1903–1920: PROGRESS IN DESIGN
BY
LIEUT-COL. W. LOCKWOOD MARSH
I
THE BEGINNINGS
Although the first actual flight of an aeroplane was made by the
Wrights on December 17th, 1903, it is necessary, in considering the
progress of design between that period and the present day, to go back
to the earlier days of their experiments with ‘gliders,’ which show the
alterations in design made by them in their step-by-step progress to a
flying machine proper, and give a clear idea of the stage at which they
had arrived in the art of aeroplane design at the time of their first
flights.
They started by carefully surveying the work of previous experimenters,
such as Lilienthal and Chanute, and from the lesson of some of the
failures of these pioneers evolved certain new principles which were
embodied in their first glider, built in 1900. In the first place,
instead of relying upon the shifting of the operator’s body to obtain
balance, which had proved too slow to be reliable, they fitted in
front of the main supporting surfaces what we now call an ‘elevator,’
which could be flexed, to control the longitudinal balance, from where
the operator lay prone upon the main supporting surfaces. The second
main innovation which they incorporated in this first glider, and the
principle of which is still used in every aeroplane in existence, was
the attainment of lateral balance by warping the extremities of the
main planes. The effect of warping or pulling down the extremity
of the wing on one side was to increase its lift and so cause that
side to rise. In the first two gliders this control was also used
for steering to right and left. Both these methods of control were
novel for other than model work, as previous experimenters, such as
Lilienthal and Pilcher, had relied entirely upon moving the legs or
shifting the position of the body to control the longitudinal and
lateral motions of their gliders. For the main supporting surfaces of
the glider the biplane system of Chanute’s gliders was adopted with
certain modifications, while the curve of the wings was founded upon
the calculations of Lilienthal as to wind pressure and consequent lift
of the plane.
This first glider was tested on the Kill Devil Hill sandhills in North
Carolina in the summer of 1900, and proved at any rate the correctness
of the principles of the front elevator and warping wings, though
its designers were puzzled by the fact that the lift was less than
they expected; whilst the ‘drag’ (as we call it), or resistance, was
also considerably lower than their predictions. The 1901 machine was,
in consequence, nearly doubled in area--the lifting surface being
increased from 165 to 308 square feet--the first trial taking place
on July 27th, 1901, again at Kill Devil Hill. It immediately appeared
that something was wrong, as the machine dived straight to the ground,
and it was only after the operator’s position had been moved nearly a
foot back from what had been calculated as the correct position that
the machine would glide--and even then the elevator had to be used far
more strongly than in the previous year’s glider. After a good deal of
thought the apparent solution of the trouble was finally found. This
consisted in the fact that with curved surfaces, while at large angles
the centre of pressure moves forward as the angle decreases, when a
certain limit of angle is reached it travels suddenly backwards and
causes the machine to dive. The Wrights had known of this tendency from
Lilienthal’s researches, but had imagined that the phenomenon would
disappear if they used a fairly lightly cambered--or curved--surface
with a very abrupt curve at the front. Having discovered what appeared
to be the cause they surmounted the difficulty by ‘trussing down’ the
camber of the wings, with the result that they at once got back to
the old conditions of the previous year and could control the machine
readily with small movements of the elevator, even being able to follow
undulations in the ground. They still found, however, that the lift was
not as great as it should have been; while the drag remained, as in
the previous glider, surprisingly small. This threw doubt on previous
figures as to wind resistance and pressure on curved surfaces; but
at the same time confirmed (and this was a most important result)
Lilienthal’s previously questioned theory that at small angles the
pressure on a curved surface instead of being normal, or at right
angles to, the chord is in fact inclined in front of the perpendicular.
The result of this is that the pressure actually tends to draw the
machine forward into the wind--hence the small amount of drag, which
had puzzled Wilbur and Orville Wright.
Another lesson which was learnt from these first two years of
experiment, was that where, as in a biplane, two surfaces are
superposed one above the other, each of them has somewhat less lift
than it would have if used alone. The experimenters were also still
in doubt as to the efficiency of the warping method of controlling
the lateral balance as it gave rise to certain phenomena which
puzzled them, the machine turning towards the wing having the greater
angle, which seemed also to touch the ground first, contrary to their
expectations. Accordingly, on returning to Dayton towards the end of
1901, they set themselves to solve the various problems which had
appeared and started on a lengthy series of experiments to check the
previous figures as to wind resistance and lift of curved surfaces,
besides setting themselves to grapple with the difficulty of lateral
control. They accordingly constructed for themselves at their home
in Dayton a wind tunnel 16 inches square by 6 feet long in which
they measured the lift and ‘drag’ of more than two hundred miniature
wings. In the course of these tests they for the first time produced
comparative results of the lift of oblong and square surfaces,
with the result that they re-discovered the importance of ‘aspect
ratio’--the ratio of length to breadth of planes. As a result, in the
next year’s glider the aspect ration of the wings was increased from
the three to one of the earliest model to about six to one, which
is approximately the same as that used in the machines of to-day.
Further than that, they discussed the question of lateral stability,
and came to the conclusion that the cause of the trouble was that the
effect of warping down one wing was to increase the resistance of,
and consequently slow down, that wing to such an extent that its lift
was reduced sufficiently to wipe out the anticipated increase in lift
resulting from the warping. From this they deduced that if the speed
of the warped wing could be controlled the advantage of increasing
the angle by warping could be utilised as they originally intended.
They therefore decided to fit a vertical fin at the rear which, if the
machine attempted to turn, would be exposed more and more to the wind
and so stop the turning motion by offering increased resistance.
As a result of this laboratory research work the third Wright glider,
which was taken to Kill Devil Hill in September, 1902, was far more
efficient aerodynamically than either of its two predecessors, and
was fitted with a fixed vertical fin at the rear in addition to the
movable elevator in front. According to Mr Griffith Brewer,[8] this
third glider contained 305 square feet of surface; though there may
possibly be a mistake here, as he states[9] the surface of the previous
year’s glider to have been only 290 square feet, whereas Wilbur Wright
himself[10] states it to have been 308 square feet. The matter is not,
perhaps, save historically, of much importance, except that the gliders
are believed to have been progressively larger, and therefore if we
accept Wilbur Wright’s own figure of the surface of the second glider,
the third must have had a greater area than that given by Mr Griffith
Brewer. Unfortunately, no evidence of the Wright Brothers themselves on
this point is available.
