flying machines-第14章

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to　experts；　is　best　obtained　with　a　large　propeller

diameter　and　reasonably　low　speed。　The　diameter　is　the

distance　from　end　to　end　of　the　blades；　which　on　the

largest　propellers　ranges　from　6　to　8　feet。　The　larger

the　blade　surface　the　greater　will　be　the　volume　of　air

displaced；　and；　following　this；　the　greater　will　be　the

impulse　which　forces　the　aeroplane　ahead。　In　all　centrifugal

motion　there　is　more　or　less　tendency　to　disintegration

in　the　form　of　〃flying　off〃　from　the　center；　and

the　larger　the　revolving　object　is　the　stronger　is　this

tendency。　This　is　illustrated　in　the　many　instances　in

which　big　grindstones　and　fly…wheels　have　burst　from

being　revolved　too　fast。　To　have　a　propeller　break

apart　in　the　air　would　jeopardize　the　life　of　the　aviator；

and　to　guard　against　this　it　has　been　found　best　to　make

its　revolving　action　comparatively　slow。　Besides　this

the　slow　motion　（it　is　only　comparatively　slow）　gives

the　atmosphere　a　chance　to　refill　the　area　disturbed　by

one　propeller　blade；　and　thus　have　a　new　surface　for

the　next　blade　to　act　upon。



Placing　of　the　Motor。



As　on　other　points；　aviators　differ　widely　in　their

ideas　as　to　the　proper　position　for　the　motor。　Wright

locates　his　on　the　lower　plane；　midway　between　the　front

and　rear　edges；　but　considerably　to　one　side　of　the　exact

center。　He　then　counter…balances　the　engine　weight　by

placing　his　seat　far　enough　away　in　the　opposite　direction

to　preserve　the　center　of　gravity。　This　leaves　a

space　in　the　center　between　the　motor　and　the　operator

in　which　a　passenger　may　be　carried　without　disturbing

the　equilibrium。



Bleriot；　on　the　contrary；　has　his　motor　directly　in

front　and　preserves　the　center　of　gravity　by　taking　his

seat　well　back；　this；　with　the　weight　of　the　aeroplane；

acting　as　a　counter…balance。



On　the　Curtiss　machine　the　motor　is　in　the　rear；　the

forward　seat　of　the　operator；　and　weight　of　the　horizontal

rudder　and　damping　plane　in　front　equalizing　the

engine　weight。



No　Perfect　Motor　as　Yet。



Engine　makers　in　the　United　States；　England；　France

and　Germany　are　all　seeking　to　produce　an　ideal　motor

for　aviation　purposes。　Many　of　the　productions　are

highly　creditable；　but　it　may　be　truthfully　said　that

none　of　them　quite　fill　the　bill　as　regards　a　combination

of　the　minimum　of　weight　with　the　maximum　of

reliable　maintained　power。　They　are　all；　in　some　respects；

improvements　upon　those　previously　in　use；　but

the　great　end　sought　for　has　not　been　fully　attained。



One　of　the　motors　thus　produced　was　made　by　the

French　firm　of　Darracq　at　the　suggestion　of　Santos　Dumont；　and　on

lines　laid　down　by　him。　Santos　Dumont

wanted　a　2…cylinder　horizontal　motor　capable　of　developing

30　horsepower；　and　not　exceeding　4　1/2　pounds　per

horsepower　in　weight。



There　can　be　no　question　as　to　the　ability　and　skill

of　the　Darracq　people；　or　of　their　desire　to　produce　a

motor　that　would　bring　new　credit　and　prominence　to

the　firm。　Neither　could　anything　radically　wrong　be

detected　in　the　plans。　But　the　motor；　in　at　least　one

important　requirement；　fell　short　of　expectations。



It　could　not　be　depended　upon　to　deliver　an　energy

of　30　horsepower　continuously　for　any　length　of　time。

Its　maximum　power　could　be　secured　only　in　〃spurts。〃



This　tends　to　show　how　hard　it　is　to　produce　an　ideal

motor　for　aviation　purposes。　Santos　Dumont；　of　undoubted

skill　and　experience　as　an　aviator；　outlined　definitely

what　he　wanted；　one　of　the　greatest　designers

in　the　business　drew　the　plans；　and　the　famous　house　of

Darracq　bent　its　best　energies　to　the　production。　But

the　desired　end　was　not　fully　attained。



Features　of　Darracq　Motor。



Horizontal　motors　were　practically　abandoned　some

time　ago　in　favor　of　the　vertical　type；　but　Santos　Dumont

had　a　logical　reason　for　reverting　to　them。　He

wanted　to　secure　a　lower　center　of　gravity　than　would

