Strangeasitmayseem，multiplicityofcylindersdoesnotalwaysaddproportionateweight.Becausea4—
　　cylindermotorweighssay100pounds，itdoesnotnecessarilyfollowthatan8—cylinderequipmentwillweigh200pounds.Thereasonofthiswillbeplainwhenitisunderstoodthatmanyofthepartsessentialtoa4—
　　cylindermotorwillfilltherequirementsofan8—cylindermotorwithoutenlargementoraddition.
　　Neitherdoesmultiplyingthecylindersalwaysincreasethehorsepowerproportionately.Ifa4—cylindermotorisratedat25horsepoweritisnotsafetotakeitforgrantedthatdoublethenumberofcylinderswillgive50horsepower.Generallyspeaking，eightcylinders，thebore，strokeandspeedbeingthesame，willgivedoublethepowerthatcanbeobtainedfromfour，butthisdoesnotalwaysholdgood.Justwhythisexceptionshouldoccurisnotexplainablebyanyacceptedrule.
　　HorsePowerandSpeed.
　　Speedisanimportantrequisiteinaflying—machinemotor，asthevelocityoftheaeroplaneisavitalfactorinflotation.Atfirstthought，thepropellerandsimilaradjunctsbeingequal，theinexperiencedmindwouldnaturallyarguethata50—horsepowerengineshouldproducejustdoublethespeedofoneof25—horsepower.
　　Thatthisisafallacyisshownbyactualperformances.
　　TheWrights，usinga25—horsepowermotor，havemade44milesanhour，whileBleriot，witha50—horsepowermotor，hasarecordofashort—distanceflightattherateof52milesanhour.Thefactisthat，sofarasspeedisconcerned，muchdependsuponthevelocityofthewind，thesizeandshapeoftheaeroplaneitself，andthesize，shapeandgearingofthepropeller.Thestrongerthewindisblowingtheeasieritwillbefortheaeroplanetoascend，butatthesametimethemoredifficultitwillbetomakeheadwayagainstthewindinahorizontaldirection.Withastrongheadwind，andproperengineforce，yourmachinewillprogresstoacertainextent，butitwillbeatanangle.Iftheaviatordesiredtokeepongoingupwardthiswouldbeallright，butthereisalimittothealtitudewhichitisdesirabletoreach——from100to500feetforexperts——andafterthatitbecomesaquestionofgoingstraightahead.
　　GreatWasteofPower.
　　Onethingiscertain——eveninthemostefficientofmodernaerialmotorsthereisagreatlossofpowerbetweenthetwopointsofproductionandeffect.TheWrightoutfit，whichisadmittedlyoneofthemosteffectiveinuse，takesonehorsepowerofforcefortheraisingandpropulsionofeach50poundsofweight.This，fora25—horsepowerengine，wouldgiveamaximumliftingcapacityof1250pounds.Itisdoubtfulifanyofthehigherratedmotorshavegreaterefficiency.Asan8—
　　cylindermotorrequiresmorefueltooperatethana4—
　　cylinder，itnaturallyfollowsthatitismoreexpensivetorunthanthesmallermotor，andanormalincreaseincapacity，takingactualperformancesasacriterion，islacking.Inotherwords，whatisthesenseofusingan8—cylindermotorwhenoneof4cylindersissufficient？
　　WhatthePropellerDoes.
　　Muchoftheefficiencyofthemotorisduetotheformandgearingofthepropeller.Hereagain，asinothervitalpartsofflying—machinemechanism，wehaveawidedivergenceofopinionastothebestform.Afishmakesprogressthroughthewaterbyusingitsfinsandtail;
　　abirdmakesitswaythroughtheairinasimilarmannerbytheuseofitswingsandtail.Inbothinstancesthemotivepowercomesfromthebodyofthefishorbird.
　　Inplaceoffinsorwingstheflyingmachineisequippedwithapropeller，theactionofwhichisfurnishedbytheengine.Finsandwingshavebeentried，buttheydon’twork.
　　Whileoperatingonthesamegeneralprinciple，aerialpropellersaremuchlargerthanthoseusedonboats.
　　Thisisbecausetheboatpropellerhasadenser，moresubstantialmediumtoworkin（water），andconsequentlycangetabetter"hold，"andproducemorepropulsiveforcethanoneofthesamesizerevolvingintheair.
　　Thisnecessitatestheaerialpropellersbeingmuchlargerthanthoseemployedformarinepurposes.Uptothispointallaviatorsagree，butastothebestformmostofthemdiffer.
　　KindsofPropellersUsed.
　　OneofthemostsimpleisthatusedbyCurtiss.Itconsistsoftwopear—shapedbladesoflaminatedwood，eachbladebeing5incheswideatitsextremepoint，taperingslightlytotheshaftconnection.Thesebladesarejoinedattheengineshaft，inadirectline.Thepropellerhasapitchof5feet，andweighs，complete，lessthan10pounds.Thelengthfromendtoendofthetwobladesis61/2feet.
　　Wrightusestwowoodenpropellers，intherearofhisbiplane，revolvinginoppositedirections.Eachpropelleristwo—bladed.
　　Bleriotalsousesatwo—bladewoodenpropeller，butitisplacedinfrontofhismachine.Thebladesareeachabout31/2feetlongandhaveanacute"twist."
　　Santos—Dumontusesatwo—bladewoodenpropeller，strikinglysimilartotheBleriot.
　　OntheAntoinettemonoplane，withwhichgoodrecordshavebeenmade，thepropellerconsistsoftwospoon—
　　shapedpiecesofmetal，joinedattheengineshaftinfront，andwiththeconcavesurfacesfacingthemachine.
　　ThepropellerontheVoisinbiplaneisalsoofmetal，consistingoftwoaluminumbladesconnectedbyaforgedsteelarm.
