the aeroplane speaks-第14部分

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Imagine　the　top　V；　illustrated　opposite；　to　be　the　front　view　of　a　surface　flying　towards　you。　The　horizontal　equivalent　（H。E。）　of　the　left　wing　is　the　same　as　that　of　the　right　wing。　Therefore；　the　lift　of　one　wing　is　equal　to　the　lift　of　the　other；　and　the　weight；　being　situated　always　in　the　centre；　is　balanced。

If　some　movement　of　the　air　causes　the　surface　to　tilt　sideways；　as　in　the　lower　illustration；　then　you　will　note　that　the　H。E。　of　the　left　wing　increases；　and　the　H。E。　of　the　right　wing　decreases。　The　left　wing　then；　having　the　greatest　lift；　rises；　and　the　surface　assumes　its　first　and　normal　position。

Unfortunately　however；　the　righting　effect　is　not　proportional　to　the　difference　between　the　right　and　left　H。E。's。



In　the　case　of　A；　the　resultant　direction　of　the　reaction　of　both　wings　is　opposed　to　the　direction　of　gravity　or　weight。　The　two　forces　R　R　and　gravity　are　then　evenly　balanced；　and　the　surface　is　in　a　state　of　equilibrium。

In　the　case　of　B；　you　will　note　that　the　R　R　is　not　directly　opposed　to　gravity。　This　results　in　the　appearance　of　M；　and　so　the　resultant　direction　of　motion　of　the　aeroplane　is　no　longer　directly　forward；　but　is　along　a　line　the　resultant　of　the　thrust　and　M。　In　other　words；　it　is；　while　flying　forward；　at　the　same　time　moving　sideways　in　the　direction　M。

In　moving　sideways；　the　keel…surface　receives；　of　course；　a　pressure　from　the　air　equal　and　opposite　to　M。　Since　such　surface　is　greatest　in　effect　towards　the　tail；　then　the　latter　must　be　pushed　sideways。　That　causes　the　aeroplane　to　turn；　and；　the　highest　wing　being　on　the　outside　of　the　turn；　it　has　a　greater　velocity　than　the　lower　wing。　That　produces　greater　lift；　and　tends　to　tilt　the　aeroplane　over　still　more。　Such　tilting　tendency　is；　however；　opposed　by　the　difference　in　the　H。E。's　of　the　two　wings。

It　then　follows　that；　for　the　lateral　dihedral　angle　to　be　effective；　such　angle　must　be　large　enough　to　produce；　when　the　aeroplane　tilts；　a　difference　in　the　H。E。's　of　the　two　wings；　which　difference　must　be　sufficient　to　not　only　oppose　the　tilting　tendency　due　to　the　aeroplane　turning；　but　sufficient　to　also　force　the　aeroplane　back　to　its　original　position　of　equilibrium。

It　is　now；　I　hope；　clear　to　the　reader　that　the　lateral　dihedral　is　not　quite　so　effective　as　would　appear　at　first　sight。　Some　designers；　indeed；　prefer　not　to　use　it；　since　its　effect　is　not　very　great；　and　since　it　must　be　paid　for　in　loss　of　H。E。　and　consequently　loss　of　lift；　thus　decreasing　the　lift…　drift　ratio；　i。e。；　the　efficiency。　Also；　it　is　sometimes　advanced　that　the　lateral　dihedral　increases　the　‘‘spill''　of　air　from　the　wing…tips　and　that　this　adversely　affects　the　lift…drift　ratio。

The　disposition　of　the　keel…surface　affects　the　lateral　stability。　It　should　be；　in　effect；　equally　divided　by　the　longitudinal　turning　axis　of　the　aeroplane。　If　there　is　an　excess　of　keel…surface　above　or　below　such　axis；　then　a　side　gust　striking　it　will　tend　to　turn　the　aeroplane　over　sideways。

The　position　of　the　centre　of　gravity　affects　lateral　stability。　If　too　low；　it　produces　a　pendulum　effect　and　causes　the　aeroplane　to　roll　sideways。

If　too　high；　it　acts　as　a　stick　balanced　vertically　would　act。　If　disturbed；　it　tends　to　travel　to　a　position　as　far　as　possible　from　its　original　position。　It　would　then　tend；　when　moved；　to　turn　the　aeroplane　over　sideways　and　into　an　upside…down　position。

