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.
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
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
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.
stiffened sides.
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.
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
with fairing bound on.
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.
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.
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.
Fuselage Struts.
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.