　　Maximumthrust，orstress——exerciseofthegreatestair—displacingforce——istheobjectsought.This，accordingtoexperts，isbestobtainedwithalargepropellerdiameterandreasonablylowspeed.Thediameteristhedistancefromendtoendoftheblades，whichonthelargestpropellersrangesfrom6to8feet.Thelargerthebladesurfacethegreaterwillbethevolumeofairdisplaced，and，followingthis，thegreaterwillbetheimpulsewhichforcestheaeroplaneahead.Inallcentrifugalmotionthereismoreorlesstendencytodisintegrationintheformof"flyingoff"fromthecenter，andthelargertherevolvingobjectisthestrongeristhistendency.Thisisillustratedinthemanyinstancesinwhichbiggrindstonesandfly—wheelshaveburstfrombeingrevolvedtoofast.Tohaveapropellerbreakapartintheairwouldjeopardizethelifeoftheaviator，andtoguardagainstthisithasbeenfoundbesttomakeitsrevolvingactioncomparativelyslow.Besidesthistheslowmotion（itisonlycomparativelyslow）givestheatmosphereachancetorefilltheareadisturbedbyonepropellerblade，andthushaveanewsurfaceforthenextbladetoactupon.
　　PlacingoftheMotor.
　　Asonotherpoints，aviatorsdifferwidelyintheirideasastotheproperpositionforthemotor.Wrightlocateshisonthelowerplane，midwaybetweenthefrontandrearedges，butconsiderablytoonesideoftheexactcenter.Hethencounter—balancestheengineweightbyplacinghisseatfarenoughawayintheoppositedirectiontopreservethecenterofgravity.Thisleavesaspaceinthecenterbetweenthemotorandtheoperatorinwhichapassengermaybecarriedwithoutdisturbingtheequilibrium.
　　Bleriot，onthecontrary，hashismotordirectlyinfrontandpreservesthecenterofgravitybytakinghisseatwellback，this，withtheweightoftheaeroplane，actingasacounter—balance.
　　OntheCurtissmachinethemotorisintherear，theforwardseatoftheoperator，andweightofthehorizontalrudderanddampingplaneinfrontequalizingtheengineweight.
　　NoPerfectMotorasYet.
　　EnginemakersintheUnitedStates，England，FranceandGermanyareallseekingtoproduceanidealmotorforaviationpurposes.Manyoftheproductionsarehighlycreditable，butitmaybetruthfullysaidthatnoneofthemquitefillthebillasregardsacombinationoftheminimumofweightwiththemaximumofreliablemaintainedpower.Theyareall，insomerespects，improvementsuponthosepreviouslyinuse，butthegreatendsoughtforhasnotbeenfullyattained.
　　OneofthemotorsthusproducedwasmadebytheFrenchfirmofDarracqatthesuggestionofSantosDumont，andonlineslaiddownbyhim.SantosDumontwanteda2—cylinderhorizontalmotorcapableofdeveloping30horsepower，andnotexceeding41/2poundsperhorsepowerinweight.
　　TherecanbenoquestionastotheabilityandskilloftheDarracqpeople，oroftheirdesiretoproduceamotorthatwouldbringnewcreditandprominencetothefirm.Neithercouldanythingradicallywrongbedetectedintheplans.Butthemotor，inatleastoneimportantrequirement，fellshortofexpectations.
　　Itcouldnotbedependedupontodeliveranenergyof30horsepowercontinuouslyforanylengthoftime.
　　Itsmaximumpowercouldbesecuredonlyin"spurts."
　　Thistendstoshowhowharditistoproduceanidealmotorforaviationpurposes.SantosDumont，ofundoubtedskillandexperienceasanaviator，outlineddefinitelywhathewanted;oneofthegreatestdesignersinthebusinessdrewtheplans，andthefamoushouseofDarracqbentitsbestenergiestotheproduction.Butthedesiredendwasnotfullyattained.
　　FeaturesofDarracqMotor.
　　Horizontalmotorswerepracticallyabandonedsometimeagoinfavoroftheverticaltype，butSantosDumonthadalogicalreasonforrevertingtothem.Hewantedtosecurealowercenterofgravitythanwouldbepossiblewithaverticalengine.Theoreticallyhisideawascorrectasthehorizontalmotorliesflat，andthereforeofferslessresistancetothewind，butitdidnotworkoutasdesired.
　　AtthesametimeitmustbeadmittedthatthisDarracqmotorisamarvelofingenuityandexquisiteworkmanship.
　　Thetwocylinders，havingaboreof51—10
　　inchesandastrokeof47—10inches，aremachinedoutofasolidbarofsteeluntiltheirweightisonly84—5
　　poundscomplete.Theheadisseparate，carryingtheseatingsfortheinletandexhaustvalves，isscrewedontothecylinder，andthenweldedinposition.Acopperwater—jacketisfitted，anditisinthisconditionthattheweightof84—5poundsisobtained.
　　Onlongtrips，especiallyinregionswheregasoleneishardtoget，theweightofthefuelsupplyisanimportantfeatureinaviation.Asanaturalconsequenceflyingmachineoperatorsfavorthemotorofgreatesteconomyingasoleneconsumption，provideditgivesthenecessarypower.
　　AnAmericaninventor，Ramseybyname，isworkingonamotorwhichissaidtopossessgreatpossibilitiesinthisline.Itsdistinctivefeaturesincludeaconnectingrodmuchshorterthanusual，andacrankshaftlocatedthelengthofthecrankfromthecentralaxisofthecylinder.Thishastheeffectofincreasingthepistonstroke，andalsoofincreasingtheproportionofthecrankcircleduringwhicheffectivepressureisappliedtothecrank.
　　Makingtheconnectingrodshorterandleavingthecrankmechanismthesamewouldintroduceexcessivecylinderfriction.ThisRamseyovercomesbythelocationofhiscrankshaft.Theeffectofthelongpistonstrokethussecured，istoincreasetheexpansionofthegases，whichinturnincreasesthepoweroftheenginewithoutincreasingtheamountoffuelused.
