aeroplanes-第3章

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！



acting　together。　Thus；　a　stone　poised　on

a　cliff；　while　it　exerts　no　power　which　can　be

utilized；　has；　nevertheless；　what　is　called　potential

energy。　When　it　is　pushed　from　its　lodging　place

kinetic　energy　is　developed。　In　both　cases；

gravity；　acting　in　conjunction　with　the　mass　of

the　stone；　produced　power。



So　in　the　case　of　gunpowder。　It　is　the　unity　of

two　or　more　substances；　that　causes　the　expansion

called　power。　The　heat　of　the　fuel　converting

water　into　steam；　is　another　illustration　of　the

unity　of　two　or　more　elements；　which　are　necessary

to　produce　energy。



MASS　AN　ELEMENT　IN　FLYING。The　boy　who

reads　this　will　smile；　as　he　tells　us　that　the　power

which　propelled　the　ball　through　the　air　came

from　the　thrower　and　not　from　the　ball　itself。

Let　us　examine　this　claim；　which　came　from　a　real

boy；　and　is　another　illustration　how　acute　his　mind

is　on　subjects　of　this　character。



We　have　two　balls　the　same　diameter；　one　of

iron　weighing　a　half　pound；　and　the　other　of　cotton

weighing　a　half　ounce。　The　weight　of　one

is；　therefore；　sixteen　times　greater　than　the　other。



Suppose　these　two　balls　are　thrown　with　the

expenditure　of　the　same　power。　What　will　be　the

result！　The　iron　ball　will　go　much　farther；　or；

if　projected　against　a　wall　will　strike　a　harder

blow　than　the　cotton　ball。



MOMENTUM　A　FACTOR。Each　had　transferred

to　it　a　motion。　The　initial　speed　was　the　same；

and　the　power　set　up　equal　in　the　two。　Why　this

difference；　The　answer　is；　that　it　is　in　the

material　itself。　It　was　the　mass　or　density　which　accounted

for　the　difference。　It　was　mass　multiplied

by　speed　which　gave　it　the　power；　called；　in

this　case；　momentum。



The　iron　ball　weighing　eight　ounces；　multiplied

by　the　assumed　speed　of　50　feet　per　second；　equals

400　units　of　work。　The　cotton　ball；　weighing　1/2

ounce；　with　the　same　initial　speed；　represents　25

units　of　work。　The　term　〃unit　of　work〃　means

a　measurement；　or　a　factor　which　may　be　used　to

measure　force。



It　will　thus　be　seen　that　it　was　not　the　thrower

which　gave　the　power；　but　the　article　itself。　A

feather　ball　thrown　under　the　same　conditions；

would　produce　a　half　unit　of　work；　and　the　iron

ball；　therefore；　produced　800　times　more　energy。



RESISTANCE。Now；　in　the　movement　of　any　body

through　space；　it　meets　with　an　enemy　at　every

step；　and　that　is　air　resistance。　This　is　much

more　effective　against　the　cotton　than　the　iron

ball：　or；　it　might　be　expressed　in　another　way：

The　momentum；　or　the　power；　residing　in　the

metal　ball；　is　so　much　greater　than　that　within　the

cotton　ball　that　it　travels　farther；　or　strikes　a

more　effective　blow　on　impact　with　the　wall。



HOW　RESISTANCE　AFFECTS　THE　SHAPE。It　is　because

of　this　counterforce；　resistance；　that　shape

becomes　important　in　a　flying　object。　The　metal

ball　may　be　flattened　out　into　a　thin　disk；　and　now；

when　the　same　force　is　applied；　to　project　it　forwardly；

it　will　go　as　much　farther　as　the　difference

in　the　air　impact　against　the　two　forms。



MASS　AND　RESISTANCE。Owing　to　the　fact　that

resistance　acts　with　such　a　retarding　force　on　an

object　of　small　mass；　and　it　is　difficult　to　set　up　a

rapid　motion　in　an　object　of　great　density；　lightness

in　flying　machine　structures　has　been　considered；

in　the　past；　the　principal　thing　necessary。



THE　EARLY　TENDENCY　TO　ELIMINATE　MOMENTUM。

Builders　of　flying　machines；　for　several

years；　sought　to　eliminate　the　very　thing

which　gives　energy　to　a　horizontally…movable

body；　namely；　momentum。



Instead　of　momentum；　something　had　to　be

substituted。　This　was　found　in　so　arranging　the

machine　that　its　weight；　or　a　portion　of　it；　would

be　sustained　in　space　by　the　very　element　which

seeks　to　retard　its　flight；　namely；　the　atmosphere。



If　there　should　be　no　material　substance；　like

air；　then　the　only　way　in　which　a　heavier…than…air

