MATERIALS OF CONSTRUCTION
The materials of construction for airplanes should be of such material,
size and form as to combine greatest strength and least weight. With
metal parts in particular it may be necessary to substitute less strong
material for the sake of getting non-corrosive qualities, ability
to withstand bending, ductility or ease of bending, etc. With wood,
absence of warping is important as well. The materials which are
considered are the following: wood, steel, including wires; special
metals as aluminum, brass, monel metal, copper, etc., and also linen
and dope.
=Strength of Materials.=—It is important in a general way to understand
the terms used in speaking of strength of materials. Thus we may have
strength in tension, strength in compression, or strength in shearing,
bending and torsion. Some material fitted to take tension will not take
compression, as for example wire; some material, as bolts, are suited
to take shear, etc.
In general all material for airplanes has been carefully tested and no
excess material is used above that necessary to give the machine the
necessary strength.
=Tension.=—This means the strength of a material which enables it to
withstand a pull. Thus wires are used where strength of this kind is
required.
=Compression.=—This refers to strength against a pressure. Wire has no
strength for this purpose, and wood or sometimes steel is used.
=Shearing.=—Refers to strength against cutting off sideways. Thus the
pull on an eyebolt tends to shear the eyebolt, or the side pull on any
bolt or pin tends to shear the pin.
=Bending.=—In bending material the fibres on the outside tend to pull
apart; those on the inside tend to go together. Thus on the outside
we have tension, and on the inside compression. Along the center line
there is neither tension or compression, it is the “neutral axis.”
=Torsion.=—Torsion is a twisting force, such as an engine propeller
shaft receives.
=Testing for Strength.=—If a wire is an inch square in cross-section
and breaks when a load of 150,000 lb. is hung on it, we say that the
strength of the wire is 150,000 per square inch. Smaller wires equally
strong have a strength of 150,000 lb. per square inch also, but they in
themselves will not support a load of 150,000 lb. but only the fraction
of that, according to the fraction of a square inch represented by
their cross-section.
In the same way, a square inch of wood under a compressive load may
break at 5000 lb. If, however, the piece of wood is long in proportion
to its thickness, it will bend easily and support much less weight.
For example, a perfectly straight walking cane could perhaps have a ton
weight put on it without breaking but if the cane were not set squarely
or if it started to bend it would immediately break under the load.
These cases illustrate the importance of having struts perfectly
straight, not too spindling and evenly bedded in their sockets. Some
training machines are built with a factor of safety of 12. That
is to say, the breaking strength of any part is twelve times the
ordinary load or stress under which the piece is placed. It should be
remembered, however, that under any unusual condition in the air, such
as banking, etc., extra strains are placed on the parts and the factor
of safety is much less than 12. Factor of safety of 12 thus does not
mean exactly what it does in other engineering work, where allowances
are made for severe conditions. The so-called factor of safety of 12 in
airplane work is probably no greater than a factor of safety of 2 or 3
in regular engineering work.
There are three all-important features in the flying machine
construction, viz., lightness, strength and extreme rigidity. Spruce is
the wood generally used for parts when lightness is desired more than
strength, oak, ash, hickory and maple are all stronger, but they are
also considerably heavier, and where the saving of weight is essential,
the difference is largely in favor of the spruce. This will be seen in
the following condensed table of U. S. Government Specifications.
———————————+—————————————+—————————————+————————————
| Weight per | Modulus of | Compression
Wood | cubic foot, | rupture, | strength,
| pounds (15% | pounds per | pounds per
| moisture) | square inch | square inch
———————————+—————————————+—————————————+————————————
Hickory | 50 | 16,300 | 7,300
White Oak | 46 | 12,000 | 5,900
Ash | 40 | 12,700 | 6,000
Walnut | 38 | 11,900 | 6,100
Spruce | 27 | 7,900 | 4,300
White Pine | 29 | 7,600 | 4,800
———————————+—————————————+—————————————+————————————
A frequently asked question is: “Why is not aluminum or some similar
metal, substituted for wood?” Wood, particularly spruce, is preferred
because, weight considered, it is much stronger than aluminum, and this
is the lightest of all metals. In this connection the following table
will be of interest.
————————————————+—————————————+——————————————+————————————————————
| Weight in | Tensile | Compression
Material | cubic feet, | strength per | strength per sq. in
| pounds | sq. in | pounds
| | pounds |
————————————————+—————————————+——————————————+————————————————————
Spruce | 27 | 7,900 | 4,300
Aluminum | 162 | 15,000 | 12,000
Brass (sheet) | 510 | 20,000 | 12,000
Steel (tool) | 490 | 100,000 | 60,000
Nickel steel | 480 | 100,000[1] |
Copper (sheet) | 548 | 30,000 | 40,000
Tobin bronze | | |
(Turnbuckles) | ... | 80,000 |
Monel metal | 540 | 90,000 | 30,000
————————————————+—————————————+——————————————+————————————————————
=Wood.=—Present practice in airplane construction is to use wood for
practically all framing, in other words, for all parts which take
pressure or compression. Although wood is not as strong for its size
as steel and therefore offers more air resistance for the same strength
yet the fact that frame parts must not be too spindling, in other
words, that they must have a certain thickness in proportion to their
unsupported length, has led to the use of wood in spite of the greater
strength of steel. Some airplanes, however, as the Sturtevant, are
constructed with practically a steel framing.
