flying machines-第27部分

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aforesaid　by　the　Aero　Club　of　America

by　granting　licenses　to　promoters　who　make　satisfactory

arrangements　with　the　company　for　its　compensation

for　the　use　of　its　patents。　At　such　licensed　meet　any

machine　of　any　make　may　participate　freely　without

securing　any　further　license　or　permit。　The　details　and

terms　of　all　meets　will　be　arranged　by　the　committee

having　in　charge　the　interests　of　both　organizations。







CHAPTER　XXIV。



HINTS　ON　PROPELLER　CONSTRUCTION。



Every　professional　aviator　has　his　own　ideas　as　to　the

design　of　the　propeller；　one　of　the　most　important　features

of　flying…machine　construction。　While　in　many

instances　the　propeller；　at　a　casual　glance；　may　appear

to　be　identical；　close　inspection　will　develop　the　fact　that

in　nearly　every　case　some　individual　idea　of　the　designer

has　been　incorporated。　Thus；　two　propellers　of　the　two…

bladed　variety；　while　of　the　same　general　size　as　to

length　and　width　of　blade；　will　vary　greatly　as　to　pitch

and　〃twist〃　or　curvature。



What　the　Designers　Seek。



Every　designer　is　seeking　for　the　same　resultthe

securing　of　the　greatest　possible　thrust；　or　air　displacement；

with　the　least　possible　energy。



The　angles　of　any　screw　propeller　blade　having　a

uniform　or　true　pitch　change　gradually　for　every　increased

diameter。　In　order　to　give　a　reasonably　clear

explanation；　it　will　be　well　to　review　in　a　primary　way

some　of　the　definitions　or　terms　used　in　connection　with

and　applied　to　screw　propellers。



Terms　in　General　Use。



Pitch。The　term　〃pitch；〃　as　applied　to　a　screw　propeller；

is　the　theoretical　distance　through　which　it　would

travel　without　slip　in　one　revolution；　and　as　applied　to

a　propeller　blade　it　is　the　angle　at　which　the　blades　are

set　so　as　to　enable　them　to　travel　in　a　spiral　path　through

a　fixed　distance　theoretically　without　slip　in　one　revolution。



Pitch　speed。The　term　〃pitch　speed〃　of　a　screw

propeller　is　the　speed　in　feet　multiplied　by　the　number　of

revolutions　it　is　caused　to　make　in　one　minute　of　time。

If　a　screw　propeller　is　revolved　600　times　per　minute；

and　if　its　pitch　is　7　ft。；　then　the　pitch　speed　of　such　a

propeller　would　be　7x600　revolutions；　or　4200　ft。　per

minute。



Uniform　pitch。A　true　pitch　screw　propeller　is　one

having　its　blades　formed　in　such　a　manner　as　to　enable

all　of　its　useful　portions；　from　the　portion　nearest　the

hub　to　its　outer　portion；　to　travel　at　a　uniform　pitch

speed。　Or；　in　other　words；　the　pitch　is　uniform　when　the

projected　area　of　the　blade　is　parallel　along　its　full

length　and　at　the　same　time　representing　a　true　sector

of　a　circle。



All　screw　propellers　having　a　pitch　equal　to　their

diameters　have　the　same　angle　for　their　blades　at　their

largest　diameter。



When　Pitch　Is　Not　Uniform。



A　screw　propeller　not　having　a　uniform　pitch；　but

having　the　same　angle　for　all　portions　of　its　blades；　or

some　arbitrary　angle　not　a　true　pitch；　is　distinguished

from　one　having　a　true　pitch　in　the　variation　of　the　pitch

speeds　that　the　various　portions　of　its　blades　are　forced

to　travel　through　while　traveling　at　its　maximum　pitch

speed。



On　this　subject　Mr。　R。　W。　Jamieson　says　in　Aeronautics：



〃Take　for　example　an　8…foot　screw　propeller　having　an

