flying machines-第20章

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air　itself。



In　this　connection　Mr。　Arthur　T。　Atherholt；　president

of　the　Aero　Club　of　Pennsylvania；　in　addressing　the

Boston　Society　of　Scientific　Research；　said：



〃Probably　the　whirlpools　of　Niagara　contain　no　more

erratic　currents　than　the　strata　of　air　which　is　now　immediately

above　us；　a　fact　hard　to　realize　on　account

of　its　invisibility。〃



Changes　In　Wind　Currents。



While　Mr。　Atherholt's　experience　has　been　mainly

with　balloons　it　is　all　the　more　valuable　on　this　account；

as　the　balloons　were　at　the　mercy　of　the　wind　and　their

varying　directions　afforded　an　indisputable　guide　as　to

the　changing　course　of　the　air　currents。　In　speaking　of

this　he　said：



〃In　the　many　trips　taken；　varying　in　distance　traversed

from　twenty…five　to　900　miles；　it　was　never　possible

except　in　one　instance　to　maintain　a　straight　course。

These　uncertain　currents　were　most　noticeable　in　the

Gordon…Bennett　race　from　St。　Louis　in　1907。　Of　the

nine　aerostats　competing　in　that　event；　eight　covered　a

more　or　less　direct　course　due　east　and　southeast；　whereas

the　writer；　with　Major　Henry　B。　Hersey；　first　started

northwest；　then　north；　northeast；　east；　east　by　south；　and

when　over　the　center　of　Lake　Erie　were　again　blown

northwest　notwithstanding　that　more　favorable　winds

were　sought　for　at　altitudes　varying　from　100　to　3；000

meters；　necessitating　a　finish　in　Canada　nearly　northeast

of　the　starting　point。



〃These　nine　balloons；　making　landings　extending　from

Lake　Ontario；　Canada；　to　Virginia；　all　started　from　one

point　within　the　same　hour。



〃The　single　exception　to　these　roving　currents　occurred

on　October　21st；　of　last　year　（1909）　when；　starting

from　Philadelphia；　the　wind　shifted　more　than　eight

degrees；　the　greatest　variation　being　at　the　lowest　altitudes；

yet　at　no　time　was　a　height　of　over　a　mile　reached。



〃Throughout　the　entire　day　the　sky　was　overcast；　with

a　thermometer　varying　from　fifty…seven　degrees　at　300

feet　to　forty…four　degrees；　Fahrenheit　at　5；000　feet；　at

which　altitude　the　wind　had　a　velocity　of　43　miles　an

hour；　in　clouds　of　a　cirro…cumulus　nature；　a　landing　finally

being　made　near　Tannersville；　New　York；　in　the

Catskill　mountains；　after　a　voyage　of　five　and　one…half

hours。



〃I　have　no　knowledge　of　a　recorded　trip　of　this　distance

and　duration；　maintained　in　practically　a　straight

line　from　start　to　finish。〃



This　wind　disturbance　is　more　noticeable　and　more

difficult　to　contend　with　in　a　balloon　than　in　a　flying

machine；　owing　to　the　bulk　and　unwieldy　character　of

the　former。　At　the　same　time　it　is　not　conducive　to

pleasant；　safe　or　satisfactory　sky…sailing　in　an　aeroplane。

This　is　not　stated　with　the　purpose　of　discouraging

aviation；　but　merely　that　the　operator　may　know　what　to

expect　and　be　prepared　to　meet　it。



Not　only　does　the　wind　change　its　horizontal　course

abruptly　and　without　notice；　but　it　also　shifts　in　a　vertical

direction；　one　second　blowing　up；　and　another

down。　No　man　has　as　yet　fathomed　the　why　and　wherefore

of　this　erratic　action；　it　is　only　known　that　it　exists。



The　most　stable　currents　will　be　found　from　50　to　100

feet　from　the　earth；　provided　the　wind　is　not　diverted

by　such　objects　as　trees；　rocks；　etc。　That　there　are

equally　stable　currents　higher　up　is　true；　but　they　are

generally　to　be　found　at　excessive　altitudes。



How　a　Bird　Meets　Currents。



Observe　a　bird　in　action　on　a　windy　day　and　you　will

find　it　continually　changing　the　position　of　its　wings。

This　is　done　to　meet　the　varying　gusts　and　eddies　of　the

air　so　that　sustentation　may　be　maintained　and　headway

made。　One　second　the　bird　is　bending　its　wings；　altering

the　angle　of　incidence；　the　next　it　is　lifting　or　depressing

one　wing　at　a　time。　Still　again　it　will　extend

one　wing　tip　in　advance　of　the　other；　or　be　spreading　or

folding；　lowering　or　raising　its　tail。



All　these　motions　have　a　meaning；　a　purpose。　They