From　the　point　of　view　of　lateral　stability；　the　best　position　for　the　centre　of　gravity　is　one　a　little　below　the　centre　of　drift。

Propeller　torque　affects　lateral　stability。　An　aeroplane　tends　to　turn　over　sideways　in　the　opposite　direction　to　which　the　propeller　revolves。　　　This　tendency　is　offset　by　increasing　the　angle　of　incidence　（and　consequently　the　lift）　of　the　side　tending　to　fall；　and　it　is　always　advisable；　if　practical　considerations　allow　it；　to　also　decrease　the　angle　upon　the　other　side。　In　that　way　it　is　not　necessary　to　depart　so　far　from　the　normal　angle　of　incidence　at　which　the　lift…drift　ratio　is　highest。

Wash…in　is　the　term　applied　to　the　increased　angle。

Wash…out　is　the　term　applied　to　the　decreased　angle。

Both　lateral　and　directional　stability　may　be　improved　by　washing　out　the　angle　of　incidence　on　both　sides　of　the　surface；　thus：

The　decreased　angle　decreases　the　drift　and　therefore　the　effect　of　gusts　upon　the　wing…tips　which　is　just　where　they　have　the　most　effect　upon　the　aeroplane；　owing　to　the　distance　from　the　turning　axis。

The　wash…out　also　renders　the　ailerons　（lateral　controlling　services）　more　effective；　as；　in　order　to　operate　them；　it　is　not　then　necessary　to　give　them　such　a　large　angle　of　incidence　as　would　otherwise　be　required。



The　less　the　angle　of　incidence　of　the　ailerons；　the　better　their　lift…drift　ratio；　i。e。；　their　efficiency。　You　will　note　that；　while　the　aileron　attached　to　the　surface　with　washed…out　angle　is　operated　to　the　same　extent　as　the　aileron　illustrated　above　it；　its　angle　of　incidence　is　considerably　less。　Its　efficiency　is　therefore　greater。

The　advantages　of　the　wash…in　must；　of　course　be　paid　for　in　some　loss　of　lift；　as　the　lift　decreases　with　the　decreased　angle。

In　order　to　secure　all　the　above　described　advantages；　a　combination　is　sometimes　effected；　thus：

BANKING。An　aeroplane　turned　off　its　course　to　right　or　left　does　not　at　once　proceed　along　its　new　course。　Its　momentum　in　the　direction　of　its　first　course　causes　it　to　travel　along　a　line　the　resultant　of　such　momentum　and　the　thrust。　In　other　words；　it　more　or　less　skids　sideways　and　away　from　the　centre　of　the　turn。　Its　lifting　surfaces　do　not　then　meet　the　air　in　their　correct　attitude；　and　the　lift　may　fall　to　such　an　extent　as　to　become　less　than　the　weight；　in　which　case　the　aeroplane　must　fall。　This　bad　effect　is　minimized　by　‘‘banking；''　i。e。；　tilting　the　aeroplane　sideways。　The　bottom　of　the　lifting　surface　is　in　that　way　opposed　to　the　air　through　which　it　is　moving　in　the　direction　of　the　momentum　and　receives　an　opposite　air　pressure。　The　rarefied　area　over　the　top　of　the　surface　is　rendered　still　more　rare；　and　this；　of　course；　assists　the　air　pressure　in　opposing　the　momentum。

The　velocity　of　the　‘‘skid；''　or　sideways　movement；　is　then　only　such　as　is　necessary　to　secure　an　air　pressure　equal　and　opposite　to　the　centrifugal　force　of　the　turn。

The　sharper　the　turn；　the　greater　the　effect　of　the　centrifugal　force；　and　therefore　the　steeper　should　be　the　‘‘bank。''　Experentia　docet。

The　position　of　the　centre　of　gravity　affects　banking。　A　low　C。G。　will　tend　to　swing　outward　from　the　centre　of　the　turn；　and　will　cause　the　aeroplane　to　bankperhaps　too　much；　in　which　case　the　pilot　must　remedy　matters　by　operating　the　ailerons。

A　high　C。G。　also　tends　to　swing　outward　from　the　centre　of　the　turn。　It　will　tend　to　make　the　aeroplane　bank　the　wrong　way；　and　such　effect　must　be　remedied　by　means　of　the　ailerons。

