THE planes of your model aeroplane need no longer be a blind experiment
whose merits or defects may only be learned by actual test. The science
of wing-building is much better understood to-day than a year ago.
Without going into the complicated equations dealing with aspect ratio,
pressure, and gravity, it is important that one bear in mind a few
definite rules in designing even the simplest planes. A great many
useless experiments may be avoided.
In a previous volume, it was pointed out that a narrow plane, or one
with a high aspect ratio, driven with its broader side forward, would
yield greater support than a square surface of the same area. (The
aspect ratio, it may be well to repeat, is the relation between the
width and depth of the plane. A wing, for instance, whose width is five
times its depth, is said to have an aspect ratio of five.)
It has been found that on small planes the center of pressure is
situated about one-third the distance back from its front or entering
edge. The center of pressure in flexed planes occupies about the same
position.
As long as a plane remains horizontal, or nearly so, a very narrow
surface,—one, that is, with a high aspect ratio,—will exert greater
lifting power than a deeper plane of the same area. An examination of
the successful model aeroplanes of 1911 will show that the depth of the
planes has been cut away. Planes are used with an aspect ratio as high
as ten. The speed at which such a plane travels is a very important
factor. As the speed increases, the efficiency of the plane surface
increases, so that a model aeroplane driven rapidly may be sustained by
less wing area than in the case of one which flies slowly.
As the front edge of a plane is raised, the center of pressure travels
backward. By the time the plane has reached an angle of about fifteen
degrees, its lifting power has diminished about one-half. A very narrow
plane, or one with a high aspect ratio, should, therefore, be set near
the horizontal. The model should, moreover, rest upon skids at as low an
angle as possible.
In starting off, the planes will thus exert their maximum lift, or
nearly so. If the narrow planes be elevated too much, the center of
pressure will come nearer the rear than the front edge, and tend to
force the aeroplane downward, or, as the phrase is, make it "sit on its
tail." As long as a plane is traveling horizontally, or at low angles,
its rear portion exerts very little sustaining power and may be cut
away.
A plane with a high aspect ratio is much more stable in flight than a
surface of greater depth. The center of gravity of a flat plane would,
of course, coincide with the center of the surface when the plane is in
motion. When the plane tilts, the center of pressure, as we have seen,
moves backward or forward. If the plane has little depth or a high
aspect ratio, this center of pressure cannot move far.
It must oscillate back and forth within very narrow limits. A very
little tilt up or down will restore it to its normal position, so that a
plane with high aspect ratio is more stable than one with a deeper
surface.
The efficiency of a curved surface over a flat plane was analyzed in a
former volume. Such a curve, when well drawn, adds to the lifting power
as well as the stability. Since a curved plane does more work than a
flat surface, its size may be reduced and its aspect ratio increased. In
other words, the curved plane may be narrower than a flat surface, and
may be made thinner in proportion to its width.
The height of the curve, or camber as it is called, has been worked out
by elaborate mathematical equations, but we may take the general results
without following the calculations. For a plane six inches in depth, the
camber should be about one-half an inch, or one in twelve, or in this
proportion. The curve should be a parabolic with the highest point well
forward, one-third the way from the front edge. The front, or entering
edge, of the plane should be the thickest point. It should be tapered
off to a thin edge in the rear.
In theory, it is possible to model a plane so delicately that it will
fly against the wind by the pressure of the wind itself. It is extremely
important that both sides of the plane be brought to this curve as
accurately as possible. An efficient plane must, therefore, be covered
smoothly on both sides. Such a plane again offers very little skin
friction to the wind.
It is difficult to lay down any hard and fast rules for the relation of
weight to wing surface, since the types of aeroplanes differ so widely.
It has been found, however, that in large models one square foot of
surface will support about one-half a pound of weight, when traveling at
a high rate of speed.
You will find that your model, if its wings have a spread of thirty
inches os thereabouts, will approach one pound in weight. The figuring
will show you that two wings, whose combined area is less than 150
square inches, will be comparatively small and certainly well under
those generally employed a year ago.
