The unerring flight of birds is, of course, the model for the builders
of heavier-than-air machines. Much of the birds’ skill in directing
their motive power remains a mystery to us, but we are learning to
analyze and, in a measure, imitate them. The builder of model aeroplanes
again must not alone imitate the methods of the birds; he must make
their system of maintaining stability automatic. A study of a variety of
successful models shows that there is great difference of opinion as to
the best plan for stabilizing the aeroplane.
Directional stability is gained by the use of horizontal elevators or
tails for controlling vertical movement, by vertical rudders or fins for
steering to right or left, and by flexible wing tips to guard against
tipping.
In designing any system of rudders, or ailerons, for gaining stability,
one should always have in mind the general principles upon which such
surfaces act. The movement of the horizontal planes or ailerons has an
important effect upon the direction of the flight, because they change
the angle of incidence.
In other words, they alter the angle of the plane with the line along
which the aeroplane is flying. If you bend the rear edge of the plane,
or aileron, downward, the angle of incidence is increased. What happens
is this. As the plane is lowered, the air is compressed beneath it,
which tends to lift the plane, throwing up the front edge and changing
the course of the flight.
This method of securing stability, which was invented and patented by
the Wright Brothers, has been widely imitated. In their later machines,
the Wrights have even abandoned the front elevating surfaces and depend
upon the movement of the main plane and a small elevating plane placed
just back of the rear rudder for their directional control.
They have thus done away with the friction encountered by the front
planes, which has resulted in giving the machine greatly increased
speed. Now in the model aeroplane, it is, of course, impossible to flex
the planes up or down during flight. Some adjustment must be hit upon
which will give the machine automatic stability. The principle of the
action of the stabilizer remains, of course, exactly the same.
In designing rudders for controlling horizontal flight, it should be
borne in mind that their stabilizing power varies largely in proportion
to their distance from the center of gravity. In most models the further
they are removed, either front or back, the greater is their leverage,
and the smaller need be their surface. By placing the rudder on an
outrigger carried far out, a very small plane will suffice.
[Illustration: A serviceable model showing excellent workmanship built
by Cecil Peoli]
[Illustration: A serviceable model showing excellent workmanship built
by Cecil Peoli]
The vertical rudders or fins, as they are sometimes called, are, of
course, intended to control the movement to right or left and keep the
model from sliding sideways. They have no counterpart in the wings of
birds, and are believed by some aviators to have little effect. At any
rate, they can do little harm since their head resistance is practically
nothing. Unlike the horizontal forward planes, these fins should not be
carried too far forward.
In practice it is found that they often get in the way, and a slight
side gust of wind striking them, with their great leverage, will knock
the aeroplane completely off its course, perhaps upset it. The best
position for such rudders is either above or below the main plane, or
behind it, where they are out of the way of cross currents. In last
year’s models, these vertical surfaces were often very large, presenting
as much surface as the planes themselves. It has been found that they
may be cut down in size, thus saving weight without losing their
efficiency.
A long vertical fin, or keel, has the disadvantage of presenting a
dangerously broad surface to any cross current of wind. The question of
the position of the rudder was taken up in a previous volume. A glance
at the successful model aeroplanes of the year shows that the vertical
rudders have been adopted very generally. Considerable ingenuity is
displayed in adjusting them.
The use of wing tips of any form is intended to control both the
horizontal and vertical movement. The general theory or equilibrium, of
course, applies in both cases. The most perfectly adjusted model is
subject to many forces which tend to tip it to one side or the other. A
gust of wind,—and the air is never perfectly quiet,—will tip one end of
the plane up or down.
[Illustration: Various Steering Devices. "a" and "b," simple aileron
forms. "A" novel fin on Vinet plane. "B" L-shaped aileron. "C" vertical
rudder (Bleriot type). "D" "Blinkers," an effective rudder. "E"
stability planes not unlike the runners of a sleigh.]
In the early models, this tendency was met by fixing the plane at a
dihedral angle. An examination of last year’s models will show how
common was this design. The dihedral angle lowers the center of gravity.
Now, after one side of the model is raised and the plane rights itself,
the center of gravity swings through a considerable arc, like a
pendulum, before it can come to rest, so that the center must swing back
and forth several times.
This tendency to tipping is fatal to a steady flight. It was first
observed by the Wright Brothers while studying the early Langeley type
of machine. The Wrights abandoned the dihedral angle entirely, as all
the world knows, and replaced it by the horizontal plane with a straight
entering edge. The keel will in a measure overcome this side motion.
