the aeroplane speaks-第13部分

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LONGITUDINAL　STABILITY。Flat　surfaces　are　longitudinally　stable　owing　to　the　fact　that　with　decreasing　angles　of　incidence　the　centre　line　of　pressure　（C。P。）　moves　forward。

The　C。P。　is　a　line　taken　across　the　surface；　transverse　to　the　direction　of　motion；　and　about　which　all　the　air　forces　may　be　said　to　balance；　or　through　which　they　may　be　said　to　act。


Imagine　A　to　be　a　flat　surface；　attitude　vertical；　travelling　through　the　air　in　the　direction　of　motion　M。　Its　C。P。　is　then　obviously　along　the　exact　centre　line　of　the　surface　as　illustrated。

In　B；　C；　and　D　the　surfaces　are　shown　with　angles　of　incidence　decreasing　to　nothing；　and　you　will　note　that　the　C。P。　moves　forward　with　the　decreasing　angle。

Now；　should　some　gust　or　eddy　tend　to　make　the　surface　decrease　the　angle；　i。e。；　dive；　then　the　C。P。　moves　forward　and　pushes　the　front　of　the　surface　up。　Should　the　surface　tend　to　assume　too　large　an　angle；　then　the　reverse　happensthe　C。P。　moves　back　and　pushes　the　rear　of　the　surface　up。

Flat　surfaces　are；　then；　theoretically　stable　longitudinally。　They　are　not；　however；　used；　on　account　of　their　poor　lift…drift　ratio。

As　already　explained；　cambered　surfaces　are　used；　and　these　are　longitudinally　unstable　at　those　angles　of　incidence　producing　a　reasonable　lift…drift　ratio；　i。e。；　at　angles　below：　about　12　degrees。

A　is　a　cambered　surface；　attitude　approximately　vertical；　moving　through　the　air　in　the　direction　M。　Obviously　the　C。　P。　coincides　with　the　transverse　centre　line　of　the　surface。

With　decreasing　angles；　down　to　angles　of　about　30　degrees；　the　C。P。　moves　forward　as　in　the　case　of　flat　surfaces　（see　B）；　but　angles　above　30　degrees　do　not　interest　us；　since　they　produce　a　very　low　ratio　of　lift　to　drift。

Below　angles　of　about　30　degrees　（see　C）　the　dipping　front　part　of　the　surface　assumes　a　negative　angle　of　incidence　resulting　in　the　DOWNWARD　air　pressure　D；　and　the　more　the　angle　of　incidence　is　decreased；　the　greater　such　negative　angle　and　its　resultant　pressure　D。　Since　the　C。P。　is　the　resultant　of　all　the　air　forces；　its　position　is　naturally　affected　by　D；　which　causes　it　to　move　backwards。　Now；　should　some　gust　or　eddy　tend　to　make　the　surface　decrease　its　angle　of　incidence；　i。e。；　dive；　then　the　C。P。　moves　backwards；　and；　pushing　up　the　rear　of　the　surface；　causes　it　to　dive　the　more。　Should　the　surface　tend　to　assume　too　large　an　angle；　then　the　reverse　happens；　the　pressure　D　decreases；　with　the　result　that　C。P。　moves　forward　and　pushes　up　the　front　of　the　surface；　thus　increasing　the　angle　still　further；　the　final　result　being　a　‘‘tail…slide。''

It　is　therefore　necessary　to　find　a　means　of　stabilizing　the　naturally　unstable　cambered　surface。　This　is　usually　secured　by　means　of　a　stabilizing　surface　fixed　some　distance　in　the　rear　of　the　main　surface；　and　it　is　a　necessary　condition　that　the　neutral　lift　lines　of　the　two　surfaces；　when　projected　to　meet　each　other；　make　a　dihedral　angle。　In　other　words；　the　rear　stabilizing　surface　must　have　a　lesser　angle　of　incidence　than　the　main　surfacecertainly　not　more　than　one…third　of　that　of　the　main　surface。　This　is　known　as　the　longitudinal　dihedral。

I　may　add　that　the　tail…plane　is　sometimes　mounted　upon　the　aeroplane　at　the　same　angle　as　the　main　surface；　but；　in　such　cases；　it　attacks　air　which　has　received　a　downward　deflection　from　the　main　surface；　thus：

