In the early nineties the air ship was engaging the attention of many
inventors, and was making important strides in the hands of Mr. Maxim.
This unrivalled mechanician, in stating the case, premises that a motive
power has to be discovered which can develop at least as much power in
proportion to its weight as a bird is able to develop. He asserts that a
heavy bird, with relatively small wings--such as a goose--carries
about 150 lb. to the horse power, while the albatross or the vulture,
possessed of proportionately greater winged surface, can carry about 250
lbs. per horse power.
Professor Langley, of Washington, working contemporaneously, but
independently of Mr. Maxim, had tried exhaustive experiments on
a rotating arm (characteristically designated by Mr. Maxim a
"merry-go-round"), thirty feet long, applying screw propellers. He used,
for the most part, small planes, carrying loads of only two or three
pounds, and, under these circumstances, the weight carried was at the
rate of 250 lbs. per horse power. His important statements with regard
to these trials are that one-horse power will transport a larger weight
at twenty miles an hour than at ten, and a still larger at forty miles
than at twenty, and so on; that "the sustaining pressure of the air on
a plane moving at a small angle of inclination to a horizontal path is
many times greater than would result from the formula implicitly given
by Newton, while, whereas in land or marine transport increased speed
is maintained only by a disproportionate expenditure of power within the
limits of experiment, in aerial horizontal transport the higher speeds
are more economical of power than the lower ones."
This Mr. Maxim is evidently ready to endorse, stating, in his own words,
that birds obtain the greater part of their support by moving forward
with sufficient velocity so as to be constantly resting on new air, the
inertia of which has not been disturbed. Mr. Maxim's trials were on a
scale comparable with all his mechanical achievements. He employed for
his experiments a rotating arm, sweeping out a circle, the circumference
of which was 200 feet. To the end of this arm he attached a cigar-shaped
apparatus, driven by a screw, and arranged in such a manner that
aero-planes could be attached to it at any angle. These planes were on
a large scale, carrying weights of from 20 lbs. to 100 lbs. With this
contrivance he found that, whatever push the screw communicated to the
aero-plane, "the plane would lift in a vertical direction from ten to
fifteen times as much as the horizontal push that it received from the
screw, and which depended upon the angle at which the plane was set, and
the speed at which the apparatus was travelling through the air." Next,
having determined by experiment the power required to perform artificial
flight, Mr. Maxim applied himself to designing the requisite motor.
"I constructed," he states, "two sets of compound engines of tempered
steel, all the parts being made very light and strong, and a steam
generator of peculiar construction, the greater part of the heating
surface consisting of small and thin copper tubes. For fuel I employed
naphtha."
This Mr. Maxim wrote in 1892, adding that he was then experimenting with
a large machine, having a spread of over 100 feet. Labour, skill, and
money were lavishly devoted henceforward to the great task undertaken,
and it was not long before the giant flying machine, the outcome of so
much patient experimenting, was completed and put to a practical trial.
Its weight was 7,500 lbs. The screw propellers were nearly 18 feet in
diameter, each with two blades, while the engines were capable of being
run up to 360 horse power. The entire machine was mounted on an inner
railway track of 9 feet and an outer of 35 feet gauge, while above there
was a reversed rail along which the machine would begin to run so soon
as with increase of speed it commenced to lift itself off the inner
track.
In one of the latest experiments it was found that when a speed of 42
miles an hour was attained all the wheels were running on the upper
track, and revolving in the opposite direction from those on the lower
track. However, after running about 1,000 feet, an axle tree doubled up,
and immediately afterwards the upper track broke away, and the machine,
becoming liberated, floated in the air, "giving those on board a
sensation of being in a boat."
The experiment proved conclusively to the inventor that a machine
could be made on a large scale, in which the lifting effect should be
considerably greater than the weight of the machine, and this, too,
when a steam engine was the motor. When, therefore, in the years shortly
following, the steam engine was for the purposes of aerial locomotion
superseded by the lighter and more suitable petrol engine, the
construction of a navigable air ship became vastly more practicable.
Still, in Sir H. Maxim's opinion, lately expressed, "those who seek
to navigate the air by machines lighter than the air have come,
practically, to the end of their tether," while, on the other hand,
"those who seek to navigate the air with machines heavier than the air
have not even made a start as yet, and the possibilities before them are
very great indeed."