　　PropellerThrustImportant.
　　Thereisonegreatprincipleinflyingmachinepropulsionwhichmustnotbeoverlooked.Nomatterhowpowerfultheenginemaybeunlessthepropellerthrustmorethanovercomesthewindpressuretherecanbenoprogressforward.Shouldtheforceofthispropellerthrustandthatofthewindpressurebeequaltheresultisobvious.Themachineisatastand—stillsofarasforwardprogressisconcernedandisdeprivedoftheessentialadvancingmovement.
　　Speednotonlyfurnishessustentationfortheairship，butaddstothestabilityofthemachine.Anaeroplanewhichmaybejerkyanduncertaininitsmovements，sofarasequilibriumisconcerned，whenmovingataslowgait，willreadilymaintainanevenkeelwhenthespeedisincreased.
　　DesignsforPropellerBlades.
　　Itistheobjectofallmenwhodesignpropellerstoobtainthemaximumofthrustwiththeminimumexpenditureofengineenergy.Withthispurposeinviewmanypeculiarformsofpropellerbladeshavebeenevolved.Intheoryitwouldseemthatthebesteffectscouldbesecuredwithbladessoshapedastopresentathin（orcutting）edgewhentheycomeoutofthewind，andthenattheclimaxofdisplacementaffordamaximumofsurfacesoastodisplaceasmuchairaspossible.
　　Whilethisistheformmostgenerallyfavoredthereareothersinsuccessfuloperation.
　　Thereisalsowidedifferenceinopinionastotheequipmentofthepropellershaftwithtwoormoreblades.Someaviatorsusetwoandsomefour.Allhavemoreorlesssuccess.Asamathematicalpropositionitwouldseemthatfourbladesshouldgivemorepropulsiveforcethantwo，buthereagaincomesinoneofthepuzzlesofaviation，asthisresultisnotalwaysobtained.
　　DifferenceinPropellerEfficiency.
　　Thatthereisagreatdifferenceinpropellerefficiencyismadereadilyapparentbythecomparisonofeffectsproducedintwoleadingmakesofmachines——theWrightandtheVoisin.
　　Intheformeraweightoffrom1，100to1，200poundsissustainedandadvanceprogressmadeattherateof40milesanhourandmore，withhalftheenginespeedofa25horse—powermotor.Thiswouldbeasustainingcapacityof48poundsperhorsepower.ButtheactualcapacityoftheWrightmachine，asalreadystated，is50
　　poundsperhorsepower.
　　TheVoisinmachine，withaviator，weighsabout1，370
　　pounds，andisoperatedwithaso—horsepowermotor.
　　AllowingitthesamespeedastheWrightwefindthat，withdoubletheengineenergy，theliftingcapacityisonly271/2poundsperhorsepower.Towhatshallwechargethisremarkabledifference？Thesurfaceoftheplanesisexactlythesameinbothmachinessothereisnoadvantageinthematterofsupportingarea.
　　ComparisonofTwoDesigns.
　　OntheWrightmachinetwowoodenpropellersoftwobladeseach（eachbladehavingadecided"twist"）
　　areused.Asone25horsepowermotordrivesbothpropellerstheengineenergyamountstojustone—halfofthisforeach，or121/2horsepower.Andthisenergyisutilizedatone—halfthenormalenginespeed.
　　OntheVoisinaradicallydifferentsystemisemployed.
　　Herewehaveonemetaltwo—bladedpropellerwithaveryslight"twist"tothebladesurfaces.Thefullenergyofa50—horsepowermotorisutilized.
　　ExpertsFailtoAgree.
　　Whyshouldtherebesuchamarkeddifferenceintheresultsobtained？Whoknows？SomeexpertsmaintainthatitisbecausetherearetwopropellersontheWrightmachineandonlyoneontheVoisin，andconsequentlydoublethepropulsivepowerisexerted.
　　Butthisisnotafairdeduction，unlessbothpropellersareofthesamesize.Propulsivepowerdependsupontheamountofairdisplaced，andtheenergyputintothethrustwhichdisplacestheair.
　　Otherexpertsarguethatthedifferenceinresultsmaybetracedtothedifferenceinbladedesign，especiallyinthematterof"twist."
　　Thefactisthatpropellerresultsdependlargelyuponthenatureoftheaeroplanesonwhichtheyareused.
　　Apropeller，forinstance，whichgivesexcellentresultsononetypeofaeroplane，willnotworksatisfactorilyonanother.
　　Therearesomefeatures，however，whichmaybesafelyadoptedinpropellerselection.Theseare：Asextensiveadiameteraspossible;bladearea10to15percentoftheareaswept;pitchfour—fifthsofthediameter;
　　rotationslow.Themaximumofthrusteffortwillbethusobtained.
　　CHAPTERX.
　　PROPERDIMENSIONSOFMACHINES.
　　Inlayingoutplansforaflyingmachinethefirstthingtodecideuponisthesizeoftheplanesurfaces.Theproportionsofthesemustbebasedupontheloadtobecarried.Thisincludesthetotalweightofthemachineandequipment，andalsotheoperator.Thiswillbearatherdifficultproblemtofigureoutexactly，butpracticalapproximatefiguresmaybereached.
　　Itiseasytogetattheweightoftheoperator，motorandpropeller，butthematterofdetermining，beforetheyareconstructed，whattheplanes，rudders，auxiliaries，etc.，willweighwhencompletedisanintricateproposition.
　　Thebestwayistotakethedimensionsofsomesuccessfulmachineandusethem，makingsuchalterationsinaminorwayasyoumaydesire.
　　DimensionsofLeadingMachines.
　　Inthefollowingtableswillbefoundthedetailsastosurfacearea，weight，power，etc.，ofthenineprincipaltypesofflyingmachineswhicharenowprominentlybeforethepublic：
　　MONOPLANES.
