MATERIALS FOR AEROPLANE CONSTRUCTION.
§ 1. The choice of materials for model aeroplane construction is more
or less limited, if the best results are to be obtained. The lightness
absolutely essential to success necessitates--in addition to skilful
building and best disposition of the materials--materials of no undue
weight relative to their strength, of great elasticity, and especially
of great resilience (capacity to absorb shock without injury).
§ 2. =Bamboo.=--Bamboo has per pound weight a greater resilience than
any other suitable substance (silk and rubber are obviously useless as
parts of the _framework_ of an aeroplane). On full-sized machines the
difficulty of making sufficiently strong connections and a liability
to split, in the larger sizes, are sufficient reasons for its not
being made more use of; but it makes an almost ideal material for
model construction. The best part to use (split out from the
centrepiece) is the strip of tough wood immediately below the hard
glazed surface. For struts, spars, and ribs it can be used in this
manner, and for the long strut supporting the rubber motor an entire
tube piece should be used of the requisite strength required; for an
ordinary rubber motor (one yard long), 30 to 50 strands, this should
be a piece 3/8 in. in diameter, and weight about 5/8 oz. per ft.
_Bamboo may be bent_ by either the "dry" heat from a spirit lamp or
stove, or it may be steamed, the latter for preference, as there is
no danger of "scorching" the fibres on the inside of the bend. When
bent (as in the case of other woods) it should be bound on to a
"former" having a somewhat greater curvature than the curve required,
because when cool and dry it will be sure to "go back" slightly. It
must be left on the former till quite dry. When bending the "tube"
entire, and not split portions thereof, it should be immersed in very
hot, or even boiling, water for some time before steaming. The really
successful bending of the tube _en bloc_ requires considerable
patience and care.
Bamboo is inclined to split at the ends, and some care is required in
making "joints." The ribs can be attached to the spars by lashing them
to thin T strips of light metal, such as aluminium. Thin thread, or
silk, is preferable to very thin wire for lashing purpose, as the
latter "gives" too much, and cuts into the fibres of the wood as well.
§ 3. =Ash=, =Spruce=, =Whitewood= are woods that are also much used by
model makers. Many prefer the last named owing to its uniform freedom
from knots and ease with which it can be worked. It is stated 15 per
cent. additional strength can be imparted by using hot size and
allowing it to soak into the wood at an increase only of 3·7 per cent.
of weight. It is less than half the weight of bamboo, but has a
transverse rupture of only 7,900 lb. per sq. in. compared to 22,500 in
the case of bamboo tubing (thickness one-eighth diameter) and a
resilience per lb. weight of slightly more than one half. Some model
makers advocate the use of =poplar= owing to its extreme lightness
(about the same as whitewood), but its strength is less in the ratio
of about 4:3; its resilience is very slightly more. It must be
remembered that wood of the same kind can differ much as to its
strength, etc., owing to what part of the tree it may have been cut
from, the manner in which it may have been seasoned, etc. For model
aeroplanes all wood used should have been at least a year in
seasoning, and should be so treated when in the structure that it
cannot absorb moisture.
If we take the resilience of ash as 1, then (according to Haswell)
relative resilience of beech is 0·86, and spruce 0·64.
The strongest of woods has a weight when well seasoned of about 40 lb.
per cub. ft. and a tenacity of about 10,000 lb. per sq. in.
[Illustration: FIG. 47A.--"AEROPLANE ALMA."
A very effective French Toy Monoplane.]
§ 4. =Steel.=--Ash has a transverse rupture of 14,300 lb. per sq. in.,
steel tubing (thickness = 1/30 its diameter) 100,000 lb. per sq. in.
Ash weighs per cub. ft. 47 lb., steel 490. Steel being more than ten
times as heavy as ash--but a transverse rupture stress seven times as
high.
Bamboo in tube form, thickness one-third of diameter, has a
transverse rupture of 22,500 lb. per sq. in., and a weight of 55 lb.
per cub. ft.
Steel then is nine times as heavy as bamboo--and has a transverse
rupture stress 4·4 times as great. In comparing these three substances
it must be carefully borne in mind that lightness and strength are not
the only things that have to be provided for in model aeroplane
building; there is the question of _resistance_--we must offer as
small a cross-section to moving through the air as possible.
Now while ash or bamboo and certain other timbers may carry a higher
load per unit of weight than steel, they will present about three to
three and a half times the cross-section, and this produces a serious
obstacle, while otherwise meeting certain requirements that are most
desirable. Steel tubing of sufficiently small bore is not, so far as
the writer knows, yet on the market in England. In France very thin
steel tubes are made of round, oval, hexagon, etc., shape, and of
accurate thickness throughout, the price being about 18s. a lb.
Although suitable steel tubing is not yet procurable under ordinary
circumstances, umbrella steel is.
