aeroplanes-第4节

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wheel　at　certain　speeds。



A　TANGENT。When　an　object　is　thrown　horizontally

the　line　of　flight　is　tangential　to　the　earth；

or　at　right　angles　to　the　force　of　gravity。　Such

a　course　in　a　flying　machine　finds　less　resistance

than　if　it　should　be　projected　upwardly；　or　directly

opposite　the　centripetal　pull。



_Fig　1。　Tangential　Flight_



TANGENTIAL　MOTION　REPRESENTS　CENTRIFUGAL

PULL。A　tangential　motion；　or　a　horizontal

movement；　seeks　to　move　matter　away　from　the

center　of　the　earth；　and　any　force　which　imparts

a　horizontal　motion　to　an　object　exerts　a　centrifugal

pull　for　that　reason。



In　Fig。　1；　let　A　represent　the　surface　of　the

earth；　B　the　starting　point　of　the　flight　of　an　object；

and　C　the　line　of　flight。　That　represents　a

tangential　line。　For　the　purpose　of　explaining

the　phenomena　of　tangential　flight；　we　will　assume

that　the　missile　was　projected　with　a　sufficient

force　to　reach　the　vertical　point　D；　which

is　4000　miles　from　the　starting　point　B。



In　such　a　case　it　would　now　be　over　5500　miles

from　the　center　of　the　earth；　and　the　centrifugal

pull　would　be　decreased　to　such　an　extent　that　the

ball　would　go　on　and　on　until　it　came　within　the

sphere　of　influence　from　some　other　celestial

body。



EQUALIZING　THE　TWO　MOTIONS。But　now　let　us

assume　that　the　line　of　flight　is　like　that　shown

at　E；　in　Fig。　2；　where　it　travels　along　parallel

with　the　surface　of　the　earth。　In　this　case　the

force　of　the　ball　equals　the　centripetal　pull；or；

to　put　it　differently；　the　centrifugal　equals　the

gravitational　pull。



The　constant　tendency　of　the　ball　to　fly　off　at

a　tangent；　and　the　equally　powerful　pull　of

gravity　acting　against　each　other；　produce　a

motion　which　is　like　that　of　the　earth；　revolving

around　the　sun　once　every　three　hundred　and

sixty…five　days。



It　is　a　curious　thing　that　neither　Langley；　nor

any　of　the　scientists；　in　treating　of　the　matter　of

flight；　have　taken　into　consideration　this　quality

of　momentum；　in　their　calculations　of　the　elements

of　flight。



_Fig。　2　Horizontal　Flight_



All　have　treated　the　subject　as　though　the

whole　problem　rested　on　the　angle　at　which　the

planes　were　placed。　At　45　degrees　the　lift　and

drift　are　assumed　to　be　equal。



LIFT　AND　DRIFT。The　terms　should　be　explained；

in　view　of　the　frequent　allusion　which

will　be　made　to　the　terms　hereinafter。　Lift

is　the　word　employed　to　indicate　the　amount

which　a　plane　surface　will　support　while　in　flight。

Drift　is　the　term　used　to　indicate　the　resistance

which　is　offered　to　a　plane　moving　forwardly

against　the　atmosphere。



_Fig。　3。　Lift　and　Drift_



In　Fig。　3　the　plane　A　is　assumed　to　be　moving

forwardly　in　the　direction　of　the　arrow　B。　This

indicates　the　resistance。　The　vertical　arrow　C

shows　the　direction　of　lift；　which　is　the　weight

held　up　by　the　plane。



NORMAL　PRESSURE。Now　there　is　another　term

much　used　which　needs　explanation；　and　that　is

normal　pressure。　A　pressure　of　this　kind

against　a　plane　is　where　the　wind　strikes　it　at

right　angles。　This　is　illustrated　in　Fig。　4；　in

which　the　plane　is　shown　with　the　wind　striking

it　squarely。



It　is　obvious　that　the　wind　will　exert　a　greater

force　against　a　plane　when　at　its　normal。　On　the

other　hand；　the　least　pressure　against　a　plane　is

when　it　is　in　a　horizontal　position；　because　then

the　wind　has　no　force　against　the　surfaces；　and

the　only　effect　on　the　drift　is　that　which　takes

place　when　the　wind　strikes　its　forward　edge。



_Fig。　4。　Normal　Air　Pressure_



_Fig。　5。　Edge　Resistance_





HEAD　RESISTANCE。Fig。　5　shows　such　a　plane；

the　only　resistance　being　the　thickness　of　the

plane　as　at　A。　This　is　called　head　resistance；

and　on　this　subject　there　has　been　much　controversy；

and　many　theories；　which　will　be　considered

under　the　proper　headings。



If　a　plane　is　placed　at　an　angle　of　45　degrees

the　lift　and　the　drift　are　the　same；　assumedly；　because；

if　we　were　to　measure　the　power　required

to　drive　it　forwardly；　it　would　be　found　to　equal

the　weight　necessary　to　lift　it。　That　is；　suppose

we　should　hold　a　plane　at　that　angle　with　a　heavy

wind　blowing　against　it；　and　attach　two　pairs　of

scales　to　the　plane；　both　would　show　the　same

pull。



_Fig。　6。　Measuring　Lift　and　Drift_



MEASURING　LIFT　AND　DRIFT。In　Fig。　6；　A　is　the

plane；　B　the　horizontal　line　which　attaches　the