Much of the advantage of the dihedral angle may be borrowed, however, by
turning up the extreme ends of the plane, without materially lowering
the center of gravity. In several of the successful models of the year,
these tips have been made equal to about one-fifth the width of the
plane, and are raised as high as forty-five degrees.
The theory is that, when the oscillation commences, these surfaces damp
out the swinging tendency, and bring the model to an even keel.
Sometimes the tips are rounded off, although in some cases they are made
triangular and brought to a point. As a rule, they are added to the rear
plane, although one notable exception is the case of the Lester Robinson
model, which carries these tips on both planes.
The tips, or ailerons, at the ends of the planes maybe made of the same
material as the planes themselves. In case you are using wire frames, it
is, of course, a very simple matter to bend up the tips to any desired
angle. When the frames of the planes are made of reed, as is generally
the case, the tips should be made separately. Bend your reeds to the
desired shape and cover them with the same material used for the planes.
It is quite as important that they be covered smoothly as in the case of
the planes. They may then be attached to the ends of the planes by
wiring rigidly in position. It will be found convenient to adjust them
so that they may be bent up or down to suit conditions. The same plan
should be followed in building and attaching the ailerons to the rear of
the main stability planes.
Some interesting experiments have been made by placing several vertical
surfaces above the main stability plane. A series of four or six
vertical rudders are sometimes spaced apart at equal distances,
extending from the front to the rear edge, with a height of about half
their length. In some cases the corners are rounded off, while others
prefer to cut away the front edge sharply.
In the Vinet monoplane, the vertical fin appears in an entirely new
form. The fin is attached near the outer edge of the main plane and the
upper edges curled inward, forming an arc of a circle. The theory of the
curve is that it tends to keep the air from slipping off the ends, after
the manner of the curtains of the Vaison biplane.
As a rule, these curled fins are about three-fourths the depth of the
plane and are attached with the front ends on a line with the entering
edge of the plane. These curled planes may be stretched on frames of
light wire or thin reed. As the model tilts to one side, the air
striking the curved surface of the outer side of the fin meets with
little resistance, while the fin at the opposite side, acting with its
concave surface against the wind, offers considerably more resistance,
thus tending to check the side motion and bring the aeroplane to an even
keel.
An effective form of aileron consists of an "L" shaped plane set closely
against the rear corner of the main wing. These ailerons are made in
pairs and hinged to a rear edge. The side should extend about half the
width of the plane. The action of the hinged plane at the rear is, of
course, familiar.
The extension at the side, which should be kept rather narrow, not more
than one-fourth the depth of the main plane, is likely to prove very
efficient. If the aileron be turned too far either up or down it will
offer considerable resistance. In one of the new English models, these
ailerons are so connected that as one rises the opposite aileron is
lowered. Here is a fascinating field for experiment in automatic
control.
The vertical rudder used on the new Bleriot, the result of an immense
amount of experiment, suggests interesting possibilities for the model
builder. The rudder is built in the form of a long triangle and is
mounted by hinging one of its shorter sides to the upper surface of the
rear plane, so that its corner will extend upward and outward. In this
position it suggests a fish’s fin. The receding angle of the front edge
offers very trifling resistance.
The new Baby Wright racer depends for its lateral control largely upon a
novel form of rudders known as "blinkers." These rudders are triangular
in shape and extend out in front of and below the forward planes with
their longer edges forward. They act much the same as the jib of a
sailing vessel, and, because of their position well in front of the
center of gravity, act with considerable leverage.
The design appeals especially to the builder of model aeroplanes, since
they can be added with very trifling weight by curving the front skids
forward and stretching the cloth across their forward corners. The
Valkyrie monoplane is equipped with similar rudders, in the form of half
circles carried in the same position.
In addition to the vertical and horizontal stability planes, many
aeroplanes are now equipped with stability planes extending diagonally
from the vertical axis. These are placed below the main planes,
extending outward not unlike the runners of a sleigh. This box-like form
tends to confine the air and affords increased support.
There is even an upward tendency from this pressure of air. These planes
are usually rectangular in shape, the forward or entering edge being cut
away sharply. By mounting these planes on the skids, their additional
weight is practically nothing. Several interesting applications of this
principle are shown in the accompanying illustrations of models.