The first glide of the 1902 season was made on September 17th of that
year, and the new machine at once showed itself an improvement on its
predecessors, though subsequent trials showed that the difficulty
of lateral balance had not been entirely overcome. It was decided,
therefore, to turn the vertical fin at the rear into a rudder by
making it movable. At the same time it was realised that there was a
definite relation between lateral balance and directional control, and
the rudder controls and wing-warping wires were accordingly connected.
This ended the pioneer gliding experiments of Wilbur and Orville
Wright--though further glides were made in subsequent years--as the
following year, 1903, saw the first power-driven machine leave the
ground.
To recapitulate--in the course of these original experiments the
Wrights confirmed Lilienthal’s theory of the reversal of the centre
of pressure on cambered surfaces at small angles of incidence: they
confirmed the importance of high aspect ratio in respect to lift:
they had evolved new and more accurate tables of lift and pressure on
cambered surfaces: they were the first to use a movable horizontal
elevator for controlling height: they were the first to adjust the
wings to different angles of incidence to maintain lateral balance: and
they were the first to use the movable rudder and adjustable wings in
combination.
They now considered that they had gone far enough to justify them in
building a power-driven ‘flier,’ as they called their first aeroplane.
They could find no suitable engine and so proceeded to build for
themselves an internal combustion engine, which was designed to give
8 horse-power, but when completed actually developed about 12–15
horse-power and weighed 240 lbs. The complete machine weighed about 750
lbs. Further details of the first Wright aeroplane are difficult to
obtain, and even those here given should be received with some caution.
The first flight was made on December 17th, 1903, and lasted 12
seconds. Others followed immediately, and the fourth lasted 59 seconds,
a distance of 852 feet being covered against a 20-mile wind.
The following year they transferred operations to a field outside
Dayton, Ohio (their home), and there they flew a somewhat larger and
heavier machine with which on September 20th, 1904, they completed the
first circle in the air. In this machine for the first time the pilot
had a seat; all the previous experiments having been carried out with
the operator lying prone on the lower wing. This was followed next
year by another still larger machine, and on it they carried out many
flights. During the course of these flights they satisfied themselves
as to the cause of a phenomenon which had puzzled them during the
previous year and caused them to fear that they had not solved the
problem of lateral control. They found that on occasions--always when
on a turn--the machine began to slide down towards the ground and
that no amount of warping could stop it. Finally it was found that if
the nose of the machine was tilted down a recovery could be effected;
from which they concluded that what actually happened was that the
machine, ‘owing to the increased load caused by centrifugal force,’
had insufficient power to maintain itself in the air and therefore
lost speed until a point was reached at which the controls became
inoperative. In other words, this was the first experience of ‘stalling
on a turn,’ which is a danger against which all embryo pilots have to
guard in the early stages of their training.
The 1905 machine was, like its predecessors, a biplane with a biplane
elevator in front and a double vertical rudder in rear. The span was
40 feet, the chord of the wings being 6 feet and the gap between
them about the same. The total area was about 600 square feet which
supported a total weight of 925 lbs.; while the motor was 12 to 15
horse-power driving two propellers on each side behind the main planes
through chains and giving the machine a speed of about 30 m.p.h. One of
these chains was crossed so that the propellers revolved in opposite
directions to avoid the torque which it was feared would be set up if
they both revolved the same way. The machine was not fitted with a
wheeled undercarriage but was carried on two skids, which also acted as
outriggers to carry the elevator. Consequently, a mechanical method of
launching had to be evolved and the machine received initial velocity
from a rail, along which it was drawn by the impetus provided by the
falling of a weight from a wooden tower or ‘pylon.’ As a result of this
the Wright aeroplane in its original form had to be taken back to its
starting rail after each flight, and could not restart from the point
of alighting. Perhaps, in comparison with French machines of more or
less contemporary date (evolved on independent lines in ignorance of
the Americans’ work), the chief feature of the Wright biplane of 1905
was that it relied entirely upon the skill of the operator for its
stability; whereas in France some attempt was being made, although
perhaps not very successfully, to make the machine automatically stable
laterally. The performance of the Wrights in carrying a loading of some
60 lbs. per horse-power is one which should not be overlooked. The wing
loading was about 1½ lbs. per square foot.
About the same time that the Wrights were carrying out their
power-driven experiments, a band of pioneers was quite independently
beginning to approach success in France. In practically every case,
however, they started from a somewhat different standpoint and took
as their basic idea the cellular (or box) kite. This form of kite,
consisting of two superposed surfaces connected at each end by a
vertical panel or curtain of fabric, had proved extremely successful
for man-carrying purposes, and, therefore, it was little wonder that
several minds conceived the idea of attempting to fly by fitting a
series of box-kites with an engine. The first to achieve success was M.
Santos-Dumont, the famous Brazilian pioneer-designer of airships, who,
on November 12th, 1906, made several flights, the last of which covered
a little over 700 feet. Santos-Dumont’s machine consisted essentially
of two box-kites, forming the main wings, one on each side of the body,
in which the pilot stood, and at the front extremity of which was
another movable box-kite to act as elevator and rudder. The curtains
at the ends were intended to give lateral stability, which was further
ensured by setting the wings slightly inclined upwards from the centre,
so that when seen from the front they formed a wide V. This feature is
still to be found in many aeroplanes to-day and has come to be known
as the ‘dihedral.’ The motor was at first of 24 horse-power, for which
later a 50 horse-power Antoinette engine was substituted; whilst a
three-wheeled undercarriage was provided, so that the machine could
start without external mechanical aid. The machine was constructed of
bamboo and steel, the weight being as low as 352 lbs. The span was 40
feet, the length being 33 feet, with a total surface of main planes of
860 square feet. It will thus be seen--for comparison with the Wright
machine--that the weight per horse-power (with the 50 horse-power
engine) was only 7 lbs., while the wing loading was equally low at ½
lb. per square foot.