machine　could　ever　fly；　would　be　by　propelling　it

through　space；　like　the　ball　was　thrown；　or　by

some　sort　of　impulse　or　reaction　mechanism　on

the　air…ship　itself。　It　could　get　no　support　from

the　atmosphere。



LIGHT　MACHINES　UNSTABLE。Gradually　the

question　of　weight　is　solving　itself。　Aviators　are

beginning　to　realize　that　momentum　is　a　wonderful

property；　and　a　most　important　element　in

flying。　The　safest　machines　are　those　which　have

weight。　The　light；　willowy　machines　are　subject

to　every　caprice　of　the　wind。　They　are　notoriously

unstable　in　flight；　and　are　dangerous　even

in　the　hands　of　experts。



THE　APPLICATION　OF　POWER。The　thing　now　to

consider　is　not　form；　or　shape；　or　the　distribution

of　the　supporting　surfaces；　but　HOW　to　apply

the　power　so　that　it　will　rapidly　transfer　a　machine

at　rest　to　one　in　motion；　and　thereby　get

the　proper　support　on　the　atmosphere　to　hold　it

in　flight。



THE　SUPPORTING　SURFACES。This　brings　us　to

the　consideration　of　one　of　the　first　great　problems

in　flying　machines；　namely；　the　supporting

surfaces；not　its　form；　shape　or　arrangement；

（which　will　be　taken　up　in　their　proper　places）；　but

the　area；　the　dimensions；　and　the　angle　necessary

for　flight。



AREA　NOT　THE　ESSENTIAL　THING。The　history

of　flying　machines；　short　as　it　is；　furnishes　many

examples　of　one　striking　fact：　That　area　has

but　little　to　do　with　sustaining　an　aeroplane　when

once　in　flight。　The　first　Wright　flyer　weighed

741　pounds；　had　about　400　square　feet　of　plane

surface；　and　was　maintained　in　the　air　with　a　12

horse　power　engine。



True；　that　machine　was　shot　into　the　air　by　a

catapult。　Motion　having　once　been　imparted　to　it；

the　only　thing　necessary　for　the　motor　was　to

maintain　the　speed。



There　are　many　instances　to　show　that　when

once　in　flight；　one　horse　power　will　sustain　over

100　pounds；　and　each　square　foot　of　supporting

surface　will　maintain　90　pounds　in　flight。



THE　LAW　OF　GRAVITY。As　the　effort　to　fly

may　be　considered　in　the　light　of　a　struggle　to

avoid　the　laws　of　nature　with　respect　to　matter；

it　may　be　well　to　consider　this　great　force　as　a

fitting　prelude　to　the　study　of　our　subject。



Proper　understanding；　and　use　of　terms　is　very

desirable；　so　that　we　must　not　confuse　them。

Thus；　weight　and　mass　are　not　the　same。　Weight

varies　with　the　latitude；　and　it　is　different　at　various

altitudes；　but　mass　is　always　the　same。



If　projected　through　space；　a　certain　mass

would　move　so　as　to　produce　momentum；　which

would　be　equal　at　all　places　on　the　earth's　surface；

or　at　any　altitude。



Gravity　has　been　called　weight；　and　weight

gravity。　The　real　difference　is　plain　if　gravity

is　considered　as　the　attraction　of　mass　for　mass。

Gravity　is　generally　known　and　considered　as　a

force　which　seeks　to　draw　things　to　the　earth。

This　is　too　narrow。



Gravity　acts　in　all　directions。　Two　balls　suspended

from　strings　and　hung　in　close　proximity

to　each　other　will　mutually　attract　each　other。

If　one　has　double　the　mass　it　will　have　twice　the

attractive　power。　If　one　is　doubled　and　the　other

tripled；　the　attraction　would　be　increased　six

times。　But　if　the　distance　should　be　doubled　the

attraction　would　be　reduced　to　one…fourth；　and

if　the　distance　should　be　tripled　then　the　pull

would　be　only　one…ninth。



The　foregoing　is　the　substance　of　the　law；

namely；　that　all　bodies　attract　all　other　bodies

with　a　force　directly　in　proportion　to　their　mass；

and　inversely　as　the　square　of　their　distance　from

one　another。



To　explain　this　we　cite　the　following　illustration：

Two　bodies；　each　having　a　mass　of　4

pounds；　and　one　inch　apart；　are　attracted　toward

each　other；　so　they　touch。　If　one　has　twice　the

mass　of　the　other；　the　smaller　will　draw　the　larger

only　one…quarter　of　an　inch；　and　the　large　one

will　draw　the　other　three…quarters　of　an　inch；

thus　confirming　the　law　that　two　bodies　will　attract

each　other　in　proportion　to　their　mass。



Suppose；　now；　that　the