the aeroplane speaks-第15部分

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ne　is　forced　into　an　increasingly　steep　and　small　spiral。

Owing　to　the　small　radius　of　such　a　spiral；　the　mass　of　the　aeroplane　may　gain　a　rotary　momentum　greater；　in　effect；　than　the　air　pressure　of　the　keel…surface　or　controlling　surfaces　opposed　to　it；　and；　when　once　such　a　condition　occurs；　it　is　difficult　to　see　what　can　be　done　by　the　pilot　to　remedy　it。　The　sensible　pilot　will　not　go　beyond　reasonable　limits　of　steepness　and　radius　when　executing　spiral　descents。

GLIDING　DESCENT　WITHOUT　PROPELLER　THRUST。All　aeroplanes　are；　or　should　be；　designed　to　assume　their　gliding　angle　when　the　power　and　thrust　is　cut　off。　This　relieves　the　pilot　of　work；　worry；　and　danger　should　he　find　himself　in　a　fog　or　cloud。　The　Pilot；　although　he　may　not　realize　it；　maintains　the　correct　attitude　of　the　aeroplane　by　observing　its　position　relative　to　the　horizon。　Flying　into　a　fog　or　cloud　the　horizon　is　lost　to　view；　and　he　must　then　rely　upon　his　instruments（1）　the　compass　for　direction；　（2）　an　inclinometer　（arched　spirit…level）　mounted　transversely　to　the　longitudinal　axis；　for　lateral　stability；　and　（3）　an　inclinometer　mounted　parallel　to　the　longitudinal　axis；　or　the　airspeed　indicator；　which　will　indicate　a　nose…down　position　by　increase　in　air　speed；　and　a　tail…down　position　by　decrease　in　air　speed。

The　pilot　is　then　under　the　necessity　of　watching　three　instruments　and　manipulating　his　three　controls　to　keep　the　instruments　indicating　longitudinal；　lateral；　and　directional　stability。　That　is　a　feat　beyond　the　capacity　of　the　ordinary　man。　If；　however；　by　the　simple　movement　of　throttling　down　the　power　and　thrust；　he　can　be　relieved　of　looking　after　the　longitudinal　stability；　he　then　has　only　two　instruments　to　watch。　That　is　no　small　job　in　itself；　but　it　is；　at　any　rate；　fairly　practicable。

Aeroplanes　are；　then；　designed；　or　should　be；　so　that　the　centre　of　gravity　is　slightly　forward　of　centre　of　lift。　The　aeroplane　is　then；　as　a　glider；　nose…heavyand　the　distance　the　C。G。　is　placed　in　advance　of　the　C。L。　should　be　such　as　to　ensure　a　gliding　angle　producing　a　velocity　the　same　as　the　normal　flying　speed　（for　which　the　strength　of　construction　has　been　designed）。

In　order　that　this　nose…heavy　tendency　should　not　exist　when　the　thrust　is　working　and　descent　not　required；　the　centre　of　thrust　is　placed　a　little　below　the　centre　of　drift　or　resistance；　and　thus　tends　to　pull　up　the　nose　of　the　aeroplane。

The　distance　the　centre　of　thrust　is　placed　below　the　centre　of　drift　should　be　such　as　to　produce　a　force　equal　and　opposite　to　that　due　to　the　C。G。　being　forward　of　the　C。L。

LOOPING　AND　UPSIDE　DOWN　FLYING。If　a　loop　is　desired；　it　is　best　to　throttle　the　engine　down　at　point　A。　The　C。G。　being　forward　of　the　C。P。；　then　causes　the　aeroplane　to　nose…　down；　and　assists　the　pilot　in　making　a　reasonably　small　loop　along　the　course　C　and　in　securing　a　quick　recovery。　If　the　engine　is　not　throttled　down；　then　the　aeroplane　may　be　expected　to　follow　the　course　D；　which　results　in　a　longer　nose　dive　than　in　the　case　of　the　course　C。

A　steady；　gentle　movement　of　the　elevator　is　necessary。　A　jerky　movement　may　change　the　direction　of　motion　so　suddenly　as　to　produce　dangerous　air　stresses　upon　the　surfaces；　in　which　case　there　is　a　possibility　of　collapse。

