the aeroplane speaks-第2部分

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he　top　Surface　a　bit　forward；　and　no　longer　is　that　suction　effect　dead　under　it。　At　the　same　time　I'm　sure　the　top　Surface　will　kindly　extend　its　Span　for　such　distance　as　its　Spars　will　support　it　without　the　aid　of　Struts。　Such　extension　will　be　quite　useful；　as　there　will　be　no　Surface　at　all　underneath　it　to　interfere　with　the　Reaction　above。''　And　the　Stagger　leaned　forward　and　picked　up　the　Chalk；　and　this　is　the　picture　he　drew：

Said　the　Blackboard；　‘‘That's　not　half　bad！　It　really　begins　to　look　something　like　the　real　thing；　eh？''

‘‘The　real　thing；　is　it？''　grumbled　Drift。　‘‘Just　consider　that　contraption　in　the　light　of　any　one　Principle；　and　I　warrant　you　will　not　find　one　of　them　applied　to　perfection。　The　whole　thing　is　nothing　but　a　Compromise。''　And　he　glared　fixedly　at　poor　Efficiency。

‘‘Oh；　dear！　Oh；　dear！''　she　cried。　‘‘I'm　always　getting　into　trouble。　What　WILL　the　Designer　say？''

‘‘Never　mind；　my　dear；''　said　the　Lift…Drift　Ratio；　consolingly。　‘‘You　are　improving　rapidly；　and　quite　useful　enough　now　to　think　of　doing　a　job　of　work。''

‘‘Well；　that's　good　news；''　and　Efficiency　wiped　her　eyes　with　her　Fabric　and　became　almost　cheerful。　‘‘Suppose　we　think　about　finishing　it　now？　There　will　have　to　be　an　Engine　and　Propeller；　won't　there？　And　a　body　to　fix　them　in；　and　tanks　for　oil　and　petrol；　and　a　tail；　and；''　archly；　‘‘one　of　those　dashing　young　Pilots；　what？''

‘‘Well；　we　are　getting　within　sight　of　those　interesting　Factors；''　said　the　Lift…Drift　Ratio；　‘‘but　first　of　all　we　had　better　decide　upon　the　Area　of　the　Surfaces；　their　Angle　of　Incidence　and　Camber。　If　we　are　to　ascend　as　quickly　as　possible　the　Aeroplane　must　be　SLOW　in　order　to　secure　the　best　possible　Lift…Drift　Ratio；　for　the　drift　of　the　struts　wires；　body；　etc。；　increases　approximately　as　the　square　of　the　speed；　but　it　carries　with　it　no　lift　as　it　does　in　the　case　of　the　Surface。　The　less　speed　then；　the　less　such　drift；　and　the　better　the　Aeroplane's　proportion　of　lift　to　drift；　and；　being　slow；　we　shall　require　a　LARGE　SURFACE　in　order　to　secure　a　large　lift　relative　to　the　weight　to　be　carried。　We　shall　also　require　a　LARGE　ANGLE　OF　INCIDENCE　relative　to　the　horizontal；　in　order　to　secure　a　proper　inclination　of　the　Surface　to　the　direction　of　motion；　for　you　must　remember　that；　while　we　shall　fly　upon　an　even　keel　and　with　the　propeller　thrust　horizontal　（which　is　its　most　efficient　attitude）；　our　flight　path；　which　is　our　direction　of　motion；　will　be　sloping　upwards；　and　it　will　therefore　be　necessary　to　fix　the　Surface　to　the　Aeroplane　at　a　very　considerable　angle　relative　to　the　horizontal　Propeller　Thrust　in　order　to　secure　a　proper　angle　to　the　upwards　direction　of　motion。　Apart　from　that；　we　shall　require　a　larger　Angle　of　Incidence　than　in　the　case　of　a　machine　designed　purely　for　speed；　and　that　means　a　correspondingly　LARGE　CAMBER。

‘‘On　the　other　hand；　if　we　are　thinking　merely　of　Speed；　then　a　SMALL　SURFACE；　just　enough　to　lift　the　weight　off　the　ground；　will　be　best；　also　a　SMALL　ANGLE　to　cut　the　Drift　down　and　that；　of　course；　means　a　relatively　SMALL　CAMBER。

‘‘So　you　see　the　essentials　for　CLIMB　or　quick　ascent　and　for　SPEED　are　diametrically　opposed。　Now　which　is　it　to　be？''

‘‘Nothing　but　perfection　for　me；''　said　Efficiency。　‘‘What　I　want　is　Maximum　Climb　and　Maximum　Speed　for　the　Power　the　Engine　produces。''

