aeroplanes- 第21部分

to　the　stopping　of　the　motor；　which　in　the

past　might　have　been　avoided　if　the　character　of

the　glide　had　been　understood。　The　only　thing

that　now　troubles　the　pilot　when　the　engine　〃goes

dead；〃　is　to　select　a　landing　place。



The　proper　course　in　such　a　case　is　to　urge

the　machine　to　descend　as　rapidly　as　possible；　in

order　to　get　a　headway；　for　the　time　being。　As

there　is　now　no　propelling　force　the　glide　is　depended

upon　to　act　as　a　substitute。　The　experienced

pilot　will　not　make　a　straight…away　glide；

but　like　the　vulture；　or　the　condor；　and　birds　of

that　class；　soar　in　a　circle；　and　thus；　by　passing

over　and　over　the　same　surfaces　of　the　earth；　enable

him　to　select　a　proper　landing　place。



THE　LANDING。The　pilot　who　can　make　a　good

landing　is　generally　a　good　flyer。　It　requires

nicety　of　judgment　to　come　down　properly。　One

thing　which　will　appear　novel　after　the　first　altitude

flights　are　attempted　is　the　peculiar　sensation

of　the　apparently　increased　speed　as　the　earth

comes　close　up　to　the　machine。



At　a　height　of　one　hundred　feet；　flying　thirty

miles　an　hour；　does　not　seem　fast；　because　the　surface

of　the　earth　is　such　a　distance　away　that　particular

objects　remain　in　view　for　some　moments；

but　when　within　ten　feet　of　the　surface　the　same

object　is　in　the　eye　for　an　instant　only。



This　lends　a　sort　of　terror　to　the　novice。　He

imagines　a　great　many　things；　but　forgets　some

things　which　are　very　important　to　do　at　this

time。　One　is；　that　the　front　of　the　machine　must

be　thrown　up　so　as　to　bank　the　planes　against　the

wind。　The　next　is　to　shut　off　the　power；　which

is　to　be　done　the　moment　the　wheels　strike　the

ground；　or　a　little　before。



Upon　his　judgment　of　the　time　of　first　touching

the　earth　depends　the　success　of　safely　alighting。

He　may　bank　too　high；　and　come　down　on　the　tail

with　disastrous　results。　If　there　is　plenty　of　field

room　it　is　better　to　come　down　at　a　less　angle；　or

even　keep　the　machine　at　an　even　keel；　and　the

elevator　can　then　depress　the　tail　while　running

over　the　ground；　and　thus　bring　the　machine　to

rest。



Frequently；　when　about　to　land　the　machine

will　rock　from　side　to　side。　In　such　a　case　it　is

far　safer　to　go　up　into　the　air　than　to　make　the

land；　because；　unless　the　utmost　care　is　exercised；

one　of　the　wing　tips　will　strike　the　earth　and

wreck　the　machine。



Another　danger　point　is　losing　headway；　as　the

earth　is　neared；　due　to　flying　at　too　flat　an　angle；

or　against　a　wind　that　happens　to　be　blowing　particularly

hard　at　the　landing　place。　If　the　motor

is　still　going　this　does　not　make　so　much　difference；

but　in　a　volplane　it　means　that　the　descent

must　be　so　steep；　at　the　last　moment　of　flight；　that

the　chassis　is　liable　to　be　crushed　by　the　impact。



FLYING　ALTITUDE。It　is　doubtful　whether　the

disturbed　condition　of　the　atmosphere；　due　to

the　contour　of　the　earth's　surface；　reaches　higher

than　500　feet。　Over　a　level　area　it　is　certain　that

it　is　much　less；　but　in　some　sections　of　the　country；

where　the　hill　ranges　extend　for　many　miles；

at　altitudes　of　three　and　four　hundred　feet；　the

upper　atmosphere　may　be　affected　for　a　thousand

feet　above。



Prof。　Lowe；　in　making　a　flight　with　a　balloon；