It should be borne in mind that any piece or kind of wood will not
answer for framing, and more especially for repair parts. There is a
tremendous difference in the strength and suitability among different
woods for the work. For instance, a piece of wood of cross or irregular
grain, one with knots, or even one which has been bored or cut or
bruised on the outside, may have only half or less the strength of the
original piece. Air drying doubles the strength of green wood, proper
oven drying is better yet.
Notice how the ends of each piece are ferruled, usually with copper
or tin. This is to prevent the bolt pulling out with the grain of the
wood, and also prevents splitting and end checking and gives a uniform
base on which the pressure comes.
It is generally advised not to paint wood as it tends to conceal
defects from inspection. So varnish only.
Wrapping wooden members with linen or cord tightly and doping this,
both to make waterproof and to still further tighten, increases the
resistance to splitting. The absence of warping tendencies determine
often what wood to choose.
The selection of lumber and detection of flaws is a matter of
experience and should be cultivated. It is, however, nothing more than
the extension of the knowledge that leads a man to pick out a good
baseball bat.
=Woods.=—1. _Spruce._—Should be clear, straight-grained, smooth and
free from knot holes and sap pockets, and carefully kiln-dried or
seasoned. It is about the lightest and for its weight the strongest
wood used. It is ordinarily used as a material for spars, struts,
landing gear, etc., as it has a proper combination of flexibility,
lightness and strength.
2. _White Pine._—A very light wood used for wing ribs, and small struts.
3. _Ash._—Springy, strong in tension, hard and tough, but is
considerably heavier than spruce. Used for longerons, rudder post, etc.
4. _Maple._—Used for small wood details, as for blocks connecting rib
pieces across a spar or for spacers in a built-up rib.
5. _Hard Pine._—Tough and uniform and recommended for long pieces, such
as the wooden braces in the wings.
6. _Walnut, Mahogany, Quarter-sawed Oak._—The strength, uniformity,
hardness and finishing qualities make these woods favorites for
propeller construction.
7. _Cedar Wood._—Is used occasionally for fuselage coverings or for
hull planking in hydroplanes as it is light, uniform and easily worked.
Veneers, or cross-glued thin layers of wood, are sometimes used for
coverings.
Laminated or built-up wooden members have been much used for framing
and for ribs and spars. The engine bearers are always of wood on
account of vibration and are also laminated. In lamination the wooden
strut is built up of several pieces of wood carefully glued together.
The grains of the different layers run in different directions,
consequently a stronger and more uniform stick often is secured. The
objection to laminated pieces comes from the weather causing ungluing.
Laminated pieces should be wrapped in linen or paper and freshened with
paint or varnish from time to time.
=Forms.=—Attention should be called to the hollowed form of many of
the wooden members. In any beam or strut, material at the center of
the cross-section is of far less value in taking the load than the
material away from the center. Therefore, to secure greatest strength
with least weight, it is permissible to lighten wooden members if done
understandingly.
=Steel.=—There is a tremendous difference in the strength, wearing
and other desirable qualities among different steels and irons. For
airplane work none but the best qualities are allowed. For this reason
the use of ordinary iron bolts (as stove bolts) or metal fastenings or
wire not standardized and of known qualities should not be permitted.
The airplane is no stronger than its weakest fitting. This does not
mean that the hardest and strongest steel must necessarily be used, as
ease of working and freedom from brittleness may be just as important
qualities, but the steel on all metal fittings should be of high-grade
uniform stock. A ductile, not too easily bent, mild carbon steel is
usually recommended for all steel plate, clips, sockets and other metal
parts. If any parts are required to be tempered or hardened it must
be remembered that they become brittle and can not afterward be bent
without annealing or softening. Tool or drill steel is a name given to
uniform or rather reliable grades of steel adapted to heat treatment as
tempering or annealing. Often the bolts, clips, nuts, pins, devices and
other fittings are of special heat-treated nickel steel which must not
be heated locally for bending or for attachment. Such work seriously
weakens the steel. The steel is often copper-or nickel-plated and
enamelled to prevent rusting. Do not forget that the proper material
may be twice as strong as other material which looks the same but which
has not received special treatment.
=Wires.=—Only the highest grade of steel wire, strand and cord is
allowable. Manufacturers, as Roebling of Trenton, N. J., manufacture
special aviator wire and cord, which is given the highest possible
combination of strength and toughness, combined with ability to
withstand bending, etc. Steel wire ropes for airplane work are divided
into three classes as follows:
1. _The solid wire_ = 1 wire (as piano-wire grade) and known as
aviation wire.
2. _The strand stay_, consisting either of 7 or 19 wires stranded
together and known as “aviator strand.” Flying and landing wires on
Curtiss.
3. _Cord or Rope Stay._—Seven strands twisted together forming a rope,
each strand being of 7 or 19 wires and known to trade as aviator cord.
