GLIDING FLIGHT
How a man may use gravity as a motor--Theory of the
“glider”--The craft Lilienthal built--A problem of
balance--The centres of gravity and pressure.
Up to this point in his research Lilienthal had moved more or less upon
the lines of other experimenters. Had he continued to follow in their
footsteps, he would have planned some large and impracticable
machine--and perhaps gone no further. But although he desired to test
the lifting power of the planes he had built, Lilienthal had always in
mind this vital fact--that a man must learn to balance himself in the
air before he can hope to fly. His own words, in summing up this
problem, were: “stability first; propulsion afterwards”; and by this he
meant a man must acquire the art of handling a craft in the air, before
he dares to fit a motor and attempt power-driven flight.
But if a man used neither wing-beats nor a motor to drive him through
the air, how was such practice to be obtained? Lilienthal solved the
problem--and made his name immortal--by devising a system in which he
used the force of gravity as his motor. His plan was this: first he
would build a pair of large, light wings--so light in fact that, even
with the woodwork that was in them and with the additional weight of a
balancing tail, he could raise them to his shoulders and run forward.
With these wings he would go to the summit of a sloping hill and face
what wind might be blowing--as he had seen the young storks do. Then he
would run forward with his wings, so as to obtain the lifting influence
necessary before they could act upon the air. And then, when the wind
was sweeping under his curved wings, he would raise his legs from the
ground and seek to soar or glide; his own weight, and that of his
machine, providing a gravity motor or downward pulling influence, while
the sustaining power of his planes, resisting this drag, would send him
gliding through the air, only a few feet from the ground, at an angle
which tended gradually earthward.
[Illustration: FIG. 21.
Paper glider, in which the cardboard weight (A.) should be 3/10 inch
wide, and 1/16 inch thick, slightly arch planes upward (B.B.). Turn up a
little flap at each end (C.C.). An eighth inch is sufficient. Hold
between finger and thumb (the cardboard weight uppermost); then allow to
dive, as indicated by the dotted line.]
This power of a weighted plane to glide, even when no motive power is
attached to it, may be demonstrated quite simply by the little paper
model seen in Fig. 21. If, when you have made this, you allow it to
flutter from your hand without any weight attached, the model plunges,
dips, and dives; it has no forward motion, therefore it has no stability
or poise. But when you gum the small cardboard weight to its fore-plane,
the action of the model is changed. By the use of this tiny strip of
cardboard you have, so to say, given it an engine; you have provided it
with means whereby it can obtain forward motion, and so glide through
the air. When you hold it as shown in the sketch, with the weighted
fore-plane tilted downward, and release it without a jerk, the tendency
for the model is to fall to the ground as it did before. But now there
is the weight to reckon with: this pulls the model forward and downward,
tending to fall more quickly, of course, than the paper by itself would
do. But there is also the plane behind the falling weight to be taken
into consideration: jerked forward and downward through the air, this
begins to exercise a sustaining influence, and so resists the falling
movement of the weight. Still the weight, actuated by the force of
gravity, pulls downward. But the plane refuses to fall sheer to the
ground; and yet the weight must have its way. So, as in most situations
of this kind, there is a compromise. The falling weight pulls; the
plane resists; and in a flash the model starts upon a graceful glide.
Its plane is fulfilling its task of bearing it through the air; and the
weight is carrying out its mission also, in causing the glide to tend
earthward. So, pulled down by its weight and yet partly sustained by its
plane, the model will pass across a room; and if its plane and its
weight are in a nice adjustment, one may see a pretty manœuvre before it
reaches the floor. As it is swept faster through the air, owing to the
increasing drag of the weight, the plane of the model acquires a greater
lifting influence; and the moment comes when this “lift,” reaching a
maximum, checks altogether the descending movement, and causes the model
actually to ascend. Up indeed it goes, for a second, in a sudden swerve.
But this ascending impulse is soon checked; gravity cannot be denied.
The model loses speed; and, as it loses speed, so does its plane lose
lift. Hence the weight is again the predominating partner; it pulls down
the fore-plane, converts the rise into a fall, and brings the model with
another dive to the floor.
But here, at all events, is a demonstration of this theory of gliding
flight--one that can be carried out without a motor. Of course such
flying has its restrictions: a man must start from the summit of a hill,
and the glide is in the form of a descent towards the ground below; but
still he is passing through the air; and above all--and this proved the
advantage of the scheme for such a pioneer as Lilienthal--there is no
need, during any such glide, to pass high above the ground. The
operator may, in fact, if the side of his hill slopes gently, skim
within only a few feet of its surface; and this means that, should he
lose his balance at first, as he may expect to do, he will not share the
fate of those who leapt from towers, but will be able to alight without
mishap.
[Illustration: FIG. 22.--Lilienthal Glider.]
