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.
[Illustration: _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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: FIG. 39.--Diagram showing flow of air across plane.]
[Illustration: 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.
[Illustration: 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
[Illustration: 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.
[Illustration: 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.
[Illustration: 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
[Illustration: 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
[Illustration: 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.