The wires are either tinned or galvanized as protection against rust,
etc. Ordinarily galvanizing is used, but hard wires and very small
wires are injured by the heat of galvanizing and they are therefore
tinned.
No. 1. The single wire is the strongest for its weight. Single wires
will not coil easily without kinking and are easily injured by a blow,
therefore their use is confined to the protected parts of the machine
such as brace wires in the fuselage and in the wings.
The strand stay (No. 2) of 7 or 19 wires is generally used for tension
wires, as it is more elastic (can be bent around smaller curve) without
injury, as the flying and landing wires on the Curtiss. The smaller
strands usually have 7 wires, the larger ones 19 wires.
No. 3. _The Tinned Aviator Cord._—The 7 by 19 cord is used for stays on
foreign machines. It is 1¾ times as elastic as a solid wire of the same
material. On the Curtiss it is used for control wires. For steering
gear and controls extra flexible aviator cord is also recommended.
This has a cotton center which gives extra flexibility and is used for
steering gear and controls. It is 2¼ times as elastic as a single wire.
Although wire strands or cords are not quite as strong for the same
size as a single wire they are preferred for general work, being easier
to handle and because a single weak spot in one wire does not seriously
injure the whole strand.
Especial care is necessary to avoid using common steel wires, or
strands which have a frayed or broken wire, or wire that has been
kinked and then straightened or wire that has been locally heated or
wire that has been bruised. All these factors weaken steel rope much
more than is supposed ordinarily.
=Wire Fastening or Terminal Connections.=—Wire terminals are of four
classes:
1. _Ferrule and dip in solder_, then bend back the end. With or without
thimble; used on single wires or on strand; 50 to 94 per cent. as
strong as the wire.
2. _Thimble and End Splicing._—The splice must be long and complete.
Used on cable; 80 to 85 per cent. as strong as the strand; breaks at
last tuck in the splice.
3. _Socket._—Nearly 100 per cent. strong.
4. _End Wrap and Solder._—Simple and serviceable; not used for hard
wire.
Present practice is rather toward elimination of acid and solder,
imperfect bends, flattening of cable on bends, and toward care in
avoiding all injury as kinking to wire, strand and cord due to
unskillful handling of material in the field.
=Other Metals.=—Other metals as aluminum, brass, bronze, copper, monel
metal (copper and nickel) are used for certain airplane fittings for
the reasons of lightness, non-corrosive qualities, or ease of bending,
etc. The trouble with these metals is that they are not uniform and
reliable in strength and in an important part the great strength
combined with minimum weight given by steel is not equalled by any of
these metals. Aluminum is used on the engine hood and also for control
levers and for the backs of the seats. In other words, for parts and
castings which require light metal construction, but which are under
no particular stress. Tin and copper are used for ferrules of wire
joints and for tankage. Copper or brass wire are used for safety wires.
Special Tobin bronze is used for turnbuckles as the part must not only
be strong but free from any tendency to rust. Monel metal (nickel 60
per cent., copper 35 per cent., iron 5 per cent.) is strong and has the
special property of being acid- and rust-resisting. It has been used
for metal fittings and even for wires and for the water jacket of the
motor. Until more strength tests show greater uniformity of strength,
it is to be recommended with caution.
In dealing with metals like steel, it should be remembered that they
are subject to crystallization and fatigue.
Repeated jarring may cause a bar of steel to break easily at a
particular point, when the metal is said to have crystallized there.
Fatigue of a metal may be defined as loss of springiness which may come
from repeated bending and which lessens the strength of metal. Above
all, however, corrosion of steel must be guarded against.
The above points should be clear, as in airplane work you are dealing
with a structure which is safe with perfect materials and workmanship.
The factor of safety, however, is not great enough to permit
carelessness, or defective material.
_Linen._—The almost universal wing covering is fine, unbleached Irish
linen, stretched rather loosely on the wing frames and then treated
with dope.
The linen used weighs 3¾ to 4¾ oz. per square yard, and should have a
strength with the length of the cloth or “warp” of at least 60 lb. per
inch of width. The strength in this direction is slightly greater than
that taken crosswise of the cloth or on the filler or weft. There is a
gain of strength and tautness by varnishing or “doping.”
In general, it is desirable to have wing material which will not sag
easily and have the fabric yield rather than break. This often reduces
stress and saves complete failure.
_Dope._—The linen must be coated with a more or less waterproof dope.
Some form of cellulose acetate or nitrate with more or less softening
material is used and to these some suitable solvent as acetone is
added.
The cellulose acetate or nitrate in the dope acts as a waterproof
sizing, shrinks the cloth tight, and prevents it from changing in
tightness due to moisture. Spar varnish protects this layer from
peeling and makes the wing more waterproof. In service, varnish or dope
must be applied every few weeks.
The U. S. Army practice calls for four coats of cellulose nitrate
dope followed by two coats of spar varnish to prevent inflammability.
Cellulose nitrate is more elastic and durable than the acetate but is
also more inflammable.
Commercial dopes with various desirable properties are: Cellon,
Novavia, Emaillite, Cavaro, Titanine, etc.
FOOTNOTES:
[1] But has very high elastic limit.