the aeroplane speaks-第12部分

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regard　to　climb　OR　velocity）　has；　so　far；　been　found　to　be　an　arrangement　of　an　inclined　surface　driven　by　a　HORIZONTAL　thrustthe　surface　lifting　the　weight；　and　the　thrust　overcoming　the　drift。　This　is；　in　practice；　a　far　more　efficient　arrangement　than　the　helicopter；　i。e。；　the　air…screw　revolving　about　a　vertical　axis　and　producing　a　thrust　opposed　to　gravity。　If；　when　climbing；　the　propeller　thrust　is　at　such　an　angle　as　to　tend　to　haul　the　aeroplane　upwards；　then　it　is；　in　a　measure；　acting　as　a　helicopter；　and　that　means　inefficiency。　The　reason　of　a　helicopter　being　inefficient　in　practice　is　due　to　the　fact　that；　owing　to　mechanical　difficulties；　it　is　impossible　to　construct　within　a　reasonable　weight　an　air…screw　of　the　requisite　dimensions。　That　being　so；　it　would　be　necessary；　in　order　to　absorb　the　power　of　the　engine；　to　revolve　the　comparatively　small…surfaced　air　screw　at　an　immensely　greater　velocity　than　that　of　the　aeroplane's　surface。　As　already　explained；　the　lift…drift　ratio　falls　with　velocity　on　account　of　the　increase　in　passive　drift。　This　applies　to　a　blade　of　a　propeller　or　air…screw；　which　is　nothing　but　a　revolving　surface　set　at　angle　of　incidence；　and　which　it　is　impossible　to　construct　without　a　good　deal　of　detrimental　surface　near　the　central　boss。

4。　The　velocity　being　low；　then　it　follows　that　for　that　reason　also　the　angle　of　incidence　should　be　comparatively　large。

5。　Camber。Since　such　an　aeroplane　would　be　of　low　velocity；　and　therefore　possess　a　large　angle　of　incidence；　a　large　camber　would　be　necessary。

Let　us　now　consider　the　essentials　for　an　aeroplane　of　maximum　velocity　for　its　power；　and　possessing　merely　enough　lift　to　get　off　the　ground；　but　no　margin　of　lift。

1。　Comparatively　HIGH　VELOCITY。

2。　A　comparatively　SMALL　SURFACE；　because；　being　of　greater　velocity　than　the　maximum　climber；　a　greater　mass　of　air　will　be　engaged　for　a　given　surface　and　time；　and　therefore　a　smaller　surface　will　be　sufficient　to　secure　the　requisit　lift。

3。　A　small　angle　relative　to　the　propeller　thrust；　since　the　latter　coincides　with　the　direction　of　motion。

4。　A　comparatively　small　angle　of　incidence　by　reason　of　the　high　velocity。

5。　A　comparatively　small　camber　follows　as　a　result　of　the　small　angle　of　incidence。


SUMMARY。

Essentials　for　Maximum　　　　　　　　　　　　Essentials　for　Maximum　　　　　　　　　Climb。　　　　　　　　　　　　　　　　　　　　　　　　　　　　Velocity

1。　Low　velocity。　　　　　　　　　　　　　　　　　　High　velocity。　2。　Large　surface。　　　　　　　　　　　　　　　　　Small　surface。　3。　Large　angle　relative　to　　　　　　　　Small　angle　relative　to　　　　propeller　thrust。　　　　　　　　　　　　　　　　　propeller　thrust。　4。　Large　angle　relative　to　　　　　　　　Small　angle　relative　to　direction　　　　direction　of　motion。　　　　　　　　　　　　　　of　motion。　5。　Large　camber。　　　　　　　　　　　　　　　　　　Small　camber。


It　is　mechanically　impossible　to　construct　an　aeroplane　of　reasonable　weight　of　which　it　would　be　possible　to　very　the　above　opposing　essentials。　Therefore；　all　aeroplanes　are　designed　as　a　compromise　between　Climb　and　Velocity。

As　a　rule　aeroplanes　are　designed　to　have　at　low　altitude　a　slight　margin　of　lift　when　the　propeller　thrust　is　horizontal。


ANGLES　OF　INCIDENCE　（INDICATED　APPROXIMATELY）　OF　AN　AEROPLANE　DESIGNED　AS　A　COMPROMISE　BETWEEN　VELOCITY　AND　CLIMB；　AND　POSSESSING　A　SLIGHT　MARGIN　OF　LIFT　AT　A　LOW　ALTITUDE　AND　WHEN　THE　THRUST　IS　HORIZONTAL

