chapter 2 section 2 why airplanes fly II

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1、WhyAirplanesFly(II)---DragTheClassificationofDragInduceddrag(诱导阻力)Parasitedrag（寄生阻力；废阻力）Totaldrag（总阻力）ThetotaldragisthecombinationoftheinduceddragandtheparasitedragInduceddrag(诱导阻力)Dragisanyforceactingparallel(平行)totheflightpathbutintheoppositedirection.Theresultantforcecreatedbyanairfoilis

2、basicallyinthecorrectdirectionforlift,butnotexactly.Theresultantforce（合力）isnotperpendiculartotherelativewind.Instead,itslantsslightlytowardthedirectionoftherelativewind.Whilelift(perpendiculartotherelativewind)iscreated,somedragisalsoinducedbythecreationoflift.Andthiscomponentofthewing’sres

3、ultantforceiscalledinduceddrag(诱导阻力).Induceddrag，liftandangleofattackInduceddragistheundesirablebutunavoidableby-productoflift,andincreasesindirectproportionto(正比)increasesinangleofattack.Thegreatertheangleofattackuptothecriticalangle,thegreatertheamountofliftdeveloped,andthegreatertheinduc

4、eddrag.Parasitedrag（寄生阻力；废阻力）★Anysolidobjectthatmovesthroughtheairmustdisturbanddisplacetheairmoleculesalongitspath.Theairmoleculesresistthisdisturbance,andtheresistanceismanifestedasadragforcecalledparasitedrag.★Parasitedragistheresistanceoftheairproducedbyanypartoftheairplanepartthatdoesn

5、otproducelift.Theamountofparasitedragdependsonanumberofinteractingfactors,suchasthesizeoftheobject,itsshape,andtheroughnessofitssurface.Severalfactorsaffectparasitedrag.Wheneachfactorisconsideredindependently,itmustbeassumedthatotherfactorsremainconstant.＊Themorestreamlined（流线型的）anobjectis,

6、thelesstheparasitedrag.＊Themoredensetheairtheairplanemovesthrough,thegreatertheparasitedrag.＊Thelargerthesizeoftheobjectintheairstream,thegreatertheparasitedrag.＊Asspeedincreases,theamountofparasitedragincreases.Ifthespeedisdoubled,fourtimesasmuchdragisproduced.Themorestreamlinedanobjectis,

7、thelesstheparasitedragTheparasitedragisgenerallydividedintothreetypes:Formdrag(形状阻力，构形阻力)SkinFrictiondrag（表面摩擦阻力）Interferencedrag（干扰阻力）＊Formdragiscausedbythefrontalarea（迎风面）oftheairplanecomponentsbeingexposedtotheairstream.＊Skinfrictiondragiscausedbyairp