The planes used on this season’s models are marvels of lightness and
strength. Much has been learned by studying the methods employed by the
builders of man-carrying aeroplanes. It is a simple matter to build a
three-foot plane which weighs complete less than one ounce, and is
strong enough to withstand many a violent shock.
[Illustration: A geared model built by Leslie V. Robinson]
[Illustration: An ingenious biplane]
It will be found a good plan first to lay out the exact form of your
plane on a smooth board. Make the depth of the plane one-fifth of its
length. It will be noticed that this plane is much more slender than
those used last year. Next draw a line at the center the entire length
of the board, and mark off one-tenth of the length of the plane from
either end. From this center describe a quarter circle at either end, on
the same side of the line. This will form your main or entering wedge.
The rear corners should be cut sharply away and only slightly rounded.
There is no better material for the main frame than a thin reed, cane or
bamboo. The longer ribs may be made of any light lath and the cross ribs
of a thin flat strip of the same material. Soak the reed overnight to
make it as pliable as possible, or heat it over a flame. Now lay the
reed over the outline of the plane, and hold it in this position by
driving thin brads on both sides and bending them over the cane. When
the outer edge is complete, mortise the ends slightly and tie and glue
firmly together.
With the outer frame held rigidly in position, it will be found a much
easier matter to introduce the ribs. If you are building a flexed plane
first, insert a stick of wood from end to end before placing your cross
ribs in position. The thickness of this temporary stick will, of course,
determine the curve of your plane. For a three-foot plane, a height of
one-half an inch will answer.
The ribs may now be bent over this obstacle and fastened securely to the
outer rim by glueing, tying, or nailing. The cross ribs may also be
raised by inserting small wedges between them and the longitudinal ribs.
When your frame is complete, loosen it from the board and you will find
it regular and rigid. Cover it with a very thin cloth pulled tightly
over the frame, and glue or sew it in position. A small plane may be
covered only on the under side.
Excellent results are being obtained in England with planes built up
entirely of wire. If aluminum wire is used, the weight of the wings is
considerably cut down, but even ordinary wire will be found lighter than
wood. For a plane thirty inches in width, or thereabouts, the wire used
should be at least one-sixteenth of an inch in diameter, and should be
soft enough to bend easily and hold its position.
It will be found a good plan to plot out the exact shape of your plane
on a sheet of paper, and then bend the wire over this outline. The ends
may be fastened together readily by binding tightly with fine wire, such
as florists use, and touching the joint with solder. Be careful, of
course, to keep the joint smooth. The cross ribs of these metal frames
may also be made of wire. Bend the ends at right angles and attach to
the inner sides of the plane with fine wire, and touch all the joints
with solder.
There are several advantages in the metal planes. It is a very simple
matter to flex the plane by bending the cross ribs and the ends upward
to the desired curve, much easier than when working with wood. Such a
frame will stand almost any amount of knocking about without injury. A
swift volplane to earth, which would smash any ordinary wooden frame to
"smithereens," would have little effect on a model plane. Such frames
again are very easy to cover.
It will be found a good plan to sew the cloth to one edge, draw tightly
across and sew fast to the opposite side, while a few stitches around
the metal cross ribs will give it any curve you desire. A metal frame
makes it possible to experiment with various curves. It is an easy
matter to bend the ribs up or down and alter the line of the plane at
will.
Small stability or guiding planes may be made of a sheet of metal,
although such construction is not advisable for the main plane. When
your front or entering plane is the smaller one, it is possible to turn
it into a very efficient motor anchorage.
The plane should be cut from a sheet of aluminum, preferably. Fasten
this securely to the front of your motor base with nails, or tying in
position. The wires of the hooks holding the ends of the motors may be
passed through the holes cut to the back of the rear edge of the plane
and bent over. Of course it is very simple to anchor double motors, or
even multiple motors, in this way.
One of the novelties in plane construction is a narrow wing with ends
brought well back. It may be built either flat or flexed, and promises
to afford unusual stability. The form is very popular among model
builders in England, where it is made very narrow, its depth often
equaling its width.
