In Chapter I it was explained that an aeroplane is fundamentally
composed of a supporting surface, divided into one or two parts, usually
the planes or wings, which cut the air in an oblique manner, driven by a
propeller and motor. Before going further it is perhaps best to
understand more exactly how the planes operate and support the machine
in the air than it was possible to explain in the first chapter without
confusion. A theoretical aeroplane consists of a flat surface or plane.
When the propeller is set into motion the plane is driven through the
air in an oblique manner and compels the gaseous molecules to glide
under its surface. Since the plane is at an angle, the front edge being
higher than the back, the air must necessarily leave at the rear in a
downward direction. The air molecules in traveling under the surface
exercise a resistance upon it which is really a pressure against the
plane. When this pressure is resolved into its components, it is found
to be made up of two forces, one horizontal, tending to retard the
forward motion, and called the _drift_; the other, vertical and tending
to _lift_ the plane.
The centre of these forces is not as might be supposed at the centre of
the plane, but at a point between the centre and the front edge called
the "centre of pressure." The centre of pressure approaches the front
edge as the angle of the plane with the horizontal becomes less.
In order to render a better idea of how it is possible for an aeroplane
to gain support in the air consider a skater moving swiftly over very
thin ice which would not bear his weight, but since he is moving so
rapidly that any one portion of the ice does not have time to bend to
the breaking point, he is supported. In somewhat the same manner, the
planes pass so rapidly on to new and undisturbed bodies of air, and stay
over one body for so brief an instant that there is no time to
completely overcome the inertia of the air and force it downwards.
[Illustration: FIG. 12. The action of the air upon a curved and a flat
plane. We have seen that by the effects of the resistance of the air, an
aeroplane may be sustained in the atmosphere. We must now see in what
manner we can use these effects to the greatest advantage.]
First of all, we have been continually speaking of a "plane" as the
supporting surface, which from the definition of the word would lead one
to believe that they were flat. If the wings of a bird are examined, it
will soon be noticed that they are concave underneath. Since the first
attempts at aviation, therefore, machines have been built with planes or
wings concave on the underside. The reason for this is very apparent
from Fig. 12. The first illustration shows the action of a flat surface
moving through the air. The air streams, as represented by the lines do
not follow the surface of the plane, but leave a considerable region of
dead air. This is the reason that a flat plane is very inefficient and
not capable of giving so great a lift as the curved plane in the next
figure where the lines follow the outline of the plane. The less
disturbance a plane causes in the surrounding air, the closer it is said
to approach to "stream line form." A correctly curved plane is
considerably more effectual than a flat one, giving at the same time
greater "lift" and less "drift."
*Built-up Planes*, that is, planes having a double curve approaching
true stream line form, come nearer being the ideal plane than any other
from some standpoints, but do not possess any advantages when used on
models of less than four feet spread.
[Illustration: FIG. 13. Section of a built-up plane showing how a rib is
made. When made small, they offer greater "drift" or head resistance
than a single curved surface plane and cannot because of the delicate
structure necessary to make them light, withstand hard knocks. They have
the further disadvantage of being from a constructional standpoint very
hard to make smooth and rigid.]
There are innumerable substances which would at first seem to recommend
themselves as material for planes, but we may immediately thrust the
greater portion aside. By all means _avoid tracing cloth or linen_, not
only because its heavy weight forever precludes it from this use, but
because it wrinkles and cockles so as to be absolutely useless when
slightly damp or wet.
*Tissue paper* wrinkles easily and is not strong enough.
*Jap silk* is an excellent material for fabric covered planes, being at
once light and strong. However, by far the most satisfactory plane of
this kind is formed by silk bolting cloth which has been coated with
collodion. The collodion is brushed on with a fine camel’s hair brush
after the fabric is in place and it is thereby rendered both waterproof
and air-tight.
Fabrics should always be stretched over the planes from end to end and
not front to back or vice versa. Make the lap joints or pockets around
the end spars as long as possible so that they will not draw "dead air"
and impede the forward motion of the machine.
*Bamboo Paper* is one of the best materials for covering the planes of a
model aeroplane and is to be highly recommended. It is made in Japan
from bamboo fibre and is very strong. It is usually stretched tightly
over the framework and then given two coats of collodion or, what is
much better, bamboo varnish.
*The framework* of the planes may be made of rattan, split bamboo,
spruce, or steel piano wire. Piano wire is excellent for small machines
since it is springy and light and able to withstand shocks. It is easily
bent to any shape and offers considerably less head resistance than
rattan because of its small diameter. Rattan can be bent into almost any
shape by wetting.
Nothing is better for the cross pieces, ribs, etc., of the planes or
framework than split bamboo. Bulk for bulk it is heavier but infinitely
stronger than other woods. It is easily worked and can be bent into all
kinds of shapes. Bamboo must always be bent while hot. The best source
of heat is a spirit lamp or a bunsen burner. Always bend _toward_ the
hottest side. When bent apply a cold wet rag to cool quickly. If bent
more than necessary, it may be straightened by applying heat again and
allowing it to straighten itself.
