the aeroplane speaks-第10部分

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een　playing　upon　the　whirling　propeller。

Now　the　distant　drone　of　the　engine　can　be　heard；　but　not　for　long；　for　suddenly　it　ceases　and；　the　nose　of　the　Aeroplane　sinking；　the　craft　commences　gliding　downwards。

‘‘Surely　too　far　away；''　says　a　subaltern。　It　will　be　a　wonderful　machine　if；　from　that　distance　and　height；　it　can　glide　into　the　Aerodrome。''　And　more　than　one　express　the　opinion　that　it　cannot　be　done；　but　the　Designer　smiles　to　himself；　yet　with　a　little　anxiety；　for　his　reputation　is　at　stake；　and　Efficiency；　the　main　reward　he　desires；　is　perhaps；　or　perhaps　not；　at　last　within　his　grasp！

Swiftly　the　machine　glides　downwards　towards　them；　and　it　can　now　be　seen　how　surprisingly　little　it　is　affected　by　the　rough　weather　and　gusts；　so　much　so　that　a　little　chorus　of　approval　is　heard。

‘‘Jolly　good　gliding　angle；''　says　someone；　and　another；　‘‘Beautifully　quick　controls；　what？''　and　from　yet　another；　‘‘By　Jove！　The　Pilot　must　be　sure　of　the　machine。　Look；　he's　stopped　the　engine　entirely。''

Then　the　Aeroplane　with　noiseless　engine　glides　over　the　boundary　of　the　Aerodrome；　and；　with　just　a　soft　soughing　sound　from　the　air　it　cleaves；　lands　gently　not　fifty　yards　from　the　onlookers。

‘‘Glad　to　see　you；''　says　the　Squadron　Commander　to　the　Pilot。　‘‘How　do　you　like　the　machine？''　And　the　Pilot　replies：

‘‘I　never　want　a　better　one；　sir。　It　almost　flies　itself！''

And　the　Designer　turns　his　face　homewards　and　towards　his　beloved　drawing…office；　well　satisfied；　but　still　dreaming　dreams　of　the　future　and　。　。　。　looking　far　ahead　whom　should　he　see　but　Efficiency　at　last　coming　towards　him！　And　to　him　she　is　all　things。　In　her　hair　is　the　morning　sunshine；　her　eyes　hold　the　blue　of　the　sky；　and　on　her　cheeks　is　the　pearly　tint　of　the　clouds　as　seen　from　above。　The　passion　of　speed；　the　lure　of　space；　the　sense　of　power；　and　the　wonder　of　the　future　。　。　。　all　these　things　she　holds　for　him。

‘‘Ah！''　he　cries。　‘‘You'll　never　leave　me　now；　when　at　last　there　is　no　one　between　us？''

And　Efficiency；　smiling　and　blushing；　but　practical　as　ever；　says：

‘‘And　you　will　never　throw　those　Compromises　in　my　face？''

‘‘My　dear；　I　love　you　for　them！　Haven't　they　been　my　life　ever　since　I　began　striving　for　you　ten　long　years　ago？''

And　so　they　walked　off　very　happily；　arm…in…arm　together；　and　if　this　hasn't　bored　you　and　you'd　like　some　more　of　the　same　sort　of　thing；　I'd　just　love　to　tell　you　some　day　of　the　wonderful　things　they　accomplish　together；　and　of　what　they　dream　the　future　holds　in　store。

And　that's　the　end　of　the　Prologue。



CHAPTER　I

FLIGHT

Air　has　weight　（about　13　cubic　feet　=　1　lb。）；　inertia；　and　momentum。　It　therefore　obeys　Newton's　laws＇＇14＇＇　and　resists　movement。　It　is　that　resistance　or　reaction　which　makes　flight　possible。


＇＇14＇＇　See　Newton's　laws　in　the　Glossary　at　the　end　of　the　book。


Flight　is　secured　by　driving　through　the　air　a　surface＇＇15＇＇　inclined　upwards　and　towards　the　direction　of　motion。


＇＇15＇＇　See　‘‘Aerofoil''　in　the　Glossary。


S　=　Side　view　of　surface。

M　=　Direction　of　motion。

CHORD。The　Chord　is；　for　practical　purposes；　taken　to　be　a　straight　line　from　the　leading　edge　of　the　surface　to　its　trailing　edge。