　　SurfaceareaSpreadinDepthinMakePassengerssq.feetlinearfeetlinearfeetSantos—Dumont111016.026.0
　　Bleriot1150.624.622.0
　　R.E.P121534.128.9
　　Bleriot223632.923.0
　　Antoinette253841.237.9
　　No.ofWeightWithoutPropellerMakeCylindersHorsePowerOperatorDiameterSantos—Dumont2302505.0
　　Bleriot3256806.9
　　R.E.P7359006.6
　　Bleriot7501，2408.1
　　Antoinette8501，0407.2
　　BIPLANES.
　　SurfaceAreaSpreadinDepthinMakePassengerssq.feetlinearfeetlinearfeetCurtiss225829.0
　　28.7
　　Wright253841.0
　　30.7
　　Farman243032.9
　　39.6
　　Voisin253837.9
　　39.6
　　No.ofWeightWithoutPropellerMakeCylindersHorsePowerOperatorDiameterCurtiss8506006.0
　　Wright4251，1008.1
　　Farman7501，2008.9
　　Voisin8501，2006.6
　　Ingivingthedepthdimensionsthelengthoverall——
　　fromtheextremeedgeofthefrontauxiliaryplanetotheextremetipoftherearisstated.ThuswhilethedimensionsofthemainplanesoftheWrightmachineare41feetspreadby61/2feetindepth，thedepthoverallis30.7.
　　FiguringOuttheDetails.
　　Withthisdataasaguideitshouldbecomparativelyeasytodecideuponthedimensionsofthemachinerequired.
　　Inarrivingatthemaximumliftingcapacitytheweightoftheoperatormustbeadded.Assumingthistoaverage170poundsthemethodofprocedurewouldbeasfollows：
　　Addtheweightoftheoperatortotheweightofthecompletemachine.ThenewWrightmachinecompleteweighs900pounds.This，plus170，theweightoftheoperator，givesatotalof1，070pounds.Thereare538
　　squarefeetofsupportingsurface，orpracticallyonesquarefootofsurfaceareatoeachtwopoundsofload.
　　Therearesomemachines，notablytheBleriot，inwhichthesupportingpowerismuchgreater.Inthislatterinstancewefindasurfaceareaof1501/2squarefeetcarryingaloadof680plus170，oranaggregateof850
　　pounds.Thisistheequivalentoffivepoundstothesquarefoot.Thisratioisphenomenallylarge，andshouldnotbetakenasaguidebyamateurs.
　　TheMatterofPassengers.
　　Thesedeductionsarebasedoneachmachinecarryingonepassenger，whichisadmittedlythelimitatpresentofthemonoplaneslikethoseoperatedforrecord—makingpurposesbySantos—DumontandBleriot.Thebiplanes，however，haveatwo—passengercapacity，andthisaddsmateriallytotheproportionoftheirweight—sustainingpowerascomparedwiththesurfacearea.Inthefollowingstatementallthemachinesarefiguredontheone—passengerbasis.CurtissandWrighthavecarriedtwopassengersonnumerousoccasions，andanextra170
　　poundsshouldthereforebeaddedtothetotalweightcarried，whichwouldmateriallyincreasethecapacity.
　　Evenwiththetwo—passengerloadthelimitisbynomeansreached，butasexperimentshavegonenofurtheritisimpossibletomakemoreaccuratefigures.
　　AverageProportionsofLoad.
　　Itwillbeinteresting，beforeproceedingtolayoutthedimensiondetails，tomakeacomparisonoftheproportionofloadeffectwiththesupportingsurfacesofvariouswell—knownmachines.Herearethefigures：
　　Santos—Dumont——Atrifleunderfourpoundspersquarefoot.
　　Bleriot——Fivepounds.
　　R.E.P.——Fivepounds.
　　Antoinette——Abouttwoandone—quarterpounds.
　　Curtiss——Abouttwoandone—halfpounds.
　　Wright——Twoandone—quarterpounds.
　　Farman——Atrifleoverthreepounds.
　　Voisin——Alittleundertwoandone—halfpounds.
　　ImportanceofEnginePower.
　　Whilethesefiguresareauthentic，theyareinawaymisleading，astheimportantfactorofenginepowerisnottakenintoconsideration.Letusrecallthefactthatitistheenginepowerwhichkeepsthemachineinmotion，andthatitisonlywhileinmotionthatthemachinewillremainsuspendedintheair.Hence，toattributethesupportsolelytothesurfaceareaiserroneous.
　　True，thatonceunderheadwaytheplanescontributelargelytothesustainingeffect，andareabsolutelyessentialinaerialnavigation——themotorcouldnotrisewithoutthem——still，whenitcomestoaquestionofweight—
　　sustainingpower，wemustalsofigureontheenginecapacity.
　　IntheWrightmachine，inwhichthereisaliftingcapacityofapproximately21/4poundstothesquarefootofsurfacearea，anengineofonly25horsepowerisused.
　　IntheCurtiss，whichhasaliftingcapacityof21/2
　　poundspersquarefoot，theengineisof50horsepower.
　　Thisisanotherofthepeculiaritiesofaerialconstructionandnavigation.Herewehaveagainof1/4poundinweight—liftingcapacitywithanexpenditureofdoublethehorsepower.ItisthisfeaturewhichenablesCurtisstogetalongwithasmallersurfaceareaofsupportingplanesattheexpenseofabigincreaseinenginepower.
　　ProperWeightofMachine.
　　Asageneralpropositionthemostsatisfactorymachineforamateurpurposeswillbefoundtobeonewithatotalweight—sustainingpowerofabout1，200pounds.