And　each　Principle　fully　agreed　with　her　beautiful　sentiments；　but　work　together　they　would　not。

The　Aspect　Ratio　wanted　infinite　Span；　and　hang　the　Chord。

The　Angle　of　Incidence　would　have　two　Angles　and　two　Cambers　in　one；　which　was　manifestly　absurd；　the　Surface　insisted　upon　no　thickness　whatever；　and　would　not　hear　of　such　things　as　Spars　and　Ribs；　and　the　Thrust　objected　to　anything　at　all　likely　to　produce　Drift；　and　very　nearly　wiped　the　whole　thing　off　the　Blackboard。

There　was；　indeed；　the　makings　of　a　very　pretty　quarrel　when　the　Letter　arrived。　It　was　about　a　mile　long；　and　began　to　talk　at　once。

‘‘I'm　from　the　Inventor；''　he　said；　and　hope　rose　in　the　heart　of　each　heated　Principle。　‘‘It's　really　absurdly　simple。　All　the　Pilot　has　to　do　is　to　touch　a　button；　and　at　his　will；　VARY　the　area　of　the　Surface；　the　Angle　of　Incidence；　and　the　Camber！　And　there　you　areMaximum　Climb　or　Maximum　Speed　as　required！　How　does　that　suit　you？''

‘‘That　suits　us　very　well；''　said　the　Surface；　‘‘but；　excuse　me　asking；　how　is　it　done　without　apparatus　increasing　the　Drift　and　the　Weight　out　of　all　reason？　You　won't　mind　showing　us　your　Calculations；　Working　Drawings；　Stress　Diagrams；　etc。；　will　you？''

Said　the　Letter　with　dignity；　‘‘I　come　from　an　Inventor　so　brilliantly　clever　as　to　be　far　above　the　unimportant　matters　you　mention。　He　is　no　common　working　man；　sir！　He　leaves　such　things　to　Mechanics。　The　point　is；　you　press　a　button　and''

‘‘Look　here；''　said　a　Strut；　rather　pointedly；　‘‘where　do　you　think　you　are　going；　anyway？''

‘‘Well；''　from　the　Letter；　‘‘as　a　matter　of　fact；　I'm　not　addressed　yet；　but；　of　course；　there's　no　doubt　I　shall　reach　the　very　highest　quarters　and　absolutely　revolutionize　Flight　when　I　get　there。''

Said　the　Chalk；　‘‘I'll　address　you；　if　that's　all　you　want；　now　drift　along　quickly！''　And　off　went　the　Letter　to　The　Technical　Editor；　‘‘Daily　Mauler；''　London。

And　a　League　was　formed；　and　there　were　Directors　with　Fees；　and　several　out…of…service　Tin　Hats；　and　the　Man…who…　takes…the…credit；　and　a　fine　fat　Guinea…pig；　and　all　the　rest　of　them。　And　the　Inventor　paid　his　Tailor　and　had　a　Hair…　Cut；　and　is　now　a　recognized　Press　Expertbut　he　is　still　waiting　for　those　Mechanics！

‘‘I'm　afraid；''　said　the　Slide…rule；　who　had　been　busy　making　those　lightning…like　automatic　calculations　for　which　he　is　so　famous；　‘‘it's　quite　impossible　to　fully　satisfy　all　of　you；　and　it　is　perfectly　plain　to　me　that　we　shall　have　to　effect　a　Compromise　and　sacrifice　some　of　the　Lift　for　Speed。''

Thud！　What　was　that？

Efficiency　had　fainted　dead　away！　The　last　blow　had　been　too　much　for　her。　And　the　Principles　gathered　mournfully　round；　but　with　the　aid　of　the　Propeller　Slip＇＇1＇＇　and　a　friendly　lift　from　the　Surface　she　was　at　length　revived　and　regained　a　more　normal　aspect。


＇＇1＇＇　Propeller　Slip：　As　the　propeller　screws　through　the　air；　the　latter　to　a　certain　extent　gives　back　to　the　thrust　of　the　propellor　blades；　just　as　the　shingle　on　the　beach　slips　back　as　you　ascend　it。　Such　‘‘give…back''　is　known　as　‘‘slip；''　and　anyone　behind　the　propellor　will　feel　the　slip　as　a　strong　draught　of　air。


Said　the　Stagger　with　a　raffish　air；　‘‘My　dear　young　lady；　I　assure　you　that　from　the　experiences　of　a　varied　career；　I　have　learned　that　perfection　is　impossible；　and　I　am　sure　the　Designer　will　be　quite　satisfied　if　you　become　the　Most　Efficient　Compromise。''

