OTTO AND GUSTAV LILIENTHAL
How two German schoolboys built wings which they
tested on moonlit nights--The beginnings of a great
and patient quest--Otto Lilienthal’s theories and
study of the birds.
Much of the ground has now been cleared, and--apart from such a story as
may be told merely from facts and figures, and is apt to prove
unsatisfying--we have striven to show the inner meaning of this great
quest: how each of these pioneers, although he may have seemed to spend
money in vain, and build models only to meet with failure, was really
playing a useful part; was in fact--although he himself did not realise
it--forging one of the links in the chain.
It has been shown how men passed from an ill-judged, haphazard stage;
how science threw upon the problem the clear, cold light of wisdom; and
then, further encouraged by the data that was to hand, how there were
engineers who were ready to build large machines and demonstrate that,
even in a crude and early form, an apparatus with curved planes would
lift itself from the ground.
But still there remained this problem: how were men to learn to balance
themselves when in the air? And, in considering the equilibrium of the
aeroplane, it must be remembered that the air in which a machine must
fly is a disturbed and turbulent sea. So, even were a man to build
himself a craft which would, without the need of a hand upon its levers,
balance itself accurately when in still air, there would still be the
problem of the wind gusts; there would, that is to say, still be the
risk of a machine being struck by an air-wave, particularly when flying
near the ground, and being thrown out of its balance and dashed to
earth.
[Illustration: FIG. 18.--Flow of the wind over hills.]
As waves roll across the surface of the sea, so in the aerial ocean are
there breakers and eddies and many dangers unknown; and men cannot see,
but only feel them. The air does not flow in regular streams over the
earth’s surface; could we follow its movements with our eyes, we should
see that it is full of whirls and eddies, with currents of warm air
flowing upward, streams of cool air moving downward; and with all the
obstructions on the face of the earth, such as hills and woods, causing
an interruption and a disturbance in the air flowing over them (Fig.
18). The face of a cliff, for instance, will deflect a current upward,
leaving a partial void at its summit; and into this void the air will
rush in the form of a whirling eddy.
The man who would learn to fly has to launch himself into a treacherous,
quickly-moving element; and one which, to add to his perils, he cannot
see. The rower in a boat, who sets out upon a stormy sea, can watch the
flow of the waves and turn the prow of his vessel to a breaker that
threatens him. But the aerial navigator moves in a medium that is
invisible; gusts that rush upon him are unseen; he is unaware of their
onslaught until his craft heels before the shock. This risk, from the
sudden sweeping up of an air-wave, was put clearly by Wilbur Wright when
he wrote:
“A gust, coming on very suddenly, will strike the front of a
machine and will throw it up before the back part is acted on at
all. Or the right wing may encounter a wind of very different
velocity and trend to the left wing.”
In the aerial sea a machine will pitch and roll as does a ship upon the
water; and the man who would fly must learn to check his craft, should
it threaten to overturn; must be ready instantly with some system of
controlling gear so as to correct the influence of each driving gust.
And his task is made the harder because his machine, when struck
suddenly by a gust, may fall towards the earth at any angle. On the
road, when one learns to ride a bicycle, the machine will topple to one
side or the other; but a craft in the air may fall forward or backward
as well as from side to side, or partly forward and partly backward--or
may slip and dive at any possible angle, either forward or backward or
upon either side. A pioneer wrote, after his first experience in
learning to fly:
“It is rather like trying to steer a motor-car along an
exceptionally greasy road; you seem to slip all ways at once; and
to slip so quickly also that, unless you make the right balancing
movements without an instant’s delay, you find your machine has
gone beyond control.”
If he were to succeed, if he were to fly like a bird, then a man had to
learn this art of balancing himself in the air. Futile it was, as has
been shown, to build some powerfully-engined machine that no one could
control; futile also, and perilous as well, to make a pair of wings and
jump from a tower. Another way must be found, or the quest abandoned and
admitted hopeless. Here was the need; and here too, as we shall tell,
came the man; a man who was not famous, who worked without reward and
struggled to find time for his experiments; who died before he could see
the final triumph; yet who won a fame that cannot die, and whom men call
“the father of the aeroplane.”
