aeroplanes-第27章

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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　at　right　angles　to　a　base　line。　In　this

case　B　is　the　base　line。



Line　C　is　made　6　feet　long；　because　we　are　trying

to　find　the　angle　of　a　6　foot　pitch。　If；　now；　a

line　D　is　drawn　from　the　ends　of　the　two　lines　B；

C；　it　will　represent　the　pitch　which；　marked　across

the　end　of　the　blank　A；　will　indicate　the　line　to　cut

the　blade。



PITCH　RULE。The　rule　may；　therefore；　be

stated　as　follows：　Multiply　the　diameter　（in

feet）　of　the　propeller　by　3。1416；　and　draw　a　line

the　length　indicated　by　the　product。　At　one　end

of　this　line　draw　a　perpendicular　line　the　length

of　the　pitch　requirement　（in　feet）；　and　join　the

ends　of　the　two　lines　by　a　diagonal　line；　and　this

line　will　represent　the　pitch　angle。



Propellers　may　be　made　of　wood　or　metal；　the

former　being　preferred　for　the　reason　that　this

material　makes　a　lighter　article；　and　is　stronger；

in　some　respects；　than　any　metal　yet　suggested。



LAMINATED　CONSTRUCTION。All　propellers

should　be　laminated；that　is；　built　up　of　layers

of　wood；　glued　together　and　thoroughly　dried；

from　which　the　propeller　is　cut。



A　product　thus　made　is　much　more　serviceable

than　if　made　of　one　piece；　even　though　the　laminated

parts　are　of　the　same　wood；　because　the

different　strips　used　will　have　their　fibers　overlapping

each　other；　and　thus　greatly　augment　the

strength　of　the　whole。



Generally　the　alternate　strips　are　of　different

materials；　black　walnut；　mahogany；　birch；　spruce；

and　maple　being　the　most　largely　used；　but　mahogany

and　birch　seem　to　be　mostly　favored。



LAYING　UP　A　PROPELLER　FORM。The　first　step

necessary　is　to　prepare　thin　strips；　each；　say；

seven　feet　long；　and　five　inches　wide；　and　three…

eighths　of　an　inch　thick。　If　seven　such　pieces　are

put　together；　as　in　Fig。　78；　it　will　make　an　assemblage

of　two　and　five…eighth　inches　high。



_Fig。　78。　A　Laminated　Blank。_



Bore　a　hole　centrally　through　the　assemblage；

and　place　therein　a　pin　B。　The　contact　faces　of

these　strips　should　be　previously　well　painted

over　with　hot　glue　liberally　applied。　When　they

are　then　placed　in　position　and　the　pin　is　in　place；

the　ends　of　the　separate　pieces　are　offset；　one　beyond

the　other；　a　half　inch；　as　shown；　for　instance；

in　Fig。　79。



This　will　provide　ends　which　are　eight　and　a

half　inches　broad；　and　thus　furnish　sufficient

material　for　the　blades。　The　mass　is　then　subjected

to　heavy　pressure；　and　allowed　to　dry　before　the

blades　are　pared　down。



_Fig。　79。　Arranging　the　Strips。_



MAKING　WIDE　BLADES。If　a　wider　blade　is　desired；

a　greater　number　of　steps　may　be　made　by

adding　the　requisite　number　of　strips；　or；　the

strips　may　be　made　thicker。　In　many　propellers；

not　to　exceed　four　different　strips　are　thus　glued

together。　The　number　is　optional　with　the

maker。



An　end　view　of　such　an　assemblage　of　strips

is　illustrated　in　Fig。　80。　The　next　step　is　to　lay

off　the　pitch；　the　method　of　obtaining　which　has

been　explained。



_Fig。　80。　End　view　of　Blank。_



Before　starting　work　the　sides；　as　well　as　the

ends；　should　be　marked；　and　care　observed　to

place　a　distinctive　mark　on　the　front　side　of　the

propeller。



Around　the　pin　B；　Fig。　81；　make　S…shaped

marks　C；　to　indicate　where　the　cuts　on　the　faces

of　the　blades　are　to　begin。　Then　on　the　ends　of

the　block；　scribe　the　pitch　angle；　which　is　indicated

by　the　diagonal　line　D；　Fig。　80。



_Fig。　81。　Marking　the　Side。_



This　line　is　on　the　rear　side　of　the　propeller；

and　is　perfectly　straight。　Along　the　front　of　this

line　is　a　bowline　E；　which　indicates　the　front　surface

of　the　propeller　blade。



PROPELLER　OUTLINE。While　the　marks　thus

given　show　the　angles；　and　are　designed　to　indicate

the　two　faces　of　the　blades；　there　is　still　another

important　element　to　be　considered；　and

that　is　the　final　outline　of　the　blades。



_Fig。　82。　Outlining。_



It　is　obvious　that　the　outline　may　be　varied

so　that　the　entire　width　at　1；　Fig。　82；　may　be　used；

or　it　may　have　an　outline；　as　represented　by　the

line　2；　in　this　figure；　so　that　the　widest　part　will

be　at　or　near　the　dotted　line　3；　say　two…thirds　of

the　distance　from　the　center　of　the　blade。



This　is　the　practice　with　most　of　the　manufacturers

at　the　present　time；　and　some　of　them

claim　that　this　form　produces　the　best　results。



FOR　HIGHER　SPEEDS。Fig。　83　shows　a　propeller

cut　from　a　blank；　4〃　x　6〃　in　cross　section；　not

laminated。



_Fig。　83。　Cut　from　a　4〃　x　6〃　Single　Blank。_



It　should　be　borne　in　mind　that　for　high　speeds

the　blades　must　be　narrow。　A　propeller　seven

feet　in　diameter　with　a　six　foot　pitch；　turning

950　revolutions　per　minute；　will　produce　a　pull　of

350　pounds；　if　properly　made。



Such　a　propeller　can　be　readily　handled　by　a

forty　horse　power　motor；　such　as　are　specially

constructed　for　flying　machine　purposes。



INCREASING　PROPELLER　EFFICIENCY。Some　experiments

have　been　made　lately；　which；　it　is

claimed；　largely　increase　the　efficiency　of　propellers。

The　improvement　is　directed　to　the　outline

shape　of　the　blade。



The　typical　propeller；　such　as　we　have　illustrated；

is　one　with　the　wide　part　of　the　blade　at

the　extremity。　The　new　type；　as　suggested；　reverses

this；　and　makes　the　wide　part　of　the　blade

near　the　hub；　so　that　it　gradually　tapers　down　to

a　narrow　tip。



Such　a　form　of　construction　is　shown　in　Fig。

84。　This　outline　has　some　advantages　from　one

standpoint；　namely；　that　it　utilizes　that　part　of

the　blade　near　the　hub；　to　produce　a　pull；　and

does　not　relegate　all　the　duty　to　the　extreme　ends

or　tips。



_Fig。　84。　A　Suggested　Form。_



To　understand　this　more　fully；　let　us　take　a

propeller　six　feet　in　diameter；　and　measure　the

pull　or　thrust　at　the　tips；　and　also　at　a　point　half

way　between　the　tip　and　the　hub。



In　such　a　propeller；　if　the　blade　is　the　same

width　and　pitch　at　the　two　p