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such　an　agreement；　should　he　agree　to　a　stipulation　of

that　sort。　The　best　any　engine　maker　will　guarantee

is　to　build　an　engine　according　to　specifications。〃







CHAPTER　XX。



SOARING　FLIGHT。



By　Octave　Chanute。



＇5＇There　is　a　wonderful　performance　daily　exhibited　in

southern　climes　and　occasionally　seen　in　northerly

latitudes　in　summer；　which　has　never　been　thoroughly

explained。　It　is　the　soaring　or　sailing　flight　of　certain

varieties　of　large　birds　who　transport　themselves　on　rigid；

unflapping　wings　in　any　desired　direction；　who　in　winds

of　6　to　20　miles　per　hour；　circle；　rise；　advance；　return　and

remain　aloft　for　hours　without　a　beat　of　wing；　save　for

getting　under　way　or　convenience　in　various　maneuvers。

They　appear　to　obtain　from　the　wind　alone　all　the　necessary

energy；　even　to　advancing　dead　against　that　wind。

This　feat　is　so　much　opposed　to　our　general　ideas　of

physics　that　those　who　have　not　seen　it　sometimes　deny

its　actuality；　and　those　who　have　only　occasionally

witnessed　it　subsequently　doubt　the　evidence　of　their　own

eyes。　Others；　who　have　seen　the　exceptional　performances；

speculate　on　various　explanations；　but　the　majority

give　it　up　as　a　sort　of　〃negative　gravity。〃



＇5＇　Aeronautics。



Soaring　Power　of　Birds。



The　writer　of　this　paper　published　in　the　〃Aeronautical

Annual〃　for　1896　and　1897　an　article　upon　the　sailing

flight　of　birds；　in　which　he　gave　a　list　of　the　authors　who

had　described　such　flight　or　had　advanced　theories　for

its　explanation；　and　he　passed　these　in　review。　He　also

described　his　own　observations　and　submitted　some　computations

to　account　for　the　observed　facts。　These　computations

were　correct　as　far　as　they　went；　but　they　were

scanty。　It　was；　for　instance；　shown　convincingly　by

analysis　that　a　gull　weighing　2。188　pounds；　with　a　total

supporting　surface　of　2。015　square　feet；　a　maximum　body

cross…section　of　0。126　square　feet　and　a　maximum　cross…

section　of　wing　edges　of　0。098　square　feet；　patrolling　on

rigid　wings　（soaring）　on　the　weather　side　of　a　steamer

and　maintaining　an　upward　angle　or　attitude　of　5　degrees

to　7　degrees　above　the　horizon；　in　a　wind　blowing　12。78

miles　an　hour；　which　was　deflected　upward　10　degrees

to　20　degrees　by　the　side　of　the　steamer　（these　all　being

carefully　observed　facts）；　was　perfectly　sustained　at　its

own　〃relative　speed〃　of　17。88　miles　per　hour　and　extracted

from　the　upward　trend　of　the　wind　sufficient　energy

to　overcome　all　the　resistances；　this　energy

amounting　to　6。44　foot…pounds　per　second。



Great　Power　of　Gulls。



It　was　shown　that　the　same　bird　in　flapping　flight　in

calm　air；　with　an　attitude　or　incidence　of　3　degrees　to　5

degrees　above　the　horizon　and　a　speed　of　20。4　miles　an

hour　was　well　sustained　and　expended　5。88　foot…pounds

per　second；　this　being　at　the　rate　of　204　pounds　sustained

per　horsepower。　It　was　stated　also　that　a　gull　in　its　observed

maneuvers；　rising　up　from　a　pile　head　on　unflapping

wings；　then　plunging　forward　against　the　wind　and

subsequently　rising　higher　than　his　starting　point；　must

either　time　his　ascents　and　descents　exactly　with　the

variations　in　wind　velocities；　or　must　meet　a　wind　billow

rotating　on　a　horizontal　axis　and　come　to　a　poise　on　its

