the aeroplane speaks-第4部分

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‘‘Well；''　he　replied；　rather　miserably；　‘‘I'm　not　nearly　so　perfect　as　the　Longitudinal　and　Directional　Stabilities。　The　Dihedral　Anglethat　is；　the　upward　inclination　of　the　Surfaces　towards　their　wing…tipsdoes　what　it　can　for　me；　but；　in　my　opinion；　it's　a　more　or　less　futile　effort。　The　Blackboard　will　show　you　the　argument。''　And　he　at　once　showed　them　two　Surfaces；　each　set　at　a　Dihedral　Angle　like　this：

‘‘Please　imagine；''　said　the　Blackboard；　‘‘that　the　top　V　is　the　front　view　of　a　Surface　flying　towards　you。　Now　if　a　gust　blows　it　into　the　position　of　the　lower　V　you　see　that　the　horizontal　equivalent　of　the　Surface　on　one　side　becomes　larger；　and　on　the　other　side　it　becomes　smaller。　That　results　in　more　Lift　on　the　lower　side　and　less　on　the　higher　side；　and　if　the　V　is　large　enough　it　should　produce　such　a　difference　in　the　Lift　of　one　side　to　the　other　as　to　quickly　turn　the　Aeroplane　back　to　its　former　and　normal　position。''

‘‘Yes；''　said　the　Dihedral　Angle；　‘‘that's　what　would　happen　if　they　would　only　make　me　large　enough；　but　they　won't　do　it　because　it　would　too　greatly　decrease　the　horizontal　equivalent；　and　therefore　the　Lift；　and　incidentally　it　would；　as　Aeroplanes　are　built　to…day；　produce　an　excess　of　Keel　Surface　above　the　turning　axis；　and　that　in　itself　would　spoil　the　Lateral　Stability。　The　Keel　Surface　should　be　equally　divided　above　and　below　the　longitudinal　turning　axis　（upon　which　the　Aeroplane　rolls　sideways）；　or　the　side　upon　which　there　is　an　excess　will　get　blown　over　by　the　gusts。　It　strikes　me　that　my　future　isn't　very　promising；　and　about　my　only　chance　is　when　the　Junior　Draughtsman　makes　a　mistake；　as　he　did　the　other　day。　And　just　think　of　it；　they　call　him　a　Designer　now　that　he's　got　a　job　at　the　Factory！　What　did　he　do？　Why；　he　calculated　the　weights　wrong　and　got　the　Centre　of　Gravity　too　high；　and　they　didn't　discover　it　until　the　machine　was　built。　Then　all　they　could　do　was　to　give　me　a　larger　Angle。　That　dropped　the　bottom　of　the　V　lower　down；　and　as　that's　the　centre　of　the　machine；　where　all　the　Weight　is；　of　course　that　put　the　Centre　of　Gravity　in　its　right　place。　But　now　there　is　too　much　Keel　Surface　above；　and　the　whole　thing's　a　Bad　Compromise；　not　at　all　like　Our　Efficiency。''

And　Efficiency；　blushing　very　prettily　at　the　compliment；　then　asked；　‘‘And　how　does　the　Centre　of　Gravity　affect　matters？''

‘‘That's　easy；''　said　Grandfather　Gravity。　‘‘I'm　so　heavy　that　if　I　am　too　low　down　I　act　like　a　pendulum　and　cause　the　Aeroplane　to　roll　about　sideways；　and　if　I　am　too　high　I'm　like　a　stick　balanced　on　your　finger；　and　then　if　I'm　disturbed；　over　I　go　and　the　Aeroplane　with　me；　and；　in　addition　to　that；　there　are　the　tricks　I　play　with　the　Aeroplane　when　it's　banked　up；＇＇6＇＇　i。e。；　tilted　sideways　for　a　turn；　and　Centrifugal　Force　sets　me　going　the　way　I'm　not　wanted　to　go。　No；　I　get　on　best　with　Lateral　Stability　when　my　Centre　is　right　on　the　centre　of　Drift；　or；　at　any　rate；　not　much　below　it。''　And　with　that　he　settled　back　into　the　Lecturer's　Chair　and　went　sound　asleep　again；　for　he　was　so　very；　very　old；　in　fact　the　father　of　all　the　Principles。


＇＇6＇＇　Banking：　When　an　aeroplane　is　turned　to　the　left　or　the　right　the　centrifugal　force　of　its　momentum　causes　it　to　skid　sideways　and　outwards　away　from　the　centre　of　the　turn。　To　minimize　such　action　the　pilot　banks；　i。e。；　tilts；　the　aeroplane　sideways　in　order　to　oppose　the　underside　of　the　planes　to　the　air。　The　aeroplane　will　not　then　skid　outwards　beyond　the　slight　skid　necessary　to　secure　a　sufficient　pressure　of　air　to　balance　the　centrifugal　force。