As to the assertion that the aerial navigators last mentioned "have not
even made a start as yet," we can only say that Sir H. Maxim speaks with
far too much modesty. His own colossal labours in the direction of that
mode of aerial flight, which he considers to be alone feasible, are
of the first importance and value, and, as far as they have gone,
exhaustive. Had his experiments been simply confined to his classical
investigations of the proper form of the screw propeller his name would
still have been handed down as a true pioneer in aeronautics. His work,
however, covers far wider ground, and he has, in a variety of ways,
furnished practical and reliable data, which must always be an
indispensable guide to every future worker in the same field.
Professor Langley, in attacking the same problem, first studied the
principle and behaviour of a well-known toy--the model invented by
Penaud, which, driven by the tension of india-rubber, sustains itself in
the air for a few seconds. He constructed over thirty modifications of
this model, and spent many months in trying from these to as certain
what he terms the "laws of balancing leading to horizontal flight." His
best endeavours at first, however, showed that he needed three or four
feet of sustaining surface to a pound of weight, whereas he calculated
that a bird could soar with a surface of less than half a foot to the
pound. He next proceeded to steam-driven models in which for a time he
found an insuperable difficulty in keeping down the weight, which, in
practice, always exceeded his calculation; and it was not till the end
of 1893 that he felt himself prepared for a fair trial. At this time he
had prepared a model weighing between nine and ten pounds, and he needed
only a suitable launching apparatus to be used over water. The model
would, like a bird, require an initial velocity imparted to it, and the
discovery of a suitable apparatus gave him great trouble. For the rest
the facilities for launching were supplied by a houseboat moored on the
Potomac. Foiled again and again by many difficulties, it was not till
after repeated failures and the lapse of many months, when, as the
Professor himself puts it, hope was low, that success finally came. It
was in the early part of 1896 that a successful flight was accomplished
in the presence of Dr. Bell, of telephone fame, and the following is
a brief epitome of the account that this accomplished scientist
contributed to the columns of Nature:--
"The flying machine, built, apparently, almost entirely of metal, was
driven by an engine said to weigh, with fuel and water, about 25 lbs.,
the supporting surface from tip to tip being 12 or 14 feet. Starting
from a platform about 20 feet high, the machine rose at first directly
in the face of the wind, moving with great steadiness, and subsequently
wheeling in large curves until steam was exhausted, when, from a height
of 80 or 100 feet, it shortly settled down. The experiment was then
repeated with similar results. Its motion was so steady that a glass of
water might have remained unspilled. The actual length of flight each
time, which lasted for a minute and a half, exceeded half a mile, while
the velocity was between twenty and twenty-five miles an hour in a
course that was constantly taking it 'up hill.' A yet more successful
flight was subsequently made."
But flight of another nature was being courageously attempted at this
time. Otto Lilienthal, of Berlin, in imitation of the motion of birds,
constructed a flying apparatus which he operated himself, and with which
he could float down from considerable elevations. "The feat," he
warns tyros, "requires practice. In the beginning the height should be
moderate, and the wings not too large, or the wind will soon show that
it is not to be trifled with." The inventor commenced with all due
caution, making his first attempt over a grass plot from a spring board
one metre high, and subsequently increasing this height to two and a
half metres, from which elevation he could safely cross the entire grass
plot. Later he launched himself from the lower ridges of a hill 250 feet
high, when he sailed to a distance of over 250 yards, and this time he
writes enthusiastically of his self-taught accomplishment:--
"To those who, from a modest beginning and with gradually increased
extent and elevation of flight have gained full control over the
apparatus, it is not in the least dangerous to cross deep and broad
ravines. It is a difficult task to convey to one who has never enjoyed
aerial flight a clear perception of the exhilarating pleasure of this
elastic motion. The elevation above the ground loses its terrors,
because we have learned by experience what sure dependence may be placed
upon the buoyancy of the air."
As a commentary to the above we extract the following:--"We have to
record the death of Otto Lilienthal, whose soaring machine, during a
gliding flight, suddenly tilted over at a height of about 60 feet,
by which mishap he met an untimely death on August 9th, 1896." Mr. O.
Chanute, C.E. of Chicago, took up the study of gliding flight at the
point where Lilienthal left it, and, later, Professor Fitzgerald
and others. Besides that invented by Penaud, other aero-plane models
demanding mention had been produced by Tatin, Moy, Stringfellow,
and Lawrence Hargrave, of Australia, the subsequent inventor of the
well-known cellular kite. These models, for the most part, aim at the
mechanical solution of the problem connected with the soaring flight of
a bird.