The main features of the Santos-Dumont machine were the box-kite form
of construction, with a dihedral angle on the main planes, and the
forward elevator which could be moved in any direction and therefore
acted in the same way as the rudder at the rear of the Wright biplane.
It had a single propeller revolving in the centre behind the wings and
was fitted with an undercarriage incorporated in the machine.
The other chief French experimenters at this period were the Voisin
Frères, whose first two machines--identical in form--were sold to
Delagrange and H. Farman, which has sometimes caused confusion, the two
purchasers being credited with the design they bought. The Voisins,
like the Wrights, based their designs largely on the experimental work
of Lilienthal, Langley, Chanute, and others, though they also carried
out tests on the lifting properties of aerofoils in a wind tunnel of
their own. Their first machines, like those of Santos-Dumont, showed
the effects of experimenting with box-kites, some of which they had
built for M. Ernest Archdeacon in 1904. In their case the machine,
which was again a biplane, had, like both the others previously
mentioned, an elevator in front--though in this case of monoplane
form--and, as in the Wright, a rudder was fitted in rear of the main
planes. The Voisins, however, fitted a fixed biplane horizontal
‘tail’--in an effort to obtain a measure of automatic longitudinal
stability--between the two surfaces of which the single rudder worked.
For lateral stability they depended entirely on end curtains between
the upper and lower surfaces of both the main planes and biplane tail
surfaces. They, like Santos-Dumont, fitted a wheeled undercarriage,
so that the machine was self-contained. The Voisin machine, then,
was intended to be automatically stable in both senses; whereas the
Wrights deliberately produced a machine which was entirely dependent
upon the pilot’s skill for its stability. The dimensions of the Voisin
may be given for comparative purposes, and were as follows: Span 33
feet with a chord (width from back to front) of main planes of 6½ feet,
giving a total area of 430 square feet. The 50 horse-power Antoinette
engine, which was enclosed in the body (or ‘nacelle’) in the front of
which the pilot sat, drove a propeller behind, revolving between the
outriggers carrying the tail. The total weight, including Farman as
pilot, is given as 1,540 lbs., so that the machine was much heavier
than either of the others; the weight per horse-power being midway
between the Santos-Dumont and the Wright at 31 lbs. per square foot,
while the wing loading was considerably greater than either at 3½ lbs.
per square foot. The Voisin machine was experimented with by Farman
and Delagrange from about June 1907 onwards, and was in the subsequent
years developed by Farman; and right up to the commencement of the
War upheld the principles of the box-kite method of construction for
training purposes. The chief modification of the original design was
the addition of flaps (or ailerons) at the rear extremities of the
main planes to give lateral control, in a manner analogous to the
wing-warping method invented by the Wrights, as a result of which the
end curtains between the planes were abolished. An additional elevator
was fitted at the rear of the fixed biplane tail, which eventually
led to the discarding of the front elevator altogether. During the
same period the Wright machine came into line with the others by the
fitting of a wheeled undercarriage integral with the machine. A fixed
horizontal tail was also added to the rear rudder, to which a movable
elevator was later attached; and, finally, the front elevator was done
away with. It will thus be seen that having started from the very
different standpoints of automatic stability and complete control by
the pilot, the Voisin (as developed in the Farman) and Wright machines,
through gradual evolution finally resulted in aeroplanes of similar
characteristics embodying a modicum of both features.
Before proceeding to the next stage of progress mention should be made
of the experimental work of Captain Ferber in France. This officer
carried out a large number of experiments with gliders contemporarily
with the Wrights, adopting--like them--the Chanute biplane principle.
He adopted the front elevator from the Wrights, but immediately went a
step farther by also fitting a fixed tail in rear, which did not become
a feature of the Wright machine until some seven or eight years later.
He built and appeared to have flown a machine fitted with a motor in
1905, and was commissioned to go to America by the French War Office
on a secret mission to the Wrights. Unfortunately, no complete account
of his experiments appears to exist, though it can be said that his
work was at least as important as that of any of the other pioneers
mentioned.
II
MULTIPLICITY OF IDEAS
In a review of progress such as this, it is obviously impossible,
when a certain stage of development has been reached, owing to the
very multiplicity of experimenters, to continue dealing in anything
approaching detail with all the different types of machines; and it is
proposed, therefore, from this point to deal only with tendencies, and
to mention individuals merely as examples of a class of thought rather
than as personalities, as it is often difficult fairly to allocate the
responsibility for any particular innovation.
During 1907 and 1908 a new type of machine, in the monoplane, began to
appear from the workshops of Louis Blériot, Robert Esnault-Pelterie,
and others, which was destined to give rise to long and bitter
controversies on the relative advantages of the two types, into which
it is not proposed to enter here; though the rumblings of the conflict
are still to be heard by discerning ears. Blériot’s early monoplanes
had certain new features, such as the location of the pilot, and in
some cases the engine, below the wing; but in general his monoplanes,
particularly the famous No. XI on which the first Channel crossing was
made on July 25th, 1909, embodied the main principles of the Wright
and Voisin types, except that the propeller was in front of instead
of behind the supporting surfaces, and was, therefore, what is called
a ‘tractor’ in place of the then more conventional ‘pusher.’ Blériot
aimed at lateral balance by having the tip of each wing pivoted,
though he soon fell into line with the Wrights and adopted the warping
system. The main features of the design of Esnault-Pelterie’s monoplane
was the inverted dihedral (or kathedral as this was called in Mr S. F.
Cody’s British Army Biplane of 1907) on the wings, whereby the tips
were considerably lower than the roots at the body. This was designed
to give automatic lateral stability, but, here again, conventional
practice was soon adopted and the R.E.P. monoplanes, which became
well-known in this country through their adoption in the early days
by Messrs Vickers, were of the ordinary monoplane design, consisting
of a tractor propeller with wire-stayed wings, the pilot being in an
enclosed fuselage containing the engine in front and carrying at its
rear extremity fixed horizontal and vertical surfaces combined with
movable elevators and rudder. Constructionally, the R.E.P. monoplane
was of extreme interest as the body was constructed of steel. The
Antoinette monoplane, so ably flown by Latham, was another very famous
machine of the 1909–1910 period, though its performance were frequently
marred by engine failure; which was indeed the bugbear of all these
early experimenters, and it is difficult to say, after this lapse
of time, how far in many cases the failures which occurred, both in
performances and even in the actual ability to rise from the ground,
were due to defects in design or merely faults in the primitive engines
available. The Antoinette aroused admiration chiefly through its
graceful, bird-like lines, which have probably never been equalled;
but its chief interest for our present purpose lies in the novel
method of wing-staying which was employed. Contemporary monoplanes
practically all had their wings stayed by wires to a post in the
centre above the fuselage, and, usually, to the undercarriage below. In
the Antoinette, however, a king post was introduced half-way along the
wing, from which wires were carried to the ends of the wings and the
body. This was intended to give increased strength and permitted of a
greater wing-spread and consequently improved aspect ratio. The same
system of construction was adopted in the British Martinsyde monoplanes
of two or three years later.