In many of the English models, these planes are placed far forward and
raised well above the main stability plane. They are built with the
entering edge either straight or slightly curved. Such front planes
behave especially well in the open air and even against a considerable
wind pressure.
There is still considerable difference of opinion as to the best
material for covering planes. Several specially prepared aeroplane
cloths have been placed upon the market which are guaranteed to be
practically airproof. The cloth is rather heavy, however, and better
suited for large machines. A thin silk answers the purpose perhaps as
well as anything.
Some model builders select the thinnest possible silk and then render it
airproof by varnishing or covering with a thin solution of wax or
paraffin. When this is neatly done, the planes are very taut and
shipshape. Several preparations are offered for sale for coating planes,
which are excellent.
In the search for the lightest possible covering, some builders have
gone a little further and use a very thin paper known as bamboo paper.
Even the thinnest paper will be found as impervious to air as a rather
heavy cloth. Its weight is practically nothing, even for a large plane.
It requires no varnishing or preparation, although it is sometimes
painted to render it more rigid.
There is, of course, a very obvious objection to paper that it is easily
punctured, but on the other hand, such accidents are very easily
repaired. A bad rip may be patched up with a touch of paste, or, the
plane may be re-covered very quickly. With this paper care must be taken
to fasten it to the frame of the plane as securely as possible, as a
loose sheet will flutter and increase the head resistance.
[Illustration: A well-proportioned model built by Reginald Overton]
[Illustration: A good model intended for long distance work built by A.
C. Odom]
In order to lighten the plane, the outer frames at the ends and rear may
be cut entirely away. An appreciable saving of weight is thus obtained
without weakening its structure. This plan is especially to be advised
in comparatively small planes. Design your plane and lay out its exact
form on a board. A thin strip of wood should be cut the width of the
front or entering edge, and similar straight lengths for the longer
ribs.
It will be found a good plan to use a heavier piece back of the front
edge or at the top of the curve. In building your plane, follow the
former directions of laying a stick on the board to give you the height
of the curve. The shorter cross ribs may then be fastened by glueing to
the longer ribs. By using a light lath or strip for the cross ribs, it
will be possible to make them sufficiently rigid merely by glueing
without the trouble of nailing. A skeleton frame of this kind has the
advantage of being very elastic.
In covering the frame, draw the cloth tightly across the upper side of
the frame and touch with glue at regular intervals along the ribs. When
dry, trim away the cloth between the points of the ribs and the open
ends. The rear edge may be held in position merely by the shorter cross
ribs. Trim the cloth along the edge.
In such a plane it is well to stiffen the cloth covering by painting
with shellac or varnish. This also lends a semi-transparent effect which
improves the general appearance of the plane. By cutting away the side
and end pieces of the frame such a plane three feet in width may be made
to weigh less than one ounce.
Since it is very important that the covering of the planes may be
perfectly smooth, it will be well to experiment with several different
methods of attaching the cloth or silk or paper. By covering with paper,
a taut surface like a drumhead may be had. Use a rice or fiber paper and
moisten the sheets by laying them between damp cloths, as was explained
in detail in a previous volume. In drying, the paper contracts and
tightens.
In covering a frame with cloth, the angle of the two sides may be
altered by stretching the covering over the raised ribs on one side and
drawing it tightly from edge to edge on the reverse side. If you have
difficulty in making your surface smooth, try lacing it to the sides.
You will need a strong hem at the edge. By using a thread, you will be
able to pull the cloth taut much the same as tent flaps are tightened.
The proper curve for a flexed plane is still a matter of dispute. The
highest part of the curve should come well forward, while the rear
surface is drawn straight. A good camber may be plotted very simply.
Draw a rectangle with a length sixteen times its height. Now mark off a
point on the upper side one-fourth of the way from the left-hand corner
and draw diagonal lines from this point to the two lower corners. Next
round off the broad angle formed by the two lines and you will have a
good curve to imitate in flexing your planes.