MINIMUM　ANGLE。

This　gives　the　greatest　velocity　during　horizontal　flight　at　a　low　altitude。　Greater　velocity　would　be　secured　if　the　surface；　angle；　and　camber　were　smaller　and　designed　to　just　maintain　horizontal　flight　with　a　horizontal　thrust。　Also；　in　such　case；　the　propeller　would　not　be　thrusting　downwards；　but　along　a　horizontal　line　which　is　obviously　a　more　efficient　arrangement　if　we　regard　the　aeroplane　merely　from　one　point　of　view；　i。e。；　either　with　reference　to　velocity　OR　climb。

OPTIMUM　ANGLE　（Thrust　horizontal）

The　velocity　is　less　than　at　the　smaller　minimum　angle；　and；　as　aeroplanes　are　designed　to…day；　the　area　and　angle　of　incidence　of　the　surface　is　such　as　to　secure　a　slight　ascent　at　a　low　altitude。　The　camber　of　the　surface　is　designed　for　this　angle　of　incidence　and　velocity。　The　lift…drift　ratio　is　best　at　this　angle。

BEST　CLIMBING　ANGLE

The　velocity　is　now　still　less　by　reason　of　the　increased　angle　producing　increase　of　drift。　Less　velocity　at　A　GIVEN　ANGLE　produces　less　lift；　but　the　increased　angle　more　or　less　offsets　the　loss　of　lift　due　to　the　decreased　velocity；　and　in　addition；　the　thrust　is　now　hauling　the　aeroplane　upwards。

MAXIMUM　ANGLE

The　greater　angle　has　now　produced　so　much　drift　as　to　lessen　the　velocity　to　a　point　where　the　combined　lifts　from　the　surface　and　from　the　thrust　are　only　just　able　to　maintain　horizontal　flight。　Any　greater　angle　will　result　in　a　still　lower　lift…drift　ratio。　The　lift　will　then　become　less　than　the　weight　and　the　aeroplane　will　consequently　fall。　Such　a　fall　is　known　as　‘‘stalling''　or　‘‘pancaking。''

NOTE。The　golden　rule　for　beginners：　Never　exceed　the　Best　Climbing　Angle。　Always　maintain　the　flying　speed　of　the　aeroplane。


By　this　means；　when　the　altitude　is　reached　where　the　margin　of　lift　disappears　（on　account　of　loss　of　engine　power）；　and　which　is；　consequently；　the　altitude　where　it　is　just　possible　to　maintain　horizontal　flight；　the　aeroplane　is　flying　with　its　thrust　horizontal　and　with　maximum　efficiency　（as　distinct　from　engine　and　propeller　efficiency）。

The　margin　of　lift　at　low　altitude；　and　when　the　thrust　is　horizontal；　should　then　be　such　that　the　higher　altitude　at　which　the　margin　of　lift　is　lost　is　that　altitude　at　which　most　of　the　aeroplane's　horizontal　flight　work　is　done。　That　ensures　maximum　velocity　when　most　required。

Unfortunately；　where　aeroplanes　designed　for　fighting　are　concerned；　the　altitude　where　most　of　the　work　is　done　is　that　at　which　both　maximum　velocity　and　maximum　margin　of　lift　for　power　are　required。

Perhaps　some　day　a　brilliant　inventor　will　design　an　aeroplane　of　reasonable　weight　and　drift　of　which　it　will　be　possible　for　the　pilot　to　vary　at　will　the　above…mentioned　opposing　essentials。　Then　we　shall　get　maximum　velocity；　or　maximum　margin　of　lift；　for　power　as　required。　Until　then　the　design　of　the　aeroplane　must　remain　a　compromise　between　Velocity　and　Climb。



CHAPTER　II

STABILITY　AND　CONTROL

STABILITY　is　a　condition　whereby　an　object　disturbed　has　a　natural　tendency　to　return　to　its　first　and　normal　position。　Example：　a　weight　suspended　by　a　cord。

INSTABILITY　is　a　condition　whereby　an　object　disturbed　has　a　natural　tendency　to　move　as　far　as　possible　away　from　its　first　position；　with　no　tendency　to　return。　Example：　a　stick　balanced　vertically　upon　your　finger。