｛illust。｝


The　angle　at　which　the　tail　surface　attacks　the　air　（the。　angle　of　incidence）　is　therefore　less　than　the　angle　of　incidence　of　the　main　surface。

I　will　now；　by　means　of　the　following　illustration；　try　to　explain　how　the　longitudinal　dihedral　secures　stability：

First；　imagine　the　aeroplane　travelling　in　the　direction　of　motion；　which　coincides　with　the　direction　of　thrust　T。　The　weight　is；　of　course；　balanced　about　a　C。P。；　the　resultant　of　the　C。P。　of　the　main　surface　and　the　C。P。　of　the　stabilizing　surface。　For　the　sake　of　illustration；　the　stabilizing　surface　has　been　given　an　angle　of　incidence；　and　therefore　has　a　lift　and　C。P。　In　practice　the　stabilizer　is　often　set　at　no　angle　of　incidence。　In　such　case　the　proposition　remains　the　same；　but　it　is；　perhaps；　a　little　easier　to　illustrate　it　as　above。

Now；　we　will　suppose　that　a　gust　or　eddy　throws　the　machine　into　the　lower　position。　It　no　longer　travels　in　the　direction　of　T；　since　the　momentum　in　the　old　direction　pulls　it　off　that　course。　M　is　now　the　resultant　of　the　Thrust　and　the　Momentum；　and　you　will　note　that　this　results　in　a　decrease　in　the　angle　our　old　friend　the　neutral　lift　line　makes　with　M；　i。e。；　a　decrease　in　the　angle　of　incidence　and　therefore　a　decrease　in　lift。

We　will　suppose　that　this　decrease　is　2　degrees。　Such　decrease　applies　to　both　main　surface　and　stabilizer；　since　both　are　fixed　rigidly　to　the　aeroplane。

The　main　surface；　which　had　12　degrees　angle；　has　now　only　10　degrees；　i。e。；　a　loss　of　ONE…SIXTH。

The　stabilizer；　which　had　4　degrees　angle；　has　now　only　2　degrees；　i。e。；　a　loss　of　ONE…HALF。

The　latter　has　therefore　lost　a　greater　PROPORTION　of　its　angle　of　incidence；　and　consequently　its　lift；　than　has　the　main　surface。　It　must　then　fall　relative　to　the　main　surface。　The　tail　falling；　the　aeroplane　then　assumes　its　first　position；　though　at　a　slightly　less　altitude。

Should　a　gust　throw　the　nose　of　the　aeroplane　up；　then　the　reverse　happens。　Both　main　surface　and　stabilizer　increase　their　angles　of　incidence　in　the　same　amount；　but　the　angle；　and　therefore　the　lift；　of　the　stabilizer　increases　in　greater　proportion　than　does　the　lift　of　the　main　surface；　with　the　result　that　it　lifts　the　tail。　The　aeroplane　then　assumes　its　first　position；　though　at　a　slightly　greater　altitude。

Do　not　fall　into　the　widespread　error　that　the　angle　of　incidence　varies　as　the　angle　of　the　aeroplane　to　the　horizontal。　It　varies　with　such　angle；　but　not　as　anything　approaching　it。　Remember　that　the　stabilizing　effect　of　the　longitudinal　dihedral　lasts　only　as　long　as　there　is　momentum　in　the　direction　of　the　first　course。

These　stabilizing　movements　are　taking　place　all　the　time；　even　though　imperceptible　to　the　pilot。

Aeroplanes　have；　in　the　past；　been　built　with　a　stabilizing　surface　in　front　of　the　main　surface　instead　of　at　the　rear　of　it。　In　such　design　the　main　surface　（which　is　then　the　tail　surface　as　well　as　the　principal　lifting　surface）　must　be　set　at　a　less　angle　than　the　forward　stabilizing　surface；　in　order　to　secure　a　longitudinal　dihedral。　The　defect　of　such　design　lies　in　the　fact　that　the　main　surface　must　have　a　certain　angle　to　lift　the　weightsay　5　degrees。　Then；　in　order　to　secure　a　sufficiency　of　longitudinal　stability；　it　is　necessary　to　set　the　forward　stabilizer　at　about　15　degrees。　Such　a　large　angle　of　incidence　results　in　a　very　poor　lift…drift　ratio　（and　consequently　great　loss　of　efficiency）；　except　at　very　low　velocities　compared　with　the　speed　of　modern　aeroplanes。　At　the　time　such　aeroplanes　were　built　velocities　were　comparatively　low；　and　this　defect　was；　for　that　reason；　not　sufficiently　appreciated。　In　the　end　it　killed　the　‘‘canard''　or　‘‘tail…first''　design。