8…foot　pitch　at　its　largest　diameter。　If　the　angle　is　the

same　throughout　its　entire　blade　length；　then　all　the　porions

of　its　blades　approaching　the　hub　from　its　outer　portion　would

have　a　gradually　decreasing　pitch。　The　2…foot

portion　would　have　a　2…foot　pitch；　the　3…foot　portion　a　3…

foot　pitch；　and　so　on　to　the　8…foot　portion　which　would

have　an　8…foot　pitch。　When　this　form　of　propeller　is

caused　to　revolve；　say　500　r。p。m。；　the　8…foot　portion　would

have　a　calculated　pitch　speed　of　8　feet　by　500　revolutions；

or　4；000　feet　per　min。；　while　the　2…foot　portion　would

have　a　calculated　pitch　speed　of　500　revolutions　by　2　feet；

or　1；000　feet　per　minute。



Effect　of　Non…Uniformity。



〃Now；　as　all　of　the　portions　of　this　type　of　screw

propeller　must　travel　at　some　pitch　speed；　which　must　have

for　its　maximum　a　pitch　speed　in　feet　below　the　calculated

pitch　speed　of　the　largest　diameter；　it　follows　that

some　portions　of　its　blades　would　perform　useful　work

while　the　action　of　the　other　portions　would　be　negative

resisting　the　forward　motion　of　the　portions　having　a

greater　pitch　speed。　The　portions　having　a　pitch　speed

below　that　at　which　the　screw　is　traveling　cease　to　perform

useful　work　after　their　pitch　speed　has　been　exceeded

by　the　portions　having　a　larger　diameter　and　a

greater　pitch　speed。



〃We　might　compare　the　larger　and　smaller　diameter

portions　of　this　form　of　screw　propeller；　to　two　power…

driven　vessels　connected　with　a　line；　one　capable　of　traveling

20　miles　per　hour；　the　other　10　miles　per　hour。　It

can　be　readily　understood　that　the　boat　capable　of　traveling

10　miles　per　hour　would　have　no　useful　effect　to

help　the　one　traveling　20　miles　per　hour；　as　its　action

would　be　such　as　to　impose　a　dead　load　upon　the　latter's

progress。〃



The　term　〃slip；〃　as　applied　to　a　screw　propeller；　is　the

distance　between　its　calculated　pitch　speed　and　the　actual

distance　it　travels　through　under　load；　depending　upon

the　efficiency　and　proportion　of　its　blades　and　the　amount

of　load　it　has　to　carry。



The　action　of　a　screw　propeller　while　performing　useful

work　might　be　compared　to　a　nut　traveling　on　a

threaded　bolt；　little　resistance　is　offered　to　its　forward

motion　while　it　spins　freely　without　load；　but　give　it　a

load　to　carry；　then　it　will　take　more　power　to　keep　up　its

speed；　if　too　great　a　load　is　applied　the　thread　will　strip；

and　so　it　is　with　a　screw　propeller　gliding　spirally　on　the

air。　A　propeller　traveling　without　load　on　to　new　air

might　be　compared　to　the　nut　traveling　freely　on　the　bolt。

It　would　consume　but　little　power　and　it　would　travel　at

nearly　its　calculated　pitch　speed；　but　give　it　work　to　do

and　then　it　will　take　power　to　drive　it。



There　is　a　reaction　caused　from　the　propeller　projecting

air　backward　when　it　slips；　which；　together　with　the　supporting

effect　of　the　blades；　combine　to　produce　useful

work　or　pull　on　the　object　to　be　carried。



A　screw　propeller　working　under　load　approaches　more

closely　to　its　maximum　efficiency　as　it　carries　its　load

with　a　minimum　amount　of　slip；　or　nearing　its　calculated

pitch　speed。



Why　Blades　Are　Curved。



It　has　been　pointed　out　by　experiment　that　certain

forms　of　curved　surfaces　as　applied　to　aeroplanes　will　lift