from　Cincinnati　to　North　Carolina；　which　lasted

a　day　and　all　of　one　night；　found　that　during　the

early　morning　the　balloon；　for　some　reason；　began

to　ascend；　and　climbed　nearly　five　thousand

feet　in　a　few　hours；　and　as　unaccountably

began　to　descend　several　hours　before　he　landed。



Before　it　began　to　ascend；　he　was　on　the　western

side　of　the　great　mountain　range　which　extends

south　from　Pennsylvania　and　terminates　in

Georgia。　He　was　actually　climbing　the　mountain

in　a　drift　of　air　which　was　moving　eastwardly；

and　at　no　time　was　he　within　four　thousand　feet

of　the　earth　during　that　period；　which　shows　that

air　movements　are　of　such　a　character　as　to　exert

their　influence　vertically　to　great　heights。



For　cross　country　flying　the　safest　altitude　is

1000　feet；　a　distance　which　gives　ample　opportunity

to　volplane；　if　necessary；　and　it　is　a　height

which　enables　the　pilot　to　make　observations　of　the

surface　so　as　to　be　able　to　judge　of　its　character。



But　explanations　and　statements；　and　the　experiences

of　pilots　might　be　detailed　in　pages；　and

still　it　would　be　ineffectual　to　teach　the　art　of　flying。

The　only　sure　course　is　to　do　the　work　on

an　actual　machine。



Many　of　the　experiences　are　valuable　to　the

learner；　some　are　merely　in　the　nature　of　cautions；

and　it　is　advisable　for　the　beginner　to　learn　what

the　experiences　of　others　have　been；　although　they

may　never　be　called　upon　to　duplicate　them。



All　agree　that　at　great　elevations　the　flying

conditions　are　entirely　different　from　those　met

with　near　the　surface　of　the　ground；　and　the　history

of　accidents　show　that　in　every　case　where

a　mishap　was　had　at　high　altitude　it　came　about

through　defect　in　the　machine；　and　not　from　gusts

or　bad　air　condition。



On　the　other　hand；　the　uptilting　of　machines；

the　accidents　due　to　the　so…called　〃Holes　in　the

air；〃　which　have　dotted　the　historic　pages　with

accidents；　were　brought　about　at　low　altitudes。



At　from　two　to　five　thousand　feet　the　air　may　be

moving　at　speeds　of　from　twenty　to　forty　miles

an　hour；great　masses　of　winds；　like　the　trade

stream；　which　are　uniform　over　vast　areas。　To

the　aviator　flying　in　such　a　field；　with　the　earth

hidden　from　him；　there　would　be　no　wind　to　indicate

that　he　was　moving　in　any　particular　direction。



He　would　fly　in　that　medium；　in　any　direction；

without　the　slightest　sense　that　he　was　in　a　gale。

It　would　not　affect　the　control　of　the　machine；

because　the　air；　though　moving　as　a　mass；　would

be　the　same　as　flying　in　still　air。　It　is　only　when

he　sees　fixed　objects　that　he　is　conscious　of　the

movement　of　the　wind。







CHAPTER　XIII



THE　PROPELLER





BY　far　the　most　difficult　problem　connected

with　aviation　is　the　propeller。　It　is　the　one　great

vital　element　in　the　science　and　art　pertaining　to

this　subject　which　has　not　advanced　in　the　slightest

degree　since　the　first　machine　was　launched。



The　engine　has　come　in　for　a　far　greater　share

of　expert　experimental　work；　and　has　advanced

most　rapidly　during　the　past　ten　years。　But；

strange　to　say；　the　propeller　is；　essentially；　the

same　with　the　exception　of　a　few　small　changes。



PROPELLER　CHANGES。The　changes　which　have

been　made　pertaining　to　the　form　of　structure；

principally；　and　in　the　use　of　new　materials。　The

