Until the summer of 1911, the longest model aeroplane flights officially
recorded in America remained under 300 feet. From England meanwhile came
disquieting reports of 1,000 foot flights and better, made by a number
of aeroplanes. A comparison of the best American and English models
showed that, both as regards form and workmanship, American boys were
holding their own against their English cousins, and utterly failed to
account for the much greater distance qualities of the foreign models.
In July, 1911, the American distance record was suddenly jumped to 1,691
feet, 6 inches, by Cecil Peoli of New York. The model used had been
flown in the regular indoor meets for very much shorter distances. This
sensational advance in the distance record was made at an outdoor
tournament at Van Courtland Park, New York City. Of the thirty-four
models entered for the contest, including the familiar models built by
Percy Pierce, H. Watkins and others, several showed a similar increase
in distance qualities. The model aeroplanes were the same as had flown
but 200 feet indoors, their rubber motors exerted no more power than
before, the pitch of the propellers remained unchanged.
What then is the secret of the suddenly acquired distance qualities?
Evidently the difference lies in the quality of the air the little ships
navigate. It is commonly said that the air indoors is dead as contrasted
to the live air found out-of-doors. The variation in the quality and
movement of the air forms a very interesting study which no aviator can
afford to neglect. To the actual navigator of the air this study is just
as important as life and death, while to the designer even of model
aeroplanes it is of course of vital importance.
Although the composition of the air and its behavior under various
conditions has been the subject of scientific examination for centuries,
"it is only within the past few years that it has been studied with the
idea of bringing it under control. The long painstaking experiments of
Langeley and Lilienthal, referred to in the previous volume, for
determining the resistance of the air and its effect on the surface of
the aeroplane, opened a new field of scientific research. Within the
past few years, even months, the advancement in our knowledge of the air
has been greater, it is safe to say, than in the previous century.
It is not generally realized by the laymen how rapid has been the
development of "aerology," nor how practical are the results obtained.
There have already been established in Germany three scientifically
equipped stations for observing air conditions for the benefit of
aeronauts, just as the weather bureau observes weather conditions and
informs ships at sea of approaching storms. One of these, established by
Dr. Polis at Aachen, is now in operation. In addition to this the Public
Weather Service Stations of Germany have been equipped recently with the
necessary apparatus for making daily observations of the upper air for
the special benefit for aerial navigators.
The resistance offered by the air to the passage of an air ship of any
type depends upon its density. The air is obviously an exceedingly
variable medium, as capricious as quicksilver, as both the sky pilot and
the flyer of model aeroplanes have learned. The density and therefore
the resistance depends upon the temperature, the pressure and the state
of equilibrium. We are in the habit of thinking of density and pressure
as affecting enormous volumes or areas miles in extent, such as are
reported in the weather forecasts from day to day. To an extent these
same conditions are found changing within a few feet. It is this
tendency to rapid change of conditions which makes the problem of
stability in aeronautics so baffling.
The pressure upon an aeroplane, whether a man-carrying machine or a
model, varies considerably between the level of the seashore and the top
of a mountain. An aeroplane in rising from the level of the sea for
several thousand feet therefore meets new and unexpected conditions. The
density is reduced fully one-half at an altitude of 18,000 feet, and
since aeroplanes have risen more than 10,000 feet this must be taken
into consideration. Even at a height of 300 feet, which is often reached
by aeroplanes in flight, the difference in the pressure calls for
skilful manipulation on the part of the sky pilot.
The density varies again according to the temperature. Let an aviator
suddenly run into a hot or a cold strait of air and the pressure upon
his planes will instantly change. The effect of temperature must be
taken into consideration by any one flying model aeroplanes. It may
happen that a draft of cool or of hot air, by changing the pressure on
the planes, will throw a model aeroplane out of balance and mar an
otherwise promising flight. A difference of a few degrees of temperature
will often affect a very sensitive model. The only way to combat this is
of course to build your model with the greatest possible stability.