The theoretical solution of the same problem had been attacked by
Professor Langley in a masterly monograph, entitled "The Internal Work
of the Wind." By painstaking experiment with delicate instruments,
specially constructed, the Professor shows that wind in general, so
far from being, as was commonly assumed, mere air put in motion with
an approximately uniform velocity in the same strata, is, in reality,
variable and irregular in its movements beyond anything which had been
anticipated, being made up, in fact, of a succession of brief pulsations
in different directions, and of great complexity. These pulsations, he
argues, if of sufficient amplitude and frequency, would be capable, by
reason of their own "internal work," of sustaining or even raising a
suitably curved surface which was being carried along by the main mean
air stream. This would account for the phenomenon of "soaring." Lord
Rayleigh, discussing the same problem, premises that when a bird is
soaring the air cannot be moving uniformly and horizontally. Then comes
the natural question, Is it moving in ascending currents? Lord Rayleigh
has frequently noticed such currents, particularly above a cliff facing
the wind. Again, to quote another eminent authority, Major Baden-Powell,
on an occasion when flying one of his own kites, found it getting to
so high an angle that it presently rose absolutely overhead, with the
string perpendicular. He then took up a heavy piece of wood, which, when
tied to the string, began to rise in the air. He satisfied himself that
this curious result was solely due to a strong uptake of the air.
But, again, Lord Rayleigh, lending support to Professor Langley's
argument, points out that the apparent cause of soaring may be the
non-uniformity of the wind. The upper currents are generally stronger
than the lower, and it is mechanically possible for a bird, taking
advantage of two adjacent air streams, different in velocity, to
maintain itself in air without effort on its own part.
Lord Rayleigh, proceeding to give his views on artificial flight,
declares the main problem of the flying machine to be the problem of the
aerial plane. He states the case thus:--"Supposing a plane surface to
be falling vertically at the rate of four miles an hour, and also moving
horizontally at the rate of twenty miles an hour, it might have been
supposed that the horizontal motion would make no difference to the
pressure on its under surface which the falling plane must experience.
We are told, however, that in actual trial the horizontal motion much
increases the pressure under the falling plane, and it is this fact on
which the possibility of natural and artificial flight depends."
Ere this opinion had been stated by Lord Rayleigh in his discourse on
"Flight," at the Royal Institution, there were already at work upon the
aero-plane a small army of inventors, of whom it will be only possible
in a future chapter to mention some. Due reference, however, should here
be made to Mr. W. F. Wenham, of Boston, U.S.A., who had been at work on
artificial flight for many years, and to whose labours in determining
whether man's power is sufficient to raise his own weight Lord Rayleigh
paid a high tribute. As far back as 1866 Mr. Wenham had published a
paper on aerial locomotion, in which he shows that any imitation by
man of the far-extended wings of a bird might be impracticable, the
alternative being to arrange the necessary length of wing as a series of
aero-planes, a conception far in advance of many theorists of his time.
But there had been developments in aerostation in other lines, and it
is time to turn from the somewhat tedious technicalities of mechanical
flight and the theory or practice of soaring, to another important means
for traversing the air--the parachute. This aerial machine, long
laid aside, was to lend its aid to the navigation of the air with a
reliability never before realised. Professor Baldwin, as he was termed,
an American aeronaut, arrived in England in the summer of 1888, and
commenced giving a series of exhibitions from the Alexandra Palace with
a parachute of his own invention, which, in actual performance, seems
to have been the most perfect instrument of the kind up to that time
devised. It was said to be about 18 feet in diameter, whereas that of
Garnerin, already mentioned, had a diameter of some 30 feet, and was
distinctly top-heavy, owing to its being thus inadequately ballasted;
for it was calculated that its enormous size would have served for the
safe descent, not of one man, but of four or five. Baldwin's parachute,
on the contrary, was reckoned to give safe descent to 250 lbs., which
would include weight of man and apparatus, and reduce the ultimate fall
to one not exceeding 8 feet. The parachute was attached to the ring of a
small balloon of 12,000 cubic feet, and the Professor ascended, sitting
on a mere sling of rope, which did duty for a car.
Mr. Thomas Moy, who investigated the mechanics of the contrivance,
estimated that after a drop of 16 feet, the upward pressure, amounting
to over 2 lb. per square foot, would act on a surface of not less than
254 square feet. There was, at the time, much foolish comment on the
great distance which the parachute fell before it opened, a complete
delusion due to the fact that observers failed to see that at the moment
of separation the balloon itself sprang upward.