NEUTRAL　INSTABILITY　is　a　condition　whereby　an　object　disturbed　has　no　tendency　to　move　farther　than　displaced　by　the　force　of　the　disturbance；　and　no　tendency　to　return　to　its　first　position。

In　order　that　an　aeroplane　may　be　reasonably　controllable；　it　is　necessary　for　it　to　possess　some　degree　of　stability　longitudinally；　laterally；　and　directionally。

LONGITUDINAL　STABILITY　in　an　aeroplane　is　its　stability　about　an　axis　transverse　to　the　direction　of　normal　horizontal　flight；　and　without　which　it　would　pitch　and　toss。

LATERAL　STABILITY　is　its　stability　about　its　longitudinal　axis；　and　without　which　it　would　roll　sideways。

DIRECTIONAL　STABILITY　is　its　stability　about　its　vertical　axis；　and　without　which　it　would　have　no　tendency　to　keep　its　course。

For　such　directional　stability　to　exist　there　must　be；　in　effect；＇＇16＇＇　more　‘‘keel…surface''　behind　the　vertical　axis　than　there　is　in　front　of　it。　By　keel…surface　I　mean　every…　thing　to　be　seen　when　looking　at　an　aeroplane　from　the　side　of　itthe　sides　of　the　body；　undercarriage；　struts；　wires；　etc。　The　same　thing　applies　to　a　weathercock。　You　know　what　would　happen　if　there　was　insufficient　keel…surface　behind　the　vertical　axis　upon　which　it　is　pivoted。　It　would　turn　off　its　proper　course；　which　is　opposite　to　the　direction　of　the　wind。　It　is　very　much　the　same　in　the　case　of　an　aeroplane。

＇＇16＇＇　‘‘In　effect''　because；　although　there　may　be　actually　the　greatest　proportion　of　keel…surface　In　front　of　the　vertical　axis；　such　surface　may　be　much　nearer　to　the　axis　than　is　the　keel…surface　towards　the　tail。　The　latter　may　then　be　actually　less　than　the　surface　in　front；　but；　being　farther　from　the　axis；　it　has　a　greater　leverage；　and　consequently　is　greater　in　effect　than　the　surface　in　front。

The　above　illustration　represents　an　aeroplane　（directionally　stable）　flying　along　the　course　B。　A　gust　striking　it　as　indicated　acts　upon　the　greater　proportion　of　keel…surface　behind　the　turning　axis　and　throws　it　into　the　new　course。　It　does　not；　however；　travel　along　the　new　course；　owing　to　its　momentum　in　the　direction　B。　It　travels；　as　long　as　such　momentum　lasts；　in　a　direction　which　is　the　resultant　of　the　two　forces　Thrust　and　Momentum。　But　the　centre　line　of　the　aeroplane　is　pointing　in　the　direction　of　the　new　course。　Therefore　its　attitude；　relative　to　the　direction　of　motion；　is　more　or　less　sideways；　and　it　consequently　receives　an　air　pressure　in　the　direction　C。　Such　pressure；　acting　upon　the　keel…surface；　presses　the　tail　back　towards　its　first　position　in　which　the　aeroplane　is　upon　its　course　B。

What　I　have　described　is　continually　going　on　during　flight；　but　in　a　well…designed　aeroplane　such　stabilizing　movements　are；　most　of　the　time；　so　slight　as　to　be　imperceptible　to　the　pilot。

If　an　aeroplane　was　not　stabilized　in　this　way；　it　would　not　only　be　continually　trying　to　leave　its　course；　but　it　would　also　possess　a　dangerous　tendency　to　‘‘nose　away''　from　the　direction　of　the　side　gusts。　In　such　case　the　gust　shown　in　the　above　illustration　would　turn　the　aeroplane　round　the　opposite　way　a　very　considerable　distance；　and　the　right　wing；　being　on　the　outside　of　the　turn；　would　travel　with　greater　velocity　than　the　left　wing。　Increased　velocity　means　increased　lift；　and　so；　the　right　wing　lifting；　the　aeroplane　would　turn　over　sideways　very　quickly。

LONGITUDINAL　STABILITY。Flat　surfaces　are　longitudinally　stable　owing　to　the　fact　that　with　decreasing　angles　of　incidence　the　centre　line　of　pressure　（C。P。）　moves　f