To Germany one turns in telling the story of this man’s work. He was an
engineer, Otto Lilienthal by name, and from the days of his boyhood he
and his brother Gustav, living in Anklam, a small German town, were
builders of model aeroplanes and students of the flight of birds. When
the boys were thirteen and fourteen years of age respectively, they
designed a flying machine; and in describing it afterwards, Gustav
Lilienthal wrote:
“Our wings consisted of beech veneer with straps on the under
sides through which we pushed our arms. It was our intention to
run down a hill and to rise against the wind like a stork. In
order to escape the gibes of our schoolmates, we experimented at
night-time on the drill-ground outside the town; but there being
no wind on these clear, star-lit summer nights, we met with no
success.”
But they were not discouraged, and continued to build simple,
easily-constructed machines--from each of which, although it would not
fly, they learned a useful lesson. One, for instance, they made with
wings of goose feathers, sewn upon tape and fixed to wooden spars. These
wings, when finished, they fastened upon hoops which were strapped to
the operator’s chest and hips; and he could, by means of a lever and a
stirrup arrangement, beat the wings up and down by movements of his
legs. This machine they hung from a beam in an attic in their house; but
although the wings _did_ flap, and actually showed some tendency to
lift, the apparatus was soon consigned to a lumber-room, and they were
busy with plans for another.
What impressed Otto Lilienthal was the fact that, even when provided by
Nature with a perfect flying apparatus, the birds of the air had to
learn to use it. They could not just leap upward and “ride the wind” as
men had tried to do; they needed to take their first fluttering
flights--beating their wings anxiously and often falling back to earth,
because they did not know as yet how to use these wings. Particularly
did Lilienthal study the flight of storks. He obtained young birds from
neighbouring villages, and fed them in his garden with meat and fish
while he watched their efforts to learn to fly, and studied that
marvellous piece of mechanism--the wing Nature had given them. Writing
of his observations in a book he afterwards prepared, called _Birdflight
as the Basis of Aviation_, Lilienthal describes the antics of young
storks upon the lawn behind his house:
“When the actual flying practice begins, the first attention is
devoted to the determination of the wind direction; all the
exercises are practised against the wind, but since the latter is
not so constant on the lawn as on the roofs, progress is some-what
slower. Frequently a sudden squall produces eddies in the air, and
it is most amusing to watch the birds dancing about with lifted
wings in order to catch the wind which changes from one side to
another, all round. Any successful short flight is announced by
joyful manifestations. When the wind blows uniformly from an open
direction over the clearing, the young stork meets it, hopping
and running; then turning round, he gravely walks back to the
starting-point and again tries to rise against the wind.
“Such exercises are continued daily: at first only one single
wing-beat succeeds, and before the wings can be raised for the
second beat, the long, cautiously placed legs are again touching
ground. But as soon as this stage is passed, _i.e._ when a second
wing-beat is possible without the legs touching the ground,
progress becomes very rapid, because the increased forward
velocity facilitates flight, and three, four, or more double beats
follow each other in one attempt, maybe awkward and unskilled, but
never attended by accident, because of the caution exercised by
the bird.”
Lilienthal was fascinated by the mechanism of the bird’s wing. He and
his brother built one machine after another to determine the exact
amount of lifting effort that a man could obtain by imitating the
wing-beat of a bird. One such apparatus is illustrated in Fig. 19. This
had a double set of wings; a wide pair in the centre and narrower ones
in front and at the rear. These wings beat alternately, by movements of
the operator’s legs; and the machine was suspended by a rope and pulleys
from a beam, being counterbalanced by a weight. The tests showed this:
that, after some practice in working the wings, a man could raise with
them just half the weight of himself and of the machine; but the
muscular effort proved so great that he could only maintain this rate
of wing-beating for a few seconds. Here, incidentally, a fact may be
mentioned: the energy a man can produce, at all events for a prolonged
effort, has been estimated at about a quarter of a horse-power; and
this--in tests so far made--has been insufficient for the purpose of
wing-flapping flight. Lilienthal himself thought that, with some perfect
form of apparatus, a man might fly with an expenditure of 1·5 h.p. of
energy; but other experimenters have put the minimum power necessary,
even if mechanism could be devised, at 2 h.p. And another fact must be
remembered: even had Lilienthal been able, with such a machine, actually
to raise himself in the air, he would still have had the problem of
balancing himself, in addition to the working of his wings.