assist　the　bird　in　preserving　its　equilibrium。　Without

them　the　bird　would　be　just　as　helpless　in　the　air　as　a

human　being　and　could　not　remain　afloat。



When　the　wind　is　still；　or　comparatively　so；　a　bird；

having　secured　the　desired　altitude　by　flight　at　an　angle；

may　sail　or　soar　with　no　wing　action　beyond　an　occasional

stroke　when　it　desires　to　advance。　But；　in　a

gusty；　uncertain　wind　it　must　use　its　wings　or　alight

somewhere。



Trying　to　Imitate　the　Bird。



Writing　in　_Fly_；　Mr。　William　E。　White　says：



〃The　bird's　flight　suggests　a　number　of　ways　in　which

the　equilibrium　of　a　mechanical　bird　may　be　controlled。

Each　of　these　methods　of　control　may　be　effected　by

several　different　forms　of　mechanism。



〃Placing　the　two　wings　of　an　aeroplane　at　an　angle　of

three　to　five　degrees　to　each　other　is　perhaps　the　oldest

way　of　securing　lateral　balance。　This　way　readily　occurs

to　anyone　who　watches　a　sea　gull　soaring。　The

theory　of　the　dihedral　angle　is　that　when　one　wing　is

lifted　by　a　gust　of　wind；　the　air　is　spilled　from　under　it；

while　the　other　wing；　being　correspondingly　depressed；

presents　a　greater　resistance　to　the　gust　and　is　lifted

restoring　the　balance。　A　fixed　angle　of　three　to　five　degrees；

however；　will　only　be　sufficient　for　very　light　puffs

of　wind　and　to　mount　the　wings　so　that　the　whole　wing

may　be　moved　to　change　the　dihedral　angle　presents

mechanical　difficulties　which　would　be　better　avoided。



〃The　objection　of　mechanical　impracticability　applies

to　any　plan　to　preserve　the　balance　by　shifting　weight

or　ballast。　The　center　of　gravity　should　be　lower　than

the　center　of　the　supporting　surfaces；　but　cannot　be

made　much　lower。　It　is　a　common　mistake　to　assume

that　complete　stability　will　be　secured　by　hanging　the

center　of　gravity　very　low　on　the　principle　of　the

parachute。　An　aeroplane　depends　upon　rapid　horizontal　motion　for

its　support；　and　if　the　center　of　gravity　be　far

below　the　center　of　support；　every　change　of　speed　or

wind　pressure　will　cause　the　machine　to　turn　about　its

center　of　gravity；　pitching　forward　and　backward　dangerously。



Preserving　Longitudinal　Balance。



〃The　birds　maintain　longitudinal；　or　fore　and　aft　balance；

by　elevating　or　depressing　their　tails。　Whether

this　action　is　secured　in　an　aeroplane　by　means　of　a

horizontal　rudder　placed　in　the　rear；　or　by　deflecting

planes　placed　in　front　of　the　main　planes；　the　principle

is　evidently　the　same。　A　horizontal　rudder　placed　well

to　the　rear　as　in　the　Antoinette；　Bleriot　or　Santos…Dumont

monoplanes；　will　be　very　much　safer　and　steadier

than　the　deflecting　planes　in　front；　as　in　the　Wright　or

Curtiss　biplanes；　but　not　so　sensitive　or　prompt　in　action。



〃The　natural　fore　and　aft　stability　is　very　much

strengthened　by　placing　the　load　well　forward。　The

center　of　gravity　near　the　front　and　a　tail　or　rudder

streaming　to　the　rear　secures　stability　as　an　arrow　is

balanced　by　the　head　and　feathering。　The　adoption　of

this　principle　makes　it　almost　impossible　for　the　aeroplane

to　turn　over。



The　Matter　of　Lateral　Balance。



〃All　successful　aeroplanes　thus　far　have　maintained

lateral　balance　by　the　principle　of　changing　the　angle

of　incidence　of　the　wings。



〃Other　ways　of　maintaining　the　lateral　balance；　suggested

by　observation　of　the　flight　of　birds　areextending

the　wing　tips　and　spilling　the　air　through　the　pinions；

or；　what　is　the　same　thing；　varying　the　area　of　the

wings　at　their　extremities。



〃Extending　the　wing　tips　seems　to　be　a　simple　and

effective　solution　of　the　problem。　The　tips　may　be　made

to　swing　outward　upon　a　vertical　axis　placed　at　the　front

edge　of　the　main　planes；　or　they　may　be　hinged　to　the

ends　of　the　main　plane　so　as　to　be　elevated　or　depressed

through　suitable　connections　by　the　aviator；　or　they　may

be　supported　from　a　horizontal　axis　parallel　with　the

ends　of　the　main　planes　so　that　they　may　swing　outward；

the　aviator　controlling　bo