民航机务英语教程完整版文档格式.docx
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民航机务英语教程完整版文档格式.docx
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Let'
splacearestrictioninthepathofmovingairinthiswindtunnel.Smokeinjectedintotheairstreamindicatesthepaththeairtakesaroundandthroughtherestriction.Therestrictioniscalledaventuri,agivenvolumeofairtravelsthroughandaroundtheventuriinagivenperiodoftime.Theentirevolumemustpassal1pointsinthesystem.Let’sconsider3moleculesofairtobeourgivenvolume,all3moleculesmustpassaroundandthroughtheventuriandmeetattheotherendatthesametime.Noticethatthemoleculepassingthroughtheventurimustfollowalongercurvedpath,travelingatgreaterdistanceinthesameperiodoftimeasthemoleculethatfollowsastraightpath.Thatmeansthemoleculestravelingthroughtheventurimusttravelfaster.AccordingtoBernoulli’sprinciple,asthevelocityincreases,pressuredecreases,thiscausesareducedpressureinthecenteroftheventuri,whilepressureremainsthesameateitherend.
Nowlet’slookatahalfsectionoftheventuri,doestheshaperemindyouanything?
Youarelookingatthecrosssectionofanairfoil,suchasawingtheprimaryliftproducingsurfaceofanaircraft.Thesameprincipleapplieshereasintheventuri,the2moleculesofairrepresentacertainvolume,bothmoleculesmustreachthetrailingedgeofthewingsectioninthesameperiodoftime,butthemoleculetravelingovertheuppersurfacemusttravelfurtherthantheonetravelingunderthewingbecauseacurvedpathitmustfollow.Again,themoleculetravelingthecurvedpathmusttraveltasterthantheonetravelingthestraightline.Thepressuredifferentialbetweentheupperandlowersurfacesofawingproducesliftthatkeepstheaircraftaloft④
Wemayillustratesimplythetremendousamountofforcethatgeneratedbylift.Supposethelowersurfaceofthewinghasanareaofabout15,000squareinches,rememberingthatpressureisaforceperunitarea,apressureof15lbs/inch²
wouldproduceatotalforceonthebottomofthewingof225,000pounds.Thecurveduppersurfaceofthewinghasagreatersurfaceareathanthelowersurface,perhaps20,000squareinches.Howeverthepressureontheuppersurfaceislesspersquareinch.Ifthepressurewe’vereducedto10lbs/inch²
ontheupperwingsurface,thetotalforcewouldbereducedto200,000pounds.Thedifferenceinpressurebetweenthelowerandupperwingsurfacesis25,000pounds.Thegreaterforceactingonthebottomwingsurface,thisisanextremelyoversimplifiedexplanationoftheproductionofliftonthewing.However,thebasicideaissound.⑤Thereareotherfactorstoconsider,suchastheshapeoftheairfoil,theangleofattack,andliftanddragcoefficientsaswellasthespeedoftheairfoilthroughtheair.We’veseenhowBernoulli’sprinciplecreatesliftonanaircraftwingwhetherinpoweredorunpoweredflight.⑥But,theprinciplealsoappliestoanyimportantfunctioninaircraftengines,specificallythefuelflowthroughanaircraftcarburetor.Let’sreturnforamomenttotheventuriinthewindtunnel,yourecallthatthepressureintherestrictionoftheventuriwaslowerthanthepressureoutsidetheventuri.Aventuriisfoundintheprimaryairpassageoffloat—typeaircraftcarburetor.Asairisdrawnintotheenginebythepistons,pressureisreducedinthroatofthecarburetor.Let’sinsertatubeinthenarrowestpartoftheventuri.andattachthetubetoafuelsupplywhichisopentoatmosphericpressure.Thedifferenceinpressurebetweenatmosphericpressureonthefuelsupplyandpressureintheventuricausesfueltoflowintotheventuriwherefuelismixedwithairbeforeenteringtheenginecombustionchambers.Venturiororificesmaybefoundinotherpartsoftheaircraft,inahydrauliclineorafuelcontrol,ameasuredorificeproducesameasuredpressuredifferentialbetweentwosidesoftheline,causingfluidtoflowatapre—determinedrate.
Let’sreviewwe’velearntabouttherelationshipofpressureandvelocityofafluidinmotion.Bernoulli’sprinciplesaysthatasthevelocityoffluidincreases,itspressuredecreases.Asthevelocityofairpassingthroughaventuriincreases,thepressureinthenarrowestpartoftheventuridecreases.Asthevelocityofairincreasesovertheuppersurfaceofawing,thepressuredecreases,theimbalanceofforcesbetweentheupperandlowarsurfacescausesthewingtorise.Bernoulli’sprincipleisalsopartlyresponsibleforfuelflowinanaircraftcarburetorwhichusesaventuriintheprimaryairpassage.Venturimayalsobeusedtocreateameasuredrateoffluidflow.Venturi-likeorificesarefoundinaircrafthydraulicsystemsandinfuelcontrols.