more　per　horse　power；　per　unit　of　square　foot；　while　on

the　other　hand　it　has　been　shown　that　a　flat　surface　will

lift　more　per　horse　power；　but　requires　more　area　of　surface

to　do　it。



As　a　true　pitch　screw　propeller　is　virtually　a　rotating

aeroplane；　a　curved　surface　may　be　advantageously　employed

when　the　limit　of　size　prevents　using　large　plane

surfaces　for　the　blades。



Care　should　be　exercised　in　keeping　the　chord　of　any

curve　to　be　used　for　the　blades　at　the　proper　pitch　angle；

and　in　all　cases　propeller　blades　should　be　made　rigid　so

as　to　preserve　the　true　angle　and　not　be　distorted　by

centrifugal　force　or　from　any　other　cause；　as　flexibility

will　seriously　affect　their　pitch　speed　and　otherwise　affect

their　efficiency。



How　to　Determine　Angle。



To　find　the　angle　for　the　proper　pitch　at　any　point　in

the　diameter　of　a　propeller；　determine　the　circumference

by　multiplying　the　diameter　by　3。1416；　which　represent

by　drawing　a　line　to　scale　in　feet。　At　the　end　of　this　line

draw　another　line　to　represent　the　desired　pitch　in　feet。

Then　draw　a　line　from　the　point　representing　the　desired

pitch　in　feet　to　the　beginning　of　the　circumference　line。

For　example：



If　the　propeller　to　be　laid　out　is　7　feet　in　diameter；　and

is　to　have　a　7…foot　pitch；　the　circumference　will　be　21。99

feet。　Draw　a　diagram　representing　the　circumference

line　and　pitch　in　feet。　If　this　diagram　is　wrapped　around

a　cylinder　the　angle　line　will　represent　a　true　thread　7

feet　in　diameter　and　7　feet　long；　and　the　angle　of　the

thread　will　be　17　3/4　degrees。



Relation　of　Diameter　to　Circumference。



Since　the　areas　of　circles　decrease　as　the　diameter

lessens；　it　follows　that　if　a　propeller　is　to　travel　at　a　uniform

pitch　speed；　the　volume　of　its　blade　displacement

should　decrease　as　its　diameter　becomes　less；　so　as　to

occupy　a　corresponding　relation　to　the　circumferences　of

larger　diameters；　and　at　the　same　time　the　projected

area　of　the　blade　must　be　parallel　along　its　full　length

and　should　represent　a　true　sector　of　a　circle。



Let　us　suppose　a　7…foot　circle　to　be　divided　into　20

sectors；　one　of　which　represents　a　propeller　blade。　If　the

pitch　is　to　be　7　feet；　then　the　greatest　depth　of　the　angle

would　be　1/20　part　of　the　pitch；　or　4　2/10　inch。　If　the

line　representing　the　greatest　depth　of　the　angle　is　kept

the　same　width　as　it　approaches　the　hub；　the　pitch　will

be　uniform。　If　the　blade　is　set　at　an　angle　so　its　projected

area　is　1/20　part　of　the　pitch；　and　if　it　is　moved

through　20　divisions　for　one　revolution；　it　would　have　a

travel　of　7　feet。







CHAPTER　XXV。



NEW　MOTORS　AND　DEVICES。



Since　the　first　edition　of　this　book　was　printed；　early　in　1910；

there　has　been　a　remarkable　advance　in　the　construction　of

aeroplane　motors；　which　has　resulted　in　a　wonderful　decrease

in　the　amount　of　surface　area　from　that　formerly　required。

Marked　gain　in　lightness　and　speed　of　the　motor　has　enabled

aviators　to　get　along；　in　some　instances；　with　one…quarter　of

the　plane　supporting　area　previously　used。　The　first　Wright

biplane；　propelled　by　a　motor　of　25　h。p。；　productive　of　a　fair

average　speed　of　30　miles　an　hour；　had　a　plane　surface　of　538

square　feet。　Now；　by　using　a　specially　designed　motor　of　65

h。　p。；　capable　of　developing　a　speed　of　from