the aeroplane speaks-第11部分

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of　struts；　wires；　etc。；　as　it　is　of　the　rarefied　area　behind。

Above　is　illustrated　the　flow　of　air　round　two　objects　moving　in　the　direction　of　the　arrow　M。

In　the　case　of　A；　you　will　note　that　the　rarefied　area　DD　is　of　very　considerable　extent；　whereas　in　the　case　of　B；　the　air　flows　round　it　in　such　a　way　as　to　meet　very　closely　to　the　rear　of　the　object；　thus　DECREASING　DD。

The　greater　the　rarefied　area　DD。　then；　the　less　the　density；　and；　consequently；　the　less　the　pressure　of　air　upon　the　rear　of　the　object。　The　less　such　pressure；　then；　the　better　is　head…resistance　D　able　to　get　its　work　in；　and　the　more　thrust　will　be　required　to　overcome　it。

The　‘‘fineness''　of　the　stream…line　shape；　i。e。；　the　proportion　of　length　to　width；　is　determined　by　the　velocitythe　greater　the　velocity；　the　greater　the　fineness。　The　best　degree　of　fineness　for　any　given　velocity　is　found　by　means　of　wind…　tunnel　research。

The　practical　application　of　all　this　is；　from　a　rigging　point　of　view；　the　importance　of　adjusting　all　stream…line　parts　to　be　dead…on　in　the　line　of　flight；　but　more　of　that　later　on。

2。　Angle　of　Incidence。The　most　efficient　angle　of　incidence　varies　with　the　thrust　at　the　disposal　of　the　designer；　the　weight　to　be　carried；　and　the　climb…velocity　ratio　desired。

The　best　angles　of　incidence　for　these　varying　factors　are　found　by　means　of　wind…tunnel　research　and　practical　trial　and　error。　Generally　speaking；　the　greater　the　velocity　the　smaller　should　be　the　angle　of　incidence；　in　order　to　preserve　a　clean；　stream…line　shape　of　rarefied　area　and　freedom　from　eddies。　Should　the　angle　be　too　great　for　the　velocity；　then　the　rarefied　area　becomes　of　irregular　shape　with　attendant　turbulent　eddies。　Such　eddies　possess　no　lift　value；　and　since　it　has　taken　power　to　produce　them；　they　represent　drift　and　adversely　affect　the　lift…　drift　ratio。

From　a　rigging　point　of　view；　one　must　presume　that　every　standard　aeroplane　has　its　lifting　surface　set　at　the　most　efficient　angle；　and　the　practical　application　of　all　this　is　in　taking　the　greatest　possible　care　to　rig　the　surface　at　the　correct　angle　and　to　maintain　it　at　such　angle。　Any　deviation　will　adversely　affect　the　lift…drift　ratio；　i。e。；　the　efficiency。

3。　Camber。（Refer　to　the　second　illustration　in　this　chapter。）　The　lifting　surfaces　are　cambered；　i。e。；　curved；　in　order　to　decrease　the　horizontal　component　of　the　reaction；　i。e。；　the　drift。

The　bottom　camber：　If　the　bottom　of　the　surface　was　flat；　every　particle　of　air　meeting　it　would　do　so　with　a　shock；　and　such　shock　would　produce　a　very　considerable　horizontal　reaction　or　drift。　By　curving　it　such　shock　is　diminished；　and　the　curve　should　be　such　as　to　produce　a　uniform　（not　necessarily　constant）　acceleration　and　compression　of　the　air　from　the　leading　edge　to　the　trailing　edge。　Any　unevenness　in　the　acceleration　and　compression　of　the　air　produces　drift。

The　top　camber：　If　this　was　flat　it　would　produce　a　rarefied　area　of　irregular　shape。　I　have　already　explained　the　bad　effect　this　has　upon　the　lift…　drift　ratio。　The　top　surface　is　then　curved　to　produce　a　rarefied　area　the　shape　of　which　shall　be　as　stream…line　and　free　from　attendant　eddies　as　possible。