WordsandPhrases
Bernoulli’sprinciple贝努利原理
carburetor汽化器n.
inverselyproportional反比于
lip唇n.
throat咽喉,狭口n.
property性质n.
vacuumcleaner真空吸尘器
suspend悬挂v.
curvedsurface弯曲外表
sewingspool缝纫机线轴
conversely相反地ad.
restriction限定,节流n.
venturi文氏管,细腰管n.
molecule分子n.
crosssectionofanairfoil翼型剖面
sound完好的,合理的a.
angleofattack攻角,迎角
piston活塞n.
orifice小孔n.
liftanddragcoefficients升力和阻力系数
hydraulic液压的a.
reinforce加强v.
concept概念,根本原理n.
Notes
①Aviationisdependentupontheprinciplediscoveredbyaneighteenthcenturyscientist,DanielBernoulli.航空学与十八世纪科学家丹诺尔·
贝努利所发现的原理密切相关。
tobedependentupon意为“依靠于…〞“取决于…〞。
discoveredby…是过去分词引导的定语修饰theprinciple。
②Thesimplestmaybeconductedwithonlyasheetofpaper.可用一张纸来进行最简单的试验。
tobeconductedwith意为“由…处理〞“由…进行〞。
asheetofpaper意为“一张纸〞,paper是不可数名词,所以一张纸必须用asheetof来表示,同样的用法还有许多。
③Theairpassingaroundthecurvedsurfaceoftheballistravelingfasterthantheairjustbeonthesurface.绕球曲面流过的气流比即将到达球外表的气流速度要快。
passingaroundthecurvedsurface是现代分词引起的定语,修饰theair;
fasterthan…意为“比…快〞,比拟的内容前后要一致。
④Thepressuredifferentialbetweentheupperandlowersurfacesofawingproducesliftthatkeepstheaircraftaloft.机翼上下外表的压差产生了升力,使飞机保持在空中。
⑤Howeverthebasicideaissound.但根本思想是正确的。
sound在这里是形容词。
⑥We’veseenhowBernoulli’sprinciplecreatesliftonanaircraftwingwhetherinpoweredorunpoweredflight.我们已经看到了,不管是有动力飞行还是无动力飞行,贝努利原理是如何在机翼上产生升力的。
whether…or…意为“不管…还是…〞。
Grammar
用it作形式主语的各种结构
常见形式有:
请看下面例句:
Itisofteninconvenienttocombinevectorsbythegeometricmethod.
用几何方法来合成矢量通常是不便当的。
Itisworthwhiletouseanultra—highvacuumsystemforsuchpurpose.
把超高真空装置用于这种场合是有价值的。
Ifabodymovesfromonepositiontoanother,itissaidtohavehaddisplacement.
如果物体从一处移到另一处,就认为它有了位移。
Itiscommonpracticetobuildrocketsinpieces.
通常的做法是把火箭分级制造。
Ittakessunlightovereightminutestotraveltotheearth.
太阳光传播到地球要8分多钟。
Itrequiresalong—termpracticetobeagoodpilot.
要成为优秀的飞行员需要长时间的实践。
Itmakessteelweakinstrengthtorust.
生锈使钢的强度下降。
Exercises
1.Answerthefollowingquestions:
1)WhatrelationshipisdescribedmainlybyBernoulli’sprinciple?
2)Howthepaperbemovedwhenyouholdthepaperuptoyourlipsandblowacrossthetopsurface?
andwhy?
3)Inwhichsectionoftheventurithepressureislowestwhentheairstreampassesthetube?
4)Whytheairtravelingovertheuppersurfacemusttravelfurtherthantheairtravelingunderthewing?
5)Listsomefactorsthatmustbeconsidertocalculatetheliftexceptthepressuredifference.
6)Whatisthefunctionofthemeasuredorificeinahydrauliclineorafuelcontr017
7)Whythepoweredflightcomeintobeing?
(TheliftcanbecreatedbyjustthewingsaccordingtoBernoulli’sprinciple.)
2.Choosethecorrectanswer:
1)Whichoneisnotcorrect?
A.Bernoulli’sprinciplehavenotappliedintheaviation.
B.Whenthevelocityincreases,pressuredecrease.
C.Bernoulli’sprincipleistheonlyscientificprinciplerelatedtoflight.
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