‘‘Well；　that　sounds　so　common　sense；''　sighed　Efficiency；　‘‘I　suppose　it　must　be　true；　and　if　the　Designer　is　satisfied；　that's　all　I　really　care　about。　Now　do　let's　get　on　with　the　job。''

So　the　Chalk　drew　a　nice　long　slim　body　to　hold　the　Engine　and　the　tanks；　etc。；　with　room　for　the　Pilot's　and　Passenger's　seats；　and　placed　it　exactly　in　the　middle　of　the　Biplane。　And　he　was　careful　to　make　its　position　such　that　the　Centre　of　Gravity　was　a　little　in　advance　of　the　Centre　of　Lift；　so　that　when　the　Engine　was　not　running　and　there　was　consequently　no　Thrust；　the　Aeroplane　should　be　‘‘nose…　heavy''　just　to　the　right　degree；　and　so　take　up　a　natural　glide　to　Earthand　this　was　to　help　the　Pilot　and　relieve　him　of　work　and　worry；　should　he　find　himself　in　a　fog　or　a　cloud。　And　so　that　this　tendency　to　glide　downwards　should　not　be　in　evidence　when　the　Engine　was　running　and　descent　not　desired；　the　Thrust　was　placed　a　little　below　the　Centre　of　Drift　or　Resistance。　In　this　way　it　would　in　a　measure　pull　the　nose　of　the　Aeroplane　up　and　counterbalance　the　‘‘nose…heavy''　tendency。

And　the　Engine　was　so　mounted　that　when　the　Propeller…　Thrust　was　horizontal；　which　is　its　most　efficient　position；　the　Angle　of　Incidence　and　the　Area　of　the　surfaces　were　just　sufficient　to　give　a　Lift　a　little　in　excess　of　the　Weight。　And　the　Camber　was　such　that；　as　far　as　it　was　concerned；　the　Lift…Drift　Ratio　should　be　the　best　possible　for　that　Angle　of　Incidence。　And　a　beautifully　simple　under…carriage　was　added；　the　outstanding　features　of　which　were　simplicity；　strength；　light…weight；　and　minimum　drift。　And；　last　of　all；　there　was　the　Elevator；　of　which　you　will　hear　more　by…and…by。　And　this　is　what　it　looked　like　then：

And　Efficiency；　smiling；　thought　that　it　was　not　such　a　bad　compromise　after　all　and　that　the　Designer　might　well　be　satisfied。

‘‘Now；''　said　she；　‘‘there's　just　one　or　two　points　I'm　a　bit　hazy　about。　It　appears　that　when　the　Propeller　shaft　is　horizontal　and　so　working　in　its　most　efficient　attitude；　I　shall　have　a　Lift　from　the　Surfaces　slightly　in　excess　of　the　Weight。　That　means　I　shall　ascend　slightly；　at　the　same　time　making　nearly　maximum　speed　for　the　power　and　thrust。　Can't　I　do　better　than　that？''

‘‘Yes；　indeed；''　spoke　up　the　Propeller；　‘‘though　it　means　that　I　must　assume　a　most　undignified　attitude；　for　helicopters＇＇2＇＇　I　never　approved　of。　In　order　to　ascend　more　quickly　the　Pilot　will　deflect　the　Elevator；　which；　by　the　way；　you　see　hinged　to　the　Tail。　By　that　means　he　will　force　the　whole　Aeroplane　to　assume　a　greater　Angle　of　Incidence。　And　with　greater　Angle；　the　Lift　will　increase；　though　I'm　sorry　to　say　the　Drift　will　increase　also。　Owing　to　the　greater　Drift；　the　Speed　through　the　air　will　lessen；　and　I'm　afraid　that　won't　be　helpful　to　the　Lift；　but　I　shall　now　be　pointing　upwards；　and　besides　overcoming　the　Drift　in　a　forward　direction　I　shall　be　doing　my　best　to　haul　the　Aeroplane　skywards。　At　a　certain　angle　known　as　the　Best　Climbing　Angle；　we　shall　have　our　Maximum　Margin　of　Lift；　and　I'm　hoping　that　may　be　as　much　as　almost　a　thousand　feet　altitude　a　minute。''


＇＇2＇＇　Helicopter。　An　air…screw　revolving　upon　a　vertical　axis。　If　driven　with　sufficient　power；　it　will　lift　vertically；　but　having　regard　to　the　mechanical　difficulties　of　such　construction；　it　is　a　most　inefficient　way　of　securing　lift　compared　with　the　arrangement　of　an　inclined　surface　driven　by　a