kind　of　wood　most　suitable　has　been　discovered；

but　the　lines　are　the　same；　and　nothing　has　been

done　to　fill　the　requirement　which　grows　out　of

the　difference　in　speed　when　a　machine　is　in　the

act　of　launching　and　when　it　is　in　full　flight。



PROPELLER　SHAPE。It　cannot　be　possible　that

the　present　shape　of　the　propeller　will　be　its　ultimate

form。　It　is　inconceivable　that　the　propeller

is　so　inefficient　that　only　one　sixty…fifth　of　the

power　of　the　engine　is　available。　The　improvement

in　propeller　efficiency　is　a　direction　which

calls　for　experimental　work　on　the　part　of　inventors

everywhere。



The　making　of　a　propeller；　although　it　appears

a　difficult　task；　is　not　as　complicated　as　would　appear；

and　with　the　object　in　view　of　making　the

subject　readily　understood；　an　explanation　will　be

given　of　the　terms　〃Diameter；〃　and　〃Pitch；〃　as

used　in　the　art。



The　Diameter　has　reference　to　the　length　of

the　propeller；　from　end　to　end。　In　calculating

propeller　pull；　the　diameter　is　that　which　indicates

the　speed　of　travel；　and　for　this　reason　is

a　necessary　element。



Thus；　for　instance；　a　propeller　three　feet　in

diameter；　rotating　500　times　a　minute；　has　a　tip

speed　of　1500　feet；　whereas　a　six　foot　propeller；

rotating　at　the　same　speed；　moves　3000　feet　at　the

tips。



PITCH。This　is　the　term　which　is　most　confusing；

and　is　that　which　causes　the　most　frequent

trouble　in　the　mind　of　the　novice。　The　term　will

be　made　clear　by　carefully　examining　the　accompanying

illustration　and　the　following　description：



In　Fig。　76　is　shown　a　side　view　of　a　propeller

A；　mounted　on　a　shaft　B；　which　is　free　to　move

longitudinally。　Suppose　we　turn　the　shaft　so　the

tip　will　move　along　on　the　line　indicated　by　the

arrow　C。



Now　the　pitch　of　the　blade　at　D　is　such　that　it

will　be　exactly　in　line　with　the　spirally…formed

course　E；　for　one　complete　turn。　As　the　propeller

shaft　has　now　advanced；　along　the　line　E；　and

stopped　after　one　turn；　at　F；　the　measure　between

the　points　F　and　G　represents　the　pitch　of　the　propeller。

Another　way　to　express　it　would　be　to

call　the　angle　of　the　blade　a　five；　or　six；　or　a　seven

foot　pitch；　as　the　pitches　are　measured　in　feet。



_Fig。　76。　Describing　the　Pitch　Line。_



In　the　illustration　thus　given　the　propeller　shaft；

having　advanced　six　feet；　we　have　what　is　called

a　six　foot　pitch。



Now；　to　lay　out　such　a　pitch　is　an　easy　matter。

Assume；　as　in　Fig。　77；　that　A　represents　the　end

of　the　blank　from　which　the　propeller　is　to　be　cut；

and　that　the　diameter　of　this　blank；　or　its　length

from　end　to　end　is　seven　feet。　The　problem　now

is　to　cut　the　blades　at　such　an　angle　that　we　shall

have　a　six　foot　pitch。



_Fig。　77。　Laying　out　the　Pitch。_



LAYING　OUT　THE　PITCH。First；　we　must　get　the

circumference　of　the　propeller；　that　is；　the　distance

the　tip　of　the　propeller　will　travel　in　making

one　complete　turn。　This　is　done　by　multiplying

7　by　3。1416。　This　equals　21。99；　or；　practically；　22

feet。



A　line　B　is　drawn；　extending　out　horizontally

along　one　side　of　the　blank　A；　this　line　being　made

on　a　scale；　to　represent　22　feet。　Secondly；　at　the

end　of　this　line　drawn　a　perpendicular　line　C；　6

feet　long。　A　perpendicular　line　is　always　one

which　is