This, then, was Lilienthal’s plan; he would build a machine with wings
and a tail, stand facing the wind as the storks had done, then seek to
glide through the air in the manner of a soaring bird. The idea
underlying his scheme was that he hoped, by a series of such gliding
flights, to learn the adjusting movements he knew would be necessary to
preserve his balance in the air. The gliding craft Lilienthal built, as
illustrated in Fig. 22, has become an historical machine. The framework
of its wings and the supports of its tail were of willow, and the wings
and tail, to give them their grip upon the air, were covered with a
smoothly stretched fabric. Then the whole structure was braced and
tightened; and though it weighed less than 50 lbs., it was strong enough
to bear its operator though the air. Lilienthal could raise the
apparatus upon his shoulders--passing his head through the aperture
between the planes, which will be noted in the sketch--and walk or run
forward; and to hold the machine, as he carried it thus, he gripped two
wooden rods. The tail was flexible, being allowed an automatic movement,
thus giving the craft a certain natural stability. The main wings had a
span of 24 feet, and the machine measured 18 feet from front to tail.
The wings were cambered, according to the curve Lilienthal had decided
most efficient, and contained about 180 square feet of lifting surface.
In giving them this area, Lilienthal was relying upon experiments he had
made; these showed that, as his machine glided through the air, each
square foot of its surface should bear a weight equal to about 1 lb.
Although enthusiastic, Lilienthal was not impatient: he had the
priceless gift of judgment, allied to common-sense. So, when he had his
glider built, he made no wild nor dangerous tests. He contented himself,
in fact, with a leap from a springboard no more than 3 feet high; and
this height he increased gradually to 8 feet. By such humble beginnings,
and without risking his life, he proved that his glider would sustain
his weight in the air; or, to be more precise, that its wings would
exercise a lift sufficient to permit him to glide rather than fall to
the ground. So now he began more elaborate tests, seeking hills which
had gently-sloping sides, so that he might glide down them. But with
many the difficulty was this: the winds near the surface, being broken
and disturbed, blew fitfully and in gusts, while what Lilienthal needed
was a steady, uniform wind. At length he found favourable conditions at
some gravel-pits at Südende; and here, on the brink of a pit, he built a
shed and housed his gliders.
[Illustration: PLATE II.--SANTOS-DUMONT’S FIRST FLIGHTS.
Here, actually in the air, and with its pilot clearly visible at the
controls, is seen the craft in which--at Bagatelle in 1906--the airman
flew a distance of 230 yards.]
Now, patient and assiduous, he began to teach himself the art of aerial
balance. Raising his wings to his shoulders he would face the
wind--which in his first tests he did not care to be blowing at more
than ten or fifteen miles an hour. Then, running against the wind to
increase the pressure beneath his wings, he would raise his legs and
begin to glide, moving forward and at the same time downward. How he
appeared when in flight is indicated by Fig. 23.
[Illustration: FIG. 23.--Lilienthal gliding.]
His first tests were brief, for the reason that his craft would either
dip too sharply, or incline its planes steeply and so check its forward
speed. In either event the result was the same: the glide came to an
end. But Lilienthal’s caution saved him from being injured in an
involuntary descent. It must be remembered he was always moving close to
the earth; therefore he had only a short distance to fall. To safeguard
himself still further he fitted below his machine a shock absorber,
which came into contact with the ground first and lessened the force of
any impact.
But the difficulties of preserving his balance were great, as he had
foreseen; for not only did his glider dip down, or rear itself up, but
also--under the influence of wind-gusts--threatened to slip sideways. It
was Wilbur Wright, lecturing afterwards upon problems of aerial
equilibrium, who said crisply:
“The balancing of a gliding or flying machine is very simple in
theory; it merely consists in causing the centre of pressure to
coincide with the centre of gravity; but in actual practice there
seems to be an almost boundless incompatibility of temperament
between the two, which prevents their remaining peaceably together
for a single instant.”
[Illustration: FIG. 24.--Centres of gravity and pressure.]
Here, in a sentence, is the problem. As a cambered plane is moved in
flight, the air-pressure upon it is not disposed equally over the
surface, but tends to locate itself at a spot to the front of the middle
line of the plane. When a plane is at a normal inclination to the air,
indeed, this centre of pressure, as it is called, is at a point upon the
surface about one-third of the distance between the front and rear
edges. As to the centre of gravity, the second factor in the problem,
this may be explained best, perhaps, by a practical illustration. Take a
small sheet of cartridge paper, cut to represent the plane of a flying
machine, and lay this along the blade of a knife. By moving it to and
fro and adjusting its equilibrium, you will be able to make it rest upon
the knife-edge without falling forward or backward; this point at which
it balances itself represents its centre of gravity. Here, then, are the
forces: the centre of pressure, which is the thrust of the air, and the
centre of gravity, which is the pull of the earth seeking to drag down a
machine when in flight. These two forces must, as we have been told, be
made to coincide. In the next illustration (Fig. 24) the problem is made
clearer. In diagram A is seen a plane A.B. which is moving through the
air in the direction indicated by the arrow. The two forces--that is to
say, the centre of pressure (C.P.) and the centre of gravity
(C.G.)--coincide with each other: therefore the plane is in equilibrium.