be　possible　with　a　vertical　engine。　Theoretically　his

idea　was　correct　as　the　horizontal　motor　lies　flat；　and

therefore　offers　less　resistance　to　the　wind；　but　it　did　not

work　out　as　desired。



At　the　same　time　it　must　be　admitted　that　this　Darracq

motor　is　a　marvel　of　ingenuity　and　exquisite　workmanship。

The　two　cylinders；　having　a　bore　of　5　1…10

inches　and　a　stroke　of　4　7…10　inches；　are　machined　out

of　a　solid　bar　of　steel　until　their　weight　is　only　8　4…5

pounds　complete。　The　head　is　separate；　carrying　the

seatings　for　the　inlet　and　exhaust　valves；　is　screwed　onto

the　cylinder；　and　then　welded　in　position。　A　copper

water…jacket　is　fitted；　and　it　is　in　this　condition　that　the

weight　of　8　4…5　pounds　is　obtained。



On　long　trips；　especially　in　regions　where　gasolene　is

hard　to　get；　the　weight　of　the　fuel　supply　is　an　important

feature　in　aviation。　As　a　natural　consequence　flying

machine　operators　favor　the　motor　of　greatest　economy

in　gasolene　consumption；　provided　it　gives　the　necessary

power。



An　American　inventor；　Ramsey　by　name；　is　working

on　a　motor　which　is　said　to　possess　great　possibilities

in　this　line。　Its　distinctive　features　include　a　connecting

rod　much　shorter　than　usual；　and　a　crank　shaft　located

the　length　of　the　crank　from　the　central　axis　of　the

cylinder。　This　has　the　effect　of　increasing　the　piston

stroke；　and　also　of　increasing　the　proportion　of　the

crank　circle　during　which　effective　pressure　is　applied

to　the　crank。



Making　the　connecting　rod　shorter　and　leaving　the

crank　mechanism　the　same　would　introduce　excessive

cylinder　friction。　This　Ramsey　overcomes　by　the　location

of　his　crank　shaft。　The　effect　of　the　long　piston

stroke　thus　secured；　is　to　increase　the　expansion　of　the

gases；　which　in　turn　increases　the　power　of　the　engine

without　increasing　the　amount　of　fuel　used。



Propeller　Thrust　Important。



There　is　one　great　principle　in　flying　machine　propulsion

which　must　not　be　overlooked。　No　matter　how

powerful　the　engine　may　be　unless　the　propeller　thrust

more　than　overcomes　the　wind　pressure　there　can　be

no　progress　forward。　Should　the　force　of　this　propeller

thrust　and　that　of　the　wind　pressure　be　equal　the　result

is　obvious。　The　machine　is　at　a　stand…still　so　far

as　forward　progress　is　concerned　and　is　deprived　of　the

essential　advancing　movement。



Speed　not　only　furnishes　sustentation　for　the　airship；

but　adds　to　the　stability　of　the　machine。　An　aeroplane

which　may　be　jerky　and　uncertain　in　its　movements；　so

far　as　equilibrium　is　concerned；　when　moving　at　a　slow

gait；　will　readily　maintain　an　even　keel　when　the　speed

is　increased。



Designs　for　Propeller　Blades。



It　is　the　object　of　all　men　who　design　propellers　to

obtain　the　maximum　of　thrust　with　the　minimum　expenditure

of　engine　energy。　With　this　purpose　in　view

many　peculiar　forms　of　propeller　blades　have　been

evolved。　In　theory　it　would　seem　that　the　best　effects

could　be　secured　with　blades　so　shaped　as　to　present　a

thin　（or　cutting）　edge　when　they　come　out　of　the　wind；

and　then　at　the　climax　of　displacement　afford　a　maximum

of　surface　so　as　to　displace　as　much　air　as　possible。

While　this　is　the　form　most　generally　favored

there　are　others　in　successful　operation。



There　is　also　wide　difference　in　opinion　as　to　the

equipment　of　the　propeller　shaft　with　two　or　more

blades。　Some　aviators　use　two　and　some　four。　All

have　more　or　less　success。　As　a　mathematical　proposition

it　would　seem　that　four　blades　should　give　more

propulsive　force　than　two；　but　here　again　comes　in　one

of　the　puzzles　of　aviation；　as　this　result　is　not　always

obtained。



Difference　in　Propeller　Efficiency。



That　there　is　a　great　difference　in　propeller　efficiency

is　made　readily　apparent　by　the　comparison　of　effects

produced　in　two　leading　makes　of　machinesthe　Wright

and　the　Voisin。



In　the　former　a　weight　of　from　1；100　to　1；200　pounds

is　sustained　and　advance　progress　made　at　the　rate　of

40　mile