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£º¡¡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¡¡points¡¡named£»¡¡the¡¡pull
at¡¡the¡¡tips¡¡will¡¡be¡¡four¡¡times¡¡greater¡¡than¡¡at¡¡the
intermediate¡¡point¡£
CHAPTER¡¡XIV
EXPERIMENTAL¡¡GLIDERS¡¡AND¡¡MODEL¡¡AEROPLANES
AN¡¡amusing¡¡and¡¡very¡¡instructive¡¡pastime¡¡is
afforded¡¡by¡¡constructing¡¡and¡¡flying¡¡gliding¡¡machines£»
and¡¡operating¡¡model¡¡aeroplanes£»¡¡the¡¡latter
being¡¡equipped¡¡with¡¡their¡¡own¡¡power¡£
Abroad¡¡this¡¡work¡¡has¡¡been¡¡very¡¡successful¡¡as
a¡¡means¡¡of¡¡interesting¡¡boys£»¡¡and£»¡¡indeed£»¡¡men
who¡¡have¡¡taken¡¡up¡¡the¡¡science¡¡of¡¡aviation¡¡are
giving¡¡this¡¡sport¡¡serious¡¡thought¡¡and¡¡study¡£
When¡¡a¡¡machine¡¡of¡¡small¡¡dimensions¡¡is¡¡made
the¡¡boy¡¡wonders¡¡why¡¡a¡¡large¡¡machine¡¡does¡¡not
bear¡¡the¡¡same¡¡relation¡¡in¡¡weight¡¡as¡¡a¡¡small¡¡machine¡£
This¡¡is¡¡one¡¡of¡¡the¡¡first¡¡lessons¡¡to¡¡learn¡£
THE¡¡RELATION¡¡OF¡¡MODELS¡¡TO¡¡FLYING¡¡MACHINES¡£
A¡¡model¡¡aeroplane£»¡¡say¡¡two¡¡feet¡¡in¡¡length£»¡¡which
has£»¡¡we¡¡will¡¡assume£»¡¡50¡¡square¡¡inches¡¡of¡¡supporting
surface£»¡¡seems¡¡to¡¡be¡¡a¡¡very¡¡rigid¡¡structure£»
in¡¡proportion¡¡to¡¡its¡¡weight¡£¡¡It¡¡may¡¡be¡¡dropped
from¡¡a¡¡considerable¡¡height¡¡without¡¡injuring¡¡it£»
since¡¡the¡¡weight¡¡is¡¡only¡¡between¡¡two¡¡and¡¡three
ounces¡£
An¡¡aeroplane¡¡twenty¡¡times¡¡the¡¡length¡¡of¡¡this
model£»¡¡however¡¡strongly¡¡it¡¡may¡¡be¡¡made£»¡¡if
dropped¡¡the¡¡same¡¡distance£»¡¡would¡¡be¡¡crushed£»¡¡and
probably¡¡broken¡¡into¡¡fragments¡£
If¡¡the¡¡large¡¡machine¡¡is¡¡twenty¡¡times¡¡the¡¡dimensions
of¡¡the¡¡small¡¡one£»¡¡it¡¡would¡¡be¡¡forty¡¡feet¡¡in
length£»¡¡and£»¡¡proportionally£»¡¡would¡¡have¡¡only
seven¡¡square¡¡feet¡¡of¡¡sustaining¡¡surface¡£¡¡But¡¡an
operative¡¡machine¡¡of¡¡that¡¡size£»¡¡to¡¡be¡¡at¡¡all¡¡rigid£»
would¡¡require¡¡more¡¡than¡¡twenty¡¡times¡¡the¡¡material
in¡¡weight¡¡to¡¡be¡¡equal¡¡in¡¡strength¡£
It¡¡would¡¡weigh¡¡about¡¡800¡¡pounds£»¡¡that¡¡is£»¡¡4800
times¡¡the¡¡weight¡¡of¡¡the¡¡model£»¡¡and¡¡instead¡¡of
having¡¡twenty¡¡times¡¡the¡¡plane¡¡surface¡¡would¡¡require
one¡¡thousand¡¡times¡¡the¡¡spread¡£
It¡¡is¡¡this¡¡peculiarity¡¡between¡¡models¡¡and¡¡the
