PRESENT DEFICIENCIES AND FUTURE POSSIBILITIES OF THE MILITARY AËROPLANE
In the portion of this handbook which especially dealt with airships,
certain advantages possessed by them over aëroplanes were noted;
several of their disadvantages were also a matter of comment. It
was hinted that in the future it might be possible to impart to
aëroplanes also those very advantages of which the airship can still
certainly make boast. Should this be done by engineering skill--and
it is well within the limits of reasonable possibility--then it would
seem that the lighter-than-air machine must entirely yield its claim
as an adjunct of war to the heavier-than-air principle. The free
balloon “mounting heavenwards,” as Carlyle said, “so beautifully,
so unguidably,” is now merely a past reminiscence, and even so, too,
will be the mammoth motor-impelled gas envelopes. When the din of war
ceases, the still greater perfection of the aëroplane should be the
object of the attention of British engineering skill. The endowment of
the aëroplane with certain qualities in which it is still deficient
appears to be merely a matter of engineering detail based on principles
that have been already elucidated.
Since the brothers Wright made their epoch motor flights which gave to
man the attribute of the bird, so long his envy, progress in flight
records has been largely made in the attempt to win a money prize.
In one sense the pilot has progressed at a faster rate than has the
evolution of the machine. He has accomplished heights, durations, and
distances on machines in which the margin of safety is indeed small.
It might be well if the next series of prizes should be devoted to the
further development of the machine itself--prizes which would, in their
turn, stimulate the genius of the aëronautical engineer.
Four essential points in the future development of flying machines
are:--
1. Variable speed.
2. Immediate rising into the air.
3. Hovering in the air.
4. Stability.
1. _Variable speed._
The aërial machine that cannot vary its speed, so as to be able to go
fast, at moderate pace, or quite slow, must from one point of view be
in a crude state of development. Yet aëroplanes are as yet in this
stage of growth.
More than one plan has been suggested for endowing the aëroplane with
the power of variable speed, which would make its use in war still
greater. One of these plans is the extension and reducing at will of
the sustaining surfaces, so that for high speeds the practical minimum
of surface may be utilised, for low speeds the practical maximum. A
machine to produce this result has been already planned by Mr. C. F.
Webb. It was described at a meeting of the Aëronautical Society of
Great Britain in 1906. At the time of the reading of the paper the
world was hardly ready to realise the importance of considering this
problem; at the present moment all military aëronautical experts agree
as to the advisability of the production of a variable speed flying
machine, though they shirk the complexity of structure the variable
speed machine would seem to necessitate. In Mr. Webb’s design is a
form of aëro-surface which, by special adaptation, can vary its area
in accordance with the requirements of, and in proportion to, the
constants, speed, and weight, and thus automatically adapt itself to
the requirements of the varying speed of the wind. In this machine the
two wings are situated on each side of the car in such a way that the
centre of support of each is some distance above the centre of the
mass of the machine. Each wing is fan-curved from front to rear, with
the outermost segment longer than the innermost. The fan wings are
opened or contracted by a hand-lever arrangement, and besides the hand
levers there is an automatic pendulum mechanism which regulates their
area to the requirements of the wind. Whether or no the inventor’s
exact arrangements may prove on trial to be successful is a matter on
which decisive opinion cannot be given; but the principle of expanding
and diminishing surface is thoroughly sound, and is worthy of lavish
expenditure and experiment. Other ways of attaining variable speed
machines have been suggested, though the method of a variable surface
would seem likely to carry the regulation of speed to a greater nicety
than do the other plans. One of these projects is to alter the angle
of the incidence of the planes while the machine is in flight; the
angle would have to be steep for slow speed, and gradually flatten for
increase of speed.
2. _Immediate rising into the air._
It is undoubtedly a disadvantage of the aëroplane that it has to run on
the ground on wheels to get the initial velocity necessary for flight.
In some of the earlier military experiments with aëroplanes the
machines were made to run over ploughed fields, for it was recognised
that machines which could only rise when running on smooth ground would
be useless for military work. But one can imagine that it may often
be expedient in military operations for machines to rise from land so
unequal that with the present method flight would be impossible.
The perfect military aëroplane should be able to rise in the air at any
time and from any place. The application of horizontal lifting screws
beneath the flying machine would make this a possibility, though it
would be necessary to have two of such screws revolving in opposite
directions. It is indeed curious that so little has been done in the
way of such experiments. It will be said that each added screw means
engine multiplication and complication; but these difficulties are
details of engineering that are not unsolvable.
In the case of such large aëroplanes as the Russian type that has been
described, it would seem specially feasible to attach the lifting
screws.
3. _Hovering in the air._
One great advantage of the lifting screws would be that by their use
the machines could hover in the air. Now, when the vertical screw
is stopped, the aëroplane must fall to earth unless the aviator
makes the “vol-plané.” This necessity brings into strong relief the
present imperfection of the flying machine. When horizontal screws
are attached to a flying machine we really have the essential feature
of sustentation, and the existence of the ordinary supporting surface
becomes superfluous. The flying machine has, in fact, become of the
“Hélicoptère” type, though doubtless for some time the supporting
surface will be retained as a means of additional security; in time it
may vanish altogether, and support as well as progression depend upon
revolving screws.