The presence of high buildings or other violent inequalities will also
affect the density of the air and in turn the pressure exerted on the
wings of an aeroplane. Among aviators it is generally believed that a
great city is one of the most dangerous possible objects to fly above.
In the case of New York with its many sky-scrapers, for instance, the
danger is vastly increased. Even in the open country the presence of a
deep valley or other depression will so affect the density of the
atmosphere that an aeroplane is likely to be drawn down from its course.
These areas are known among aviators as "pockets," and are often large
enough to swallow up a large man-carrying craft, at times with
disastrous effects. The chance of your model aeroplane running into such
a pocket is of course considerable.
A striking illustration of the effects of these chance currents was
afforded during a recent model aero tournament in New York. A model
aeroplane which had flown with remarkable steadiness for more than 150
feet chanced to pass over the head of a boy who was walking slowly
across the course. This moving object served to set up a small whirlpool
of air. The model on striking it was instantly checked, when it turned,
skirted the column of air and passed on. In an indoor flying, an open
window, merely by changing the temperature slightly in its vicinity,
will often cause a model to be seriously deflected, perhaps to be thrown
completely off its course. It will often be noticed in outdoor flying
that a model, in passing over a stream or body of water or a mass of
dense foliage, will encounter a change of temperature which will
appreciably affect its course.
In flying model aeroplanes the performance of a machine will often vary
unaccountably from day to day. With the same motor and winding, the
model will fly much higher and more freely at different seasons of the
year. It is well to bear in mind that in the summer months the heat
causes a low density. The pressure exerted by the atmosphere is
therefore correspondingly small; the model, or for that matter a large
machine, travels much faster. A dry day also tends to cause low density.
This will account for the excellent flights on warm, dry days and the
crankiness of the machine when the weather is damp.
Cold weather on the other hand tends to increase the density of the air.
The speed of an aeroplane is materially reduced but on the other hand
the air will be found more buoyant. Such weather is better for heavy
models, although they fly much more slowly. It will be seen that at high
altitudes, where the density is least, models should fly both faster and
higher than they will nearer the sea level.
One of the curiosities of the air is the effect of eddies created by the
passage of aeroplanes. Let an aeroplane move fast enough through the air
and it will create high density above its course, and low density below
its line of flight. This condition may also be found in the wake of
model aeroplanes. It will often be noticed when two models are flying
near together that the disturbance caused by one will seriously
interfere with the second machine. The contrast between the air above
and below the models in flight in especially noticeable when the planes
are flexed.
[Illustration: Two of the earlier Peoli models]
A great deal has been learned of the action of the air upon aeroplanes
by photographing the air currents. The smallest eddies of the air have
been made visible by taking instantaneous photographs of thin smoke as
it passes obstacles of various size and form. It has been found that a
square object causes a great deal of disturbance when in an air current.
The air is compressed in front of it and eddies for some distance in its
wake before finally coming to rest. An elliptical object causes less
disturbance, but the air continues to splash in its wake for some
distance. Even a perfectly spherical object offers a surprising amount
of resistance to the air. In the case of a long narrow ellipse the
disturbance is considerably reduced. A curved surface however, such as
is used for the planes of an aeroplane, cuts the air with practically no
resistance, and the air flows smoothly about and joins behind it with
very little wake or splashing. A form which suits the air in this way or
a "stream line" body as it is called is obviously just the right design
for the wings of an aeroplane.
Even the experts in aeronautics differ so widely, however, that it is
impossible to lay down any definite rules. One of the greatest
authorities on the science of aviation, Mr. Horatio Philips, of England,
believes that aeroplanes gain more support from the entering edges of
their planes than from the rest of their surface. He argues that it is
this edge, meeting the air currents, which serves to hold the aeroplane
suspended.
