the aeroplane speaks-第16部分

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ng　places　on　the　outside；　with　the　result　that　the　stress　will　not　be　evenly　disposed　about　the　centre　of　strength；　and　a　bending　stress　may　be　produced。


3。　Struts；　spars；　etc。；　must　be　undamaged。　Remember　that；　from　what　I　have　already　explained　about　bending　stresses；　the　outside　fibres　of　the　wood　are　doing　by　far　the　most　work。　If　these　get　bruised　or　scored；　then　the　strut　or　spar　suffers　in　strength　much　more　than　one　might　think　at　first　sight；　and；　if　it　ever　gets　a　tendency　to　bend；　it　is　likely　to　collapse　at　that　point。


4。　The　wood　must　have　a　good；　clear　grain　with　no　cross…　grain；　knots；　or　shakes。　Such　blemishes　produce　weak　places　and；　if　a　tendency　to　bend　appears；　then　it　may　collapse　at　such　a　point。

5。　The　struts；　spars；　etc。；　must　be　properly　bedded　into　their　sockets　or　fittings。　To　begin　with；　they　must　be　of　good　pushing　or　gentle　tapping　fit。　They　must　never　be　driven　in　with　a　heavy　hammer。　Then　again；　a　strut　must　bed　well　down　all　over　its　cross…sectional　area　as　illustrated　above；　otherwise　the　stress　of　compression　will　not　be　evenly　disposed　about　the　centre　of　strength；　and　that　may　produce　a　bending　stress。　The　bottom　of　the　strut　or　spar　should　be　covered　with　some　sort　of　paint；　bedded　into　the　socket　or　fitting；　and　then　withdrawn　to　see　if　the　paint　has　stuck　all　over　the　bed。


6。　The　atmosphere　is　sometimes　much　damper　than　at　other　times；　and　this　causes　wood　to　expand　and　contract　appreciably。　This　would　not　matter　but　for　the　fact　that　it　does　not　expand　and　contract　uniformly；　but　becomes　unsymmetrical；　i。e。；　distorted。　I　have　already　explained　the　danger　of　that　in　condition　2。　This　should　be　minimized　by　WELL　VARNISHING　THE　WOOD　to　keep　the　moisture　out　of　it。


FUNCTION　OF　INTERPLANE　STRUTS。These　struts　have　to　keep　the　lifting　surfaces　or　‘‘planes''　apart；　but　this　is　only　part　of　their　work。　They　must　keep　the　planes　apart；　so　that　the　latter　are　in　their　correct　attitude。　That　is　only　so　when　the　spars　of　the　bottom　plane　are　parallel　with　those　of　the　top　plane。　Also；　the　chord　of　the　top　plane　must　be　parallel　with　the　chord　of　the　bottom　plane。　If　that　is　not　so；　then　one　plane　will　not　have　the　same　angle　of　incidence　as　the　other　one。　At　first　sight　one　might　think　that　all　that　is　necessary　is　to　cut　all　the　struts　to　be　the　same　length；　but　that　is　not　the　case。

Sometimes；　as　illustrated　above；　the　rear　spar　is　not　so　thick　as　the　main　spar；　and　it　is　then　necessary　to　make　up　for　that　difference　by　making　the　rear　struts　correspondingly　longer。　If　that　is　not　done；　then　the　top　and　bottom　chords　will　not　be　parallel；　and　the　top　and　bottom　planes　will　have　different　angles　of　incidence。　Also；　the　sockets　or　fittings；　or　even　the　spars　upon　which　they　are　placed；　sometimes　vary　in　thickness　owing　to　faulty　manufacture。　This　must　be　offset　by　altering　the　length　of　the　struts。　The　best　way　to　proceed　is　to　measure　the　distance　between　the　top　and　bottom　spars　by　the　side　of　each　strut；　and　if　that　distance；　or　‘‘gap''　as　it　is　called；　is　not　as　stated　in　the　aeroplane's　specifications；　then　make　it　correct　by　changing　the　length　of　the　strut。　This　applies　to　both　front　and　rear　interplane　struts。　When　measuring　the　gap；　always　be　careful　to　measure　from　the　centre　of　the　spar；　as　it　may　be　set　at　an　angle；　and　the　rear　of　it　may　be　considerably　lower　than　its　front。