4. _Stability._
It has been stated that the properly constructed airship is stable
when in the air; it has not got to fear the more treacherous side gust
which over and over again has brought the aëroplane to earth, and
coupled its name with tragedy. The vexed problem of the stability and
equilibrium of aëroplanes is the most important that has yet to be
solved; until this is done it is not likely the airship will completely
disappear as an instrument of war. In speaking of the remarkable
exploits of Pégoud, it was said that they were an object-lesson on
the materiality of the air, and we have yet to learn how to use this
materiality to the best advantage, so as to afford us continual
stability. Until the problem is solved, man cannot be said to have
brought himself to the level of the soaring bird; the latter, indeed,
makes good use of the very attributes of the wind which at present
tend to upset the aëroplanist--the vertical component of the wind,
its internal work, _i.e._, its gustiness; its non-uniformity, _i.e._,
its different velocities at different levels. Every light, therefore,
that can be thrown experimentally or mathematically on the difficult
subject of equilibrium and stability should be eagerly sought.
Professor G. H. Bryan’s mathematical researches are indeed
epoch-making, and their study by the aëronautical engineer should be
prolific of practical result. He does much to elucidate points of
the problem of stability that before had been imperfectly grasped.
For instance, take the case of his remarks as to distinction between
equilibrium and stability.
We say that the motion of a flying machine is steady when the
resultant velocity is constant in direction and magnitude, and when
the angle of the machine to the horizontal is constant. If this
motion is slightly disturbed the machine may either return after a
time to the original motion, or it may take up a new and altogether
different mode of motion. In the first case, the steady motion is
said to be stable, and in the second unstable.
It is evidently necessary for steady motion of any kind that there
should be equilibrium--_i.e._, that there should be no forces
acting on the machine (apart from accidental disturbances) which
tend to vary the motion, and hence it follows that the number
of modes of steady motion of which a machine is capable is, in
general, limited, and that when an unstable, steady motion is
disturbed, the new mode of motion taken up is entirely different
from the old.
It is necessary to distinguish carefully between equilibrium and
stability, as the two are very often confused together. Equilibrium
is necessary to secure the existence of a mode of steady motion,
but is not sufficient to ensure the stability of the motion. The
question of the stability of a rigid body moving under the action
of any forces has been solved by Routh. In order to apply his
results to the stability of flying machines, it is necessary to
know the moment of inertia of the machine about its centre of
gravity, the resistance of the air on the supporting surfaces as a
function of the velocity and angle of incidence, and also the point
of application of this force--_i.e._, the centre of pressure for
different angles of incidence. If these are known for the surfaces
constituting any machine, then the problem of its stability for
small oscillations can be completely solved. Unfortunately, our
knowledge of these points is very unsatisfactory. Several valuable
series of experiments have been made to determine the resistance
on planes, but there is still some doubt as to the position of the
centre of pressure at small angles of incidence, especially for
oblong planes, and very little indeed is known as to the movement
of the centre of pressure on concave surfaces. Until experiments
are made on this point it will be impossible to solve the problem
of stability for machines supported on concave surfaces.
The subject of the stability of aëroplanes falls under two heads:--
1. Automatic stability.
2. Inherent stability.
Attempts have been made to produce the first by the aid of moving
gyroscopes and pendulums without much success, and Professor Bryan has
pointed out, apart from the fact that movable parts are likely to get
out of order, they also increase the degree of the friction of the
machine, thus further adding to the number of conditions that have to
be satisfied for stability.
It would seem, therefore, that the desideratum is inherent stability.
Professor Bryan considers that there is hope of attaining longitudinal
and lateral stability by the use of exhaustive mathematical researches;
these will result in the fixing of independent auxiliary surfaces
in aëroplanes in such happy positions as will secure stability in
all conditions of atmosphere. Or it may well be that through some
unlooked-for observation or simple experiment the answer will come. In
the shape of the aëroplane surfaces alone may be the solution of the
problem. But if the aëroplane be still an imperfect instrument, it is
sufficiently developed to be already one of the greatest factors of
modern warfare.
_Printed by Hazell, Watson & Viney, Ld., London and Aylesbury._--1414573
Transcriber’s Notes
Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in the original book; otherwise they
were not changed.
Simple typographical errors were corrected; unbalanced quotation
marks were remedied when the change was obvious, and otherwise left
unbalanced.
Illustrations in this eBook have been positioned between paragraphs
and outside quotations. In versions of this eBook that support
hyperlinks, the page references in the List of Illustrations lead to
the corresponding illustrations.
Page 2: “the battle of Fleurus, 1794” was misprinted as “the battle of
Fleurus, 1784”.
Page 86: “the longest being 22½ feet” was printed that way.