N　=　A　line　through　the　surface　starting　from　its　trailing　edge。　The　position　of　this　line；　which　I　call　the　Neutral　Lift　Line；　is　found　by　means　of　wind…tunnel　research；　and　it　varies　with　differences　in　the　camber　（curvature）　of　surfaces。　In　order　to　secure　flight；　the　inclination　of　the　surface　must　be　such　that　the　neutral　lift　line　makes　an　angle　with　and　ABOVE　the　line　of　motion。　If　it　is　coincident　with　M；　there　is　no　lift。　If　it　makes　an　angle　with　M　and　BELOW　it；　then　there　is　a　pressure　tending　to　force　the　surface　down。

I　=　Angle　of　Incidence。　This　angle　is　generally　defined　as　the　angle　the　chord　makes　with　the　direction　of　motion；　but　that　is　a　bad　definition；　as　it　leads　to　misconception。　The　angle　of　incidence　is　best　described　as　the　angle　the　neutral　lift　line　makes　with　the　direction　of　motion　relative　to　the　air。　You　will；　however；　find　that　in　nearly　all　rigging　specifications　the　angle　of　incidence　is　taken　to　mean　the　angle　the　chord　makes　with　a　line　parallel　to　the　propeller　thrust。　This　is　necessary　from　the　point　of　view　of　the　practical　mechanic　who　has　to　rig　the　aeroplane；　for　he　could　not　find　the　neutral　lift　line；　whereas　he　can　easily　find　the　chord。　Again；　he　would　certainly　be　in　doubt　as　to　‘‘the　direction　of　motion　relative　to　the　air；''　whereas　he　can　easily　find　a　line　parallel　to　the　propeller　thrust。　It　is　a　pity；　however；　that　these　practical　considerations　have　resulted　in　a　bad　definition　of　the　angle　of　incidence　becoming　prevalent；　a　consequence　of　which　has　been　the　widespread　fallacy　that　flight　may　be　secured　with　a　negative　inclination　of　the　surface。　Flight　may　conceivably　be　secured　with　a　negative　angle　of　chord；　but　never　with　a　negative　inclination　of　the　surface。　All　this　is　only　applicable　to　cambered　surfaces。　In　the　case　of　flat　surfaces　the　neutral　lift　line　coincides　with　the　chord　and　the　definition　I　have　criticised　adversely　is　then　applicable。　Flat　lifting　surfaces　are；　however；　never　used。

The　surface　acts　upon　the　air　in　the　following　manner：


As　the　bottom　of　the　surface　meets　the　air；　it　compresses　it　and　accelerates　it　DOWNWARDS。　As　a　result　of　this　definite　action　there　is；　of　course；　an　equal　and　opposite　reaction　UPWARDS。

The　top　surface；　in　moving　forward；　tends　to　leave　the　air　behind　it；　thus　creating　a　semi…vacuum　or　rarefied　area　over　the　top　of　the　surface。　Consequently　the　pressure　of　air　on　the　top　of　the　surface　is　decreased；　thus　assisting　the　reaction　below　to　lift　the　surface　UPWARDS。

The　reaction　increases　approximately　as　the　square　of　the　velocity。　It　is　the　result　of　（1）　the　mass　of　air　engaged；　and　（2）　the　velocity　and　consequent　force　with　which　the　surface　engages　the　air。　If　the　reaction　was　produced　by　only　one　of　those　factors　it　would　increase　in　direct　proportion　to　the　velocity；　but；　since　it　is　the　product　of　both　factors；　it　increases　as　V。

Approximately　three…fifths　of　the　reaction　is　due　to　the　decrease　of　density　（and　consequent　decrease　of　downward　pressure）　on　the　top　of　the　surface；　and　only　some　two…　fifths　is　due　to　the　upward　reaction　secured　by　the　action　of　the　bottom　surface　upon　the　air。　A　practical　point　in　respect　of　this　is　that；　in　the　event　of　the　fabric　covering　the　surface　getting　into　bad　condition；　it　is　more　likely　to　strip　off　the　top　than　off　the　bottom。

The　direction　of　the　reaction　is　approximately　at　right…　angles　to　the　chord　of　the　surface；　as　illustrated　above；　and　it　is；　in　considering　flight；　convenient　to　divide　it　into　two　component　parts　or　values；　thus：