[Illustration: Latham’s Antoinette 29.]
This period also saw the production of the first triplane, which was
built by A. V. Roe in England and was fitted with a J.A.P. engine of
only 9 horse-power--an amazing performance which remains to this day
unequalled. Mr Roe’s triplane was chiefly interesting otherwise for
the method of maintaining longitudinal control, which was achieved
by pivoting the whole of the three main planes so that their angle
of incidence could be altered. This was the direct converse of the
universal practice of elevating by means of a subsidiary surface either
in front or rear of the main planes.
Recollection of the various flying meetings and exhibitions which
one attended during the years from 1909 to 1911, or even 1912, are
chiefly notable for the fact that the first thought on seeing any new
type of machine was not as to what its ‘performance’--in speed, lift,
or what not--would be; but speculation as to whether it would leave
the ground at all when eventually tried. This is perhaps the best
indication of the outstanding characteristic of that interim period
between the time of the first actual flights and the later period,
commencing about 1912, when ideas had become settled and it was at last
becoming possible to forecast on the drawing-board the performance of
the completed machine in the air. Without going into details, for
which there is no space here, it is difficult to convey the correct
impression of the chaotic state which existed as to even the elementary
principles of aeroplane design. All the exhibitions contained large
numbers--one had almost written a majority--of machines which embodied
the most unusual features and which never could, and in practice
never did, leave the ground. At the same time, there were few who
were sufficiently hardy to say certainly that this or that innovation
was wrong; and consequently dozens of inventors in every country were
conducting isolated experiments on both good and bad lines. All kinds
of devices, mechanical and otherwise, were claimed as the solution of
the problem of stability, and there was even controversy as to whether
any measure of stability was not undesirable; one school maintaining
that the only safety lay in the pilot having the sole say in the
attitude of the machine at any given moment, and fearing danger from
the machine having any mind of its own, so to speak. There was, as
in most controversies, some right on both sides, and when we come to
consider the more settled period from 1912 to the outbreak of the War
in 1914 we shall find how a compromise was gradually effected.
At the same time, however, though it was at the time difficult to pick
out, there was very real progress being made, and, though a number of
‘freak’ machines fell out by the wayside, the pioneer designers of
those days learnt by a process of trial and error the right principles
to follow and gradually succeeded in getting their ideas crystallised.
In connection with stability mention must be made of a machine which
was evolved in the utmost secrecy by Mr J. W. Dunne in a remote part
of Scotland under subsidy from the War Office. This type, which was
constructed in both monoplane and biplane form, showed that it was
in fact possible in 1910 and 1911 to design an aeroplane which could
definitely be left to fly itself in the air. One of the Dunne machines
was, for example, flown from Farnborough to Salisbury Plain without any
control other than the rudder being touched; and on another occasion
it flew a complete circle with all controls locked, automatically
assuming the correct bank for the radius of turn. The peculiar form
of wing used, the camber of which varied from the root to the tip,
gave rise, however, to a certain loss in efficiency, and there was
also a difficulty in the pilot assuming adequate control when desired.
Other machines designed to be stable--such as the German Etrich and
the British Weiss gliders and Handley-Page monoplanes--were based on
the analogy of a wing attached to a certain seed found in Nature (the
‘Zanonia’ leaf), on the righting effect of back-sloped wings combined
with upturned (or ‘negative’) tips. Generally speaking, however, the
machines of the 1909–1912 period relied for what automatic stability
they had on the principle of the dihedral angle, or flat V, both
longitudinally and laterally. Longitudinally this was obtained by
setting the tail at a slightly smaller angle than the main planes.
The question of reducing the resistance by adopting ‘stream-line’
forms, along which the air could flow uninterruptedly without the
formation of eddies, was not at first properly realised, though credit
should be given to Edouard Nieuport, who in 1909 produced a monoplane
with a very large body which almost completely enclosed the pilot and
made the machine very fast, for those days, with low horse-power. On
one of these machines C. T. Weymann won the Gordon-Bennett Cup for
America in 1911, and another put up a fine performance in the same
race with only a 30 horse-power engine. The subject, was however,
early taken up by the British Advisory Committee for Aeronautics,
which was established by the Government in 1909, and designers began
to realise the importance of streamline struts and fuselages towards
the end of this transition period. These efforts were at first not
always successful and showed at times a lack of understanding of the
problems involved, but there was a very marked improvement during the
year 1912. At the Paris Aero Salon held early in that year there was
a notable variety of ideas on the subject; whereas by the time of the
one held in October designs had considerably settled down, more than
one exhibitor showing what were called ‘monocoque’ fuselages completely
circular in shape and having very low resistance, while the same show
saw the introduction of rotating cowls over the propeller bosses, or
‘spinners,’ as they came to be called during the War. A particularly
fine example of stream-lining was to be found in the Deperdussin
monoplane on which Védrines won back the Gordon-Bennett Aviation Cup
from America at a speed of 105·5 m.p.h.--a considerable improvement on
the 78 m.p.h. of the preceding year, which was by no means accounted
for by the mere increase in engine power from 100 horse-power to 140
horse-power. This machine was the first in which the refinement of
‘stream-lining’ the pilot’s head, which became a feature of subsequent
racing machines, was introduced. This consisted of a circular padded
excresence above the cockpit immediately behind the pilot’s head,
which gradually tapered off into the top surface of the fuselage. The
object was to give the air an uninterrupted flow instead of allowing
it to be broken up into eddies behind the head of the pilot, and it
also provided a support against the enormous wind-pressure encountered.