BORING　HOLES　IN　WOOD。It　should　be　a　strict　rule　that　no　spar　be　used　which　has　an　unnecessary　hole　in　it。　Before　boring　a　hole；　its　position　should　be　confirmed　by　whoever　is　in　charge　of　the　workshop。　A　bolt…hole　should　be　of　a　size　to　enable　the　bolt　to　be　pushed　in；　or；　at　any　rate；　not　more　than　gently　tapped　in。　Bolts　should　not　be　hammered　in；　as　that　may　split　the　spar。　On　the　other　hand；　a　bolt　should　not　be　slack　in　its　hole；　as；　in　such　a　case；　it　may　work　sideways　and　split　the　spar；　not　to　speak　of　throwing　out　of　adjustment　the　wires　leading　from　the　lug　or　socket　under　the　bolt…head。


WASHERS。Under　the　bolt…head；　and　also　under　the　nut；　a　washer　must　be　placeda　very　large　washer　compared　with　the　size　which　would　be　used　in　all…metal　construction。　This　is　to　disperse　the　stress　over　a　large　area；　otherwise　the　washer　may　be　pulled　into　the　wood　and　weaken　it；　besides　possibly　throwing　out　of　adjustment　the　wires　attached　to　the　bolt　or　the　fitting　it　is　holding　to　the　spar。


LOCKING。Now　as　regards　locking　the　bolts。　If　split　pins　are　used；　be　sure　to　see　that　they　are　used　in　such　a　way　that　the　nut　cannot　possibly　unscrew　at　all。　The　split　pin　should　be　passed　through　the　bolt　as　near　as　possible　to　the　nut。　It　should　not　be　passed　through　both　nut　and　bolt。

If　it　is　locked　by　burring　over　the　edge　of　the　bolt；　do　not　use　a　heavy　hammer　and　try　to　spread　the　whole　head　of　the　bolt。　That　might　damage　the　woodwork　inside　the　fabric…covered　surface。　Use　a　small；　light　hammer；　and　gently　tap　round　the　edge　of　the　bolt　until　it　is　burred　over。


TURNBUCKLES。A　turnbuckle　is　composed　of　a　central　barrel　into　each　end　of　which　is　screwed　an　eye…bolt。　Wires　are　taken　from　the　eyes　of　the　eye…bolt；　and　so；　by　turning　the　barrel；　they　can　be　adjusted　to　their　proper　tension。　Eye…bolts　must　be　a　good　fit　in　the　barrel；　that　is　to　say；　not　slack　and　not　very　tight。　Theoretically　it　is　not　neces…　sary　to　screw　the　eye…bolt　into　the　barrel　for　a　distance　greater　than　the　diameter　of　the　bolt；　but；　in　practice；　it　is　better　to　screw　it　in　for　a　considerably　greater　distance　than　that　if　a　reasonable　degree　of　safety　is　to　be　secured。

Now　about　turning　the　barrel　to　secure　the　right　adjustment。　The　barrel　looks　solid；　but；　as　a　matter　of　fact；　it　is　hollow　and　much　more　frail　than　it　appears。　For　that　reason　it　should　not　be　turned　by　seizing　it　with　pliers；　as　that　may　distort　it　and　spoil　the　bore　within　it。　The　best　method　is　to　pass　a　piece　of　wire　through　the　hole　in　its　centre；　and　to　use　that　as　a　lever。　When　the　correct　adjustment　has　been　secured；　the　turnbuckle　must　be　locked　to　prevent　it　from　unscrewing。　It　is　quite　possible　to　lock　it　in　such　a　way　as　to　allow　it　to　unscrew　a　quarter　or　a　half　turn；　and　that　would　throw　the　wires　out　of　the　very　fine　adjustment　necessary。　The　proper　way　is　to　use　the　locking　wire　so　that　its　direction　is　such　as　to　oppose　the　tendency　of　the　barrel　to　unscrew；　thus：


WIRES。The　following　points　should　be　carefully　observed　where　wire　is　concerned：

1。　Quality。It　must　not　be　too　hard　or　too　soft。　An　easy　practical　way　of　learning　to　know　the　approximate　quality　of　wire　is　as　follows：

