aeroplanes-第12章

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be　used。　Lightness　and　strength　for　the　wing

itself　are　the　first　requirements。　Then　rigidity

in　the　joint　and　in　the　main　rib　of　the　wing；　are

the　next　considerations。



In　these　respects　the　ability　of　man　is　limited。

The　wing　ligatures　of　flying　creatures　is　exceedingly

strong；　and　flexible；　the　hollow　bone　formation

and　the　feathers　are　extremely　light；　compared

with　their　sustaining　powers。



THE　HELICOPTER　MOTION。The　helicopter；　or

helix…wing；　is　a　form　of　flying　machine　which　depends

on　revolving　screws　to　maintain　it　in　the

air。　Many　propellers　are　now　made；　six　feet　in

length；　which　have　a　pull　of　from　400　to　500

pounds。　If　these　are　placed　on　vertically…disposed

shafts　they　would　exert　a　like　power　to

raise　a　machine　from　the　earth。



Obviously；　it　is　difficult　to　equip　such　a　machine

with　planes　for　sustaining　it　in　flight；　after　it　is

once　in　the　air；　and　unless　such　means　are　provided

the　propellers　themselves　must　be　the

mechanism　to　propel　it　horizontally。



This　means　a　change　of　direction　of　the　shafts

which　support　the　propellers；　and　the　construction

is　necessarily　more　complicated　than　if　they

were　held　within　non…changeable　bearings。



This　principle；　however；　affords　a　safer　means

of　navigating　than　the　orthopter　type；　because

the　blades　of　such　an　instrument　can　be　forced

through　the　air　with　infinitely　greater　speed　than

beating　wings；　and　it　devolves　on　the　inventor　to

devise　some　form　of　apparatus　which　will　permit

the　change　of　pull　from　a　vertical　to　a　horizontal

direction　while　in　flight。







CHAPTER　VI



THE　LIFTING　SURFACES　OF　AEROPLANES





THIS　subject　includes　the　form；　shape　and　angle

of　planes；　used　in　flight。　It　is　the　direction　in

which　most　of　the　energy　has　been　expended　in

developing　machines；　and　the　true　form　is　still

involved　in　doubt　and　uncertainty。



RELATIVE　SPEED　AND　ANGLE。The　relative

speed　and　angle；　and　the　camber；　or　the　curved

formation　of　the　plane；　have　been　considered　in

all　their　aspects；　so　that　the　art　in　this　respect　has

advanced　with　rapid　strides。



NARROW　PLATES　MOST　EFFECTIVE。It　was

learned；　in　the　early　stages　of　the　development

by　practical　experiments；　that　a　narrow　plane；

fore　and　aft；　produces　a　greater　lift　than　a　wide

one；　so　that；　assuming　the　plane　has　100　square

feet　of　sustaining　surface；　it　is　far　better　to　make

the　shape　five　feet　by　twenty　than　ten　by　ten。



However；　it　must　be　observed；　that　to　use　the

narrow　blade　effectively；　it　must　be　projected

through　the　air　with　the　long　margin　forwardly。

Its　sustaining　power　per　square　foot　of　surface

is　much　less　if　forced　through　the　air　lengthwise。



Experiments　have　shown　why　a　narrow　blade

has　proportionally　a　greater　lift；　and　this　may

be　more　clearly　understood　by　examining　the

illustrations　which　show　the　movement　of　planes

through　the　air　at　appropriate　angles。



_Fig。　22。　Stream　lines　along　a　plane。_



STREAM　LINES　ALONG　A　PLANE。In　Fig。　22；　A

is　a　flat　plane；　which　we　will　assume　is　10　feet

from　the　front　to　the　rear　margin。　For　convenience

seven　stream　lines　of　air　are　shown；

which　contact　with　this　inclined　surface。　The　first

line　1；　after　the　contact　at　the　forward　end；　is

driven　downwardly　along　the　surface；　so　that　it

forms　what　we　might　term　a　moving　film。



The　second　air　stream　2；　strikes　the　first　stream；

followed　successively　by　the　other　streams；　3；　4；

and　so　on；　each　succeeding　stream　being　compelled

to　ride　over；　or　along　on　the　preceding　mass　of

cushioned　air；　the　last　lines；　near　the　lower　end；

being；　therefore；　at　such　angles；　and　contacting

with　such　a　rapidly…moving　column；　that　it　produces

but　little　lift　in　comparison　with　the　1st；

2d　and　3d　stream　lines。　These　stream　lines　are

taken　by　imagining　that　the　air　approaches　and

contacts　with　the　plane　only　along　the　lines　indicated