[Illustration: FIG. 19.]
[Illustration: FIG. 20.--Lilienthal Kite.]
After many tests such as these, carried out over a number of years,
during which the brothers grew from boys to men, Lilienthal decided that
no good results could be obtained unless a machine was made to move
forward through the air, instead of seeking to rise straight upward. By
such forward motion if rapidly made, and with a suitably shaped wing or
surface, he calculated that a definite support might be obtained from
the air, and without any great output of energy. The value of forward
motion is seen when a large bird seeks to rise. The first few flaps are
heavy and laboured; but after this, as soon as it begins to travel
forward, the wings exercise a lifting influence apart from their beat;
and, as the bird flies faster, so its wing-beats become less violent. An
instance of the need for a bird to move forward when it begins to fly,
is provided in the case, say, of a sparrow imprisoned in a chimney: even
if the chimney is wide, and there is plenty of room for the bird to fly
straight upward and escape, it has not the power to lift itself
vertically for any appreciable distance, because it cannot obtain the
lifting assistance of a leap forward through the air; it is in fact a
prisoner within the chimney.
Lilienthal studied the gliding or soaring flight of many birds; that
form of flight in which, with its wings outstretched and held almost
motionless, a bird such as the falcon will hover in the air, using no
apparent effort and yet supporting itself with ease; diving, rising
again, and wheeling in a perfect mastery of the medium in which it
moves. Lilienthal built and flew kites, to which he gave curved wings in
imitation of those of birds (Fig. 20). With one of these he obtained,
although only for a few moments, an actual gliding flight. The incident
is described by his brother Gustav:
“It (the kite) was held by three persons, one of whom took hold
of the two lines which were fastened to the front cane and to the
tail respectively, whilst the other two persons each held the
line which was fastened to each wing. In this way it was possible
to regulate the floating kite, as regards its two axes. Once,
in the autumn of 1874, during a very strong wind, we were able
to so direct the kite that it moved against the wind. As soon
as its long axis was approximately horizontal the kite did not
come down, but moved forward at the same level. I held the cords
controlling the longitudinal axis, and my brother and my sister
each one of the cords for the adjustment of the cross-axis. As the
kite maintained its lateral equilibrium, they let go the cords;
the kite then stood almost vertically above me and I also had to
free it. After another thirty steps forward my cords got entangled
in some bushes, the kite lost its balance, and in coming down was
destroyed. Yet, having gained another experience, we easily got
over the loss.”
From kites, in quest of a curved surface which should give a maximum
lift with a minimum resistance to its own passage through the air,
Lilienthal embarked upon a series of tests with wing shapes; setting
these up in the wind upon suitable recording machines, and noting
patiently the data that could be procured. There are many problems to be
considered when planning a wing for flight. If it is given a deep curve
or camber from front to back this may, while exercising a powerful lift,
offer too high a resistance as it passes through the air, and thus waste
the energy needed to propel the craft; or, if its front edge is dipped
too sharply, this may cause the air to act upon its upper surface, and
send a machine diving headlong to the ground. The planning of a
successful wing becomes a compromise, having for its object a surface
which shall give the greatest lifting influence with the least
resistance. Lilienthal, after much experiment and the examination of the
wings of many birds, decided that the curve, camber, or upward arch of a
plane should measure, at its maximum depth, about one-twelfth of
whatever width the plane might have from front to back.