This true stream-line form of fuselage owed its introduction to the
Paulhan-Tatin ‘Torpille’ monoplane of the Paris Salon of early 1912.
Altogether the end of the year 1912 began to see the disappearance of
‘freak’ machines with all sorts of original ideas for the increase
of stability and performance. Designs had by then gradually become
to a considerable extent standardised, and it had become unusual to
find a machine built which would fail to fly. The Gnome engine held
the field owing to its advantages, as the first of the rotary type,
in lightness and ease of fitting into the nose of a fuselage. The
majority of machines were tractors (propeller in front) although a
preference, which died down subsequently, was still shown for the
monoplane over the biplane. This year also saw a great increase in the
number of seaplanes, although the ‘flying boat’ type had only appeared
at intervals and the vast majority were of the ordinary aeroplane type
fitted with floats in place of the land undercarriage; which type was
at that time commonly called ‘hydro-aeroplane.’ The usual horse-power
was 50--that of the smallest Gnome engine--although engines of 100
to 140 horse-power were also fitted occasionally. The average weight
per horse-power varied from 18 to 25 lbs., while the wing-loading
was usually in the neighbourhood of 5 to 6 lbs. per square foot. The
average speed ranged from 65–75 miles per hour.
[Illustration: Bristol Fighter, rear view.]
III
PROGRESS ON STANDARDISED LINES
In the last section an attempt has been made to show how, during what
was from the design standpoint perhaps the most critical period, order
gradually became evident out of chaos, ill-considered ideas dropped out
through failure to make good, and, though there was still plenty of
room for improvement in details, the bulk of the aeroplanes showed a
general similarity in form and conception. There was still a great deal
to be learnt in finding the best form of wing section, and performances
were still low; but it had become definitely possible to say that
flying had emerged from the chrysalis stage and had become a science.
The period which now began was one of scientific development and
improvement--in performance, manœuvrability, and general airworthiness
and stability.
The British Military Aeroplane Competition held in the summer of 1912
had done much to show the requirements in design by giving possibly
the first opportunity for a definite comparison of the performance
of different machines as measured by impartial observers on standard
lines--albeit the methods of measuring were crude. These showed that
a high speed--for those days--of 75 miles an hour or so was attended
by disadvantages in the form of an equally fast low speed, of 50 miles
per hour or more, and generally may be said to have given designers an
idea what to aim for and in what direction improvements were required.
In fact, the most noticeable point perhaps of the machines of this
time was the marked manner in which a machine that was good in one
respect would be found to be wanting in others. It had not yet been
possible to combine several desirable attributes in one machine. The
nearest approach to this was perhaps to be found in the much discussed
Government B.E.2 machine, which was produced from the Royal Aircraft
Factory at Farnborough, in the summer of 1912. Though considerably
criticised from many points of view it was perhaps the nearest approach
to a machine of all-round efficiency that had up to that date appeared.
The climbing rate, which subsequently proved so important for military
purposes, was still low, seldom, if ever, exceeding 400 feet per
minute; while gliding angles (ratio of descent to forward travel over
the ground with engine stopped) little exceeded 1 in 8.
The year 1912 and 1913 saw the subsequently all-conquering tractor
biplane begin to come into its own. This type, which probably
originated in England, and at any rate attained to its greatest
excellence prior to the War from the drawing offices of the Avro
Bristol and Sopwith firms, dealt a blow at the monoplane from which the
latter never recovered.
The two-seater tractor biplane produced by Sopwith and piloted by H. G.
Hawker, showed that it was possible to produce a biplane with at least
equal speed to the best monoplanes, whilst having the advantage of
greater strength and lower landing speeds. The Sopwith machine had a
top speed of over 80 miles an hour while landing as slowly as little
more than 30 miles an hour; and also proved that it was possible
to carry 3 passengers with fuel for 4 hours’ flight with a motive
power of only 80 horse-power. This increase in efficiency was due to
careful attention to detail in every part, improved wing sections,
clean fuselage-lines, and simplified undercarriages. At the same
time, in the early part of 1913 a tendency manifested itself towards
the four-wheeled undercarriage, a pair of smaller wheels being added
in front of the main wheels to prevent overturning while running on
the ground; and several designs of oleo-pneumatic and steel-spring
undercarriages were produced in place of the rubber shock-absorber type
which had up till then been almost universal.
These two statements as to undercarriage designs may appear to be
contradictory, but in reality they do not conflict as they both showed
a greater attention to the importance of good springing, combined with
a desire to avoid complication and a mass of struts and wires which
increased head resistance.
The Olympia Aero Show of March, 1913, also produced a machine which,
although the type was not destined to prove the best for the purpose
for which it was designed, was of interest as being the first to be
designed specially for war purposes. This was the Vickers ‘Gun-bus,’ a
‘pusher’ machine, with the propeller revolving behind the main planes
between the outriggers carrying the tail, with a seat right in front
for a gunner who was provided with a machine gun on a swivelling mount
which had a free field of fire in every direction forward. The device
which proved the death-blow for this type of aircraft during the war
will be dealt with in the appropriate place later, but the machine
should not go unrecorded.
As a result of a number of accidents to monoplanes the Government
appointed a Committee at the end of 1912 to inquire into the causes
of these. The report, which was presented in March, 1913, exonerated
the monoplane by coming to the conclusion that the accidents were not
caused by conditions peculiar to monoplanes, but pointed out certain
desiderata in aeroplane design generally which are worth recording.
They recommended that the wings of aeroplanes should be so internally
braced as to have sufficient strength in themselves not to collapse
if the external bracing wires should give way. The practice, more
common in monoplanes than biplanes, of carrying important bracing
wires from the wings to the undercarriage was condemned owing to the
liability of damage from frequent landings. They also pointed out the
desirability of duplicating all main wires and their attachments, and
of using stranded cable for control wires. Owing to the suspicion
that one accident at least had been caused through the tearing of the
fabric away from the wing, it was recommended that fabric should be
more securely fastened to the ribs of the wings, and that devices for
preventing the spreading of tears should be considered. In the last
connection it is interesting to note that the French Deperdussin firm
produced a fabric wing-covering with extra strong threads run at right
angles through the fabric at intervals in order to limit the tearing to
a defined area.