　　Deducting170poundsastheweightoftheoperator，thiswillleave1，030poundsforthecompletemotor—
　　equippedmachine，anditshouldbeeasytoconstructonewithinthislimit.Thisimplies，ofcourse，thatduecarewillbetakentoeliminateallsuperfluousweightbyusingthelightestmaterialcompatiblewithstrengthandsafety.
　　Thisplanwilladmitof686poundsweightintheframework，coverings，etc.，and344forthemotor，propeller，etc.，whichwillbeample.Justhowtodistributetheweightoftheplanesisamatterwhichmustbelefttotheingenuityofthebuilder.
　　ComparisonofBirdPower.
　　Thereisaninterestingstudyintheaccompanyingillustration.Notethatthesurfaceareaofthealbatrossismuchsmallerthanthatofthevulture，althoughthewingspreadisaboutthesame.Despitethisthealbatrossaccomplishesfullyasmuchinthewayofflightandsoaringasthevulture.Why？Becausethealbabossisquickerandmorepowerfulinaction.Itistheapplicationofthissameprincipleinflyingmachineswhichenablesthoseofgreatspeedandpowertogetalongwithlesssupportingsurfacethanthoseofslowermovement.
　　MeasurementsofCurtissMachine.
　　SomeideaofframeworkproportionmaybehadfromthefollowingdescriptionoftheCurtissmachine.Themainplaneshaveaspread（width）of29feet，andare41/2feetdeep.Thefrontdoublesurfacehorizontalrudderis6x2feet，withanareaof24squarefeet.Totherearofthemainplanesisasinglesurfacehorizontalplane6x2feet，withanareaof12squarefeet.Inconnectionwiththisisaverticalrudder21/2feetsquare.
　　Twomovableailerons，orbalancingplanes，areplacedattheextremeendsoftheupperplanes.Theseare6x2
　　feet，andhaveacombinedareaof24squarefeet.Thereisalsoatriangularshapedverticalsteadyingsurfaceinconnectionwiththefrontrudder.
　　Thuswehaveatotalof195squarefeet，butastheofficialfiguresare258，andthesizeofthetriangular—
　　shapedsteadyingsurfaceisunknown，wemusttakeitforgrantedthatthismakesupthedifference.Inthematterofproportionthehorizontaldouble—planerudderisaboutone—tenththesizeofthemainplane，countingthesurfaceareaofonlyoneplane，theverticalrudderone—fortieth，andtheaileronsone—twentieth.
　　CHAPTERXI.
　　PLANEANDRUDDERCONTROL.
　　Havingconstructedandequippedyourmachine，thenextthingistodecideuponthemethodofcontrollingthevariousruddersandauxiliaryplanesbywhichthedirectionandequilibriumandascendinganddescendingofthemachinearegoverned.
　　Theoperatormustbeinpositiontoshiftinstantaneouslythepositionofruddersandplanes，andalsotocontroltheactionofthemotor.Thislatterissupposedtoworkautomaticallyandasageneralthingdoessowithentiresatisfaction，buttherearetimeswhenthesupplyofgasolenemustberegulated，andsimilarthingsdone.
　　Airshipnavigationcallsforquickaction，andforthisreasonthematterofcontrolisanimportantone——itismorethanimportant;itisvital.
　　SeveralMethodsofControl.
　　Someaviatorsuseasteeringwheelsomewhatafterthestyleofthatusedinautomobiles，andbythisnotonlymanipulatetherudderplanes，butalsotheflowofgasolene.Othersemployfootlevers，andstillothers，liketheWrights，dependuponhandlevers.
　　Curtisssteershisaeroplanebymeansofawheel，butsecuresthedesiredstabilizingeffectwithaningeniousjointedchair—back.Thisissoarrangedthatbyleaningtowardthehighpointofhiswingplanestheaeroplaneisrestoredtoanevenkeel.Thesteeringpostofthewheelismovablebackwardandforward，andbythismotionelevationisobtained.
　　TheWrightsforsometimeusedtwohandlevers，onetosteerbyandwarptheflexibletipsoftheplanes，theothertosecureelevation.Theyhavenowconsolidatedallthefunctionsinonelever.Bleriotalsousesthesinglelevercontrol.
　　Farmanemploysalevertoactuatetherudders，butmanipulatesthebalancingplanesbyfootlevers.
　　Santos—Dumontusestwohandleverswithwhichtosteerandelevate，butmanipulatestheplanesbymeansofanattachmenttothebackofhisoutercoat.
　　ConnectionWiththeLevers.
　　Nomatterwhichparticularmethodisemployed，theconnectionbetweentheleversandtheobjecttobemanipulatedisalmostinvariablybywire.Forinstance，fromthesteeringlevers（orlever）twowiresconnectwithoppositesidesoftherudder.Asaleverismovedsoastodrawintheright—handwiretherudderisdrawntotherightandviceversa.Theoperationisexactlythesameasinsteeringaboat.Itisthesamewayinchangingthepositionofthebalancingplanes.Amovementofthehandsorfeetandthemachinehaschangeditscourse，or，iftheequilibriumisthreatened，isbackonanevenkeel.
　　Simpleasthisseemsitcallsforacoolhead，quickeye，andsteadyhand.Theleasthesitationorafalsemovement，andbothaviatorandcraftareindanger.
　　WhichMethodisBest？
　　Itwouldbeaboldmanwhowouldattempttopickoutanyoneofthesemethodsofcontrolandsayitwasbetterthantheothers.Asinothersectionsofaeroplanemechanismeachmethodhasitsadvocateswhodwelllearnedlyuponitsadvantages，butthefactremainsthatallthevariousplansworkwellandgivesatisfaction.
　　Whatthenoviceisinterestedinknowingishowthecontroliseffected，andwhetherhehasbecomeproficientenoughinhismanipulationofittobeabsolutelydependableintimeofemergency.Noamateurshouldattemptaflightalone，untilhehasthoroughlymasteredthesteeringandplanecontrol.Iftheservicesandadviceofanexperiencedaviatorarenottobehadthenoviceshouldmounthismachineonsomesuitablesupportssoitwillbewellclearoftheground，and，gettingintotheoperator’sseat，proceedtomakehimselfwellacquaintedwiththeoperationofthesteeringwheelandlevers.