actual¡¡flyers¡¡that¡¡for¡¡years¡¡made¡¡the¡¡question¡¡of
flying¡¡a¡¡problem¡¡which£»¡¡on¡¡the¡¡basis¡¡of¡¡pure¡¡calculation
alone£»¡¡seemed¡¡to¡¡offer¡¡a¡¡negative£»¡¡and
many¡¡scientific¡¡men¡¡declared¡¡that¡¡practical¡¡flying
was¡¡an¡¡impossibility¡£
LESSONS¡¡FROM¡¡MODELS¡£Men£»¡¡and¡¡boys£»¡¡too£»
can¡¡learn¡¡a¡¡useful¡¡lesson¡¡from¡¡the¡¡model¡¡aeroplanes
in¡¡other¡¡directions£»¡¡however£»¡¡and¡¡the¡¡principal
thing¡¡is¡¡the¡¡one¡¡of¡¡stability¡£
When¡¡everything¡¡is¡¡considered¡¡the¡¡form¡¡or
shape¡¡of¡¡a¡¡flying¡¡model¡¡will¡¡serve¡¡to¡¡make¡¡a¡¡large
flyer¡£¡¡The¡¡manner¡¡of¡¡balancing¡¡one¡¡will¡¡be¡¡a
good¡¡criterion¡¡for¡¡the¡¡other¡¡in¡¡practice£»¡¡and
experimenting¡¡with¡¡these¡¡small¡¡devices¡¡is£»¡¡therefore£»
most¡¡instructive¡£
The¡¡difference¡¡between¡¡gliders¡¡and¡¡model¡¡aeroplanes
is£»¡¡that¡¡gliders¡¡must¡¡be¡¡made¡¡much¡¡lighter
because¡¡they¡¡are¡¡designed¡¡to¡¡be¡¡projected¡¡through
the¡¡air¡¡by¡¡a¡¡kick¡¡of¡¡some¡¡kind¡£
FLYING¡¡MODEL¡¡AEROPLANES¡£Model¡¡aeroplanes
contain¡¡their¡¡own¡¡power¡¡and¡¡propellers¡¡which£»
while¡¡they¡¡may¡¡run¡¡for¡¡a¡¡few¡¡seconds¡¡only£»¡¡serve
the¡¡purpose¡¡of¡¡indicating¡¡how¡¡the¡¡propeller¡¡will
act£»¡¡and¡¡in¡¡what¡¡respect¡¡the¡¡sustaining¡¡surfaces
are¡¡efficient¡¡and¡¡properly¡¡arranged¡£
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subject¡¡but¡¡to¡¡confine¡¡this¡¡chapter¡¡to¡¡an¡¡exposition
of¡¡a¡¡few¡¡of¡¡the¡¡gliders¡¡and¡¡model¡¡forms¡¡which
are¡¡found¡¡to¡¡be¡¡most¡¡efficient¡¡for¡¡experimental
work¡£
AN¡¡EFFICIENT¡¡GLIDER¡£Probably¡¡the¡¡simplest
and¡¡most¡¡efficient¡¡glider£»¡¡and¡¡one¡¡which¡¡can¡¡be
made¡¡in¡¡a¡¡few¡¡moments£»¡¡is¡¡to¡¡make¡¡a¡¡copy¡¡of¡¡the
deltoid¡¡kite£»¡¡previously¡¡referred¡¡to¡£
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paper£»¡¡or¡¡stiff¡¡cardboard¡¡A£»¡¡Fig¡£¡¡84£»¡¡creased¡¡in
the¡¡middle£»¡¡along¡¡the¡¡dotted¡¡line¡¡B£»¡¡the¡¡side¡¡wings
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THE¡¡DELTOID¡¡FORMATION¡£This¡¡same¡¡form¡¡may
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in¡¡that¡¡case¡¡the¡¡broad¡¡end¡¡should¡¡be¡¡foremost¡£
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