In spite, however, of the whitewashing of the monoplane by the
Government Committee just mentioned, considerable stir was occasioned
later in the year by the decision of the War Office not to order any
more monoplanes; and from this time forward until the War period the
British Army was provided exclusively with biplanes. Even prior to
this the popularity of the monoplane had begun to wane. At the Olympia
Aero Show in March, 1913, biplanes for the first time outnumbered the
‘single-deckers’ (as the Germans call monoplanes); which had the effect
of reducing the wing-loading. In the case of the biplanes exhibited
this averaged about 4½ lbs. per square foot, while in the case of the
monoplanes in the same exhibition the lowest was 5½ lbs., and the
highest over 8½ lbs. per square foot of area. It may here be mentioned
that it was not until the War period that the importance of loading
per horse-power was recognised as the true criterion of aeroplane
efficiency, far greater interest being displayed in the amount of
weight borne per unit area of wing.
An idea of the state of development arrived at about this time may be
gained from the fact that the Commandant of the Military Wing of the
Royal Flying Corps in a lecture before the Royal Aeronautical Society
read in February, 1913, asked for single-seater scout aeroplanes
with a speed of 90 miles an hour and a landing speed of 45 miles
an hour--a performance which even two years later would have been
considered modest in the extreme. It serves to show that, although
higher performances were put up by individual machines on occasion,
the general development had not yet reached the stage when such
performances could be obtained in machines suitable for military
purposes. So far as seaplanes were concerned, up to the beginning of
1913 little attempt had been made to study the novel problems involved,
and the bulk of the machines at the Monaco Meeting in April, 1913,
for instance, consisted of land machines fitted with floats, in many
cases of a most primitive nature, without other alterations. Most of
those which succeeded in leaving the water did so through sheer pull
of engine power; while practically all were incapable of getting off
except in a fair sea, which enabled the pilot to jump the machine into
the air across the trough between two waves. Stability problems had not
yet been considered, and in only one or two cases was fin area added
at the rear high up, to counterbalance the effect of the floats low
down in front. Both twin and single-float machines were used, while
the flying boat was only just beginning to come into being from the
workshops of Sopwith in Great Britain, Borel-Denhaut in France, and
Curtiss in America. In view of the approaching importance of amphibious
seaplanes, mention should be made of the flying boat (or ‘bat boat’ as
it was called, following Rudyard Kipling) which was built by Sopwith in
1913 with a wheeled landing-carriage which could be wound up above the
bottom surface of the boat so as to be out of the way when alighting on
water.
During 1913 the (at one time almost universal) practice originated by
the Wright Brothers, of warping the wings for lateral stability, began
to die out and the bulk of aeroplanes began to be fitted with flaps
(or ‘ailerons’) instead. This was a distinct change for the better, as
continually warping the wings by bending down the extremities of the
rear spars was bound in time to produce ‘fatigue’ in that member and
lead to breakage; and the practice became completely obsolete during
the next two or three years.
The Gordon-Bennett race of September, 1913, was again won by a
Deperdussin machine, somewhat similar to that of the previous year,
but with exceedingly small wings, only 107 square feet in area. The
shape of these wings was instructive as showing how what, from the
general utility point of view, may be disadvantageous can, for a
special purpose, be turned to account. With a span of 21 feet, the
chord was 5 feet, giving the inefficient ‘aspect ratio’ of slightly
over 4 to 1 only. The object of this was to reduce the lift, and
therefore the resistance, to as low a point as possible. The total
weight was 1,500 lbs., giving a wing-loading of 14 lbs. per square
foot--a hitherto undreamt-of figure. The result was that the machine
took an enormously long run before starting; and after touching the
ground on landing ran for nearly a mile before stopping; but she beat
all records by attaining a speed of 126 miles per hour. Where this
performance is mainly interesting is in contrast to the machines of
1920, which with an even higher speed capacity would yet be able to
land at not more than 40 or 50 miles per hour, and would be thoroughly
efficient flying machines.
[Illustration: R.A.F. Aeroplane.]
The Rheims Aviation Meeting, at which the Gordon-Bennett race was
flown, also saw the first appearance of the Morane ‘Parasol’ monoplane.
The Morane monoplane had been for some time an interesting machine
as being the only type which had no fixed surface in rear to give
automatic stability, the movable elevator being balanced through being
hinged about one-third of the way back from the front edge. This made
the machine difficult to fly except in the hands of experts, but it was
very quick and handy on the controls and therefore useful for racing
purposes. In the ‘Parasol’ the modification was introduced of raising
the wing above the body, the pilot looking out beneath it, in order
to give as good a view as possible.
Before passing to the year 1914 mention should be made of the feat
performed by Nesteroff, a Russian, and Pégoud, a French pilot, who were
the first to demonstrate the possibilities of flying upside-down and
looping the loop. Though perhaps not coming strictly within the purview
of a chapter on design (though certain alterations were made to the
top wing-bracing of the machine for this purpose) this performance was
of extreme importance to the development of aviation by showing the
possibility of recovering, given reasonable height, from any position
in the air; which led designers to consider the extra stresses to which
an aeroplane might be subjected and to take steps to provide for them
by increasing strength where necessary.
When the year 1914 opened a speed of 126 miles per hour had been
attained and a height of 19,600 feet had been reached. The Sopwith
and Avro (the forerunner of the famous training machine of the War
period) were probably the two leading tractor biplanes of the world,
both two-seaters with a speed variation from 40 miles per hour up to
some 90 miles per hour with 80 horse-power engines. The French were
still pinning their faith mainly to monoplanes, while the Germans were
beginning to come into prominence with both monoplanes and biplanes
of the ‘Taube’ type. These had wings swept backward and also upturned
at the wing-tips which, though it gave a certain measure of automatic
stability, rendered the machine somewhat clumsy in the air, and their
performances were not on the whole as high as those of either France or
Great Britain.