crest；　thus　availing　of　an　ascending　trend。



But　the　observations　failed　to　demonstrate　that　the

variations　of　the　wind　gusts　and　the　movements　of　the

bird　were　absolutely　synchronous；　and　it　was　conjectured

that　the　peculiar　shape　of　the　soaring　wing　of　certain

birds；　as　differentiated　from　the　flapping　wing；　might；

when　experimented　upon；　hereafter　account　for　the　performance。



Mystery　to　be　Explained。



These　computations；　however　satisfactory　they　were

for　the　speed　of　winds　observed；　failed　to　account　for　the

observed　spiral　soaring　of　buzzards　in　very　light　winds

and　the　writer　was　compelled　to　confess：　〃Now；　this

spiral　soaring　in　steady　breezes　of　5　to　10　miles　per　hour

which　are　apparently　horizontal；　and　through　which　the

bird　maintains　an　average　speed　of　about　20　miles　an

hour；　is　the　mystery　to　be　explained。　It　is　not　accounted

for；　quantitatively；　by　any　of　the　theories　which　have

been　advanced；　and　it　is　the　one　performance　which　has

led　some　observers　to　claim　that　it　was　done　through

'aspiration。'　i；　e。；　that　a　bird　acted　upon　by　a　current；

actually　drew　forward　into　that　current　against　its　exact

direction　of　motion。〃



Buzzards　Soar　in　Dead　Calm。



A　still　greater　mystery　was　propounded　by　the　few

observers　who　asserted　that　they　had　seen　buzzards　soaring

in　a　dead　calm；　maintaining　their　elevation　and　their

speed。　Among　these　observers　was　Mr。　E。　C。　Huffaker；

at　one　time　assistant　experimenter　for　Professor　Langley。

The　writer　believed　and　said　then　that　he　must　in　some

way　have　been　mistaken；　yet；　to　satisfy　himself；　he　paid

several　visits　to　Mr。　Huffaker；　in　Eastern　Tennessee　and

took　along　his　anemometer。　He　saw　quite　a　number　of

buzzards　sailing　at　a　height　of　75　to　100　feet　in　breezes

measuring　5　or　6　miles　an　hour　at　the　surface　of　the

ground；　and　once　he　saw　one　buzzard　soaring　apparently

in　a　dead　calm。



The　writer　was　fairly　baffled。　The　bird　was　not　simply

gliding；　utilizing　gravity　or　acquired　momentum；　he　was

actually　circling　horizontally　in　defiance　of　physics　and

mathematics。　It　took　two　years　and　a　whole　series　of

further　observations　to　bring　those　two　sciences　into

accord　with　the　facts。



Results　of　Close　Observations。



Curiously　enough　the　key　to　the　performance　of　circling

in　a　light　wind　or　a　dead　calm　was　not　found

through　the　usual　way　of　gathering　human　knowledge；

i。　e。；　through　observations　and　experiment。　These　had

failed　because　I　did　not　know　what　to　look　for。　The

mystery　was；　in　fact；　solved　by　an　eclectic　process　of

conjecture　and　computation；　but　once　these　computations

indicated　what　observations　should　be　made；　the　results

gave　at　once　the　reasons　for　the　circling　of　the　birds；　for

their　then　observed　attitude；　and　for　the　necessity　of　an

independent　initial　sustaining　speed　before　soaring　began。

Both　Mr。　Huffaker　and　myself　verified　the　data

many　times　and　I　made　the　computations。



These　observations　disclosed　several　facts：



1st。That　winds　blowing　five　to　seventeen　miles　per

hour　frequently　had　rising　trends　of　10　degrees　to　15

degrees；　and　that　upon　occasions　when　there　seemed　to　be

absolutely　no　wind；　there　was　often　nevertheless　a　local

rising　of　the　air　estimated　at　a　rate　of　four　to　eight　miles

