aeroplanes-第4章
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each other in proportion to their mass。
Suppose; now; that these balls are placed two
inches apart;that is; twice the distance。 As
each is; we shall say; four pounds in weight; the
square of each would be 16。 This does not mean
that there would be sixteen times the attraction;
but; as the law says; inversely as the square of
the distance; so that at two inches there is only
one…sixteenth the attraction as at one inch。
If the cord of one of the balls should be cut; it
would fall to the earth; for the reason that the
attractive force of the great mass of the earth is
so much greater than the force of attraction in
its companion ball。
INDESTRUCTIBILITY OF GRAVITATION。Gravity
cannot be produced or destroyed。 It acts between
all parts of bodies equally; the force being
proportioned to their mass。 It is not affected by
any intervening substance; and is transmitted
instantaneously; whatever the distance may be。
While; therefore; it is impossible to divest matter
of this property; there are two conditions
which neutralize its effect。 The first of these is
position。 Let us take two balls; one solid and
the other hollow; but of the same mass; or density。
If the cavity of the one is large enough to receive
the other; it is obvious that while gravity is still
present the lines of attraction being equal at
all points; and radially; there can be no pull which
moves them together。
DISTANCE REDUCES GRAVITATIONAL PULL。Or
the balls may be such distance apart that the attractive
force ceases。 At the center of the earth
an object would not weigh anything。 A pound
of iron and an ounce of wood; one sixteen times
the mass of the other; would be the same;absolutely
without weight。
If the object should be far away in space it
would not be influenced by the earth's gravity;
so it will be understood that position plays an
important part in the attraction of mass for mass。
HOW MOTION ANTAGONIZES GRAVITY。The second
way to neutralize gravity; is by motion。 A
ball thrown upwardly; antagonizes the force of
gravity during the period of its ascent。 In like
manner; when an object is projected horizontally;
while its mass is still the same; its weight is less。
Motion is that which is constantly combating
the action of gravity。 A body moving in a circle
must be acted upon by two forces; one which tends
to draw it inwardly; and the other which seeks to
throw it outwardly。
The former is called centripetal; and the latter
centrifugal motion。 Gravity; therefore; represents
centripetal; and motion centrifugal force。
If the rotative speed of the earth should be retarded;
all objects on the earth would be increased
in weight; and if the motion should be accelerated
objects would become lighter; and if sufficient
speed should be attained all matter would fly off
the surface; just as dirt dies off the rim of a
wheel at certain speeds。
A TANGENT。When an object is thrown horizontally
the line of flight is tangential to the earth;
or at right angles to the force of gravity。 Such
a course in a flying machine finds less resistance
than if it should be projected upwardly; or directly
opposite the centripetal pull。
_Fig 1。 Tangential Flight_
TANGENTIAL MOTION REPRESENTS CENTRIFUGAL
PULL。A tangential motion; or a horizontal
movement; seeks to move matter away from the
center of the earth; and any force which imparts
a horizontal motion to an object exerts a centrifugal
pull for that reason。
In Fig。 1; let A represent the surface of the
earth; B the starting point of the flight of an object;
and C the line of flight。 That represents a
tangential line。 For the purpose of explaining
the phenomena of tangential flight; we will assume
that the missile was projected with a sufficient
force to reach the vertical point D; which
is 4000 miles from the starting point B。
In such a case it would now be over 5500 miles
from the center of the earth; and the centrifugal
pull would be decreased to such an extent that the
ball would go on and on until it came within the
sphere of influence from some other celestial
body。
EQUALIZING THE TWO MOTIONS。But now let us
assume that the line of flight is like that shown
at E; in Fig。 2; where it travels along parallel
with the surface of the earth。 In this case the
force of the ball equals the centripetal pull;or;
to put it differently; the centrifugal equals the
gravitational pull。
The constant tendency of the ball to fly off at
a tangent; and the equally powerful pull of
gravity acting against each other; produce a
motion which is like that of the earth; revolving
around the sun once every three hundred and
sixty…five days。
It is a curious thing that neither Langley; nor
any of the scientists; in treating of the matter of
flight; have taken into consideration this quality
of momentum; in their calculations of the elements
of flight。
_Fig。 2 Horizontal Flight_
All have treated the subject as though the
whole problem rested on the angle at which the
planes were placed。 At 45 degrees the lift and
drift are assumed to be equal。
LIFT AND DRIFT。The terms should be explained;
in view of the frequent allusion which
will be made to the terms hereinafter。 Lift
is the word employed to indicate the amount
which a plane surface will support while in flight。
Drift is the term used to indicate the resistance
which is offered to a plane moving forwardly
against the atmosphere。
_Fig。 3。 Lift and Drift_
In Fig。 3 the plane A is assumed to be moving
forwardly in the direction of the arrow B。 This
indicates the resistance。 The vertical arrow C
shows the direction of lift; which is the weight
held up by the plane。
NORMAL PRESSURE。Now there is another term
much used which needs explanation; and that is
normal pressure。 A pressure of this kind
against a plane is where the wind strikes it at
right angles。 This is illustrated in Fig。 4; in
which the plane is shown with the wind striking
it squarely。
It is obvious that the wind will exert a greater
force against a plane when at its normal。 On the
other hand; the least pressure against a plane is
when it is in a horizontal position; because then
the wind has no force against the surfaces; and
the only effect on the drift is that which takes
place when the wind strikes its forward edge。
_Fig。 4。 Normal Air Pressure_
_Fig。 5。 Edge Resistance_
HEAD RESISTANCE。Fig。 5 shows such a plane;
the only resistance being the thickness of the
plane as at A。 This is called head resistance;
and on this subject there has been much controversy;
and many theories; which will be considered
under the proper headings。
If a plane is placed at an angle of 45 degrees
the lift and the drift are the same; assumedly; because;
if we were to measure the power required
to drive it forwardly; it would be found to equal
the weight necessary to lift it。 That is; suppose
we should hold a plane at that angle with a heavy
wind blowing against it; and attach two pairs of
scales to the plane; both would show the same
pull。
_Fig。 6。 Measuring Lift and Drift_
MEASURING LIFT AND DRIFT。In Fig。 6; A is the
plane; B the horizontal line which attaches the
plane to a scale C; and D the line attaching it to
the scale E。 When the wind is of sufficient force
to hold up the plane; the scales will show the same
pull; neglecting; of course; the weight of the
plane itself。
PRESSURE AT DIFFERENT ANGLES。What every
one wants to know; and a subject on which a
great deal of experiment and time have been expended;
is to determine what the pressures are at
the different angles between the horizontal; and
laws have been formulated which enable the pressures
to be calculated。
DIFFERENCE BETWEEN LIFT AND DRIFT IN MOTION。The
first observation is directed to