flying machines-第31章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！



and　the　speaker；　in　spite　of　newspaper　reports　to　the

contrary；　had　never　seen　either　flown　in　more　than　a

gentle　breeze。　The　damping　out　of　the　oscillations　of

the　flight　path；　discussed　in　the　last　lecture；　increased

with　the　fourth　power　of　the　natural　velocity　of　flight；

and　rapid　damping　formed　the　easiest；　and　sometimes

the　only；　defense　against　dangerous　oscillations。　A

machine　just　stable　at　35　miles　per　hour　would　have

reasonably　rapid　damping　if　its　speed　were　increased　to

60　miles　per　hour。



Thinks　Use　Is　Limited。



It　was；　the　lecturer　proceeded；　inconceivable　that　any

very　extended　use　should　be　made　of　the　aeroplane　unless

the　speed　was　much　greater　than　that　of　the　motor　car。

It　might　in　special　cases　be　of　service；　apart　from　this

increase　of　speed；　as　in　the　exploration　of　countries

destitute　of　roads；　but　it　would　have　no　general　utility。

With　an　automobile　averaging　25　to　35　miles　per　hour；

almost　any　part　of　Europe；　Russia　excepted；　was　attainable

in　a　day's　journey。　A　flying　machine　of　but

equal　speed　would　have　no　advantages；　but　if　the　speed

could　be　raised　to　90　or　100　miles　per　hour；　the　whole

continent　of　Europe　would　become　a　playground；　every

part　being　within　a　daylight　flight　of　Berlin。　Further；

some　marine　craft　now　had　speeds　of　40　miles　per　hour；

and　efficiently　to　follow　up　and　report　movements　of

such　vessels　an　aeroplane　should　travel　at　60　miles　per

hour　at　least。　Hence　from　all　points　of　view　appeared

the　imperative　desirability　of　very　high　velocities　of

flight。　The　difficulties　of　achievement　were；　however；

great。



Weight　of　Lightest　Motors。



As　shown　in　the　first　lecture　of　his　course；　the　resistance

to　motion　was　nearly　independent　of　the　velocity；

so　that　the　total　work　done　in　transporting　a　given

weight　was　nearly　constant。　Hence　the　question　of　fuel

economy　was　not　a　bar　to　high　velocities　of　flight；　though

should　these　become　excessive；　the　body　resistance　might

constitute　a　large　proportion　of　the　total。　The　horsepower

required　varied　as　the　velocity；　so　the　factor　governing

the　maximum　velocity　of　flight　was　the　horsepower

that　could　be　developed　on　a　given　weight。　At

present　the　weight　per　horsepower　of　feather…weight

motors　appeared　to　range　from　2　1/4　pounds　up　to　7

pounds　per　brake　horsepower；　some　actual　figures　being

as　follows：



Antoinette。。。。。。。。　5　lbs。

Fiat。。。。。。。。。。。。。。　3　lbs。

Gnome。。。。。。。　Under　3　lbs。

Metallurgic。。。。。。。　8　lbs。

Renault。。。。。。。。。。。　7　lbs。

Wright。。。。。。。。。。。。。6　lbs。



Automobile　engines；　on　the　other　hand；　commonly

weighed　12　pounds　to　13　pounds　per　brake　horsepower。



For　short　flights　fuel　economy　was　of　less　importance

than　a　saving　in　the　weight　of　the　engine。　For　long

flights；　however；　the　case　was　different。　Thus；　if　the

gasolene　consumption　was　1/2　pound　per　horsepower　hour；

and　the　engine　weighed　3　pounds　per　brake　horsepower；

the　fuel　needed　for　a　six…hour　flight　would　weigh　as　much

as　the　engine；　but　for　half　an　hour's　flight　its　weight

would　be　unimportant。



Best　Means　of　Propulsion。



The　best　method　of　propulsion　was　by　the　screw；

which　acting　in　air　was　subject　to　much　the　same　conditions

as　obtained　in　marine　work。　Its　efficiency　depended

on　its　diameter　and　pitch　and　on　its　position；

whether　in　front　of　or　behind　the　body　propelled。　From

this　theory　of　dynamic　support；　Mr。　Lanchester　proceeded；

the　efficiency　of　each　element　of　a　screw　propeller

could　be　represented　by　curves　such　as　were　given

in　his　first　lecture　before　the　society；　and　from　these

curves　the　over…all　efficiency　of　any　proposed　propeller

could　be　computed；　by　mere　inspection；　with　a　fair　degree

of　accuracy。　These　curves　showed　that　the　tips　of

long…bladed　propellers　were　inefficient；　as　was　also　the

portion　of　the　blade　near　the　root。　In　actual　marine

practice　the　blade　from　boss　to　tip　was　commonly　of