The　camber　varies　with　the　angle　of　incidence；　the　velocity；　and　the　thickness　of　the　surface。　Generally　speaking；　the　greater　the　velocity；　the　less　the　camber　and　angle　of　incidence。　With　infinite　velocity　the　surface　would　be　set　at　no　angle　of　incidence　（the　neutral　lift　line　coincident　with　the　direction　of　motion　relative　to　the　air）；　and　would　be；　top　and　bottom；　of　pure　streamline　formi。e。；　of　infinite　fineness。　This　is；　of　course；　carrying　theory　to　absurdity　as　the　surface　would　then　cease　to　exist。

The　best　cambers　for　varying　velocities；　angles　of　incidence；　and　thicknesses　of　surface；　are　found　by　means　of　wind…tunnel　research。　The　practical　application　of　all　this　is　in　taking　the　greatest　care　to　prevent　the　surface　from　becoming　distorted　and　thus　spoiling　the　camber　and　consequently　the　lift…drift　ratio。

4。　Aspect　Ratio。This　is　the　proportion　of　span　to　chord。　Thus；　if　the　span　is；　for　instance；　50　feet　and　the　chord　5　feet；　the　surface　would　be　said　to　have　an　aspect　ratio　of　10　to　1。

For　A　GIVEN　VELOCITY　and　A　GIVEN　AREA　of　surface；　the　greater　the　aspect　ratio；　the　greater　the　reaction。　It　is　obvious；　I　think；　that　the　greater　the　span；　the　greater　the　mass　of　air　engaged；　and；　as　already　explained；　the　reaction　is　partly　the　result　of　the　mass　of　air　engaged。

Not　only　that；　but；　PROVIDED　the　chord　is　not　decreased　to　an　extent　making　it　impossible　to　secure　the　best　camber　owing　to　the　thickness　of　the　surface；　the　greater　the　aspect　ratio；　the　better　the　lift…drift　ratio。　The　reason　of　this　is　rather　obscure。　It　is　sometimes　advanced　that　it　is　owing　to　the　‘‘spill''　of　air　from　under　the　wing…　tips。　With　a　high　aspect　ratio　the　chord　is　less　than　would　otherwise　be　the　case。　Less　chord　results　in　smaller　wing…tips　and　consequently　less　‘‘spill。''　This；　however；　appears　to　be　a　rather　inadequate　reason　for　the　high　aspect　ratio　producing　the　high　lift…drift　ratio。　Other　reasons　are　also　advanced；　but　they　are　of　such　a　contentious　nature　I　do　not　think　it　well　to　go　into　them　here。　They　are　of　interest　to　designers；　but　this　is　written　for　the　practical　pilot　and　rigger。

5。　Stagger。This　is　the　advancement　of　the　top　surface　relative　to　the　bottom　surface；　and　is　not；　of　course；　applicable　to　a　single　surface；　i。e。；　a　monoplane。　In　the　case　of　a　biplane　having　no　stagger；　there　will　be　‘‘interference''　and　consequent　loss　of　Efficiency　unless　the　gap　between　the　top　and　bottom　surfaces　is　equal　to　not　less　than　1　1/2　　times　the　chord。　If　less　than　that；　the　air　engaged　by　the　bottom　of　the　top　surface　will　have　a　tendency　to　be　drawn　into　the　rarefied　area　over　the　top　of　the　bottom　surface；　with　the　result　that　the　surfaces　will　not　secure　as　good　a　reaction　as　would　otherwise　be　the　case。

It　is　not　practicable　to　have　a　gap　of　much　more　than　a　distance　equal　to　the　chord；　owing　to　the　drift　produced　by　the　great　length　of　struts　and　wires　such　a　large　gap　would　necessitate。　By　staggering　the　top　surface　forward；　however；

it　is　removed　from　the　action　of　the　lower　surface　and　engages　undisturbed　air；　with　the　result　that　the　efficiency　can　in　this　way　be　increased　by　about　5　per　cent。　Theoretically　the　top　plane　should　be　staggered　forward　for　a　distance　equal　to　about　30　per　cent。　of　the　chord；　the　exact　distance　depending　upon　the　velocity　and　angle　of　incidence；　but　this　is　not　always　possible　to　arrange　in　designing　an　aeroplane；　owing　to　difficulties　of　balance；　desired　position；　and　view　of　pilot；　observer；　etc。

6。　Horizontal　Equivalent。The　vertical　component　of　the　reaction；　i。e。；　lift；　varies　as　the　horizontal　equivalent　（H。E。）　of　the　surface；　but　the　drift　remains　the　same。　Then　it　follows　that　if　H。E。　grows　less；　the　ratio　of　lift　to　drift　must　do　the　same。