plane　to　a　scale　C；　and　D　the　line　attaching　it　to

the　scale　E。　When　the　wind　is　of　sufficient　force

to　hold　up　the　plane；　the　scales　will　show　the　same

pull；　neglecting；　of　course；　the　weight　of　the

plane　itself。



PRESSURE　AT　DIFFERENT　ANGLES。What　every

one　wants　to　know；　and　a　subject　on　which　a

great　deal　of　experiment　and　time　have　been　expended；

is　to　determine　what　the　pressures　are　at

the　different　angles　between　the　horizontal；　and

laws　have　been　formulated　which　enable　the　pressures

to　be　calculated。



DIFFERENCE　BETWEEN　LIFT　AND　DRIFT　IN　MOTION。The

first　observation　is　directed　to　the　differences

that　exist　between　the　lift　and　drift；

when　the　plane　is　placed　at　an　angle　of　less　than

45　degrees。　A　machine　weighing　1000　pounds

has　always　the　same　lift。　Its　mass　does　not

change。　Remember；　now；　we　allude　to　its　mass；

or　density。



We　are　not　now　referring　to　weight；　because

that　must　be　taken　into　consideration；　in　the

problem。　As　heretofore　stated；　when　an　object

moves　horizontally；　it　has　less　weight　than　when

at　rest。　If　it　had　the　same　weight　it　would　not

move　forwardly；　but　come　to　rest。



When　in　motion；　therefore；　while　the　lift；　so

far　as　its　mass　is　concerned；　does　not　change；　the

drift　does　decrease；　or　the　forward　pull　is　less

than　when　at　45　degrees；　and　the　decrease　is　less

and　less　until　the　plane　assumes　a　horizontal　position；

where　it　is　absolutely　nil；　if　we　do　not　consider

head　resistance。



TABLES　OF　LIFT　AND　DRIFT。All　tables　of　Lift

and　Drift　consider　only　the　air　pressures。　They

do　not　take　into　account　the　fact　that　momentum

takes　an　important　part　in　the　translation　of　an

object；　like　a　flying　machine。



A　mass　of　material；　weighing　1000　pounds　while

at　rest；　sets　up　an　enormous　energy　when　moving

through　the　air　at　fifty；　seventy…five；　or　one　hundred

miles　an　hour。　At　the　latter　speed　the　movement

is　about　160　feet　per　second；　a　motion　which

is　nearly　sufficient　to　maintain　it　in　horizontal

flight；　independently　of　any　plane　surface。



Such　being　the　case；　why　take　into　account　only

the　angle　of　the　plane？　It　is　no　wonder　that

aviators　have　not　been　able　to　make　the　theoretical

considerations　and　the　practical　demonstrations

agree。



WHY　TABLES　OF　LIFT　AND　DRIFT　ARE　WRONG。

A　little　reflection　will　show　why　such　tables　are

wrong。　They　were　prepared　by　using　a　plane

surface　at　rest；　and　forcing　a　blast　of　air　against

the　plane　placed　at　different　angles；　and　for　determining

air　pressures；　this　is；　no　doubt；　correct。

But　it　does　not　represent　actual　flying　conditions。

It　does　not　show　the　conditions　existing

in　an　aeroplane　while　in　flight。



To　determine　this；　short　of　actual　experiments

with　a　machine　in　horizontal　translation；　is　impossible；

unless　it　is　done　by　taking　into　account

the　factor　due　to　momentum　and　the　element

attributable　to　the　lift　of　the　plane　itself　due　to　its

impact　against　the　atmosphere。



LANGLEY'S　LAW。The　law　enunciated　by

Langley　is；　that　the　greater　the　speed　the　less　the

power　required　to　propel　it。　Water　as　a　propelling

medium　has　over　seven　hundred　times

more　force　than　air。　A　vessel　having；　for　instance；

twenty　horse　power；　and　a　speed　of　ten

miles　per　hour；　would　require　four　times　that

power　to　drive　it　through　the　water　at　double　the

speed。　The　power　is　as　the　square　of　the　speed。



With　air　the　conditions　are　entirely　different。

The　boat　submergence　in　the　water　is　practically

the　same；　whether　going　ten　or　twenty　miles　an

hour。　The　head　resistance　is　the　same；　substantially；

at　all　times　in　the　case　of　the　boat；　with　the

flying　machine　the　resistance　of　its　sustaining

surfaces　decreases。



Without　going　into　a　too　technical　description

of　the　reasoning　which　led　to　the　discovery　of　the

law　of　air　pressures；　let　us　try　and　understand

it　by　examining　the　diagram；　Fig。　7。



A　represents　a　plane　at　an　angle　of　45　degrees；

moving　forwardly　into　the　atmosphere　in　the

direction　of　the　arrows　B。　The　measurement

across　the　plane　vertically；　along　the　line　B；

which　is　called　the　sine　of　the　angle；　represents

the　surface　impact　of　air　against　the　plane。



In　Fig。　8　the　plane　is　at　an　angle　of　27　degrees；

which　makes　the　distance　in　height　across　the　line

C　just　one…half　the　length　of　the　line　B　of　Fig。　7；

hence　the　surface　impact　of　the　air　is　one…half　that

of　Fig。　7；　and　the　drift　is　correspondingly　decreased。



_Fig。　7。　Equal　Lift　and　Drift　in　Flig