PRINCIPLES OF AEROPLANE FLIGHT
FROM the foregoing chapter, while it may be
rightly inferred that power is the true secret of
aeroplane flight, it is desirable to point out certain
other things which must be considered.
SPEED AS ONE OF THE ELEMENTS--Every boy,
probably, has at some time or other thrown small
flat stones, called "skippers." He has noticed
that if they are particularly thin, and large in
diameter, that there is a peculiar sailing motion,
and that they move through the air in an undulating
or wave-like path.
Two things contribute to this motion; one is the
size of the skipper, relative to its weight, and the
other is its speed. If the speed is slow it will
quickly wend its way to the earth in a gradual
curve. This curved line is called its trajectory.
If it is not very large diametrically, in proportion
to its weight, it will also make a gradual curve in
descending, without "skimming" up and down
in its flight.
SHAPE AND SPEED.--It has been observed, also,
that a round ball, or an object not flattened out,
will make a regular curved path, whatever the
speed may be.
It may be assumed, therefore, that the shape
alone does not account for this sinuous motion;
but that speed is the element which accounts for
it. Such being the case it may be well to inquire
into the peculiar action which causes a skipper
to dart up and down, and why the path thus
formed grows more and more accentuated as the
speed increases.
As will be more fully described in a later chapter,
the impact of air against a moving body does
not increase in proportion to its speed, but in the
ratio of the square of the speed.
WHAT SQUARE OF THE SPEED MEANS.--In mathematics
a figure is squared when it is multiplied
by itself. Thus, 4 X 4= 16; 5 X 5 = 25; and so
on, so that 16 is the square of 4, and 25 the square
of 5. It has been found that a wind moving at the
speed of 20 miles an hour has a striking or pushing
force of 2 pounds on every square foot of surface.
If the wind travels twice as fast, or 40 miles
an hour, the pushing force is not 4 pounds, but
8 pounds. If the speed is 60 miles an hour the
pushing force increases to 18 pounds.
ACTION OF A SKIPPER.--When the skipper leaves
the hands of the thrower it goes through the air
in such a way that its fiat surface is absolutely
on a line with the direction in which it is projected.
At first it moves through the air solely by force
of the power which impels it, and does not in any
way depend on the air to hold it up. See Fig.
1, in which A represents the line of projection,
and B the disk in its flight.
_Fig. 11. A Skipper in Flight._
After it has traveled a certain distance, and
the force decreases, it begins to descend, thus describing
the line C, Fig. 1, the disk B, in this case
descending, without changing its position, which
might be described by saying that it merely settles
down to the earth without changing its plane.
The skipper still remains horizontal, so that as
it moves toward the earth its flat surface, which
is now exposed to the action of the air, meets
with a resistance, and this changes the angle of
the disk, so that it will not be horizontal. Instead
it assumes the position as indicated at D,
and this impinging effect against the air causes
the skipper to move upwardly along the line E,
and having reached a certain limit, as at, say E,
it automatically again changes its angle and moves
downwardly along the path F, and thus continues
to undulate, more or less, dependent on the combined
action of the power and weight, or momentum,
until it reaches the earth.
It is, therefore, clear that the atmosphere has
an action on a plane surface, and that the extent
of the action, to sustain it in flight, depends on two
things, surface and speed.
Furthermore, the greater the speed the less the
necessity for surface, and that for gliding purposes
speed may be sacrificed, in a large measure,
where there is a large surface.
This very action of the skipper is utilized by
the aviator in volplaning,--that is, where the
power of the engine is cut off, either by accident,
or designedly, and the machine descends to the
earth, whether in a long straight glide, or in a
great circle.
As the machine nears the earth it is caused to
change the angle of flight by the control mechanism
so that it will dart upwardly at an angle, or downwardly,
and thus enable the pilot to sail to another
point beyond where he may safely land.
This changing the course of the machine so that
it will glide upwardly, means that the incidence
of the planes has been changed to a positive
angle.
ANGLE OF INCIDENCE.--In aviation this is a term
given to the position of a plane, relative to the
air against which it impinges. If, for instance,
an aeroplane is moving through the air with the
front margin of the planes higher than their rear
margins, it is said to have the planes at a positive
angle of incidence. If the rear margins are
higher than the front, then the planes have a negative
angle of incidence.
The word incidence really means, a falling
upon, or against; and it will be seen, therefore,
that the angle of incidence means the tilt of the
planes in relation to the air which strikes it.
Having in view, therefore, that the two qualities,
namely, speed and surface, bear an intimate
relation with each other, it may be understood
wherein mechanical flight is supposed to be analogous
to bird flight.
SPEED AND SURFACE.--Birds which poise in the
air, like the humming bird, do so because they
beat their wings with great rapidity. Those
which soar, as stated, can do so only by moving
through the atmosphere rapidly, or by having a
large wing spread relative to the weight. It will
thus be seen that speed and surface become the
controlling factors in flight, and that while the
latter may be entirely eliminated from the problem,
speed is absolutely necessary under any and
all conditions.
By speed in this connection is not meant high
velocity, but that a movement, produced by power
expressed in some form, is the sole and most necessary
requisite to movement through the air with
all heavier-than-air machines.
If sufficient power can be applied to an aeroplane,
surface is of no consequence; shape need
not be considered, and any sort of contrivance
will move through the air horizontally.
CONTROL OF THE DIRECTION OF FLIGHT.--But the
control of such a body, when propelled through
space by force alone, is a different matter. To
change the machine from a straight path to a
curved one, means that it must be acted upon by
some external force.
We have explained that power is something
which is inherent in the thing itself. Now, in order
that there may be a change imparted to a
moving mass, advantage must be taken of the medium
through which it moves,--the atmosphere.
VERTICAL CONTROL PLANES.--If vertically-arranged
planes are provided, either fore or aft of
the machine, or at both ends, the angles of incidence
may be such as to cause the machine to
turn from its straight course.
In practice, therefore, since it is difficult to supply
sufficient power to a machine to keep it in motion
horizontally, at all times, aeroplanes are provided
with supporting surfaces, and this aid in
holding it up grows less and less as its speed increases.
But, however strong the power, or great the
speed, its control from side to side is not dependent
on the power of the engine, or the speed
at which it travels through the air.
Here the size of the vertical planes, and their
angles, are the only factors to be considered, and
these questions will be considered in their proper
places.