　　SomeThingstoBeLearned.
　　Hewillsoonlearnthatcertainmovementsofthesteeringgearproducecertaineffectsontherudders.If，forinstance，hismachineisequippedwithasteeringwheel，hewillfindthatturningthewheeltotherightturnstheaeroplaneinthesamedirection，becausethetillerisbroughtaroundtotheleft.Inthesamewayhewilllearnthatagivenmovementoftheleverthrowstheforwardedgeofthemainplaneupward，andthatthemachine，gettingtheimpetusofthewindundertheconcavesurfacesoftheplanes，willascend.Inthesamewayitwillquicklybecomeapparenttohimthatanoppositemovementoftheleverwillproduceanoppositeeffect——theforwardedgesoftheplaneswillbelowered，theairwillbe"spilled"outtotherear，andthemachinewilldescend.
　　Thetimeexpendedinthesepreliminarylessonswillbewellspent.Itwouldbeanactoffollytoattempttoactuallysailthecraftwithoutthem.
　　CHAPTERXII.
　　HOWTOUSETHEMACHINE.
　　Itisamistakenideathatflyingmachinesmustbeoperatedatextremealtitudes.True，undertheimpetusofhandsomeprizes，andtheincentivetoadvancescientificknowledge，professionalaviatorshaveascendedtoconsiderableheights，flightsatfrom500to1，500feetbeingnowcommonwithsuchexpertsasFarman，Bleriot，Latham，Paulhan，WrightandCurtiss.Thealtituderecordatthistimeisabout4，165feet，heldbyPaulhan.
　　Oneoftheinstructionsgivenbyexperiencedaviatorstopupils，andforwhichtheyinsistuponimplicitobeyance，is：
　　"Ifyourmachinegetsmorethan30feethigh，orcomesclosertothegroundthan6feet，descendatonce."SuchmenasWrightandCurtisswillnottolerateaviolationofthisrule.Iftheirinstructionsarenotstrictlycompliedwiththeydeclinetogivetheoffenderfurtherlessons.
　　WhyThisRulePrevails.
　　Thereisgoodreasonforthisprecaution.Thehigherthealtitudethemorerarefied（thinner）becomestheair，andthelesssustainingpowerithas.Consequentlythemoredifficultitbecomestokeepinsuspensionagivenweight.Whensailingwithin30feetofthegroundsustentationiscomparativelyeasyand，shouldafalloccur，theresultsarenotlikelytobeserious.Ontheotherhand，sailingtoonearthegroundisalmostasobjectionableinmanywaysasgettinguptoohigh.Ifthecraftisnavigatedtooclosetothegroundtrees，shrubs，fencesandotherobstructionsareliabletobeencountered.
　　Thereisalsothehandicapofcontraryaircurrentsdivertedbytheobstructionsreferredto，andwhichwillbeexplainedmorefullyfurtheron.
　　HowtoMakeaStart.
　　Takingitforgrantedthatthebeginnerhasfamiliarizedhimselfwiththemanipulationofthemachine，andespeciallythecontrolmechanism，thenextthinginorderisanactualflight.Itisprobablethathismachinewillbeequippedwithawheeledalightinggear，astheskidsusedbytheWrightsnecessitatetheuseofaspecialstartingtrack.Inthisrespectthewheeledmachineismucheasiertohandlesofarasnovicesareconcernedasitmaybeeasilyrolledtothetrialgrounds.This，asinthecaseoftheinitialexperiments，shouldbeaclear，reasonablylevelplace，freefromtrees，fences，rocksandsimilarobstructionswithwhichtheremaybedangerofcolliding.
　　Thebeginnerwillneedtheassistanceofthreemen.
　　Oneoftheseshouldtakehispositionintherearofthemachine，andoneateachend.Onreachingthetrialgroundtheaviatortakeshisseatinthemachineand，whilethemenattheendsholditsteadytheoneintherearassistsinretainingituntiltheoperatorisready.Inthemeantimetheaviatorhasstartedhismotor.Liketheglidertheflyingmachine，inordertoaccomplishthedesiredresults，shouldbeheadedintothewind.
　　WhentheMachineRises.
　　Undertheimpulseofthepushingmovement，andassistedbythemotoraction，themachinewillgraduallyrisefromtheground——providedithasbeenproperlyproportionedandputtogether，andeverythingisinworkingorder.Thisisthetimewhentheaviatorrequiresacoolhead，Atamodestdistancefromthegroundusethecontrollevertobringthemachineonahorizontallevelandovercomethetendencytorise.Theexactmanipulationofthisleverdependsuponthemethodofcontroladopted，andwiththistheaviatorissupposedtohavethoroughlyfamiliarizedhimselfaspreviouslyadvisedinChapterXI.
　　Itisatthisjuncturethattheoperatormustactpromptly，butwiththeperfectcomposurebegottenofconfidence.Oneofthegreatdrawbacksinaviationbynovicesisthetendencytobecomerattled，andthisismuchmoreprevalentthanonemightsuppose，evenamongmenwho，underotherconditions，arecoolandconfidentintheiractions.
　　Thereissomethinginthesensationofbeingsuddenlyliftedfromtheground，andsuspendedintheairthatisdisconcertingatthestart，butthiswillsoonwearoffiftheexperimenterwillkeepcool.Afewsuccessfulflightsnomatterhowshorttheymaybe，willputalotofconfidenceintohim.
　　MakeYourFlightsShort.
