aeroplanes-第11章

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is　the　comparatively　small　head　resistance；　due

to　the　entire　absence　of　vertical　supporting　posts；

which　latter　are　necessary　with　the　biplane　type。

The　bracing　supports　which　hold　the　outer　ends

of　the　planes　are　composed　of　wires；　which　offer

but　little　resistance；　comparatively；　in　flight。



ITS　ADVANTAGES。Then　the　vertical　height　of

the　machine　is　much　less　than　in　the　biplane。　As

a　result　the　weight；　which　is　farther　below　the

supporting　surface　than　in　the　biplane；　aids　in

maintaining　the　lateral　stability；　particularly

since　the　supporting　frame　is　higher。



Usually；　for　the　same　wing　spread；　the　monoplane

is　narrower；　laterally；　which　is　a　further

aid　to　prevent　tilting。



ITS　DISADVANTAGES。But　it　also　has　disadvantages

which　must　be　apparent　from　its　structure。

As　all　the　supporting　surface　is　concentrated

in　half　the　number　of　planes；　they　must

be　made　of　greater　width　fore　and　aft；　and　this；

as　we　shall　see；　later　on；　proves　to　be　a　disadvantage。



It　is　also　doubted　whether　the　monoplane　can

be　made　as　strong　structurally　as　the　other　form；

owing　to　the　lack　of　the　truss　formation　which　is

the　strong　point　with　the　superposed　frame。　A

truss　is　a　form　of　construction　where　braces　can

be　used　from　one　member　to　the　next；　so　as　to

brace　and　stiffen　the　whole。



THE　BIPLANE。Nature　does　not　furnish　a　type

of　creature　which　has　superposed　wings。　In　this

particular　the　inventor　surely　did　not　follow　nature。

The　reasons　which　led　man　to　employ　this

type　may　be　summarized　as　follows：



In　experimenting　with　planes　it　is　found　that

a　broad　fore　and　aft　surface　will　not　lift　as　much

as　a　narrow　plane。　This　subject　is　fully　explained

in　the　chapter　on　The　Lifting　Surfaces　of

Planes。　In　view　of　that　the　technical　descriptions

of　the　operation　will　not　be　touched　upon

at　this　place；　except　so　far　as　it　may　be　necessary

to　set　forth　the　present　subject。



This　peculiarity　is　due　to　the　accumulation　of

a　mass　of　moving　air　at　the　rear　end　of　the　plane；

which　detracts　from　its　lifting　power。　As　it

would　be　a　point　of　structural　weakness　to　make

the　wings　narrow　and　very　long；　Wenham　many

years　ago　suggested　the　idea　of　placing　one　plane

above　the　other；　and　later　on　Chanute；　an

engineer；　used　that　form　almost　exclusively；　in

experimenting　with　his　gliders。



It　was　due　to　his　influence　that　the　Wrights

adopted　that　form　in　their　gliding　experiments；

and　later　on　constructed　their　successful　flyers

in　that　manner。　Originally　the　monoplane　was

the　type　generally　employed　by　experimenters；

such　as　Lilienthal；　and　others。



STABILITY　IN　BIPLANES。Biplanes　are　not　naturally

as　stable　laterally　as　the　monoplane。

The　reason　is；　that　a　downward　tilt　has　the　benefit

of　only　a　narrow　surface；　comparable　with　the

monoplane；　which　has　broadness　of　wing。



To　illustrate　this；　let　us　assume　that　we　have

a　biplane　with　planes　five　feet　from　front　to　rear；

and　thirty…six　feet　in　length。　This　would　give

two　planes　with　a　sustaining　surface　of　360　square

feet。　The　monoplane　would；　probably；　divide

this　area　into　one　plane　eight　and　a　half　feet　from

front　to　rear；　and　42　feet　in　length。



In　the　monoplane　each　wing　would　project　out

about　three　feet　more　on　each　side；　but　it　would

have　eight　and　a　half　feet　fore　and　aft　spread

to　the　biplane's　five　feet；　and　thus　act　as　a　greater

support。



THE　ORTHOPTER。The　term　orthopter；　or　ornithopter；

meaning　bird　wing；　is　applied　to　such

flying　machines　as　depend　on　wing　motion　to　support

them　in　the　air。



Unquestionably；　a　support　can　be　obtained　by

beating　on　the　air　but　to　do　so　it　is　necessary　to

adopt　the　principle　employed　by　nature　to　secure

an　upward　propulsion。　As　pointed　out　elsewhere；

it　cannot　be　the　concaved　type　of　wing；

or　its　shape；　or　relative　size　to　the　weight　it　must

carry。



As　nature　has　furnished　such　a　variety　of　data