Take　three　pieces；　all　of　the　same　gauge；　and　each　about　a　foot　in　length。　One　piece　should　be　too　soft；　another　too　hard；　and　the　third　piece　of　the　right　quality。　Fix　them　in　a　vice；　about　an　inch　apart　and　in　a　vertical　position；　and　with　the　light　from　a　window　shining　upon　them。　Burnish　them　if　necessary；　and　you　will　see　a　band　of　light　reflected　from　each　wire。

Now　bend　the　wires　over　as　far　as　possible　and　away　from　the　light。　Where　the　soft　wire　is　concerned；　it　will　squash　out　at　the　bend；　and　this　will　be　indicated　by　the　band　of　light；　which　will　broaden　at　that　point。　In　the　case　of　the　wire　which　is　too　hard；　the　band　of　light　will　broaden　very　little　at　the　turn；　but；　if　you　look　carefully；　you　will　see　some　little　roughnesses　of　surface。　In　the　case　of　the　wire　of　the　right　quality；　the　band　of　light　may　broaden　a　very　little　at　the　turn；　but　there　will　be　no　roughnesses　of　surface。

By　making　this　experiment　two　or　three　times　one　can　soon　learn　to　know　really　bad　wire　from　good；　and　also　learn　to　know　the　strength　of　hand　necessary　to　bend　the　right　quality。

2。　It　must　not　be　damaged。　That　is　to　say；　it　must　be　unkinked；　rustless；　and　unscored。

3。　Now　as　regards　keeping　wire　in　good　condition。　Where　outside　wires　are　concerned；　they　should　be　kept　WELL　GREASED　OR　OILED；　especially　where　bent　over　at　the　ends。　Internal　bracing　wires　cannot　be　reached　for　the　purpose　of　regreasing　them；　as　they　are　inside　fabric…covered　surfaces。　They　should　be　prevented　from　rusting　by　being　painted　with　an　anti…rust　mixture。　Great　care　should　be　taken　to　see　that　the　wire　is　perfectly　clean　and　dry　before　being　painted。　A　greasy　finger…mark　is　sufficient　to　stop　the　paint　from　sticking　to　the　wire。　In　such　a　case　there　will　be　a　little　space　between　the　paint　and　the　wire。　Air　may　enter　there　and　cause　the　wire　to　rust。

4。　Tension　of　Wires。The　tension　to　which　the　wires　are　adjusted　is　of　the　greatest　importance。　All　the　wires　should　be　of　the　same　tension　when　the　aeroplane　is　supported　in　such　a　way　as　to　throw　no　stress　upon　them。　If　some　wires　are　in　greater　tension　than　others；　the　aeroplane　will　quickly　become　distorted　and　lose　its　efficiency。

In　order　to　secure　the　same　tension　of　all　wires；　the　aeroplane；　when　being　rigged；　should　be　supported　by　packing　underneath　the　lower　surfaces　as　well　as　by　packing　underneath　the　fuselage　or　nacelle。　In　this　way　the　anti…lift　wires　are　relieved　of　the　weight；　and　there　is　no　stress　upon　any　of　the　wires。

As　a　general　rule　the　wires　of　an　aeroplane　are　tensioned　too　much。　The　tension　should　be　sufficient　to　keep　the　framework　rigid。　Anything　more　than　that　lowers　the　factor　of　safety；　throws　various　parts　of　the　framework　into　undue　compression；　pulls　the　fittings　into　the　wood；　and　will；　in　the　end；　distort　the　whole　framework　of　the　aeroplane。

Only　experience　will　teach　the　rigger　what　tension　to　employ。　Much　may　be　done　by　learning　the　construction　of　the　various　types　of　aeroplanes；　the　work　the　various　parts　do；　and　in　cultivating　a　touch　for　tensioning　wires　by　constantly　handling　them。

5。　Wires　with　no　Opposition　Wires。In　some　few　cases　wires　will　be　found　which　have　no　opposition　wires　pulling　in　the　opposite　direction。　For　instance；　an　auxiliary　lift　wire　may　run　from　the　bottom　of　a　strut　to　a　spar　in　the　top　plane　at　a　point　between　struts。　In　such　a　case　great　care　should　be　taken　not　to　tighten　the　wire　beyond　barely　taking　up　the　slack。

Such　a　wire　must　be　a　little　slack；　or；　as　illustrated　above；　it　will　distort　the　framework。　That；　in　the　example　given；　will　spoil　the　camber　（curvature）　of　the　surface；　and　result　in　changing　both　the　lift　and　the　drift　at　that