in　the　sketch；　although　they　also　in　practice

are　active　against　every　part　of　the　plane。



THE　CENTER　OF　PRESSURE。In　such　a　plane　the

center　of　pressure　is　near　its　upper　end；　probably

near　the　line　3；　so　that　the　greater　portion　of　the

lift　is　exerted　by　that　part　of　the　plane　above

line　3。



AIR　LINES　ON　THE　UPPER　SIDE　OF　THE　PLANE。

Now；　another　factor　must　be　considered；　namely；

the　effect　produced　on　the　upper　side　of　the　plane；

over　which　a　rarefied　area　is　formed　at　certain

points；　and；　in　practice；　this　also　produces；　or

should　be　utilized　to　effect　a　lift。



RAREFIED　AREA。What　is　called　a　rarefied　area；

has　reference　to　a　state　or　condition　of　the　atmosphere

which　has　less　than　the　normal　pressure　or

quantity　of　air。　Thus；　the　pressure　at　sea　level；

is　about　14　3/4　per　square　inch



As　we　ascend　the　pressure　grows　less；　and　the

air　is　thus　rarer；　or；　there　is　less　of　it。　This　is　a

condition　which　is　normally　found　in　the　atmosphere。

Several　things　tend　to　make　a　rarefied

condition。　One　is　altitude；　to　which　we　have　just

referred。



Then　heat　will　expand　air；　making　it　less　dense；

or　lighter；　so　that　it　will　move　upwardly；　to　be

replaced　by　a　colder　body　of　air。　In　aeronautics

neither　of　these　conditions　is　of　any　importance

in　considering　the　lifting　power　of　aeroplane　surfaces。



RAREFACTION　PRODUCED　BY　MOTION。The　third

rarefied　condition　is　produced　by　motion；　and　generally

the　area　is　very　limited　when　brought　about

by　this　means。　If；　for　instance；　a　plane　is　held

horizontally　and　allowed　to　fall　toward　the　earth；

it　will　be　retarded　by　two　forces；　namely；　compression

and　rarefaction；　the　former　acting　on　the

under　side　of　the　plane；　and　the　latter　on　the　upper

side。



Of　the　two　rarefaction　is　the　most　effectual；

and　produces　a　greater　effect　than　compression。

This　may　be　proven　by　compressing　air　in　a　long

pipe；　and　noting　the　difference　in　gauge　pressure

between　the　ends；　and　then　using　a　suction　pump

on　the　same　pipe。



When　a　plane　is　forced　through　the　air　at　any

angle；　a　rarefied　area　is　formed　on　the　side　which

is　opposite　the　one　having　the　positive　angle　of

incidence。



If　the　plane　can　be　so　formed　as　to　make　a　large

and　effective　area　it　will　add　greatly　to　the　value

of　the　sustaining　surface。



Unfortunately；　the　long　fiat　plane　does　not　lend

any　aid　in　this　particular；　as　the　stream　line　flows

down　along　the　top；　as　shown　in　Fig。　23；　without

being　of　any　service。



_Fig。　23。　Air　lines　on　the　upper　side　of　a　Plane。_



THE　CONCAVED　PLANE。These　considerations

led　to　the　adoption　of　the　concaved　plane　formation；

and　for　purposes　of　comparison　the　diagram；

Fig。　24；　shows　the　plane　B　of　the　same　length　and

angle　as　the　straight　planes。



In　examining　the　successive　stream　lines　it　will

be　found　that　while　the　1st；　2d　and　3d　lines　have

a　little　less　angle　of　impact　than　the　corresponding

lines　in　the　straight　plane；　the　last　lines；　5；　6

and　7；　have　much　greater　angles；　so　that　only　line

4　strikes　the　plane　at　the　same　angle。



Such　a　plane　structure　would；　therefore；　have

its　center　of　pressure　somewhere　between　the

lines　3　and　4；　and　the　lift　being　thus；　practically；

uniform　over　the　surface；　would　be　more　effective。



THE　CENTER　OF　PRESSURE。This　is　a　term　used

to　indicate　the　place　on　the　plane　where　the　air

acts　with　the　greatest　force。　It　has　reference　to

a　point　between　the　front　and　rear　margins　only

of　the　plane。



_Fig。　24。　Air　lines　below　a　concaved　Plane。_



UTILIZING　THE　RAREFIED　AREA。This　structure；

however；　has　another　important　advantage；　as　it

utilizes　the　rarefied　area　which　is　produced；　and

which　may　be　understood　by　reference　to　Fig。　25。



The　plane　B；　with　its　upward　curve；　and　at　the

same　angle　as　the　straight　plane；　has　its　lower

end　so　curved；　with　relation　to　the　forward　movement；

that　the　air；　in　rushing　past　the　upper　end；

cannot　follow　the　curve　rapidly　enough　to　maintain

the　same　density　along　C