In order to make long bends, such as the ends of planes, alighting
skids, etc., first wind a strip of wet rag around, the bamboo and allow
it to remain on for ten or fifteen minutes. Then remove the rag, heat
the bamboo in a flame and bend slowly.
[Illustration: PLATE III.]
With a little care, strips several feet long may be easily split from
bamboo rods. The best method of accomplishing this is to use a fine saw,
but a sharp knife will often be successful.
[Illustration: FIG. 14. How ribs may be joined to the long members.]
*Planes* of any considerable size require ribs to support and hold the
fabric in shape. Split bamboo is one of the best materials for this
purpose. Two very good methods of joining the ribs to the long members
of the planes are illustrated in Fig. 14. In the first, a strip of thin
sheet aluminum is bent around the rib and spar and fastened by lashing
with silk thread. Care must be taken to file off all sharp edges on the
aluminum which might otherwise cut the thread. The second method is the
neatest and probably the best, since the rib cannot so easily twist or
slip out of place.
*Wood Planes.* In spite of the many advantages of fabric planes they
cannot approach wooden planes for efficiency on a small machine. Wood is
strong, light and does not change its adjustment.
Whitewood and spruce are the best materials for the purpose. Do not
endeavor to saw out the wood. Use a carpenter’s plane as much as
possible in the work. A saw tears the fibres of the wood and will make
the finished plane full of tiny splits.
The wood, however, may be sawed down to a thickness of 5/32 of an inch
and then planed down from that. The finished plane should be about 1/16
of an inch thick.
When planing down the wood do not butt one end against a bench stop,
because as the wood becomes thin, the pressure exerted by the plane
against the wood will cause it to rise in the middle and thereby become
thinner at that part. Instead, use a clamp to fasten the wood at one end
to the bench and _plane away_ from the clamp—Plane down to a smooth
surface and _avoid_ the use of sand-paper.
[Illustration: FIG. 15. Form for bending the planes.]
*Forming the Curve* by steaming and bending the wood is a very poor
method. It soon becomes distorted and warped.
[Illustration: FIG. 16. A good method of building a wooden plane.]
The best method is illustrated in Fig 16. A piece of wood of the same
length as the completed plane and having a cross section like that at A
is glued to the forward under edge of a flat plane B. After the glue has
hardened, the plane is worked down to the shape shown at D which is very
close to the stream line form. The plane is then varnished to prevent it
from absorbing moisture and losing its shape. The ends may be covered
with thin Jap silk, carefully glued on to prevent splitting. The Wright
brothers cover the blades of the propellers on their aeroplanes with
silk for the same purpose.
Air does not flow smoothly when changing from an interrupted flow to an
uninterrupted flow around a square corner and so by rounding the ends of
the planes, the disturbance at that point is somewhat eliminated.
Planes having rattan or piano wire edges cannot very well be of any
other shape than those which are illustrated in Fig. 17.
[Illustration: FIG. 17. Various shapes a plane may take.]
It is a good plan to give wooden planes the shape shown by 3 and 4 in
Fig. 17, as the disturbances mentioned above are not so marked.
[Illustration: FIG. 18. An edgewise view of several planes showing the
different ways they may be bent to secure stability.]
The planes of large man-carrying machines possess the same
characteristics, but not to such an alarming extent as in a model. The
Voisin aeroplanes overcome the objection by the use of vertical panels
set between the planes.
The angles at which the planes are set may vary from 1 in 6 to 1 in 20.
One in ten might be called the "happy medium." If the planes are given
too great an angle, the drift becomes so great that the propeller thrust
is severely taxed. The smaller the angle, the less will be the drift and
consequently the greater the speed. However, if the surface is curved
the angle must not be made too small or not much lift will result.
[Illustration: FIG. 19. The various ways two planes may be combined to
secure stability or form a biplane.]
The angle of the tail planes should be adjustable. If too great, the
machine will slow down and the tail will drop, destroying the
equilibrium of the machine and consequently the flight. If the lift of
the tail is too great, however, it will cause that part to rise and the
machine will dive downwards.
*Elevators and Tails* are usually made of thin wood or fabric stretched
over a rattan or wire framework. They are usually rectangular or
elliptical in shape.
In case they are made of wood one of the best methods of attachment is
to fasten the plane to a small stick by means of two or three small
rivets. The stick is secured to the framework of the machine by two
small rubber bands. Then in case the machine strikes head on in
alighting, the band will absorb the shock and permit the elevator to
move so that it is not damaged by the fall.
*Vertical Fins*. It is a much mooted question whether or not a vertical
fin is of any value on a model aeroplane since a good model should be so
designed that it will fly in a straight line without the use of a
rudder. It has been the author’s experience that it is often of decided
advantage in correcting the flight of an "erratic machine" or in
compensating any little difference that there may result in the drift of
the two halves of the planes.
[Illustration: FIG. 20. Fins.]
The fin should be placed well toward the rear of the machine and,
whenever possible, stretched both above and below the centre line of the
machine, so that the pressure due to cross winds will be equal both
above and below and there will be no tendency for the machine to twist
about its longitudinal axis.
When it is not possible to place the fin both above and below the centre
line it should be placed above rather than below.
Fins may be made out of thin wood, sheet aluminum or fabric stretched
over a wire or rattan framework.