A；　B；　and　C　are　front　views　of　three　surfaces。

A　has　its　full　H。E。；　and　therefore；　from　the　point　of　view　from　which　we　are　at　the　moment　considering　efficiency；　it　has　its　best　lift…drift　ratio。

B　and　C　both　possess　the　same　surface　as　A；　but　one　is　inclined　upwards　from　its　centre　and　the　other　is　straight　but　tilted。　For　these　reasons　their　H。E。's　are；　as　illustrated；　less　than　in　the　case　of　A。　That　means　less　vertical　lift；　and；　the　drift　remaining　the　same　（for　there　is　the　same　amount　of　surface　as　in　A　to　produce　it）；　the　lift…drift　ratio　falls。

THE　MARGIN　OF　POWER　is　the　power　available　above　that　necessary　to　maintain　horizontal　flight。

THE　MARGIN　OF　LIFT　is　the　height　an　aeroplane　can　gain　in　a　given　time　and　starting　from　a　given　altitude。　As　an　example；　thus：　1；000　feet　the　first　minute；　and　starting　from　an　altitude　of　500　feet　above　sea…level。

The　margin　of　lift　decreases　with　altitude；　owing　to　the　decrease　in　the　density　of　the　air；　which　adversely　affects　the　engine。　Provided　the　engine　maintained　its　impulse　with　altitude；　then；　if　we　ignore　the　problem　of　the　propeller；　which　I　will　go　into　later　on；　the　margin　of　lift　would　not　disappear。　Moreover；　greater　velocity　for　a　given　power　would　be　secured　at　a　greater　altitude；　owing　to　the　decreased　density　of　air　to　be　overcome。　After　reading　that；　you　may　like　to　light　your　pipe　and　indulge　in　dreams　of　the　wonderful　possibilities　which　may　become　realities　if　some　brilliant　genius　shows　us　some　day　how　to　secure　a　constant　power　with　increasing　altitude。　I　am　afraid；　however；　that　will　always　remain　impossible；　but　it　is　probable　that　some　very　interesting　steps　may　be　taken　in　that　direction。

THE　MINIMUM　ANGLE　OF　INCIDENCE　is　the　smallest　angle　at　which；　for　a　given　power；　surface　（including　detrimental　surface）；　and　weight；　horizontal　flight　can　be　maintained。

THE　MAXIMUM　ANGLE　OF　INCIDENCE　is　the　greatest　angle　at　which；　for　a　given　power；　surface　（including　detrimental　surface）；　and　weight；　horizontal　flight　can　be　maintained。

THE　OPTIMUM　ANGLE　OF　INCIDENCE　is　the　angle　at　which　the　lift…drift　ratio　is　highest。　In　modern　aeroplanes　it　is　that　angle　of　incidence　possessed　by　the　surface　when　the　axis　of　the　propeller　is　horizontal。

THE　BEST　CLIMBING　ANGLE　is　approximately　half…way　between　the　maximum　and　the　optimum　angles。

All　present…day　aeroplanes　are　a　compromise　between　Climb　and　horizontal　Velocity。　We　will　compare　the　essentials　for　two　aeroplanes；　one　designed　for　maximum　climb；　and　the　other　for　maximum　velocity。

ESSENTIALS　FOR　MAXIMUM　CLIMB：

1。　Low　velocity；　in　order　to　secure　the　best　lift…drift　ratio。

2。　Having　a　low　velocity；　a　large　surface　will　be　necessary　in　order　to　engage　the　necessary　mass　of　air　to　secure　the　requisite　lift。

3。　Since　（1）　such　a　climbing　machine　will　move　along　an　upward　sloping　path；　and　（2）　will　climb　with　its　propeller　thrust　horizontal；　then　a　large　angle　relative　to　the　direction　of　the　thrust　will　be　necessary　in　order　to　secure　the　requisite　angle　relative　to　the　direction　of　motion。

The　propeller　thrust　should　be　always　horizontal；　because　the　most　efficient　flying…machine　（having　regard　to　climb　OR　velocity）　has；　so　far；　been　found　to　be　an　arrangement　of　an　inclined　surface　dri