aeroplanes-第6章

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supplies　the　true　answer。　The　answer　lies　in　the

angle　of　movement　of　every　wing　motion；　which

is　at　the　control　of　the　bird；　and　if　this　is　examined

it　will　be　found　that　it　supplies　the　correct

answer　to　every　type　of　wing　which　nature　has

made。



AN　INITIAL　IMPULSE　OR　MOVEMENT　NECESSARY。

Let　A；　Fig。　9；　represent　the　section　of　a　bird's

wing。　All　birds；　whether　of　the　soaring　or　the

flapping　kind；　must　have　an　initial　forward　movement

in　order　to　attain　flight。　This　impulse　is

acquired　either　by　running　along　the　ground；　or

by　a　leap；　or　in　dropping　from　a　perch。　Soaring

birds　cannot；　by　any　possibility；　begin　flight；

unless　there　is　such　a　movement　to　change　from　a

position　of　rest　to　one　of　motion。



_Fig。　9。　Wing　Movement　in　Flight。_



In　the　diagram；　therefore；　the　bird；　in　moving

forwardly；　while　raising　the　wing　upwardly；　depresses

the　rear　edge　of　the　wing；　as　in　position

1；　and　when　the　wing　beats　downwardly　the　rear

margin　is　raised；　in　relation　to　its　front　margin；

as　shown　in　position　2。



A　WEDGING　MOTION。Thus　the　bird；　by　a

wedge…like　motion；　gives　a　forwardly…propelling

action；　and　as　the　rear　margin　has　more　or　less

flexure；　its　action　against　the　air　is　less　during　its

upward　beat；　and　this　also　adds　to　the　upward　lift

of　the　body　of　the　bird。



NO　MYSTERY　IN　THE　WAVE　MOTION。There　is

no　mystery　in　the　effect　of　such　a　wave…like　motion；

and　it　must　be　obvious　that　the　humming

bird；　and　like　flyers；　which　poise　at　one　spot；　are

able　to　do　so　because；　instead　of　moving　forwardly；

or　changing　the　position　of　its　body　horizontally；

in　performing　the　undulatory　motion　of


the　wing；　it　causes　the　body　to　rock；　so　that　at　the

point　where　the　wing　joins　the　body；　an　elliptical

motion　is　produced。



_Fig。　10。　Evolution　of　Humming…Bird's　Wing。_





HOW　BIRDS　POISE　WITH　FLAPPING　WINGS。This

is　shown　in　Fig。　10；　in　which　eight　successive　positions

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