§ 5. =Umbrella Section Steel= is a section 5/32 in. by 1/8 in. deep, 6
ft. long weighing 2·1 oz., and a section 3/32 in. across the base by
1/8 in. deep, 6 ft. long weighing 1·95 oz.
It is often stated that umbrella ribs are too heavy--but this entirely
depends on the length you make use of, in lengths of 25 in. for small
aerofoils made from such lengths it is so; but in lengths of 48 in.
(two such lengths joined together) the writer has used it with great
success; often making use of it now in his larger models; the
particular size used by him weighs 13½ grammes, to a length of 25
in. He has never had one of these aerofoils break or become
kinked--thin piano wire is used to stay them and also for spars when
employed--the front and ends of the aerofoil are of umbrella steel,
the trailing edge of steel wire, comparatively thin, kept taut by
steel wire stays.
§ 6. =Steel Wire.=--Tensile strength about 300,000 lb. per sq. in. For
the aerofoil framework of small models and for all purposes of
staying, or where a very strong and light tension is required, this
substance is invaluable. Also for framework of light fabric covered
propellers as well as for skids and shock absorber--also for hooks to
hold the rubber motor strands, etc. No model is complete without it in
some form or another.
§ 7. =Silk.=--This again is a _sine qua non_. Silk is the strongest of
all organic substances for certain parts of aeroplane construction. It
has, in its best form, a specific gravity of 1·3, and is three times
as strong as linen, and twice as strong in the thread as hemp. Its
finest fibres have a section of from 0·0010 to 0·0015 in diameter. It
will sustain about 35,000 lb. per sq. in. of its cross section; and
its suspended fibre should carry about 150,000 ft. of its own
material. This is six times the same figure for aluminium, and equals
about 75,000 lb. steel tenacity, and 50 more than is obtained with
steel in the form of watch springs or wire. For aerofoil surface no
substance can compare with it. But it must be used in the form of an
"oiled" or specially treated silk. Several such are on the market.
Hart's "fabric" and "radium" silk are perhaps the best known. Silk
weighs 62 lb. per cub. ft., steel has, we have seen, 490 lb., thus
paying due regard to this and to its very high tensile strength it is
superior to even steel wire stays.
§ 8. =Aluminium and Magnalium.=--Two substances about which a great
deal has been heard in connection with model aeroplaning; but the
writer does not recommend their use save in the case of fittings for
scale models, not actual flyers, unless especially light ones meant
to fly with the wind. Neither can compare with steel. Steel, it is
true, is three times as heavy as aluminium, but it has four or five
times its strength; and whereas aluminium and magnalium may with
safety be given a permissible breaking strength of 60 per cent. and 80
per cent. respectively, steel can easily be given 80 per cent. Being
also less in section, resistance to air travel is again less as in the
case of wood. In fact, steel scores all round. Weight of magnalium :
weight of aluminium :: 8:9.
§ 9. =Alloys.=--During recent years scores, hundreds, possibly
thousands of different alloys have been tried and experimented on, but
steel still easily holds its own. It is no use a substance being
lighter than another volume for volume, it must be _lighter and
stronger weight for weight_, to be superior for aeronautical purpose,
and if the difference be but slight, question of _bulk_ may decide it
as offering _less resistance_.
§ 10. =Sheet Ebonite.=--This substance is sometimes useful for
experiments with small propellers, for it can be bent and moulded in
hot water, and when cold sets and keeps its shape. _Vulcanized fibre_
can be used for same purpose. _Sheet celluloid_ can be used in the
same way, but in time it becomes brittle and shrinks. _Mica_ should be
avoided. _Jointless cane_ in various sizes is a very useful
material--the main aerofoil can be built of it, and it is useful for
skids, and might be made more use of than it is.[38] _Three ply wood_,
from 1/50 in. in thickness, is now on the market. Four or five ply
wood can also be obtained. To those desiring to build models having
wooden aerofoils such woods offer the advantage of great strength and
extreme lightness.
Referring to Table V. (Timber) at the end of the book, apparently the
most suitable wood is Lombardy poplar; but its light weight means
increased bulk, i.e. additional air resistance. Honduras mahogany is
really a better all-round wood, and beech is not far behind.
Resilience is an important factor. Ash heads the list; but mahogany's
factor is also good, and in other respects superior.
Lombardy poplar ought to be a very good wood for propellers, owing to
its lightness and the ease with which it can be worked.
_Hollow reeds_, and even _porcupine quills_, have been pressed into
the service of the model maker, and owing to their great strength and
extreme lightness, more especially the latter, are not without their
uses.
FOOTNOTES:
[38] The chief advantage of cane--its want of stiffness, or facility
in bending--is for some parts of the machine its chief disadvantage,
where stiffness with resilience is most required.