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.
[Illustration: FIG. 1.--Heart shake.]
[Illustration: FIG. 2.--Star shake.]
[Illustration: 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
[Illustration: 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.
[Illustration: FIG. 5.--Shrinkage of boards due to position in log.]
[Illustration: 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.