　　Bemodestinyourinitialflights.Don’tattempttomatchtherecordsofexperiencedmenwhohavedevotedyearstomasteringthedetailsofaviation.Paulhan，Farman，Bleriot，Wright，Curtiss，andalltherestofthembegan，andpracticedforyears，inthemannerheredescribed，beingcontenttomakejustalittleadvancementateachattempt.Aflightof150feet，cleanlyandsafelymade，isbetterasabeginningthanoneof400
　　yardsfullofbunglingmishaps.
　　Andyettheselatterhavetheiruses，providedtheoperatorisofadiscerningmindandcantakeadvantageofthemasobjectlessons.But，itisnotwelltoinvitethem.Theywilloccurfrequentlyenoughunderthemostfavorableconditions，anditisbesttohavethemcomelaterwhenthefeelingoftrepidationanduncertaintyastowhattodohaswornoff.
　　Aboveall，don’tattempttoflytoohigh.Keepwithinareasonabledistancefromtheground——about25or30
　　feet.Thisadviceisnotgivensolelytolessentheriskofseriousaccidentincaseofcollapse，butmainlybecauseitwillassisttoinstillconfidenceintheoperator.
　　Itiscomparativelyeasytolearntoswiminshallowwater，buttheknowledgethatoneistemptingdeathindeepwaterbegetstimidity.
　　PreservingtheEquilibrium.
　　Afterlearninghowtostartandstop，toascendanddescend，thenextthingtomasteristheartofpreservingequilibrium，theknackofkeepingthemachineperfectlylevelintheair——onan"evenkeel，"asasailorwouldsay.Thissimileisparticularlyappropriateasallaviatorsareinrealitysailors，andmuchmoredaringonesthanthosewhocoursetheseas.Thelatterareincraftwhicharekeptafloatbythebuoyancyofthewater，whetherinmotionorotherwiseand，solongasnormalconditionsprevail，willnotsink.Aviatorssailtheairincraftinwhichconstantmotionmustbemaintainedinordertoensureflotation.
　　Themanwhohasriddenabicycleormotorcyclearoundcurvesatanythinglikehighspeed，willhaveaverygoodideaastotheprincipleofmaintainingequilibriuminanairship.Heknowsthatinroundingcurvesrapidlythereisamarkedtendencytochangethedirectionofthemotionwhichwillresultinanupsetunlessheovercomesitbyaninclinationofhisbodyinanoppositedirection.Thisiswhyweseeracersleanwelloverwhentakingthecurves.Itsimplymustbedonetopreservetheequilibriumandavoidaspill.
　　HowItWorksIntheAir.
　　Iftheequilibriumofanairshipisdisturbedtoanextentwhichcompletelyovercomesthecenterofgravityitfallsaccordingtothelocationofthedisplacement.
　　Ifthisdisplacement，forinstance，isateitherendtheapparatusfallsendways;ifitistothefrontorrear，thefallisinthecorrespondingdirection.
　　Owingtouncertainaircurrents——theairiscontinuallyshiftingandeddying，especiallywithinahundredfeetorsooftheearth——theequilibriumofanairshipisalmostconstantlybeingdisturbedtosomeextent.Evenifthisdisturbanceisnotseriousenoughtobringonafallitinterfereswiththeprogressofthemachine，andshouldbeovercomeatonce.Thisisoneofthethingsconnectedwithaerialnavigationwhichcallsforprompt，intelligentaction.
　　Frequently，whenthedisplacementisveryslight，itmaybeovercome，andthecraftimmediatelyrightedbyamereshiftingoftheoperator’sbody.Take，forillustration，acaseinwhichtheextremerightendofthemachinebecomesloweredatriflefromthenormallevel.
　　Itispossibletobringitbackintoproperpositionbyleaningovertotheleftfarenoughtoshifttheweighttothecounter—balancingpoint.Thesameholdsgoodastominorfrontorreardisplacements.
　　WhenPlanesMustBeUsed.
　　Thereareotherdisplacements，however，andthesearethemostfrequent，whichcanbeonlyovercomebymanipulationofthestabilizingplanes.Themethodofproceduredependsupontheformofmachineinuse.TheWrightmachine，aspreviouslyexplained，isequippedwithplaneendswhicharesocontrivedastoadmitoftheirbeingwarped（positionchanged）bymeansofthelevercontrol.Theseflexibletipplanesmovesimultaneously，butinoppositedirections.Asthoseononeendrise，thoseontheotherendfallbelowthelevelofthemainplane.Bythismeansairisdisplacedatonepoint，andanincreasedamountsecuredinanother.
　　Thismayseemlikeacomplicatedsystem，butitsworkingsaresimplewhenonceunderstood.Itisbythemanipulationorwarpingoftheseflexibletipsthattransversestabilityismaintained，andanytendencytodisplacementendwaysisovercome.Longitudinalstabilityisgovernedbymeansofthefrontrudder.
　　Stabilizingplanesofsomeformareafeature，andanecessaryfeature，onallflyingmachines，butthemethodsofapplicationandmanipulationvaryaccordingtotheindividualideasoftheinventors.Theyalltend，however，towardthesameend——thekeepingofthemachineperfectlylevelwhenbeingnavigatedintheair.
　　WhentoMakeaFlight.
　　Abeginnershouldneverattempttomakeaflightwhenastrongwindisblowing.Thefiercerthewind，themorelikelyitistobegustyanduncertain，andthemoredifficultitwillbetocontrolthemachine.Eventhemostexperiencedanddaringofaviatorsfindthereisalimittowindspeedagainstwhichtheydarenotcompete.Thisisnotbecausetheylackcourage，buthavethesensetorealizethatitwouldbesillyanduseless.
　　Thenovicewillfindacomparativelystillday，oronewhenthewindisblowingatnottoexceed15milesanhour，thebestforhisexperiments.Themachinewillbemoreeasilycontrolled，thetripwillbesafer，andalsocheaperastheconsumptionoffuelincreaseswiththespeedofthewindagainstwhichtheaeroplaneisforced.
