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-bystep 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 sand-hills 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,[*] this third glider
contained 305 square feet of surface; though there may possibly be a
mistake here, as he states[**] the surface of the previous year's glider
to have been only 290 square feet, whereas Wilbur Wright himself[***]
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.
[*] Fourth Wilbur Wright Memorial Lecture, Aeronautical Journal, Vol.
XX, No. 79, page 75.
[**] Ibid. page 73.
[***] Ibid. pp. 91 and 102.
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 1 1/2 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 1/2 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
Freres, 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 6 1/2 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 3 1/2 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 Bleriot, 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. Bleriot'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.' Bleriot 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, birdlike 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.
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. Weyman 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 Vedrines 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 1917. 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.
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, manoeuvrability, 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
criticized 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 4 1/2 lbs. per square foot, while in the case of
the monoplanes in the same exhibition the lowest was 5 1/2 lbs., and
the highest over 8 1/2 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.
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 Pegoud, 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 of
f 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 mail 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' Bleriot. Most of you know
that there is very little difference between that machine and the 50
horse-power Bleriot 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 manoeuvrability. 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 3 1/2-4 lbs. per horse-power to an
average of 2 1/2 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 1 1/2 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.
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 Junker 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 interplane
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 subjectto
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.