Early in 1914 it became known that the experimental work of Edward
Busk--who was so lamentably killed during an experimental flight
later in the year--following upon the researches of Bairstow and
others had resulted in the production at the Royal Aircraft Factory at
Farnborough of a truly automatically stable aeroplane. This was the
‘R.E.’ (Reconnaissance Experimental), a development of the B.E. which
has already been referred to. The remarkable feature of this design
was that there was no particular device to which one could point out
as the cause of the stability. The stable result was attained simply
by detailed design of each part of the aeroplane, with due regard to
its relation to, and effect on, other parts in the air. Weights and
areas were so nicely arranged that under practically any conditions
the machine tended to right itself. It did not, therefore, claim to be
a machine which it was impossible to upset, but one which if left to
itself would tend to right itself from whatever direction a gust might
come. When the principles were extended to the ‘B.E. 2c’ type (largely
used at the outbreak of the War) the latter machine, if the engine were
switched off at a height of not less than 1,000 feet above the ground,
would after a few moments assume its correct gliding angle and glide
down to the ground.
The Paris Aero Salon of December, 1913, had been remarkable chiefly
for the large number of machines of which the chassis and bodywork had
been constructed of steel-tubing; for the excess of monoplanes over
biplanes; and (in the latter) predominance of ‘pusher’ machines (with
propeller in rear of the main planes) compared with the growing British
preference for ‘tractors’ (with air-screw in front). Incidentally, the
Maurice Farman, the last relic of the old type box-kite with elevator
in front appeared shorn of this prefix, and became known as the
‘short-horn’ in contradistinction to its front-elevatored predecessor
which, owing to its general reliability and easy flying capabilities,
had long been affectionately called the ‘mechanical cow.’ The 1913
Salon also saw some lingering attempts at attaining automatic stability
by pendulum and other freak devices.
Apart from the appearance of ‘R.E.1,’ perhaps the most notable
development towards the end of 1913 was the appearance of the Sopwith
‘Tabloid’ tractor biplane. This single-seater machine, evolved from
the two-seater previously referred to, fitted with a Gnome engine of
80 horse-power, had the, for those days, remarkable speed of 92 miles
an hour; while a still more notable feature was that it could remain
in level flight at not more than 37 miles per hour. This machine is of
particular importance because it was the prototype and forerunner of
the successive designs of single-seater scout fighting machines which
were used so extensively from 1914 to 1918. It was also probably the
first machine to be capable of reaching a height of 1,000 feet within
one minute. It was closely followed by the ‘Bristol Bullet,’ which was
exhibited at the Olympia Aero Show of March, 1914. This last pre-war
show was mainly remarkable for the good workmanship displayed--rather
than for any distinct advance in design. In fact, there was a notable
diversity in the types displayed, but in detailed design considerable
improvements were to be seen, such as the general adoption of stranded
steel cable in place of piano wire for the main bracing.
IV
THE WAR PERIOD
Up to this point an attempt has been made to give some idea of the
progress that was made during the eleven years that had elapsed since
the days of the Wrights’ first flights. Much advance had been made
and aeroplanes had settled down, superficially at any rate, into more
or less standardised forms in three main types--tractor monoplanes,
tractor biplanes, and pusher biplanes. Through the application of
the results of experiments with models in wind tunnels to full-scale
machines, considerable improvements had been made in the design of wing
sections, which had greatly increased the efficiency of aeroplanes by
raising the amount of ‘lift’ obtained from the wing compared with the
‘drag’ (or resistance to forward motion) which the same wing would
cause. In the same way the shape of bodies, interplane struts, etc.,
had been improved to be of better stream-line shape, for the further
reduction of resistance; while the problems of stability were beginning
to be tolerably well understood. Records (for what they are worth)
stood at 21,000 feet as far as height was concerned, 126 miles per hour
for speed, and 24 hours duration. That there was considerable room for
development is, however, evidenced by a statement made by the late
B. C. Hucks (the famous pilot) in the course of an address delivered
before the Royal Aeronautical Society in July, 1914. ‘I consider,’
he said, ‘that the present day standard of flying is due far more to
the improvement in piloting than to the improvement in machines....
I consider those (early 1914) machines are only slight improvements
on the machines of three years ago, and yet they are put through
evolutions which, at that time, were not even dreamed of. I can take
a good example of the way improvement in piloting has outdistanced
improvement in machines--in the case of myself, my ‘looping’ Blériot.
Most of you know that there is very little difference between that
machine and the 50 horse-power Blériot of three years ago.’ This
statement was, of course, to some extent an exaggeration and was by no
means agreed with by designers, but there was at the same time a germ
of truth in it. There is at any rate little doubt that the theory and
practice of aeroplane design made far greater strides towards becoming
an exact science during the four years of War than it had done during
the six or seven years preceding it.
It is impossible in the space at disposal to treat of this development
even with the meagre amount of detail that has been possible while
covering the ‘settling down’ period from 1911 to 1914, and it is
proposed, therefore, to indicate the improvements by sketching briefly
the more noticeable difference in various respects between the average
machine of 1914 and a similar machine of 1918.
In the first place, it was soon found that it was possible to obtain
greater efficiency and, in particular, higher speeds, from tractor
machines than from pusher machines with the air-screw behind the
main planes. This was for a variety of reasons connected with the
efficiency of propellers and the possibility of reducing resistance to
a greater extent in tractor machines by using a ‘stream-line’ fuselage
(or body) to connect the main planes with the tail. Full advantage of
this could not be taken, however, owing to the difficulty of fixing
a machine-gun in a forward direction owing to the presence of the
propeller. This was finally overcome by an ingenious device (known as
an ‘Interrupter gear’) which allowed the gun to fire only when none of
the propeller blades was passing in front of the muzzle. The monoplane
gradually fell into desuetude, mainly owing to the difficulty of
making that type adequately strong without it becoming prohibitively
heavy, and also because of its high landing speed and general lack of
manœuvrability. The triplane was also little used except in one or two
instances, and, practically speaking, every machine was of the biplane
tractor type.