such　a　length　that　the　over…all　efficiency　was　95　per　cent

of　that　of　the　most　efficient　element　of　it。



Advocates　Propellers　in　Rear。



From　these　curves　the　diameter　and　appropriate　pitch

of　a　screw　could　be　calculated；　and　the　number　of

revolutions　was　then　fixed。　Thus；　for　a　speed　of　80　feet

per　second　the　pitch　might　come　out　as　8　feet；　in　which

case　the　revolutions　would　be　600　per　minute；　which

might；　however；　be　too　low　for　the　motor。　It　was　then

necessary　either　to　gear　down　the　propeller；　as　was　done

in　the　Wright　machine；　or；　if　it　was　decided　to　drive　it

direct；　to　sacrifice　some　of　the　efficiency　of　the　propeller。

An　analogous　case　arose　in　the　application　of　the　steam

turbine　to　the　propulsion　of　cargo　boats；　a　problem　as

yet　unsolved。　The　propeller　should　always　be　aft；　so

that　it　could　abstract　energy　from　the　wake　current；　and

also　so　that　its　wash　was　clear　of　the　body　propelled。

The　best　possible　efficiency　was　about　70　per　cent；　and

it　was　safe　to　rely　upon　66　per　cent。



Benefits　of　Soaring　Flight。



There　was；　Mr。　Lanchester　proceeded；　some　possibility

of　the　aeronaut　reducing　the　power　needed　for　transport

by　his　adopting　the　principle　of　soaring　flight；　as

exemplified　by　some　birds。　There　were；　he　continued；　two

different　modes　of　soaring　flight。　In　the　one　the　bird

made　use　of　the　upward　current　of　air　often　to　be　found

in　the　neighborhood　of　steep　vertical　cliffs。　These　cliffs

deflected　the　air　upward　long　before　it　actually　reached

the　cliff；　a　whole　region　below　being　thus　the　seat　of

an　upward　current。　Darwin　has　noted　that　the　condor

was　only　to　be　found　in　the　neighborhood　of　such　cliffs。

Along　the　south　coast　also　the　gulls　made　frequent　use

of　the　up　currents　due　to　the　nearly　perpendicular　chalk

cliffs　along　the　shore。



In　the　tropics　up　currents　were　also　caused　by

temperature　differences。　Cumulus　clouds；　moreover；　were

nearly　always　the　terminations　of　such　up　currents　of

heated　air；　which；　on　cooling　by　expansion　in　the　upper

regions；　deposited　their　moisture　as　fog。　These　clouds

might；　perhaps；　prove　useful　in　the　future　in　showing

the　aeronaut　where　up　currents　were　to　he　found。　An…

other　mode　of　soaring　flight　was　that　adopted　by　the

albatross；　which　took　advantage　of　the　fact　that　the　air

moved　in　pulsations；　into　which　the　bird　fitted　itself；

being　thus　able　to　extract　energy　from　the　wind。

Whether　it　would　be　possible　for　the　aeronaut　to　employ

a　similar　method　must　be　left　to　the　future　to　decide。



Main　Difficulties　in　Aviation。



In　practical　flight　difficulties　arose　in　starting　and　in

alighting。　There　was　a　lower　limit　to　the　speed　at

which　the　machine　was　stable；　and　it　was　inadvisable　to

leave　the　ground　till　this　limit　was　attained。　Similarly；

in　alighting　it　was　inexpedient　to　reduce　the　speed　below

the　limit　of　stability。　This　fact　constituted　a　difficulty

in　the　adoption　of　high　speeds；　since　the　length　of　run

needed　increased　in　proportion　to　the　square　of　the

velocity。　This　drawback　could；　however；　be　surmounted

by　forming　starting　and　alighting　grounds　of　ample　size。

He　thought　it　quite　likely　in　the　future　that　such　grounds

would　be　considered　as　essential　to　the　flying　machine

as　a　seaport　was　to　an　ocean…going　steamer　or　as　a　road

was　to　the　automobile。



Requisites　of　Flying　Machine。



Flying　machines　were　commonly　divided　into　monoplanes

and　biplanes；　according　as　they　had　one　or　two

supporting　surfaces。　The　distinction　was　not；　however；

fundamental。　To　get　the　requisite　strength　some　form

of　girder　framework　was　necessary；　and　it　was　a　mere

question　of　convenience　whether　the　supporting　surface

was　arranged　along　both　the　top　and　the　bottom　of　this

girder；　or　along　the　bottom　only。　The　framework　adopted

universally　was　of　wood　braced　by　ties　of　pianoforte

wire；　an　arrangement　giving　the　stiffness　desired　with

the　least　possible　weight。　Some　kind　of　chassis　was　also

necessary。







CHAPTER　XXIII。



AMATEURS　MAY　USE