarding the small wash-out at the extreme tip, are generally parallel in depth from root to tip, the amount of material at the point of intersection of the plane-strut being in excess of that necessary for the stresses concerned. Another attachment achieving similar results is shown in the diagram ([Fig. 55]), forming an example of the fitting employed on the pre-war Avro biplane. It will be noticed that in this case two bolts only are used for the connection, the pull of the flying or lift-wires being counteracted by the duplicated wires taken from the washer-plate to a fitting located on the single central skid of the undercarriage.

Fig. 55.—Interplane strut attachment.

Head Resistance of Strut Sockets.

A point calling for comment is the apparent oversight or neglect of the amount of head resistance offered by the average strut fitting, although great care is taken to ensure the strut and wing sections being of correct form. It seems probable that some difference must occur, especially at the high speeds now prevalent, between the air flow across the plane and that which meets the strut terminal. Anyway, some discontinuity of flow exists, and whether or no the aggregate resistance of all the fittings is of any great moment provides matter for discussion. It is quite possible to fair off any irregularities in air-flow due to the strut connections by the attachment of sheet-aluminium fairings, which could be beaten, pressed, or spun with little difficulty. Although examples of this practice are very little in evidence, the writer inclines to the belief that the additional weight would be negligible compared with the ensuing reduction in head resistance.

The foregoing examples cannot be said to constitute the latest practice, nor is it possible under present conditions to give such details, but sufficient has been said to indicate the progress and trend of design.

CHAPTER VII.
WING-TRUSSING SYSTEMS.

Although the trussing of aeroplanes is carried out along certain well-defined lines, there are occasional divergences from the orthodox. The differences now existing are not nearly so great as those of former days, this being explained by the fact that the progress of any science or industry tends towards uniformity of method, while practical experience eliminates the undesirable systems. This does not necessarily mean that the present methods in vogue are incapable of improvement, but merely denotes their suitability for present requirements.

The Pratt Truss.

Fig. 56.—The Pratt truss.

The basis of all modern trussing systems, with modifications, is the Pratt truss ([Fig. 56]), familiar in bridge-building circles, the basic principle of which is that the compression members are disposed vertically, and while of minimum length are most favourably placed for obtaining the maximum efficiency. There are other types of trusses used in structural engineering, as, for instance, the Howe truss, in which the compression members are arranged diagonally, and the Warren lattice-type girder; but for various reasons these are not applicable to the needs of aeronautical engineering. But a brief consideration of the chief features of the Pratt or box-girder system of trussing will suffice to illustrate its great advantages for aircraft work, particularly for machines exceeding a certain span; and it is this limiting span to which a monoplane can safely and efficiently be built which is largely responsible for its present spell of unpopularity.

Monoplane Trussing.

From the standpoint of simplicity, the monoplane equals the biplane. As each wing of the former may be considered as a cantilever, it is the difficulty of adequately staying the wings above a certain span which forms the deterrent feature, for it is obvious that, as the span increases, in order to obtain a reasonable angle for the wires, the king post, or cabane, must be increased in height. This would necessitate an ungainly undercarriage, less able to withstand rough landings, with a consequent increase in both weight and head resistance. However, it seems that the monoplane will have a future for sporting purposes, where the span will not exceed 30 ft., and will probably be nearer 20 ft.

Fig. 57.—Monoplane wing bracing.

Various attempts have been made to obviate this inherent defect of the monoplane system of trussing, the first and most popular being the king-post system ([Fig. 57]), in which short masts are incorporated in the wing structure and wire-braced to the spars. From the points formed by the crossing of the mast and spar the main bracing-wires are taken. That this system is of real use is demonstrated by the fact that, amongst others, the Antoinette, Flanders, and Martinsyde monoplanes incorporated this system. It is worthy of note that this system also characterized the huge Martinsyde trans-Atlantic ’bus, the wing-spread being in the neighbourhood of 70 ft. Another original attempt at improvement, the wing-bracing of the Deperdussin hydro-monoplane, is of interest ([Fig. 58]). As regards the bracing, the machine was virtually a biplane, the wings being stayed by a steel tube running parallel with the wings, and connected to it at intervals by steel tubular struts, with cross-bracing between, as in a biplane. The abolition of the top wires rendered the machine of greater value for war purposes than other tractor machines of that period. The logical conclusion of this system is exemplified by the Nieuport scouting biplane, the lower plane of which corresponds to the streamlined steel boom of the Dep.

Fig. 58.—Deperdussin monoplane bracing.

Wireless Wing Structure.

Superficially, it would appear that the abolition of external trussing and wiring would make for greater aerodynamical efficiency; and, constructionally, it would be quite possible to build wings devoid of external staying, and at the same time of sufficient strength. But when it is considered that this would entail an excessive depth of spar at the root of the wing, with a resultant increase of head resistance, it is doubtful whether any appreciable advantage would accrue. In the event of the wing becoming deformed or out of alignment, re-truing up would be almost impossible, and would certainly require the uncovering of the wing and partial reconstruction. Contrast this with the orthodox wire bracing. It is simple of attachment, of relatively low cost, and offers the utmost facility for truing up. A monoplane of note, built without external trussing, was the special Antoinette, produced for the French military trials of 1911. This had a span of approximately 46 ft., and the depth of spar at the root was about 2 ft. 3 ins., and at the tip 9 ins., the consequent weight alone being abnormal.

Anchorage of Lift Wires.

The one-time practice of anchoring lift wires to various parts of the undercarriage is bad in principle, as there is a distinct possibility that a rough landing may damage the wire or its attachment, and ultimately cause failure in flight. This practice undoubtedly arose from a desire to obtain a good angle for the lift wires, a subsequent improvement being the addition of a separate pylon or cabane.