Aeroplanes　of　the　Dunne　and　similar　types　possess　no　stabilizing　surface　distinct　from　the　main　surface；　but　they　have　a　longitudinal　dihedral　which　renders　them　stable。

The　main　surface　towards　the　wing…tips　is　given　a　decreasing　angle　of　incidence　and　corresponding　camber。　The　wing…tips　then　act　as　longitudinal　stabilizers。

This　design　of　aeroplane；　while　very　interesting；　has　not　proved　very　practicable；　owing　to　the　following　disadvantages：　（1）　The　plan　design　is　not；　from　a　mechanical　point　of　view；　so　sound　as　that　of　the　ordinary　aeroplane　surface；　which　is；　in　plan；　a　parallelogram。　It　is；　then；　necessary　to　make　the　strength　of　construction　greater　than　would　otherwise　be　the　case。　That　means　extra　weight。　（2）　The　plan　of　the　surface　area　is　such　that　the　aspect　ratio　is　not　so　high　as　if　the　surface　was　arranged　with　its　leading　edges　at　right　angles　to　the　direction　of　motion。　The　lower　the　aspect　ratio；　then；　the　less　the　lift。　This　design；　then；　produces　less　lift　for　weight　of　surface　than　would　the　same　surface　if　arranged　as　a　parallelogram。　（3）　In　order　to　secure　the　longitudinal　dihedral；　the　angle　of　incidence　has　to　be　very　much　decreased　towards　the　wing…tips。　Then；　in　order　that　the　lift…drift　ratio　may　be　preserved；　there　must　be　a　corresponding　decrease　in　the　camber。　That　calls　for　surface　ribs　of　varying　cambers；　and　results　in　an　expensive　and　lengthy　job　for　the　builder。　（4）　In　order　to　secure　directional　stability；　the　surface　is；　in　the　centre；　arranged　to　dip　down　in　the　form　of　a　V；　pointing　towards　the　direction　of　motion。　Should　the　aeroplane　turn　off　its　course；　then　its　momentum　in　the　direction　of　its　first　course　causes　it　to　move　in　a　direction　the　resultant　of　the　thrust　and　the　momentum。　It　then　moves　in　a　more　or　less　sideways　attitude；　which　results　in　an　air　pressure　upon　one　side　of　the　V；　and　which　tends　to　turn　the　aeroplane　back　to　its　first　course。　This　arrangement　of　the　surface　results　in　a　bad　drift。　Vertical　surfaces　at　the　wing…tips　may　also　be　set　at　an　angle　producing　the　same　stabilizing　effect；　but　they　also　increase　the　drift。

The　gyroscopic　action　of　a　rotary　engine　will　affect　the　longitudinal　stability　when　an　aeroplane　is　turned　to　right　or　left。　In　the　case　of　a　Gnome　engine；　such　gyroscopic　action　will　tend　to　depress　the　nose　of　the　aeroplane　when　it　is　turned　to　the　left；　and　to　elevate　it　when　it　is　turned　to　the　right。　In　modern　aeroplanes　this　tendency　is　not　sufficiently　important　to　bother　about。　In　the　old　days　of　crudely　designed　and　under…powered　aeroplanes　this　gyroscopic　action　was　very　marked；　and　led　the　majority　of　pilots　to　dislike　turning　an　aeroplane　to　the　right；　since；　in　doing　so；　there　was　some　danger　of　‘‘stalling。''


LATERAL　STABILITY　is　far　more　difficult　for　the　designer　to　secure　than　is　longitudinal　or　directional　stability。　Some　degree　of　lateral　stability　may　be　secured　by　means　of　the　‘‘lateral　dihedral；''　i。e。；　the　upward　inclination　of　the　surface　towards　its　wing…tips　thus：

Imagine　the　top　V；　illustrated　opposite；　to　be　the　front　view　of　a　surface　flying　towards　you。　Th