If　an　upside…down　flight　is　desired；　the　engine　may；　or　may　not；　be　throttled　down　at　point　A。　If　not　throttled　down；　then　the　elevator　must　be　operated　to　secure　a　course　approximately　in　the　direction　B。　If　it　is　throttled　down；　then　the　course　must　be　one　of　a　steeper　angle　than　B；　or　there　will　be　danger　of　stalling。


Diagram　p。　88。This　is　not　set　at　quite　the　correct　angle。　Path　B　should　slope　slightly　downwards　from　Position　A。



CHAPTER　III

RIGGING

In　order　to　rig　an　aeroplane　intelligently；　and　to　maintain　it　in　an　efficient　and　safe　condition；　it　is　necessary　to　possess　a　knowledge　of　the　stresses　it　is　called　upon　to　endure；　and　the　strains　likely　to　appear。


STRESS　is　the　load　or　burden　a　body　is　called　upon　to　bear。　It　is　usually　expressed　by　the　result　found　by　dividing　the　load　by　the　number　of　superficial　square　inches　contained　in　the　cross…sectional　area　of　the　body。

Thus；　if；　for　instance；　the　object　illustrated　above　contains　4　square　inches　of　cross…sectional　area；　and　the　total　load　it　is　called　upon　to　endure　is　10　tons；　the　stress　would　be　expressed　as　2　1/2　tons。


STRAIN　is　the　deformation　produced　by　stress。


THE　FACTOR　OF　SAFETY　is　usually　expressed　by　the　result　found　by　dividing　the　stress　at　which　it　is　known　the　body　will　collapse；　by　the　maximum　stress　it　will　be　called　upon　to　endure。　For　instance；　if　a　control　wire　be　called　upon　to　endure　a　maximum　stress　of　2　cwts。；　and　the　known　stress　at　which　it　will　collapse　is　10　cwts。；　the　factor　of　safety　is　then　5。

＇cwts。　=　centerweights　=　100　pound　units　as　in　cent　&　century。　Interestinly　enough；　this　word　only　exists　today　in　abbreviation　form；　probably　of　centreweights；　but　the　dictionary　entries；　even　from　a　hundred　years　ago　do　not　list　this　as　a　word；　but　do　list　c。　or　C。　as　the　previous　popular　abbreviation　as　in　Roman　Numerals＇　The　word　listed　is　〃hundredweight。　　Michael　S。　Hart；　1997＇


COMPRESSION。The　simple　stress　of　compression　tends　to　produce　a　crushing　strain。　Example：　the　interplane　and　fuselage　struts。


TENSION。The　simple　stress　of　tension　tends　to　produce　the　strain　of　elongation。　Example：　all　the　wires。


BENDING。The　compound　stress　of　bending　is　a　combination　of　compression　and　tension。

The　above　sketch　illustrates　a　straight　piece　of　wood　of　which　the　top；　centre；　and　bottom　lines　are　of　equal　length。　We　will　now　imagine　it　bent　to　form　a　circle；　thus：

The　centre　line　is　still　the　same　length　as　before　being　bent；　but　the　top　line；　being　farther　from　the　centre　of　the　circle；　is　now　longer　than　the　centre　line。　That　can　be　due　only　to　the　strain　of　elongation　produced　by　the　stress　of　tension。　The　wood　between　the　centre　line　and　the　top　line　is　then　in　tension；　and　the　farther　from　the　centre；　the　greater　the　strain；　and　consequently　the　greater　the　tension。

The　bottom　line；　being　nearest　to　the　centre　of　the　circle；　is　now　shorter　than　the　centre　line。　That　can　be　due　only　to　the　strain　of　crushing　produced　by　the　stress　of　compression。　The　wood　between　the　centre　and　bottom　lines　is　then　in　compression；　and　the　nearer　the　centre　of　the　circle；　the　greater　the　strain；　and　consequently　the　greater　the　compression。

It　then　follows　that　there　is　neither　tension　nor　compression；　i。e。；　no　stress；　at　the　centre　line；　and　that　the　wood　immediately　surrounding　it　is　under　considerably　less　stress　than　the　wood　farther　away。　This　being　so；　the　wood　in　the　centre　may　be　hollowed　out　without　unduly　weakening　struts　and　spars。　In　this　way　25　to　33　per　cent。　is　saved　in　the　weight　of　wood　in　an　aeroplane。

