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.
[Illustration: 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
[Illustration: 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
[Illustration: 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.
[Illustration: FIG. 10.--Hollow spar with stiffened sides.]
[Illustration: 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.
[Illustration: 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
[Illustration: FIG. 13.--Steel tube strut with fairing bound _on_.]
[Illustration: 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.
[Illustration: FIG. 16.--Interplane support from body.]
[Illustration: 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.
[Illustration: 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
[Illustration: 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.
[Illustration: FIG. 23.--Showing inefficiency of pointed section in a
side wind.]
Fuselage Struts.
[Illustration: FIG. 24.--Channel-section fuselage strut.]
[Illustration: 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.