But now suppose a gust of wind strikes the plane. This tends to tilt it
upward; and the result is that the centres of pressure and gravity show
that “boundless incompatibility of temperament” of which Wilbur Wright
complained. The impact of the gust, making the plane rear up, throws the
centre of pressure farther back along its surface, as is shown in
diagram B. The plane is at once out of balance. Or it may be argued
that, as it passes through the air, the wind pressure under the plane is
suddenly lessened. This would cause its front edge to drop; whereupon
the centre of pressure would, as is seen in diagram C, move immediately
forward upon the plane--and so throw it out of balance again.
This is the problem of the man who would navigate the air. He launches
himself in a treacherous, unstable element: constantly, beneath his
wings, the air pressure changes and varies in its strength; constantly
is he losing his balance--and as constantly must he regain it. Imagine a
man walking a tight-rope, and seeking incessantly to keep himself in
equilibrium, and you have a notion of what the first man faced when he
strove to fly. And his case, really, was worse than that of the
tight-rope walker. The latter is concerned mainly with the danger of
falling to one side or the other; he need not trouble himself unduly
with the problem of his fore and aft stability. But the aerial acrobat
may fall forward or backward, or from side to side. Hence his trick,
once he masters it, is the more skilful.
The art, as has been shown, is to bring together these centres of
gravity and pressure; and it can be done in either of two ways. One is
to alter the centre of gravity should the machine begin to fall, and the
other to move the centre of pressure. Lilienthal, and others of the
early gliders, adopted the plan first mentioned; they shifted the centre
of gravity. But others who followed them, and notably the Wright
brothers, finding that they needed to build larger craft, made use of
movable planes by which they could shift the centre of pressure; but
this, of course, will be dealt with in its place.
To alter the centre of gravity of a machine it is necessary to move in
some way the weight it carries--to shift the load forward or backward,
say, or from side to side. In Lilienthal’s glider the load was the
weight of his own body, and he learned to move this when wind-gusts
struck his craft. His body, as he passed through the air in flight, hung
free from the shoulders below the wings of his machine; he was therefore
able to swing himself forward or backward, or from side to side. And
this he did, counteracting the rolling movements of his machine, and
seeking always to prolong the glide. Should his craft be struck by a
sudden gust, for example, and heel to one side, he swung the weight of
his body towards the rising wing; should he dive abruptly, or threaten
to rise at an acute angle, he was ready with a movement of his body to
check the falling tendency and restore the machine to an even keel. But
the point to be considered is this: all these movements, when a craft is
in flight, have to be made with a lightning rapidity. There is not an
instant to lose; not a fraction of a second to be wasted while a man
thinks what he is to do. His balancing, if he would glide through the
air with wings, must be instinctive--instantaneous; as, indeed, is the
balancing of the birds. Here, then, is the difficulty: to learn to make
these balancing movements with sufficient quickness; and this Lilienthal
found to be the stumbling-block. Time after time, while gliding close to
the ground, his machine lost its balance and, before he could correct
the slip or dive, had come to earth. But these falls did not hurt him,
nor did they damage his machine; so he was able, like the storks, to try
again and again.
It is not easy to realise this difficulty of learning to fly. The first
airmen found their rate of thinking too slow. For all earthly actions
they could think quickly enough; but when they came to pass through the
air they found the sending of a command from their brains to their limbs
was not done fast enough. They found they could not rely upon _thinking_
what to do when a craft threatened to fall. They had to practise until
they acquired the power of making a balancing movement without thinking
at all; they learned, that is to say, to keep their equilibrium by
sub-conscious movements--or, to use a simpler word, by “instinct”; to
balance themselves as they passed through the air, like a man balances
himself when he rides a bicycle, without giving the action a thought.
Lilienthal probed all these difficulties, and saw that--as with other
problems--it was not so much brilliant daring that would bring him
success, as a painstaking course of practice, along right and sensible
lines. So, whenever the weather was favourable and the wind not too
high, he made his running leaps down the sides of hills, being content
as a rule, in all his early trials, if he remained only a second or so
in the air. Here, indeed, was another difficulty of learning to fly. No
experience was possible unless a machine was in flight; and yet, in
making his first tests, Lilienthal had to be content with a second’s
practice here and a second there; to be glad in fact if, after a whole
month’s work, he had been for one clear minute in the air.