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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

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