1。　The　vertical　component　of　the　reaction；　i。e。；　Lift；　which　is　opposed　to　Gravity；　i。e。；　the　weight　of　the　aeroplane。

2。　The　horizontal　component；　i。e。；　Drift　（sometimes　called　Resistance）；　to　which　is　opposed　the　thrust　of　the　propeller。

The　direction　of　the　reaction　is；　of　course；　the　resultant　of　the　forces　Lift　and　Drift。

The　Lift　is　the　useful　part　of　the　reaction；　for　it　lifts　the　weight　of　the　aeroplane。

The　Drift　is　the　villain　of　the　piece；　and　must　be　overcome　by　the　Thrust　in　order　to　secure　the　necessary　velocity　to　produce　the　requisite　Lift　for　flight。

DRIFT。The　drift　of　the　whole　aeroplane　（we　have　considered　only　the　lifting　surface　heretofore）　may　be　conveniently　divided　into　three　parts；　as　follows：

Active　Drift；　which　is　the　drift　produced　by　the　lifting　surfaces。

Passive　Drift；　which　is　the　drift　produced　by　all　the　rest　of　the　aeroplanethe　struts；　wires；　fuselage；　under…carriage；　etc。；　all　of　which　is　known　as　‘‘detrimental　surface。''

Skin　Friction；　which　is　the　drift　produced　by　the　friction　of　the　air　with　roughnesses　of　surface。　The　latter　is　practically　negligible　having　regard　to　the　smooth　surface　of　the　modern　aeroplane；　and　its　comparatively　slow　velocity　compared　with；　for　instance；　the　velocity　of　a　propeller　blade。

LIFT…DRIFT　RATIO。The　proportion　of　lift　to　drift　is　known　as　the　lift…drift　ratio；　and　is　of　paramount　importance；　for　it　expresses　the　efficiency　of　the　aeroplane　（as　distinct　from　engine　and　propeller）。　A　knowledge　of　the　factors　governing　the　lift…drift　ratio　is；　as　will　be　seen　later；　an　absolute　necessity　to　anyone　responsible　for　the　rigging　of　an　aeroplane；　and　the　maintenance　of　it　in　an　efficient　and　safe　condition。

Those　factors　are　as　follows：

1。　Velocity。The　greater　the　velocity　the　greater　the　proportion　of　drift　to　lift；　and　consequently　the　less　the　efficiency。　Considering　the　lifting　surfaces　alone；　both　the　lift　and　the　（active）　drift；　being　component　parts　of　the　reaction；　increase　as　the　square　of　the　velocity；　and　the　efficiency　remains　the　same　at　all　speeds。　But；　considering　the　whole　aeroplane；　we　must　remember　the　passive　drift。　It　also　increases　as　the　square　of　the　velocity　（with　no　attendant　lift）；　and；　adding　itself　to　the　active　drift；　results　in　increasing　the　proportion　of　total　drift　（active　＋　passive）　to　lift。

But　for　the　increase　in　passive　drift　the　efficiency　of　the　aeroplane　would　not　fall　with　increasing　velocity；　and　it　would　be　possible；　by　doubling　the　thrust；　to　approximately　double　the　speed　or　lifta　happy　state　of　affairs　which　can　never　be；　but　which　we　may；　in　a　measure；　approach　by　doing　everything　possible　to　diminish　the　passive　drift。

Every　effort　is　then　made　to　decrease　it　by　‘‘stream…lining；''　i。e。；　by　giving　all　‘‘detrimental''　parts　of　the　aeroplane　a　form　by　which　they　will　pass　through　the　air　with　the　least　possible　drift。　Even　the　wires　bracing　the　aeroplane　together　are；　in　many　cases；　stream…lined；　and　with　a　markedly　good　effect　upon　the　lift…drift　ratio。　In　the　case　of　a　certain　well…known　type　of　aeroplane　the　replacing　of　the　ordinary　wires　by　stream…lined　wires　added　over　five　miles　an　hour　to　the　flight　speed。

Head…resistance　is　a　term　often　applied　to　passive　drift；　but　it　is　apt　to　convey　a　wrong　impression；　as　the　drift　is　not　nearly　so　much　the　result　of　the　head　or　forward　part　of　struts；　wires；　etc。；　as　it　is　of　the　rarefied　area　behind。

Above　is　illustrated　the　flow　of　ai