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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: FIG. 29.]
[Illustration: FIG. 30.]
[Illustration: FIG. 31.]
[Illustration: A FIG. 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.
[Illustration: 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.
[Illustration: _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.