aeroplanes-第5节

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　Fig。　7；　and　the　drift　is　correspondingly　decreased。



_Fig。　7。　Equal　Lift　and　Drift　in　Flight。_



_Fig。　8。　Unequal　Lift　and　Drift。_





MOVING　PLANES　VS。　WINDS。In　this　way　Boisset；

Duchemin；　Langley；　and　others；　determined

the　comparative　drift；　and　those　results　have　been

largely　relied　upon　by　aviators；　and　assumed　to

be　correct　when　applied　to　flying　machines。



That　they　are　not　correct　has　been　proven　by

the　Wrights　and　others；　the　only　explanation　being

that　some　errors　had　been　made　in　the　calculations；

or　that　aviators　were　liable　to　commit　errors

in　observing　the　true　angle　of　the　planes

while　in　flight。



MOMENTUM　NOT　CONSIDERED。The　great　factor

of　momentum　has　been　entirely　ignored；　and　it　is

our　desire　to　press　the　important　point　on　those

who　begin　to　study　the　question　of　flying　machines。



THE　FLIGHT　OF　BIRDS。Volumes　have　been

written　concerning　observations　on　the　flight　of

birds。　The　marvel　has　been　why　do　soaring　birds

maintain　themselves　in　space　without　flapping

their　wings。　In　fact；　it　is　a　much　more　remarkable

thing　to　contemplate　why　birds　which　depend

on　flapping　wings　can　fly。



THE　DOWNWARD　BEAT。It　is　argued　that　the

downward　beat　of　the　wings　is　so　much　more

rapid　than　the　upward　motion；　that　it　gets　an　action

on　the　air　so　as　to　force　the　body　upwardly。

This　is　disposed　of　by　the　wing　motion　of　many

birds；　notoriously　the　crow；　whose　lazily…flapping

wings　can　be　readily　followed　by　the　eye；　and　the

difference　in　movement；　if　any；　is　not　perceptible。



THE　CONCAVED　WING。It　is　also　urged　that　the

concave　on　the　under　side　of　the　wing　gives　the

quality　of　lift。　Certain　kinds　of　beetles；　and　particularly

the　common　house　fly；　disprove　that　theory；

as　their　wings　are　perfectly　flat。



FEATHER　STRUCTURE　CONSIDERED。Then　the

feather　argument　is　advanced；　which　seeks　to

show　that　as　each　wing　is　made　up　of　a　plurality

of　feathers；　overlapping　each　other；　they　form　a

sort　of　a　valved　surface；　opening　so　as　to　permit

air　to　pass　through　them　during　the　period　of

their　upward　movement；　and　closing　up　as　the

wing　descends。



It　is　difficult　to　perform　this　experiment　with

wings；　so　as　to　show　such　an　individual　feather

movement。　It　is　certain　that　there　is　nothing　in

the　structure　of　the　wing　bone　and　the　feather

connection　which　points　to　any　individual　feather

movement；　and　our　observation　is；　that　each

feather　is　entirely　too　rigid　to　permit　of　such　an

opening　up　between　them。



It　is　obvious　that　the　wing　is　built　up　in　that

way　for　an　entirely　different　reason。　Soaring

birds；　which　do　not　depend　on　the　flapping　motion；

have　the　same　overlapping　feather　formation。



WEBBED　WINGS。Furthermore；　there　are　numerous

flying　creatures　which　do　not　have

feathered　wings；　but　web…like　structures；　or　like　the

house　fly；　in　one　continuous　and　unbroken

plane。



That　birds　which　fly　with　flapping　wings　derive

their　support　from　the　air；　is　undoubtedly　true；

and　that　the　lift　produced　is　due；　not　to　the　form；

or　shape；　or　area　of　the　wing；　is　also　beyond　question。

The　records　show　that　every　conceivable

type　of　outlined　structure　is　used　by　nature；　the

material　and　texture　of　the　wings　themselves　differ

to　such　a　degree　that　there　is　absolutely　no

similarity；　some　have　concaved　under　surfaces；

and　others　have　not；　some　fly　with　rapidly　beating

wings；　and　others　with　slow　and　measured

movements；　many　of　them　fly　with　equal　facility

without　flapping　movements；　and　the　proportions

of　weight　to　wing　surface　vary　to　such　an　extent

that　it　is　utterly　impossible　to　use　such　data　as　a

guide　in　calculating　what　the　proper　surface

should　be　for　a　correct　flying　machine。



THE　ANGLE　OF　MOVEMENT。How；　then；　it　may

be　asked；　do　they　get　their　support？　There　must

be　something；　in　all　this　variety　and　diversity　of

form；　of　motion；　and　of　characteristics；　which

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。　I