“THE BIRD MAN”
Construction of an artificial hill--The building of
larger craft--Peril of gusty winds--The accident
which caused Lilienthal’s death.
So determined was Lilienthal to obtain the best conditions for his
gliding that, finding no natural slope to meet his purpose, he ordered
the construction of an artificial hill. This was built at
Gross-Lichterfelde, near Berlin, and was 50 feet high and had gently
sloping sides from which, at any direction of the wind, he could make a
soaring flight. On the top of the hill, which is illustrated in Fig. 25,
Lilienthal had a roomy chamber, and in it he stored his craft. In this
illustration, also, the airman may be observed standing upon the
hilltop, ready for a trial. By means of dotted lines, and
representations of machines in flight, it is possible to show how he
glided through the air.
In the upper of the two flights shown, profiting by a day when there
were rising currents in the wind, Lilienthal had allowed himself to be
lifted, for a moment or so, to a point in the air actually higher than
that from which he started. Then, in order to obtain forward speed, he
dived, only to incline his wings more steeply again, and allow the wind
to bear him upward. In this way, by exercising skill in the balancing
of his machine, he was able to prolong a glide, and under favourable
conditions to traverse, before touching ground, a distance through the
air of nearly 1000 feet.
[Illustration: FIG. 25.--Lilienthal’s artificial hill.]
In the second of the glides shown in Fig. 25, Lilienthal is making a
swift, low flight--one of those during which he was never far from the
ground, and with which he contented himself in early tests. On glancing
again at this sketch it will be noted that the upper of the craft shown
has two main sustaining wings, placed one over the other in the girder
construction already described. It is in fact a biplane, whereas the
lower machine is a monoplane--such a craft as was illustrated in Fig.
22. It was when Lilienthal became expert at balancing himself in the air
that he built a machine on the biplane principle. His reason for doing
so was that he required more surface in the sustaining wings, so that
they might carry him farther through the air. What he wanted to do in
each of his glides was to remain in the air as long as possible, and
thus gain a maximum of experience. The difficulty, as explained, was to
obtain enough practice. In five years, for instance, although assiduous
in his experiments, Lilienthal was not more than five hours in the air.
[Illustration: FIG. 26.--Lilienthal’s Biplane Glider.]
Thus it was that he built a biplane, each of the wings being 18 feet in
span, and containing 100 square feet of surface. The craft is shown in
Fig. 26. Lilienthal did with it what he expected he would be able to do;
he increased materially the length of his glides. But there were
drawbacks in the use of this machine, and one introduced an element of
danger. With the monoplane, so soon as he made balancing movements by
instinct, Lilienthal found he could control his craft quite well; it was
small, and responded quickly when he threw the weight of his body from
side to side. But the biplane, being considerably larger, and having
more surface upon which the wind could act, was sluggish in its response
to his controlling movements. In the case of both monoplane and biplane,
Lilienthal relied merely upon the weight of his body to counteract
falling movements. In the biplane, therefore, although it required a
greater leverage to restore its balance, he was unable to increase the
correcting influence. This difficulty, in the use of a large machine,
was faced subsequently by the Wrights, and how they solved it will be
shown.
Lilienthal recognised the position, of course, and saw there might be
peril in the use of a biplane; but he was content, none the less, to
rely upon his skill. In each glide he made he became more expert;
instead of allowing his machine to slip to the ground when struck by a
gust, he restored its equilibrium by an instantaneous movement of his
body: he was, in fact, like a man who had learned to ride a
bicycle--balancing himself without pausing to think what he should do.
But Lilienthal, in his navigation of the air, was facing a danger the
cyclist has not to fear. He was braving dangerous wind-gusts; and he did
not know, and had no means of knowing, with just what strength these
gusts would strike his craft. Also--and this too was a danger no man on
earth need fear--he had empty air below him should he fall. The peril
grew greater as his skill increased, because he soared higher, and left
greater distances between him and the ground below. In another way,
also, he courted greater risk; and this was through gliding in stronger
winds. At first, when he was unskilled, he had cared only to brave a
wind of a velocity, say, of 10 or 15 miles an hour. But soon, feeling
his balancing power grow greater, he ventured into the air when there
was a wind of from 20 to 25 miles an hour.
In regard to this question of the strength of the wind, uncertainty
often exists. What, for example, is a “stiff breeze”? What is a “strong
wind”? And at what velocity must the wind blow before it is called a
gale? Such questions are often asked, and the table below should prove
instructive:
Velocity of wind Description given
(miles per hour). such strength.
0-5 Calm.
6-10 Light air.
11-15 Light breeze.
16-20 Gentle breeze.
21-25 Moderate breeze.
26-30 Fresh breeze.
31-36 Strong breeze.
37-44 Moderate gale.
45-53 Fresh gale.
54-60 Strong gale.
61-69 Whole gale.
70-80 Storm.
80 upwards Hurricane.
[Illustration: FIG. 27.--Lilienthal combating a wind-gust.]
In gliding in a breeze, say of 25 miles an hour, Lilienthal had to face
this danger, and it is one all airmen meet: whereas the average strength
of a wind may be maintained at 25 miles an hour, there is no assurance
that there will not be a sudden and heavy gust of a greater force than
this. Sometimes, when the wind is uncertain, there will come a gust
which has double the force of the normal pressure; and such a gust,
sweeping unexpectedly against an aircraft, threatens to blow it over and
send it headlong to the ground. Thus Lilienthal, having no more control
over his machine than could be brought to bear by movements of his
body, was running a considerable risk when he soared in gusty
winds--particularly if using the biplane form of craft. Sometimes, when
struck by a gust, his glider would heel and assume a dangerous position
in the air such as is illustrated in Fig. 27. Here the craft threatens
to fall backwards and partly sideways; and the operator can be seen
throwing his body and legs forward, in a quick effort to check this
overturning impulse.
