the aeroplane speaks-第20部分

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At　every　point　tested　the　pitch　angle　is　wrong；　for　nowhere　does　the　actual　pitch　coincide　with　the　specified　pitch。　Angles　A；　C；　and　D；　are　too　large；　and　B　is　too　small。　The　angle　should　be　correct　to　half　a　degree　if　reasonable　efficiency　is　to　be　maintained。

A　fault　in　the　pitch　angle　may　be　due　to　（1）　faulty　manufacture；　（2）　distortion；　or　（3）　the　shaft　hole　through　the　boss　being　out　of　position。


2。　STRAIGHTNESS。To　test　for　straightness　the　propeller　must　be　mounted　upon　a　shaft。　Now　bring　the　tip　of　one　blade　round　to　graze　some　fixed　object。　Mark　the　point　it　grazes。　Now　bring　the　other　tip　round；　and　it　should　come　within　1/8　inch　of　the　mark。　If　it　does　not　do　so；　it　is　due　to　（1）　faulty　manufacture；　（2）　distortion；　or　（3）　to　the　hole　through　the　boss　being　out　of　position。


3。　LENGTH。The　blades　should　be　of　equal　length　to　inch。


4。　BALANCE。The　usual　method　of　testing　a　propeller　for　balance　is　as　follows：　Mount　it　upon　a　shaft；　which　must　be　on　ball…bearings。　Place　the　propeller　in　a　horizontal　position；　and　it　should　remain　in　that　position。　If　a　weight　of　a　trifle　over　an　ounce　placed　in　a　bolt…hole　on　one　side　of　the　boss　fails　to　disturb　the　balance；　then　the　propeller　is　usually　regarded　as　unfit　for　use。

The　above　method　is　rather　futile；　as　it　does　not　test　for　the　balance　of　centrifugal　force；　which　comes　into　play　as　soon　as　the　propeller　revolves。　It　can　be　tested　as　follows：


The　propeller　must　be　in　a　horizontal　position；　and　then　weighed　at　fixed　points；　such　as　A；　B；　C；　D；　E；　and　F；　and　the　weights　noted。　The　points　A；　B；　and　C　must；　of　course；　be　at　the　same　fixed　distances　from　the　centre　of　the　propeller　as　the　points　D；　E；　and　F。　Now　reverse　the　propeller　and　weigh　at　each　point　again。　Note　the　results。　The　first　series　of　weights　should　correspond　to　the　second　series；　thus：

　　　　　　Weight　A　should　equal　weight　F。　　　　　　　　　　‘‘　　B　　　‘‘　　　　　‘‘　　　　　‘‘　　E。　　　　　　　　　　‘‘　　C　　　‘‘　　　　　‘‘　　　　　‘‘　　D。


There　is　no　standard　practice　as　to　the　degree　of　error　permissible；　but　if　there　are　any　appreciable　differences　the　propeller　is　unfit　for　use。


5。　SURFACE　AREA。The　surface　area　of　the　blades　should　be　equal。　Test　with　callipers　thus：

The　points　between　which　the　distances　are　taken　must；　of　course；　be　at　the　same　distance　from　the　centre　in　the　case　of　each　blade。

There　is　no　standard　practice　as　to　the　degree　of　error　permissible。　If；　however；　there　is　an　error　of　over　1/8　inch；　the　propeller　is　really　unfit　for　use。


6。　CAMBER。The　camber　（curvature）　of　the　blades　should　be　（1）　equal；　（2）　decrease　evenly　towards　the　tips　of　the　blades；　and　（3）　the　greatest　depth　of　the　curve　should；　at　any　point　of　the　blade；　be　approximately　at　the　same　percentage　of　the　chord　from　the　leading　edge　as　at　other　points。

It　is　difficult　to　test　the　top　camber　without　a　set　of　templates；　but　a　fairly　accurate　idea　of　the　concave　camber　can　be　secured　by　slowly　passing　a　straight…edge　along　the　blade；　thus：

The　camber　can　now　be　easily　seen；　and　as　the　straight…　edge　is　passed　along　the　blade；　the　observer　should　look　for　any　irregularities　of　the　curvature；　which　should　gradually　and　evenly　decrease　towards　the　tip　of　the　blade。