　　CHAPTERXIII.
　　PECULIARITIESOFAIRSHIPPOWER.
　　Asageneralpropositionittakesmuchmorepowertopropelanairshipagivennumberofmilesinacertaintimethanitdoesanautomobilecarryingafarheavierload.Automobileswithagrossloadof4，000pounds，andequippedwithenginesof30horsepower，havetravelledconsiderabledistancesattherateof50milesanhour.Thisisanequivalentofabout134poundsperhorsepower.Foranaveragemodernflyingmachine，withatotalload，machineandpassengers，of1，200
　　pounds，andequippedwitha50—horsepowerengine，50
　　milesanhouristhemaximum.Herewehavetheequivalentofexactly24poundsperhorsepower.Whythisgreatdifference？
　　NolessanauthoritythanMr.OctaveChanuteanswersthequestioninaplain，easilyunderstoodmanner.Hesays：
　　"Inthecaseofanautomobilethegroundfurnishesastablesupport;inthecaseofaflyingmachinetheenginemustfurnishthesupportandalsovelocitybywhichtheapparatusissustainedintheair."
　　PressureoftheWind.
　　Airpressureisabigfactorinthematterofaeroplanehorsepower.Allowingthatadeadcalmexists，abodymovingintheatmospherecreatesmoreorlessresistance.
　　Thefasteritmoves，thegreateristhisresistance.
　　Movingattherateof60milesanhourtheresistance，orwindpressure，isapproximately50poundstothesquarefootofsurfacepresented.Ifthemovingobjectisadvancingatarightangletothewindthefollowingtablewillgivethehorsepowereffectoftheresistancepersquarefootofsurfaceatvariousspeeds.
　　HorsePowerMilesperHourpersq.foot100.013
　　150044
　　200.105
　　250.205
　　300.354
　　400.84
　　501.64
　　602.83
　　806.72
　　10013.12
　　Whilethepressurepersquarefootat60milesanhour，isonly1.64horsepower，at100miles，lessthandoublethespeed，ithasincreasedto13.12horsepower，orexactlyeighttimesasmuch.Inotherwordsthepressureofthewindincreaseswiththesquareofthevelocity.
　　Windat10milesanhourhasfourtimesmorepressurethanwindat5milesanhour.
　　HowtoDetermineUponPower.
　　Thiselementofairresistancemustbetakenintoconsiderationindeterminingtheenginehorsepowerrequired.
　　Whenthemachineisunderheadwaysufficienttoraiseitfromtheground（about20milesanhour），eachsquarefootofsurfaceresistance，willrequirenearlynine—tenthsofahorsepowertoovercomethewindpressure，andpropelthemachinethroughtheair.Asshowninthetabletheratioofpowerrequiredincreasesrapidlyasthespeedincreasesuntilat60milesanhourapproximately3horsepowerisneeded.
　　InamachineliketheCurtisstheareaofwind—exposedsurfaceisabout15squarefeet.Onthebasisofthisresistancemovingthemachineat40milesanhourwouldrequire12horsepower.Thiscomputationcoversonlythemachine’spowertoovercomeresistance.Itdoesnotcoverthepowerexertedinpropellingthemachineforwardaftertheairpressureisovercome.TomeetthisimportantrequirementMr.Curtissfindsitnecessarytousea50—horsepowerengine.Ofthispower，ashasbeenalreadystated，12horsepowerisconsumedinmeetingthewindpressure，leaving38horsepowerforthepurposeofmakingprogress.
　　Theflyingmachinemustmovefasterthantheairtowhichitisopposed.Unlessitdoesthistherecanbenodirectprogress.Ifthetwoforcesareequalthereisnostraight—aheadadvancement.Take，forsakeofillustration，acaseinwhichanaeroplane，whichhasdevelopedaspeedof30milesanhour，meetsawindvelocityofequalforcemovinginanoppositedirection.Whatistheresult？Therecanbenoadvancebecauseitisacontestbetweentwoevenlymatchedforces.Theaeroplanestandsstill.Theonlywaytogetoutofthedifficultyisfortheoperatortowaitformorefavorableconditions，orbringhismachinetothegroundintheusualmannerbymanipulationofthecontrolsystem.
　　Takeanothercase.Anaeroplane，capableofmaking50milesanhourinacalm，ismetbyaheadwindof25
　　milesanhour.Howmuchprogressdoestheaeroplanemake？Obviouslyitis25milesanhourovertheground.
　　Putthepropositioninstillanotherway.Ifthewindisblowingharderthanitispossiblefortheenginepowertoovercome，themachinewillbeforcedbackward.
　　WindPressureaNecessity.
　　Whileallthisistrue，thefactremainsthatwindpressure，uptoacertainstage，isanabsolutenecessityinaerialnavigation.Theatmosphereitselfhasverylittlerealsupportingpower，especiallyifinactive.Ifabodyheavierthanairistoremainafloatitmustmoverapidlywhileinsuspension.
　　Oneofthebestillustrationsofthisistobefoundinskatingoverthinice.Everyschoolboyknowsthatifhemoveswithspeedhemayskateorglideinsafetyacrossathinsheetoficethatwouldnotbegintobearhisweightifhewerestandingstill.Exactlythesamepropositionobtainsinthecaseoftheflyingmachine.
　　Thenon—technicalreasonwhythesupportofthemachinebecomeseasierasthespeedincreasesisthatthesustainingpoweroftheatmosphereincreaseswiththeresistance，andthespeedwithwhichtheobjectismovingincreasesthisresistance.Withavelocityof12milesanhourtheweightofthemachineispracticallyreducedby230pounds.Thus，ifunderaconditionofabsolutecalmitwerepossibletosustainaweightof770pounds，thesameatmospherewouldsustainaweightof1，000
　　poundsmovingataspeedof12milesanhour.Thissustainingpowerincreasesrapidlyasthespeedincreases.