And　the　Blackboard　had　been　busy；　and　now　showed　them　a　picture　of　the　Aeroplane　as　far　as　they　knew　it；　and　you　will　see　that　there　is　a　slight　Dihedral　Angle；　and　also；　fixed　to　the　tail；　a　vertical　Keel　Surface　or　fin；　as　is　very　often　the　case　in　order　to　ensure　the　greater　effect　of　such　surface　being　behind　the　vertical　turning　axis。

But　Efficiency；　growing　rather　critical　with　her　newly　gained　knowledge；　cried　out：　‘‘But　where's　the　horizontal　Tail　Surface？　It　doesn't　look　right　like　that！''

‘‘This　is　when　I　have　the　pleasure　of　meeting　you；　my　dear；''　said　Longitudinal　Stability。　‘‘Here's　the　Tail　Surface；''　he　said；　‘‘and　in　order　to　help　me　it　must　be　set　IN　EFFECT　at　a　much　less　Angle　of　Incidence　than　the　Main　Surface。　To　explain　we　must　trouble　the　Blackboard　again；''　and　this　was　his　effort：

‘‘I　have　tried　to　make　that　as　clear　as　possible；''　he　said。　‘‘It　may　appear　a　bit　complicated　at　first；　but　if　you　will　take　the　trouble　to　look　at　it　for　a　minute　you　will　find　it　quite　simple。　A　is　the　normal　and　proper　direction　of　motion　of　the　Aeroplane；　but；　owing　to　a　gust　of　air；　it　takes　up　the　new　nose…down　position。　Owing　to　Momentum；　however；　it　does　not　fly　straight　along　in　that　direction；　but　moves　more　or　less　in　the　direction　B；　which　is　the　resultant　of　the　two　forces；　Momentum　and　Thrust。　And　so　you　will　note　that　the　Angle　of　Incidence；　which　is　the　inclination　of　the　Surfaces　to　the　Direction　of　Motion；　has　decreased；　and　of　course　the　Lift　decreases　with　it。　You　will　also　see；　and　this　is　the　point；　that　the　Tail　Surface　has　lost　a　higher　proportion　of　its　Angle；　and　consequently　its　Lift；　than　has　the　Main　Surface。　Then；　such　being　the　case；　the　Tail　must　fall　and　the　Aeroplane　assume　its　normal　position　again；　though　probably　at　a　slightly　lower　altitude。''

‘‘I'm　afraid　I'm　very　stupid；''　said　Efficiency；　‘‘but　please　tell　me　why　you　lay　stress　upon　the　words　‘IN　EFFECT。'　''

‘‘Ah！　I　was　wondering　if　you　would　spot　that；''　he　replied。　‘‘And　there　is　a　very　good　reason　for　it。　You　see；　in　some　Aeroplanes　the　Tail　Surface　may　be　actually　set　at　the　same　Angle　on　the　machine　as　the　Main　Surface；　but　owing　to　the　air　being　deflected　downwards　by　the　front　Main　Surface　it　meets　the　Tail　Surface　at　a　lesser　angle；　and　indeed　in　some　cases　at　no　angle　at　all。　The　Tail　is　then　for　its　surface　getting　less　Lift　than　the　Main　Surface；　although　set　at　the　same　angle　on　the　machine。　It　may　then　be　said　to　have　IN　EFFECT　a　less　Angle　of　Incidence。　I'll　just　show　you　on　the　Blackboard。''

‘‘And　now；''　said　Efficiency；　‘‘I　have　only　to　meet　the　Ailerons　and　the　Rudder；　haven't　I？''

‘‘Here　we　are；''　replied　the　Ailerons；　or　little　wings。　‘‘Please　hinge　us　on　to　the　back　of　the　Main　Surfaces；　one　of　us　at　each　Wing…tip；　and　join　us　up　to　the　Pilot's　joystick　by　means　of　the　control　cables。　When　the　Pilot　wishes　to　tilt　the　Aeroplane　sideways；　he　will　move　the　stick　and　depress　us　upon　one　side；　thus　giving　us　a　larger　Angle　of　Incidence　and　so　creating　more　Lift　on　that　side　of　the　Aeroplane；　and；　by　means　of　a　cable　connecting　us　with　the　Ailerons　on　the　other　side　of　the　Aeroplane；　we　shall；　as　we　are　depressed；　pull　them　up　and　give　them　a　reverse　or　negative　Angle　of　Incidence；　and　that　side　will　then　get　a　reverse　Lift　or　downward　thrust；　and　so　we　are　able　to　tilt　the　Aeroplane　sideways。