7。　THE　JOINTS。The　usual　method　for　testing　the　glued　joints　is　by　revolving　the　propeller　at　greater　speed　than　it　will　be　called　upon　to　make　during　flight；　and　then　carefully　examining　the　joints　to　see　if　they　have　opened。　It　is　not　likely；　however；　that　the　reader　will　have　the　opportunity　of　making　this　test。　He　should；　however；　examine　all　the　joints　very　carefully；　trying　by　hand　to　see　if　they　are　quite　sound。　Suspect　a　propeller　of　which　the　joints　appear　to　hold　any　thickness　of　glue。　Sometimes　the　joints　in　the　boss　open　a　little；　but　this　is　not　dangerous　unless　they　extend　to　the　blades；　as　the　bolts　will　hold　the　laminations　together。


8。　CONDITION　OF　SURFACE。The　surface　should　be　very　smooth；　especially　towards　the　tips　of　the　blades。　Some　propeller　tips　have　a　speed　of　over　30；000　feet　a　minute；　and　any　roughness　will　produce　a　bad　drift　or　resistance　and　lower　the　efficiency。


9。　MOUNTING。Great　care　should　be　taken　to　see　that　the　propeller　is　mounted　quite　straight　on　its　shaft。　Test　in　the　same　way　as　for　straightness。　If　it　is　not　straight；　it　is　possibly　due　to　some　of　the　propeller　bolts　being　too　slack　or　to　others　having　been　pulled　up　too　tightly。


FLUTTER。Propeller　‘‘flutter；''　or　vibration；　may　be　due　to　faulty　pitch　angle；　balance；　camber；　or　surface　area。　It　causes　a　condition　sometimes　mistaken　for　engine　trouble；　and　one　which　may　easily　lead　to　the　collapse　of　the　propeller。


CARE　OF　PROPELLERS。The　care　of　propellers　is　of　the　greatest　importance；　as　they　become　distorted　very　easily。


1。　Do　not　store　them　in　a　very　damp　or　a　very　dry　place。


2。　Do　not　store　them　where　the　sun　will　shine　upon　them。


3。　Never　leave　them　long　in　a　horizontal　position　or　leaning　up　against　a　wall。


4。　They　should　be　hung　on　horizontal　pegs；　and　the　position　of　the　propellers　should　be　vertical。


If　the　points　I　have　impressed　upon　you　in　these　notes　are　not　attended　to；　you　may　be　sure　of　the　following　results：


1。　Lack　of　efficiency；　resulting　in　less　aeroplane　speed　and　climb　than　would　otherwise　be　the　case。


2。　Propeller　‘‘flutter''　and　possible　collapse。


3。　A　bad　stress　upon　the　propeller　shaft　and　its　bearings。


TRACTOR。A　propeller　mounted　in　front　of　the　main　surface。


PUSHER。A　propeller　mounted　behind　the　main　surface。


FOUR…BLADED　PROPELLERS。Four…　bladed　propellers　are　suitable　only　when　the　pitch　is　comparatively　large。

For　a　given　pitch；　and　having　regard　to　‘‘interference；''　they　are　not　so　efficient　as　two…bladed　propellers。

The　smaller　the　pitch；　the　less　the　‘‘gap；''　i。e。；　the　distance；　measured　in　the　direction　of　the　thrust；　between　the　spiral　courses　of　the　blades。

If　the　gap　is　too　small；　then　the　following　blade　will　engage　air　which　the　preceding　blade　has　put　into　motion；　with　the　result　that　the　following　blade　will　not　secure　as　good　a　reaction　as　would　otherwise　be　the　case。　It　is　very　much　the　same　as　in　the　case　of　the　aeroplane　gap。

For　a　given　pitch；　the　gap　of　a　four…bladed　propeller　is　only　half　that　of　a　two…bladed　one。　Therefore　the　four…　bladed　propeller　is　only　suitable　for　large　pitch；　as　such　pitch　produces　spirals　with　a　large　gap；　thus　offsetting　the　decrease　in　gap　caused　by　the　numerous　blades。

The　greater　the　speed　of　rotation；　the　less　the　pitch　for　a　given　aeroplane　speed。　Then；　in　order　to　secure　a　large　pitch　and　consequently　a　good　gap；　the　four…bladed　propeller　is　usually　geared　to　rotate　at　a　lower　speed　than　would　be　the　case　if　directly　attached　to　the　engine　crank…shaft。