on　these　points；　all　varying　to　such　a　remarkable

degree；　we　must　look　elsewhere　to　find　the　secret。

Only　one　other　direction　offers　any　opportunity；

and　that　is　in　the　individual　wing　movement。



NATURE'S　TYPE　NOT　UNIFORM。When　this　is

examined；　the　same　obscurity　surrounds　the　issue。

Even　the　speeds　vary　to　such　an　extent　that　when

it　is　tried　to　differentiate　them；　in　comparison

with　form；　shape；　and　construction；　the　experimenter

finds　himself　wrapt　in　doubt　and　perplexity。



But　birds　do　fly；　notwithstanding　this　wonderful

array　of　contradictory　exhibitions。　Observation

has　not　enabled　us　to　learn　why　these　things

are　so。　High　authorities；　and　men　who　are　expert

aviators；　tell　us　that　the　bird　flies　because

it　is　able　to　pick　out　ascending　air　currents。



THEORIES　ABOUT　FLIGHT　OF　BIRDS。Then　we

are　offered　the　theory　that　the　bird　has　an　instinct

which　tells　it　just　how　to　balance　in　the

air　when　its　wings　are　once　set　in　motion。

Frequently；　what　is　taken　for　instinct；　is　something

entirely　different。



It　has　been　assumed；　for　instance；　that　a　cyclist

making　a　turn　at　a　rapid　speed；　and　a　bird　flying

around　a　circle　will　throw　the　upper　part　of　the

body　inwardly　to　counteract　the　centrifugal　force

which　tends　to　throw　it　outwardly。



Experiments　with　the　monorail　car；　which　is

equipped　with　a　gyroscope　to　hold　it　in　a　vertical

position；　show　that　when　the　car　approaches　a

curve　the　car　will　lean　inwardly；　exactly　the　same

as　a　bird；　or　a　cyclist；　and　when　a　straight　stretch

is　reached；　it　will　again　straighten　up。



INSTINCT。Now；　either　the　car；　so　equipped

possesses　instinct；　or　there　must　be　a　principle

in　the　laws　of　nature　which　produces　the　similarity

of　action。



In　like　manner　there　must　be　some　principle

that　is　entirely　independent　of　the　form　of　matter；

or　its　arrangement；　which　enables　the　bird

to　perform　its　evolutions。　We　are　led　to　believe

from　all　the　foregoing　considerations　that　it　is

the　manner　or　the　form　of　the　motion。



MODE　OF　MOTION。In　this　respect　it　seems　to

be　comparable　in　every　respect　to　the　great　and

universal　law　of　the　motions　in　the　universe。

Thus；　light；　heat　and　electricity　are　the　same；　the

manifestations　being　unlike　only　because　they

have　different　modes　of　motion。



Everything　in　nature　manifests　itself　by　motion。

It　is　the　only　way　in　which　nature　acts。

Every　transformation　from　one　thing　to　another；

is　by　way　of　a　movement　which　is　characteristic

in　itself。



Why；　then；　should　this　great　mystery　of　nature；

act　unlike　the　other　portions　of　which　it　is

a　part？



THE　WING　STRUCTURE。The　wing　structure　of

every　flying　creature　that　man　has　examined；　has

one　universal　point　of　similarity；　and　that　is　the

manner　of　its　connection　with　the　body。　It　is　a

sort　of　universal　joint；　which　permits　the　wing

to　swing　up　and　down；　perform　a　gyratory　movement

while　doing　so；　and　folds　to　the　rear　when

at　rest。



Some　have　these　movements　in　a　greater　or

less　degree；　or　capable　of　a　greater　range；　but

the　joint　is　the　same；　with　scarcely　an　exception。

When　the　stroke　of　the　wing　is　downwardly　the

rear　margin　is　higher　than　the　front　edge；　so

that　the　downward　beat　not　only　raises　the　body

upwardly；　but　also　propels　it　forwardly。



THE　WING　MOVEMENT。The　moment　the　wing

starts　to　swing　upwardly　the　rear　end　is

depressed；　and　now；　as　the　bird　is　moving　forwardly；

the　wing　surface　has　a　positive　angle　of

incidence；　and　as　the　wing　rises　while　the　forward

motion　is　taking　place；　there　is　no　resistance

which　is　effective　enough　to　counteract　the

momentum　which　has　been　set　up。



The　great　problem　is　to　put　this　motion　into　a

mechanical　form。　The　trouble　is　not　ascribable

to　the　inability　of　the　mechanic　to　describe　this

movement。　It　is　an　exceedingly　simple　one。

The　first　difficulty　is　in　the　material　that　must

be　used。　Lightness　and　strength　for　the　wing

itself　are　the　first　requirements。　Then　rigidi