The first words which may well be said upon this subject are to
emphasize caution. But by this I do not wish to imply that gliding is
exceedingly dangerous. Neither do I by caution mean timidity but rather
judgment and common sense.
Canoeing is generally considered a safe sport, but who would think of
canoeing on the ocean in a storm. It is exactly the same extreme to
glide from a very high object, or experiment in a high wind.
*The atmosphere* near the earth is a mass of whirling and swirling
currents which are constantly rising and falling and become very
pronounced in a high wind. Even in a comparative calm these eddy
currents exist but of course not to a dangerous degree. Evidence of this
may be seen by watching the little dust particles floating in the air
and made visible by a sunbeam coming through the window of a quiet room.
Although the sense of feeling cannot detect the smallest air current,
these little particles are whirling around and constantly changing their
direction.
When the wind strikes some natural object such as a tree or a stone, the
streams of air divide, part of them passing to the sides and part going
over the top. The air begins to divide some distance before it reaches
the object and the result is a rising current on one side and a falling
current on the other.
These currents are the bugbears of aviators for when one end of their
machine passes into such a current that end rises or falls depending
whether or not the current is rising or falling.
[Illustration: Fig. 26.—Top view, showing how streams of air divide.]
Other rising and falling currents are caused by the sun passing behind
clouds. Portions of the atmosphere are thus chilled and commence to fall
while others upon which the sun is reappearing are heated and rise.
Balloonists constantly encounter these changes in temperature and the
gas in the bag expands or contracts so rapidly that it often requires a
skillful pilot to prevent disaster.
These rising and falling currents caused by changes of temperature may
be clearly seen on the surface of a lake if the observer is stationed at
a height where he may look down on the water. In some places the water
is covered with smooth glassy streaks which run in various directions.
These smooth streaks are evidence of rising currents of air at those
places. The rough spots which suddenly spread out and run across the
water are caused by descending currents.
Therefore it is not good judgment to attempt gliding over ground broken
by trees or other natural objects or when the wind is blowing over 12-15
miles per hour.
[Illustration: Fig. 27.—Showing how air currents pass over objects.]
Do not under any consideration jump off from a height which rises
prominently from surrounding objects. Otto Lilienthal, the brilliant
German investigator and engineer who made over two thousand gliding
flights specifically warned experimenters against starting glides from
precipitous cliffs or buildings. There are two excellent reasons for
this. First, because when jumping from such an elevation, a gust of wind
rebounds from the sides and strikes the machine so that it requires
great skill to counteract its influence. Second, because, the operator
and machine are suddenly suspended high in the air.
Be satisfied at first by running against the wind on level ground and
making short jumps. After some practice, operations may be transferred
to a gentle slope and the length of the glides considerably increased.
If the experimenter thus proceeds slowly without impatience, there is no
danger in gliding. It is said that the Wright brothers never so much as
turned an ankle in the hundreds of flights they made, before building a
power driven machine.
[Illustration: Fig. 28—Action of aeroplane.]
*Action of an Aeroplane*. Before starting to glide it is perhaps well to
understand how the machine operates and supports its passenger. The
illustration shows the cross section of an aeroplane moving forward
through the air in the direction indicated by the arrow. The front edge
of the aeroplane is elevated so that the surfaces form an angle with the
horizontal. The front edge enters practically still air and causes it to
follow the curve of the planes and leave at the rear in a downward
direction. Since the action and reaction of two forces are always equal
and opposite, there is a force exerted against the aeroplane causing it
to rise.
A sky-rocket is caused to ascend by the reaction of gases formed by
burning powder escaping downwards through a small hole. The aeroplane,
by means of its curvature directs the air downwards and so rises itself.
The planes pass so rapidly on to new and undisturbed bodies of air, and
stay over one body for so brief an instant, that there is no time to
completely overcome the inertia of the air and force it downwards. This
may be likened to a skater moving swiftly over very thin ice which would
not bear his weight were he standing still, but since he is moving so
rapidly, that any one portion of the ice does not have time to bend to
the breaking point, is supported.
*Equilibrium.* A glider will remain in perfect equilibrium only so long
as the centre of gravity of the machine and operator fall in the same
vertical line as the pressure exerted by the air. If the former is
forward of the latter, the machine will incline forward and travel
downwards. If the centre of gravity is to the rear of the centre of
upward thrust exerted by the air, the head of the machine will rise,
while if it is to either the right or left side, the machine will lean
or turn over respectively to the right or left.
[Illustration: Fig. 29—Ready to Start]
The centre of pressure on the plane is somewhat in advance of the actual
dimensional centre of the plane. This is due to the curvature of the
plane and also to the disturbing action upon the air of the front edge.
*To make a glide*, carry the machine to the top of a slope. Have two
assistants hold the ends of the lower plane. Get in underneath and stand
up between the arm pieces. Grasp the front horizontal beam of the lower
plane and lift the machine until the arm sticks are snugly under the arm
pits as in the illustration.
If necessary have the two assistants prepared to run a short distance
with the machine, but as soon as you are in motion you will be relieved
of all weight and surprised at the lift exerted.
After getting the machine snugly up under the arm pits, face the wind,
elevate the front of the machine slightly, run a short, distance and
leap into the air. If you are in the right position you will sail to the
foot of the slope in free flight. To land, push yourself towards the
back of the machine, so that the glider tips upward slightly in front.
It will then rise slightly but loose its momentum and slowly settle so
that you drop gently on your feet.
*Balancing* is accomplished in flight by moving the legs and body
towards that side which is highest.
[Illustration: Fig. 30—Lines of Flight]
*Shifting the centre of gravity* by swinging, the legs forward or moving
the body in the same direction, will naturally cause the centre of
gravity to assume a forward position, and being a force exerted
downwards, the machine will dip and descend. A reverse movement of the
centre of gravity will cause the front of the machine to tip up and
ascend. But if the upward slant is continued too long the glider will
loose its forward velocity and settle.
The tendency is always to place the weight of the body too far to the
rear. After a little experience the experimenter will learn how to dip
his machine to acquire velocity for a rise and to otherwise handle it.
Fig. 29 illustrates two lines of flight in their successive stages. At 1
the operator is running along the top of the hill and the dotted line
from 1 to 2 represents his course immediately after leaving the ground.
In case the weight is back slightly too far and is not shifted much
during the glide, the machine will follow the upper line indicated by 3,
4, 5 and land at 6. If instead, at 2, the body is moved forward, the
machine will travel down as shown by 7 and approach the earth. Having
attained considerable velocity at _S_, the operator moves back and the
machine rises, travels upwards as at 9 and then settles about at the
point 6. This latter line of flight is to be preferred since the machine
does not rise quite so high in the air and moreover has more velocity so
that the operator may rise if necessary.
If during a flight a gust of wind strikes the machine from the front, it
will accelerate its vertical motion in regard to the earth. That is, if
the machine is already rising it will rise higher and if descending will
fall more quickly. A gust of wind from the rear will cause the machine
to drop suddenly and so always glide _into the_ wind.