The　strength　of　wood　is　in　its　fibres；　which　should；　as　far　as　possible；　run　without　break　from　one　end　of　a　strut　or　spar　to　the　other　end。　A　point　to　remember　is　that　the　outside　fibres；　being　farthest　removed　from　the　centre　line；　are　doing　by　far　the　greatest　work。


SHEAR　STRESS　IS　such　that；　when　material　collapses　under　it；　one　part　slides　over　the　other。　Example：　all　the　locking　pins。

Some　of　the　bolts　are　also　in　shear　or　‘‘sideways''　stress；　owing　to　lugs　under　their　heads　and　from　which　wires　are　taken。　Such　a　wire；　exerting　a　sideways　pull　upon　a　bolt；　tries　to　break　it　in　such　a　way　as　to　make　one　piece　of　the　bolt　slide　over　the　other　piece。

TORSION。This　is　a　twisting　stress　compounded　of　compression；　tension；　and　shear　stresses。　Example：　the　propeller　shaft。


NATURE　OF　WOOD　UNDER　STRESS。Wood；　for　its　weight；　takes　the　stress　of　compression　far　better　than　any　other　stress。　For　instance：　a　walking…stick　of　less　than　1　lb。　in　weight　will；　if　kept　perfectly　straight；　probably　stand　up　to　a　compression　stress　of　a　ton　or　more　before　crushing；　whereas；　if　the　same　stick　is　put　under　a　bending　stress；　it　will　probably　collapse　to　a　stress　of　not　more　than　about　50　lb。　That　is　a　very　great　difference；　and；　since　weight　is　of　the　greatest　importance；　the　design　of　an　aeroplane　is　always　such　as　to；　as　far　as　possible；　keep　the　various　wooden　parts　of　its　construction　in　direct　compression。　Weight　being　of　such　vital　importance；　and　designers　all　trying　to　outdo　each　other　in　saving　weight；　it　follows　that　the　factor　of　safety　is　rather　low　in　an　aeroplane。　The　parts　in　direct　compression　will；　however；　take　the　stresses　safely　provided　the　following　conditions　are　carefully　observed。

CONDITIONS　TO　BE　OBSERVED：


1。　All　the　spars　and　struts　must　be　perfectly　straight。

The　above　sketch　illustrates　a　section　through　an　interplane　strut。　If　the　strut　is　to　be　kept　straight；　i。e。；　prevented　from　bending；　then　the　stress　of　compression　must　be　equally　disposed　about　the　centre　of　strength。　If　it　is　not　straight；　then　there　will　be　more　compression　on　one　side　of　the　centre　of　strength　than　on　the　other　side。　That　is　a　step　towards　getting　compression　on　one　side　and　tension　on　the　other　side；　in　which　case　it　may　be　forced　to　take　a　bending　stress　for　which　it　is　not　designed。　Even　if　it　does　not　collapse　it　will；　in　effect；　become　shorter；　and　thus　throw　out　of　adjustment　the　gap　and　all　the　wires　attached　to　the　top　and　bottom　of　the　strut；　with　the　result　that　the　flight　efficiency　of　the　aeroplane　will　be　spoiled。

The　only　exception　to　the　above　condition　is　what　is　known　as　the　Arch。　For　instance；　in　the　case　of　the　Maurice　Farman；　the　spars　of　the　centre…　section　plane；　which　have　to　take　the　weight　of　the　nacelle；　are　arched　upwards。　If　this　was　not　done；　it　is　possible　that　rough　landings　might　result　in　the　weight　causing　the　spars　to　become　slightly　distorted　downwards。　That　would　produce　a　dangerous　bending　stress；　but；　as　long　as　the　wood　is　arched；　or；　at　any　rate；　kept　from　bending　downwards；　it　will　remain　in　direct　compression　and　no　danger　can　result。


2。　Struts　and　spars　must　be　symmetrical。　By　that　I　mean　that　the　cross…sectional　dimensions　must　be　correct；　as　otherwise　there　will　be　bulging　places　on　the　outside；　with　the　result　that　the　stress　will　not　be　evenly　disposed　about　the　cent