flying machines-第34章

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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　70　to　80　miles　an

hour；　the　Wrights　are　enabled　to　successfully　navigate　a　machine

the　plane　area　of　which　is　about　130　square　feet。　This

apparatus　is　intended　to　carry　only　one　person　（the　operator）。

At　Belmont　Park；　N。　Y。；　the　Wrights　demonstrated　that　the

small…surfaced　biplane　is　much　faster；　easier　to　manage　in　the

hands　of　a　skilled　manipulator；　and　a　better　altitude　climber

than　the　large　and　cumbersome　machines　with　538　square　feet

of　surface　heretofore　used　by　them。



In　this　may　be　found　a　practical　illustration　of　the　principle

that　increased　speed　permits　of　a　reduction　in　plane　area　in

mathematical　ratio　to　the　gain　in　speed。　The　faster　any　object

can　be　made　to　move　through　the　air；　the　less　will　be　the

supporting

surface　required　to　sustain　a　given　weight。　But；　there

is　a　limit　beyond　which　the　plane　surface　cannot　be　reduced

with　safety。　Regard　must　always　be　had　to　the　securing　of

an　ample　sustaining　surface　so　that　in　case　of　motor　stoppage

there　will　be　sufficient　buoyancy　to　enable　the　operator　to

descend　safely。



The　baby　Wright　used　at　the　Belmont　Park　（N。　Y。）　aviation

meet　in　the　fall　of　1910；　had　a　plane　length　of　19　feet　6　inches；

and　an　extreme　breadth　of　21　feet　6　inches；　with　a　total　surface

area　of　146　square　feet。　It　was　equipped　with　a　new　Wright

8…cylinder　motor　of　60　h。　p。；　and　two　Wright　propellers　of　8

feet　6　inches　diameter　and　500　r。　p。　m。　It　was　easily　the　fastest

machine　at　the　meet。　After　the　tests；　Wilbur　Wright　said：



〃It　is　our　intention　to　put　together　a　machine　with　specially

designed　propellers；　specially　designed　gears　and　a　motor　which

will　give　us　65　horsepower　at　least。　We　will　then　be　able；

after　some　experimental　work　we　are　doing　now；　to　send　forth

a　machine　that　will　make　a　new　speed　record。〃



In　the　new　Wright　machines　the　front　elevating　planes　for

up…and…down　control　have　been　eliminated；　and　the　movements

of　the　apparatus　are　now　regulated