按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
of the wing are shown； and wherein four
of the position； namely； 1； 2； 3； and 4； represent
the downward movement； and 6； 7； 8； and 9； the
upward beat。
All the wing angles are such that whether the
suspension point of each wing is moving downwardly；
or upwardly； a support is found in some
part of the wing。
NARROW…WINGED BIRDS。Birds with rapid flapping
motions have comparatively narrow wings；
fore and aft。 Those which flap slowly； and are
not swift flyers； have correspondingly broader
wings。 The broad wing is also typical of the
soaring birds。
But how do the latter overcome gravitation
without exercising some sort of wing movement？
INITIAL MOVEMENT OF SOARING BIRDS。Acute
observations show that during the early stages
of flight； before speed is acquired； they depend
on the undulating movement of the wings； and
some of them acquire the initial motion by flapping。
When speed is finally attained it is difficult
for the eye to note the motion of the wings。
SOARING BIRDS MOVE SWIFTLY。Now； the first
observation is； that soaring birds are swiftly…
moving creatures。 As they sail overhead
majestically they seem to be moving slowly。 But
distance is deceptive。 The soaring bird travels
at great speeds； and this in itself should be sufficient
to enable us to cease wondering； when it is
remembered that swift translation decreases
weight； so that this factor does not； under those
conditions； operate against flight。
MUSCULAR ENERGY EXERTED BY SOARING BIRDS。
It is not conceivable that the mere will of the
bird would impel it forwardly； without it exerted
some muscular energy to keep up its speed。 The
distance at which the bird performs this wonderful
evolution is at such heights from the observer
that the eye cannot detect a movement。
WINGS NOT MOTIONLESS。While the wings appear
to be absolutely motionless； it is more reasonable
to assume that a slight sinuous movement；
or a rocking motion is constantly kept up； which
wedges forwardly with sufficient speed to compel
momentum to maintain it in flight。 To do so requires
but a small amount of energy。 The head
resistance of the bird formation is reduced to a
minimum； and at such high speeds the angle of
incidence of the wings is very small； requiring but
little aid to maintain it in horizontal flight。
CHAPTER II
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 ELEMENTSEvery 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