‘‘And　we　work　best　when　the　Angle　of　Incidence　of　the　Surface　in　front　of　us　is　very　small；　for　which　reason　it　is　sometimes　decreased　or　washed…out　towards　the　Wing…tips。　The　reason　of　that　is　that　by　the　time　the　air　reaches　us　it　has　been　deflected　downwardsthe　greater　the　Angle　of　Incidence　the　more　it　is　driven　downwardsand　in　order　for　us　to　secure　a　Reaction　from　it；　we　have　to　take　such　a　large　Angle　of　Incidence　that　we　produce　a　poor　proportion　of　Lift　to　Drift；　but　the　smaller　the　Angle　of　the　Surface　in　front　of　us　the　less　the　air　is　deflected　downwards；　and　consequently　the　less　Angle　is　required　of　us；　and　the　better　our　proportion　of　Lift　to　Drift；　which；　of　course；　makes　us　much　more　effective　Controls。''

‘‘Yes；''　said　the　Lateral　and　Directional　Stabilities　in　one　voice；　‘‘that's　so；　and　the　wash…out　helps　us　also；　for　then　the　Surfaces　towards　their　Wing…tips　have　less　Drift　or　‘Head…Resistance；'　and　consequently　the　gusts　will　affect　them　and　us　less；　but　such　decreased　Angle　of　Incidence　means　decreased　Lift　as　well　as　Drift；　and　the　Designer　does　not　always　care　to　pay　the　price。''

‘‘Well；''　said　the　Ailerons；　‘‘if　it's　not　done　it　will　mean　more　work　for　the　Rudder；　and　that　won't　please　the　Pilot。''

‘‘Whatever　do　you　mean？''　asked　Efficiency。　‘‘What　can　the　Rudder　have　to　do　with　you？''

‘‘It's　like　this；''　they　replied：　‘‘when　we　are　deflected　downwards　we　gain　a　larger　Angle　of　Incidence　and　also　enter　an　area　of　compressed　air；　and　so　produce　more　Drift　than　those　of　us　on　the　other　side　of　the　Aeroplane；　which　are　deflected　upwards　into　an　area　of　rarefied　air　due　to　the　SUCTION　effect　（though　that　term　is　not　academically　correct）　on　the　top　of　the　Surface。　If　there　is　more　Drift；　i。e。；　Resistance；　on　one　side　of　the　Aeroplane　than　on　the　other　side；　then　of　course　it　will　turn　off　its　course；　and　if　that　difference　in　Drift　is　serious；　as　it　will　very　likely　be　if　there　is　no　wash…out；　then　it　will　mean　a　good　deal　of　work　for　the　Rudder　in　keeping　the　Aeroplane　on　its　course；　besides　creating　extra　Drift　in　doing　so。''

‘‘I　think；　then；''　said　Efficiency；　‘‘I　should　prefer　to　have　that　wash…out；＇＇7＇＇　and　my　friend　the　Designer　is　so　clever　at　producing　strength　of　construction　for　light　weight；　I'm　pretty　sure　he　won't　mind　paying　the　price　in　Lift。　And　now　let　me　see　if　I　can　sketch　the　completed　Aeroplane。''


＇＇7＇＇　An　explanation　of　the　way　in　which　the　wash…out　is　combined　with　a　wash…in　to　offset　propellor　torque　will　be　found　on　p。　82。


‘‘Well；　I　hope　that's　all　as　it　should　be；''　she　concluded；　‘‘for　to…morrow　the　Great　Test　in　the　air　is　due。''



PART　III

THE　GREAT　TEST

It　is　five　o'clock　of　a　fine　calm　morning；　when　the　Aeroplane　is　wheeled　out　of　its　shed　on　to　the　greensward　of　the　Military　Aerodrome。　There　is　every　promise　of　a　good　flying　day；　and；　although　the　sun　has　not　yet　risen；　it　is　light　enough　to　discern　the　motionless　layer　of　fleecy　clouds　some　five　thousand　feet　high；　and　far；　far　above　that　a　few　filmy　mottled　streaks　of　vapour。　Just　the　kind　of　morning　beloved　of　pilots。

A　brand　new；　rakish；　up…to…date　machine　it　is；　of　highly　polished；　beautifully　finished　wood；　fabric　as　tight　as　a　drum；　polished　metal；　and　every　part　so　perfectly　‘‘streamlined''　to　m