One incident, indicating the risks Lilienthal ran, should be mentioned:
he was gliding 50 feet high one day, in a fresh wind, when one of the
wooden arm supports, which he gripped while in flight, broke suddenly
and threw his craft out of balance. The machine, before he could right
it, fell heavily to the ground; but, thanks to the shock-absorber below
the wings, Lilienthal escaped with nothing worse than bruises.
He had set himself to master this art of balance, and master it he did,
and was ready to risk his life in so doing. In the year 1896 he was bold
enough to glide from hills 250 feet high; and from such a height he
would come sweeping through the air, often traversing before alighting a
distance of 750 feet. Sometimes, too, on a day when the wind was high,
he would stand upon the hilltop and allow the wind pressure under his
wings to raise him in the air; then, throwing his weight forward, he
would start his craft on a downward glide. Frequently, when
experimenting in strong winds, he would find himself higher than his
starting-point, and would hang almost motionless for a moment or so,
soaring in the air. But such hovering flight, though he practised
assiduously, he found difficult to maintain. He could not keep his
machine poised in an ascending current of wind; he had not that instinct
of the birds which enables them to profit instantly by each rising gust,
and hold themselves in it as they allow it to bear them upward. Soaring
flight has a fascination for those who study the navigation of the air;
but no man, as yet, has been able to indulge in it for more than the
briefest space of time. It is only possible to hover thus, without
effort or the use of a motor, when the forces that govern a machine are
exactly in balance; that is to say, the power of gravity which is
pulling downwards must be balanced perfectly by the strength of the
wind, which is blowing under the planes of the machine and tending to
force it upward. Should the wind fail in its thrust, then the craft will
move forward and downward; should the wind blow more strongly, then it
will drive the machine backward, and tend to throw it out of
equilibrium. Some birds, profiting by the skill they have in the minute
adjustment of their wings, are able to hover over a given spot at will,
remaining motionless in the air, without flap or visible effort.
But Lilienthal, although he never attained such proficiency as might
enable him to soar indefinitely above the hill from which he sprang, was
always confident that some perfect glider would be invented, and men
thus be able to imitate the birds. It was when writing upon this problem
of soaring flight that he expressed the thought:
“It is not to be wondered at that birds are able to perceive the
slightest variations in the movements of the air, because the
whole of their body surface acts as an organ for this sensation;
the long and widely extended wings constitute a sensitive feeling
lever, and minute sensibility will be particularly concentrated in
the follicles from which the feathers issue, just as is the case
with our finger-tips.... Should it ever become possible for man
to imitate the splendid sailing movements of birds, he will not
require to use steam engines or electro-motors for the purpose;
a light, properly shaped, and sufficiently moveable wing, and
the necessary practice in its manipulation is all that will be
required of him. He should, unconsciously, be able to draw the
greatest advantages from whatever wind may be blowing, by properly
presenting the wings.”
When three years of gliding lay behind him, Lilienthal thought he could
go little farther in this research. He was able to balance himself in
the air; he could glide in high winds; but always, seeing that he had no
motive power with which to drive his craft, he must start from a hilltop
and descend to the ground. Now he sought longer and bolder flight; and
so he and his brother discussed the building of an engine which might
propel a glider through the air. No petrol motor, unfortunately for
Lilienthal, was then available; so he planned to construct a carbonic
acid motor, and make this drive his craft by flapping the ends of its
wings.
During the summer of 1896 he was busy with plans for this motor, while
still continuing his flights; but in August he decided to cease gliding
for a while, and await a test of the power-driven craft. So on Sunday,
August 9th, he said he would travel out to Stollen, make one or two
final flights, and pack up his machine. His brother Gustav was to have
accompanied him as usual, but had a mishap with his bicycle, and so
remained behind. What happened is described, in few but expressive
words, by Gustav Lilienthal:
“Our families, whom we had intended to take with us, remained
at home, and my brother drove out, accompanied by a servant.
He intended to make some change on the rudder, but at the very
first glide, the wind being uncertain, the apparatus, when at a
considerable height, lost its balance. Unfortunately my brother
had not fitted the shock-absorber, and the full shock of the fall
took effect, so that the apprehension of our uncle was fulfilled.
My brother fell, a victim to the great idea which--although at
that time so little recognised--is now acknowledged in its full
bearing by the whole civilised world.”
At the moment his machine lost balance, Lilienthal was more than 100
feet in the air. Striking the ground with great violence, he sustained
injuries which were almost immediately fatal. In his work, to which he
sacrificed his life, he had met with no encouragement or recognition. He
suffered the fate of pioneers; his theories were so far ahead of his
time that folk did not grasp their significance. Little interest seems
to have been taken in his glides; there was no sensation; there were no
crowds. Nobody, in fact, realised what he was doing, or appreciated the
vast importance of these seemingly simple tests. But in the years that
followed, when other men came to grips with the problem as Lilienthal
had done, when they were able to use the data he had compiled and to
profit by his experiences in actual flight, then this pioneer came into
his own.
His work, summarised, may be said to lie in this: he provided a
stepping-stone to power-driven flight. He showed men that they should
learn to balance themselves in the air _before_, and not _after_, they
had built themselves costly craft. How his example acted as a spur upon
others, and how the work he had begun was carried to its triumph, will
be the purpose of our next chapters to show.[1]
[1] In addition to the work of the Wright brothers, which was carried to
the triumph of actual flight--and with which we shall concern
ourselves--valuable gliding tests were made in England by Filcher, and
in America by Chanute.