A careful consideration of the salient features leading to maximum
efficiency in aeroplanes--particularly in regard to speed and climb,
which were the two most important military requirements--showed that
a vital feature was the reduction in the amount of weight lifted per
horse-power employed; which in 1914 averaged from 20 to 25 lbs. This
was effected both by gradual increase in the power and size of the
engines used and by great improvement in their detailed design (by
increasing compression ratio and saving weight whenever possible);
with the result that the motive power of single-seater aeroplanes
rose from 80 and 100 horse-power in 1914 to an average of 200 to 300
horse-power, while the actual weight of the engine fell from 3½-4 lbs.
per horse-power to an average of 2½ lbs. per horse-power. This meant
that while a pre-war engine of 100 horse-power would weigh some 400
lbs., the 1918 engine developing three times the power would have less
than double the weight. The result of this improvement was that a scout
aeroplane at the time of the Armistice would have 1 horse-power for
every 8 lbs. of weight lifted, compared with the 20 or 25 lbs. of its
1914 predecessors. This produced a considerable increase in the rate of
climb, a good postwar machine being able to reach 10,000 feet in about
5 minutes and 20,000 feet in under half an hour. The loading per square
foot was also considerably increased; this being rendered possible both
by improvement in the design of wing sections and by more scientific
construction giving increased strength. It will be remembered that in
the machine of the very early period each square foot of surface had
only to lift a weight of some 1½ to 2 lbs., which by 1914 had been
increased to about 4 lbs. By 1918 aeroplanes habitually had a loading
of 8 lbs. or more per square foot of area; which resulted in great
increase in speed. Although a speed of 126 miles per hour had been
attained by a specially designed racing machine over a short distance
in 1914, the average at that period little exceeded, if at all, 100
miles per hour; whereas in 1918 speeds of 130 miles per hour had become
a commonplace, and shortly afterwards a speed of over 166 miles an hour
was achieved.
In another direction, also, that of size, great developments were made.
Before the War a few machines fitted with more than one engine had been
built (the first being a triple Gnome-engined biplane built by Messrs
Short Bros. at Eastchurch in 1913), but none of large size had been
successfully produced, the total weight probably in no case exceeding
about 2 tons. In 1916, however, the twin engine Handley-Page biplane
was produced, to be followed by others both in this country and abroad,
which represented a very great increase in size and, consequently,
load-carrying capacity. By the end of the War period several types
were in existence weighing a total of 10 tons when fully loaded, of
which some 4 tons or more represented ‘useful load’ available for crew,
fuel, and bombs or passengers. This was attained through very careful
attention to detailed design, which showed that the material could be
employed more efficiently as size increased, and was also due to the
fact that a large machine was not liable to be put through the same
evolutions as a small machine, and therefore could safely be built
with a lower factor of safety. Owing to the fact that a wing section
which is adopted for carrying heavy loads usually has also a somewhat
low lift to drag ratio, and is not therefore productive of high speed,
these machines are not as fast as light scouts; but, nevertheless, they
proved themselves capable of achieving speeds of 100 miles an hour or
more in some cases; which was faster than the average small machine of
1914.
[Illustration: Bristol Fighters in formation.]
In one respect the development during the War may perhaps have proved
to be somewhat disappointing, as it might have been expected that
great improvements would be effected in metal construction, leading
almost to the abolition of wooden structures. Although, however, a
good deal of experimental work was done which resulted in overcoming
at any rate the worst of the difficulties, metal-built machines were
little used (except to a certain extent in Germany) chiefly on account
of the need for rapid production and the danger of delay resulting
from switching over from known and tried methods to experimental
types of construction. The Germans constructed some large machines,
such as the giant Siemens-Schukhert machine, entirely of metal except
for the wing covering, while the Fokker and Jünker firms about the
time of the Armistice in 1918 both produced monoplanes with very deep
all-metal wings (including the covering) which were entirely unstayed
externally, depending for their strength on internal bracing. In Great
Britain cable bracing gave place to a great extent to ‘stream-line
wires,’ which are steel rods rolled to a more or less oval section,
while tie-rods were also extensively used for the internal bracing
of the wings. Great developments in the economical use of material
were also made in the direction of using built-up main spars for the
wings and inter-plane struts; spars composed of a series of layers (or
‘laminations’) of different pieces of wood also being used.
Apart from the metallic construction of aeroplanes an enormous amount
of work was done in the testing of different steels and light alloys
for use in engines, and by the end of the War period a number of
aircraft engines were in use of which the pistons and other parts were
of such alloys; the chief difficulty having been not so much in the
design as in the successful heat-treatment and casting of the metal.
An important development in connection with the inspection and
testing of aircraft parts, particularly in the case of metal, was
the experimental application of X-ray photography, which showed up
latent defects, both in the material and in manufacture, which would
otherwise have passed unnoticed. This method was also used to test the
penetration of glue into the wood on each side of joints, so giving a
measure of the strength; and for the effect of ‘doping’ the wings,
dope being a film (of cellulose acetate dissolved in acetone with other
chemicals) applied to the covering of wings and bodies to render the
linen taut and weatherproof, besides giving it a smooth surface for the
lessening of ‘skin friction’ when passing rapidly through the air.
An important result of this experimental work was that it in many cases
enabled designers to produce aeroplane parts from less costly material
than had previously been considered necessary, without impairing the
strength. It may be mentioned that it was found undesirable to use
welded joints on aircraft in any part where the material is subject
to a tensile or bending load, owing to the danger resulting from bad
workmanship causing the material to become brittle--an effect which
cannot be discovered except by cutting through the weld, which, of
course, involves a test to destruction. Written, as it has been, in
August, 1920, it is impossible in this chapter to give any conception
of how the developments of War will be applied to commercial
aeroplanes, as few truly commercial machines have yet been designed,
and even those still show distinct traces of the survival of war
mentality. When, however, the inevitable recasting of ideas arrives, it
will become evident, whatever the apparent modification in the relative
importance of different aspects of design, that enormous advances were
made under the impetus of War which have left an indelible mark on
progress.
We have, during the seventeen years since aeroplanes first took the
air, seen them grow from tentative experimental structures of unknown
and unknowable performance to highly scientific products, of which not
only the performances (in speed, load-carrying capacity, and climb)
are known, but of which the precise strength and degree of stability
can be forecast with some accuracy on the drawing board. For the
rest, with the future lies--apart from some revolutionary change in
fundamental design--the steady development of a now well-tried and
well-found engineering structure.