CHAPTER　V

MAINTENANCE

CLEANLINESS。The　fabric　must　be　kept　clean　and　free　from　oil；　as　that　will　rot　it。　To　take　out　dirt　or　oily　patches；　try　acetone。　If　that　will　not　remedy　matters；　then　try　petrol；　but　use　it　sparingly；　as　otherwise　it　will　take　off　an　unnecessary　amount　of　dope。　If　that　will　not　remove　the　dirt；　then　hot　water　and　soap　will　do　so；　but；　in　that　case；　be　sure　to　use　soap　having　no　alkali　in　it；　as　otherwise　it　may　injure　the　fabric。　Use　the　water　sparingly；　or　it　may　get　inside　the　planes　and　rust　the　internal　bracing　wires；　or　cause　some　of　the　wooden　framework　to　swell。

The　wheels　of　the　undercarriage　have　a　way　of　throwing　up　mud　on　to　the　lower　surface。　This　should；　if　possible；　be　taken　off　while　wet。　It　should　never　be　scraped　off　when　dry；　as　that　may　injure　the　fabric。　If　dry；　then　it　should　be　moistened　before　being　removed。

Measures　should　be　taken　to　prevent　dirt　from　collecting　upon　any　part　of　the　aeroplane；　as；　otherwise；　excessive　skin…　friction　will　be　produced　with　resultant　loss　of　flight　speed。　The　wires；　being　greasy；　collect　dirt　very　easily。


CONTROL　CABLES。After　every　flight　the　rigger　should　pass　his　hand　over　the　control　cables　and　carefully　examine　them　near　pulleys。　Removal　of　grease　may　be　necessary　to　make　a　close　inspection　possible。　If　only　one　strand　is　broken　the　wire　should　be　replaced。　Do　not　forget　the　aileron　balance　wire　on　the　top　surface。

Once　a　day　try　the　tension　of　the　control　cables　by　smartly　moving　the　control　levers　about　as　explained　elsewhere。


WIRES。All　the　wires　should　be　kept　well　greased　or　oiled；　and　in　the　correct　tension。　When　examining　the　wires；　it　is　necessary　to　place　the　aeroplane　on　level　ground；　as　otherwise　it　may　be　twisted；　thus　throwing　some　wires　into　undue　tension　and　slackening　others。　The　best　way；　if　there　is　time；　is　to　pack　the　machine　up　into　its　‘‘flying　position。''

If　you　see　a　slack　wire；　do　not　jump　to　the　conclusion　that　it　must　be　tensioned。　Perhaps　its　opposition　wire　is　too　tight；　in　which　case　slacken　it；　and　possibly　you　will　find　that　will　tighten　the　slack　wire。

Carefully　examine　all　wires　and　their　connections　near　the　propeller；　and　be　sure　that　they　are　snaked　round　with　safety　wire；　so　that　the　latter　may　keep　them　out　of　the　way　of　the　propeller　if　they　come　adrift。

The　wires　inside　the　fuselage　should　be　cleaned　and　regreased　about　once　a　fortnight。


STRUTS　AND　SOCKETS。These　should　be　carefully　examined　to　see　if　any　splitting　has　occurred。


DISTORTION。Carefully　examine　all　surfaces；　including　the　controlling　surfaces；　to　see　whether　any　distortion　has　occurred。　If　distortion　can　be　corrected　by　the　adjustment　of　wires；　well　and　good；　but　if　not；　then　some　of　the　internal　framework　probably　requires　replacement。


ADJUSTMENTS。Verify　the　angles　of　incidence；　dihedral；　and　stagger；　and　the　rigging　position　of　the　controlling…　surfaces；　as　often　as　possible。


UNDERCARRIAGE。Constantly　examine　the　alignment　and　fittings　of　the　undercarriage；　and　the　condition　of　tyres　and　shock　absorbers。　The　latter；　when　made　of　rubber；　wear　quickest　underneath。　Inspect　axles　and　skids　to　see　if　there　are　any　signs　of　them　becoming　bent。　The　wheels　should　be　taken　off　occasionally　and　greased。


LOCKING　ARRANGEMENTS。Constantly　inspect　the　locking　arrangements　of　turnbuckles；　bolts；　etc。　Pay　particular　attention　to　the　control　cable　connections；　and　to　all　moving　parts　in　respect　of　the　controls。


LUBRICATION。