The　pleasantest　machine　from　a　banking　point　of　view　is　one　in　which　the　C。G。　is　a　little　below　the　centre　of　drift。　It　tends　to　bank　the　aeroplane　the　right　way　for　the　turn；　and　the　pilot　can；　if　necessary；　perfect　the　bank　by　means　of　the　ailerons。

The　disposition　of　the　keel…surface　affects　banking。　It　should　be；　in　effect；　evenly　divided　by　the　longitudinal　axis。　An　excess　of　keel…surface　above　the　longitudinal　axis　will；　when　banking；　receive　an　air　pressure　causing　the　aeroplane　to　bank；　perhaps　too　much。　An　excess　of　keel…surface　below　the　axis　has　the　reverse　effect。


SIDE…SLIPPING。This　usually　occurs　as　a　result　of　over…　banking。　It　is　always　the　result　of　the　aeroplane　tilting　sideways　and　thus　decreasing　the　horizontal　equivalent；　and　therefore　the　lift；　of　the　surface。　An　excessive　‘‘bank；''　or　sideways　tilt；　results　in　the　H。E。；　and　therefore　the　lift；　becoming　less　than　the　weight；　when；　of　course；　the　aeroplane　must　fall；　i。e。；　side…slip。


When　making　a　very　sharp　turn　it　is　necessary　to　bank　very　steeply　indeed。　If；　at　the　same　time；　the　longitudinal　axis　of　the　aeroplane　remains　approximately　horizontal；　then　there　must　be　a　fall；　and　the　direction　of　motion　will　be　the　resultant　of　the　thrust　and　the　fall　as　illustrated　above　in　sketch　A。　The　lifting　surfaces　and　the　controlling　surfaces　are　not　then　meeting　the　air　in　the　correct　attitude；　with　the　result　that；　in　addition　to　falling；　the　aeroplane　will　probably　become　quite　unmanageable。

The　Pilot；　however；　prevents　such　a　state　of　affairs　from　happening　by　‘‘nosing…down；''　i。e。；　by　operating　the　rudder　to　turn　the　nose　of　the　aeroplane　downward　and　towards　the　direction　of　motion　as　illustrated　in　sketch　B。　This　results　in　the　higher　wing；　which　is　on　the　outside　of　the　turn；　travelling　with　greater　velocity；　and　therefore　securing　a　greater　reaction　than　the　lower　wing；　thus　tending　to　tilt　the　aeroplane　over　still　more。　The　aeroplane　is　now　almost　upside…down；　but　its　attitude　relative　to　the　direction　of　motion　is　correct　and　the　controlling　surfaces　are　all　of　them　working　efficiently。　The　recovery　of　a　normal　attitude　relative　to　the　Earth　is　then　made　as　illustrated　in　sketch　C。

The　Pilot　must　then　learn　to　know　just　the　angle　of　bank　at　which　the　margin　of　lift　is　lost；　and；　if　a　sharp　turn　necessitates　banking　beyond　that　angle；　he　must　‘‘nose…down。''

In　this　matter　of　banking　and　nosing…down；　and；　indeed；　regarding　stability　and　control　generally；　the　golden　rule　for　all　but　very　experienced　pilots　should　be：　Keep　the　aeroplane　in　such　an　attitude　that　the　air　pressure　is　always　directly　in　the　pilot's　face。　The　aeroplane　is　then　always　engaging　the　air　as　designed　to　do　so；　and　both　lifting　and　controlling　surfaces　are　acting　efficiently。　The　only　exception　to　this　rule　is　a　vertical　dive；　and　I　think　that　is　obviously　not　an　attitude　for　any　but　very　experienced　pilots　to　hanker　after。

SPINNING。This　is　the　worst　of　all　predicaments　the　pilot　can　find　himself　in。　Fortunately　it　rarely　happens。

It　is　due　to　the　combination　of　（1）　a　very　steep　spiral　descent　of　small　radius；　and　（2）　insufficiency　of　keel…surface　behind　the　vertical　axis；　or　the　jamming　of　the　rudder　end　or　elevator　into　a　position　by　which　the　aeroplane　is　forced　into　an　increasingly　s