This theory is founded on the experiments made with a machine of
original design flown by Mr. Philips as far back as 1890. It consisted
of a series of planes mounted one above the other at regular intervals,
much the same as the strips of a Venetian blind. The lifting power of
this model in proportion to its surface and the power exerted was
enormous. This theory is borne out in part by the success of the model
aeroplanes with very narrow wings which have been flown with great
success during the year. The beautiful model built by Stewart Easter,
for instance, which is illustrated on another page, depends for its
support on planes which are no wider than ordinary window blinds.
There is an immense difference of opinion again as regards skin
friction. Some writers believe the air has a tendency to stick to
certain materials more than to others, and that this difference is so
great as to materially retard some machines in their passage through the
air. A complicate series of tables has been worked out in great detail
to show the exact amount of this friction on various bodies. Some
aviators go to great pains to make every part of their aeroplanes as
slippery as possible. This is done by polishing the surfaces exposed to
the air and in some cases enclosing the foreward part of the aeroplane,
like a ship’s prow, to diminish friction.
On the other hand we find some of the greatest authorities on aviation
disregarding this question almost entirely. In the Wright machines, for
instance, the surface of the wings is usually left comparatively rough,
and the sticks and wires are placed without any attention to diminishing
friction. This is true as well of the Delegrange, Voison and Farman
machines. Still other aviators design every detail of their machines to
cut down this so-called skin friction. The uprights, for instance, are
made elliptical in shape, with the sharp edges turned forward so that
they will cut their way the more smoothly through the air.
Several interesting attempts have been made to design a prow for an
aeroplane which will cut the air with the least possible amount of
friction. It is noticeable in these designs that the prows are very
blunt, like the prow of a canal boat, and not as might be expected,
sharp and narrow like that of a racing craft. The blunt-nosed prow is
considered best for the air ship, because the air being one eight
hundredth as dense as water offers very little resistance to its
entrance. It is so much easier, in other words, to push an air ship
through the air than a boat through water that there is no object in
sharpening its nose.
The air, however, in flowing along the sides of a rapidly moving body
sticks to it, and retards its progress relatively more than water
retards a boat. It is important, therefore, that the body of the
aeroplane be made very short, so that the sides will offer as little
friction as possible. This reverses the proportions of a water-borne
ship. We are so accustomed to see fast boats with long, narrow hulls,
that it comes as a surprise to find that a fast air ship must have a
very broad beam and as short a hull as possible.
It is probable that, as air ships develop, this general characteristic
will become more marked. As aeroplanes become larger and faster they
will therefore depart further and further from the conventional ideas
concerning water-borne craft. It is impossible to prophesy at present
what form the great passenger-carrying air ships of the future will
take, but it is certain that their hulls or the closed-in portion
carrying the machinery and passengers will be very short, snubbed-nose
affairs. The world will be obliged to change its mind as to what
constitutes a speedy-looking craft.
Man has learned to fly. More than 1,000 aeroplanes have been built which
have successfully risen above the earth. It is estimated that these
aeroplanes have flown in all more than 250,000 miles, or a distance
equal to ten times the circumference of the earth. But no machine has
yet been made which will fly alone, without the skilful manipulation of
planes and rudders. The model aeroplane which soars gracefully aloft,
suiting itself to the varying air conditions, perhaps comes nearest to
the automatic flying machine.
A great advance will be made in aviation with the appearance of some
practical contrivance for securing automatic stability. The aeroplane as
it stands to-day shows a wonderful advance in the improvement of its
general lines, and the mechanical perfection of its parts, but the
question of stability remains practically the same as it was when the
Wright Brothers made their first flights. The machine has been brought
under a remarkable control, but only as it is directed by the practised
hand of the sky pilot. Let him take his hand even for a moment from the
levers, which control the planes and rudders, and there is danger of a
bad spill, perhaps a fatal accident. Practically nothing has been
accomplished in building machines which will fly unaided.
