QUESTION OF RESISTANCE. WEIGHT. STABILITY.
*The first requirement* of a model aeroplane is that it shall fly. The
first essential for a machine to fly well is that it must be simple.
Simplicity usually insures success and is synonymous with efficiency. A
complicated scale model having as its prototype one of the most
successful man-carrying machines usually will not fly. If it does fly,
_it does not do so well_. Miniature steam engines, motors, etc., can be
constructed to exact scale and will justify their existence by actually
working and performing duty, but in most cases a model aeroplane made to
scale will not fly well until it begins to approach full size.
The next indispensable feature might be called _lightness_, but at the
same time it must be borne in mind that strength is also "second to
none" and it would be fatal to sacrifice the one for the other. The hard
knocks and battering which a model usually receives at the hands of a
novice will soon wreck any flimsy construction.
*To design model aeroplanes will at first seem like* "robbing Peter to
pay Paul," that is, no one part can be developed to an extreme without
seriously affecting the efficiency of the other parts. The successful
machine is a sort of "happy medium" arrived at solely through
experiment. A thorough understanding, however, of the part played by
each individual member of a model and its characteristics will make it
possible to avoid much unnecessary work in that connection. It is
therefore well to carefully read the following chapters before
commencing to carry out any original ideas or to make any radical
departure from the designs offered in this book.
From these statements it must not be inferred that the successful model
aeroplane builder is necessarily an individual possessed of consummate
skill in the handling of tools or a person of unusual judgment. A few
simple tools and trifling mechanical ability will enable any one to
build the simple little machines herein described. The greatest asset
required in the work is patience, patience spelled with a capital "P."
Not only patience in building the machines, but patience in adjusting
them and patience in flying them. Making haste with a model aeroplane is
poor policy. It never pays to use slipshod methods. Take the time to
make sure every part is the best that you can make it. Care with the
little details will insure success.
Model aeroplanes are exasperating to the extreme. A new model will
swerve to the right and left or dive with unerring precision to the
ground or nearest object. They seem to defy all attempts to make them
behave and in the first few flights usually perform a "new one" every
time. This is the point where success comes to the model aeroplanist who
possesses patience and perseverance. One must learn to adjust and fly a
model aeroplane by practice just as he must also learn to swim or ride a
bicycle by repeated trials. A little persuasion will soon make a model
soar in a surprising manner.
*The question of resistance* is the first consideration of the model
aeroplane designer. An aeroplane should pass through the air in such a
manner that it leaves that medium in as motionless a state as possible.
All motion of the surrounding air represents so much power wasted. It is
obvious that a boat with a square prow will offer more resistance than a
ship having a sharp bow. The latter causes considerably less disturbance
of the fluid in which it moves than the former.
[Illustration: FIG. 5. The disturbance created in the air by a square
object. The arrow points in the direction of motion. The space in the
rear of the object is the scene of violent eddy.]
The resistance of an aeroplane is made up of:
1. _Aerodynamic_ resistance.
2. Head resistance.
3. Surface resistance.
The first is offered by the planes of the machine itself and results
directly from the pressure of the air supporting the model during
flight.
The second is set up by the framework, the edges of the planes, the
wires, etc., while the last is caused solely by the air in traveling
over the surfaces of the various members composing the machines.
[Illustration: FIG. 6. The disturbance caused by a triangular body
moving through the atmosphere.]
The head and surface or _skin_ resistance, as it is sometimes called,
can be reduced, but the aerodynamical resistance cannot.
Air is no less a fluid than water, and the same considerations apply to
it, subject, of course, to certain conditions and with due regard for
such factors as density, viscoscity, etc.
[Illustration: Plate II.]
When an object, such as a square stick of wood, is moved through the
air, the latter flows around it leaving behind a region of "dead air."
The dead air represents so much waste energy or unnecessary resistance
to overcome because it requires an expenditure of power to drag it
along.
[Illustration: FIG. 7. Showing the disturbance created by a small spar
on the back of a plane.]
It is obvious then that bodies which are to move through the air with
the least resistance possible should be given such a shape that the
stream lines of air will flow around it smoothly and not leave a dead
region behind. In other words, the stream line flow of the air shall
keep the same contour as the surface.
*The ichthyoid or fish-like form* is of such a shape. This is
illustrated in Fig. 8. Its greatest diameter should be about two-fifths
of its entire length from the head. All struts, stanchions, etc., should
be given this shape.
[Illustration: FIG. 8. Diagram illustrating the ichthyoid shape and how
smoothly it slips through the air without creating an eddy.]
This shape is very interesting because of its probable origin, for a
glance is sufficient to tell that it not only resembles a fish but also
the body of a bird.
*Weight* is an all-important item in model aeroplaning. How to obtain
the maximum strength with the minimum of weight is undoubtedly the most
difficult problem which the aviator has to solve. Weight is a much more
important factor in model aeroplanes than in the case of full-size
machines because models do not fly fast enough to possess a high
weight-carrying capacity.
[Illustration: FIG. 9. Of the three shapes shown above, the round one
will slip through the air with the least disturbance and resistance. A
bar of wood like (A), 2 inches square, showed a "drift" of 5.16 lbs.
when placed in a breeze blowing 49 miles per hour. Turning it as shown
by (B) changed the "drift" to 5.47 lbs. A round bar, 2 inches in
diameter, like (C) showed 2.97 lbs. "drift" under the same conditions.]
It is only by the constant use of a pair of scales and an accurate
knowledge of materials with the ability to combine them in the most
efficient manner that the weight and strength may be kept in harmony.
Such knowledge and experience come only with practice. They may,
however, be acquired by any one. In this regard, a notebook forms an
almost indispensable aid to the experimenter. After a machine has been
built an accurate record of every flight and of every alteration or
change in material should be made.
[Illustration: FIG. 10. The figures given above each shape show the
"drift" in lbs. of wooden bars of those shapes when placed in a wind
blowing 40 miles an hour. The bars experimented with had a depth of 9
inches in the direction of the arrows and were 2 inches wide.]
*Automatic stability* without doubt has attracted more attention from
engineers and aviators than any other one problem connected with
aviation. Since it is not possible for a model aeroplane to carry a
pilot it is much more important that it should be naturally stable than
any of its man-carrying prototypes. Automatic stability in a model of
only two or three feet spread at the most, is quite a different
proposition from that offered by a full-size machine.
[Illustration: FIG. 11. Flat and dihedral planes.]
It would at first seem, that by placing the centre of gravity of the
machine very low such stability could be secured. This is accomplished
to a certain extent by setting the wings or planes at a dihedral angle.
But if the angle is excessive, the aeroplane will fly with a pitching
motion known as accentricity.
The centres of gravity, of pressure and of head resistance _should be at
the same point_. The centre of thrust of the propeller should also pass
through this point. _In this will be found the secret of the successful
model aeroplane_. It is only arrived at by careful experiment and
calculation.
Head resistance increases stability while weight and speed lessen it.
When an aeroplane is gliding (traveling downwards) its stability is
greater than when it is rising or flying horizontally. It is the _least
stable_ when _rising_.