or　more　per　hour。　This　was　ascertained　by　watching

thistledown；　and　rising　fogs　alongside　of　trees　or　hills　of

known　height。　Everyone　will　readily　realize　that　when

walking　at　the　rate　of　four　to　eight　miles　an　hour　in　a

dead　calm　the　〃relative　wind〃　is　quite　inappreciable　to

the　senses　and　that　such　a　rising　air　would　not　be　noticed。



2nd。That　the　buzzard；　sailing　in　an　apparently　dead

horizontal　calm；　progressed　at　speeds　of　fifteen　to　eighteen

miles　per　hour；　as　measured　by　his　shadow　on　the

ground。　It　was　thought　that　the　air　was　then　possibly

rising　8。8　feet　per　second；　or　six　miles　per　hour。



3rd。That　when　soaring　in　very　light　winds　the　angle

of　incidence　of　the　buzzards　was　negative　to　the　horizon

i。　e。；　that　when　seen　coming　toward　the　eye；　the　afternoon

light　shone　on　the　back　instead　of　on　the　breast；

as　would　have　been　the　case　had　the　angle　been　inclined

above　the　horizon。



4th。That　the　sailing　performance　only　occurred　after

the　bird　had　acquired　an　initial　velocity　of　at　least　fifteen

or　eighteen　miles　per　hour；　either　by　industrious　flapping

or　by　descending　from　a　perch。



An　Interesting　Experiment。



5th。That　the　whole　resistance　of　a　stuffed　buzzard；

at　a　negative　angle　of　3　degrees　in　a　current　of　air　of

15。52　miles　per　hour；　was　0。27　pounds。　This　test　was

kindly　made　for　the　writer　by　Professor　A。　F。　Zahm　in

the　〃wind　tunnel〃　of　the　Catholic　University　at　Washington；

D。　C。；　who；　moreover；　stated　that　the　resistance

of　a　live　bird　might　be　less；　as　the　dried　plumage　could

not　be　made　to　lie　smooth。



This　particular　buzzard　weighed　in　life　4。25　pounds；

the　area　of　his　wings　and　body　was　4。57　square　feet；　the

maximum　cross…section　of　his　body　was　0。110　square　feet；

and　that　of　his　wing　edges　when　fully　extended　was

0。244　square　feet。



With　these　data；　it　became　surprisingly　easy　to　compute

the　performance　with　the　coefficients　of　Lilienthal

for　various　angles　of　incidence　and　to　demonstrate　how

this　buzzard　could　soar　horizontally　in　a　dead　horizontal

calm；　provided　that　it　was　not　a　vertical　calm；　and　that

the　air　was　rising　at　the　rate　of　four　or　six　miles　per

hour；　the　lowest　observed；　and　quite　inappreciable　without

actual　measuring。



Some　Data　on　Bird　Power。



The　most　difficult　case　is　purposely　selected。　For　if

we　assume　that　the　bird　has　previously　acquired　an　initial

minimum　speed　of　seventeen　miles　an　hour　（24。93

feet　per　second；　nearly　the　lowest　measured）；　and　that

the　air　was　rising　vertically　six　miles　an　hour　（8。80　feet

per　second）；　then　we　have　as　the　trend　of　the　〃relative

wind〃　encountered：



　6

　　=　0。353；　or　the　tangent　of　19　degrees　26'。

　17



which　brings　the　case　into　the　category　of　rising　wind

effects。　But　the　bird　was　observed　to　have　a　negative

angle　to　the　horizon　of　about　3　degrees；　as　near　as　could　be

guessed；　so　that　his　angle　of　incidence　to　the　〃relative

wind〃　was　reduced　to　16　degrees　26'。



The　relative　speed　of　his　soaring　was　therefore：



Velocity　=　square　root　of　（17　squared　＋　6　squared）　=　18。03　miles

per　hour。



At　this　speed；　using　the　Langley　co…efficient　recently

practically　confirmed　by　the　accurate　experiments　of　Mr。

Eiffel；　the　air　pressure　would　be：



18。03　squared　X　0。00327　=　1。063　pounds　per　square　foot。



If　we　apply　Lilienthal's　co…eff