It has been pointed out, elsewhere in this volume, that the experiments
with the model aeroplanes are certain to have an important influence
upon the development of aeronautics as a whole, because they address
themselves particularly to this problem of stability.
It is believed by many aviators that the problem of automatic stability
will be solved by some form of the gyroscope. A great many experiments
are being made with various forms of the gyroscope, although no machine
has as yet been actually fitted and flown with such a device. The
general principle of the gyroscope is very simple. It consists of a
wheel which is made to revolve at very high speed. When such a wheel
turns fast enough it will remain in a fixed position. Every one is
familiar for instance with the gyroscope top. You wind it up and place
it at any angle, and it will support itself and retain the position
until it has run down.
The gyroscope has been applied for instance to railroad trains and has
worked, experimentally at least, with remarkable success. By installing
a gyroscope on a car running on a single track the car may be kept
upright by the stabilizing force exerted by the revolving wheel. The
same principle has been applied to steamships to prevent their rolling
in heavy seas. The tipping of a car running on a single track, or of a
ship at sea, is of course very much like the rolling of an aeroplane in
flight; and it would seem that such a stabilizing device might solve the
problem. The gyroscope has a serious disadvantage, however. Since it
revolves at enormous speed any breakage would tend to throw the detached
part to one side with dangerous force. In the case of the gyroscope used
to steady ships at sea this force would be sufficient to send a piece of
metal through the hull. On so delicate a craft as an aeroplane such an
accident might readily prove disastrous.
It has been found again in experiments with gyroscopes on steamers that
the frames have been seriously strained, even when the gyroscope worked
smoothly. When a ship rolls and pitches to the motion of the waves, it
is of course under a great strain but this balances itself. When it is
held in a rigid horizontal position the pitching and rolling exert an
exaggerated strain on the hull. In one instance a vessel actually broke
its back under such a strain and was lost. An air ship, being at best a
very frail structure, could scarcely be expected to stand the strain
which has wrecked a steel ship. At the recent Paris Aeronautical show
several extremely ingenious combinations of the pendulum and gyroscope
principles were illustrated. It is of course possible that some modified
form of the gyroscope may solve the problem.
A remarkable series of tests has been made this year in France with an
automatic stability device based upon an entirely new principle. The
rudders are operated automatically, in this case by a vertical fin
opposed to the wind. As the pressure of the air varies, the rudder is
forced up or down, thus bringing the aeroplane to an even keel without
the assistance of the aviator. It is reported that model aeroplanes
equipped with this device have been flown in three hundred experiments
without a single accident.
The day is approaching when the air conditions will be observed and
announced for the benefit of aviators exactly as the weather is foretold
to-day. An aviator who is about to start on a cross-country flight will
thus be able to study the conditions of the air lanes and lay out his
route just as an automobilist looks up good-roads maps. In crossing a
particular piece of country he will therefore know whether to take a low
or a high air lane, that is, one of few hundred feet above the earth or
the one several thousand feet aloft. The pressure of the air on the
regular air lines will be announced as well. In this way aviation will
be made much safer than it is to-day, when the aviator must venture
without any knowledge of conditions aloft except those he may gain from
the ground surface weather maps. During the year 1910, fully a score of
lives might have been saved had aviators had such information.
The first of these observation stations is actually in operation in
Germany to-day. Each of these stations is supplied with a number of
rubber balloons equipped with automatic apparatus for recording
atmospheric conditions in the upper air lanes. The stations also contain
the proper apparatus for measuring the ascensional force of the
balloons, with the gas generators used for inflating the balloons. When
the balloons are sent up for great heights their altitude will be
measured by means of the theodolite.
The soundings of the air are taken twice daily at eight o’clock in the
morning and two in the afternoon whenever the weather permits. In the
summer the morning observations are made much earlier. The movements of
the balloons are then carefully observed at various altitudes until they
are lost. These observations are then telegraphed to the central station
at Lindenberg and sent out much the same as the regular weather
forecast.