雅思阅读:No more whirly-splat
ALTHOUGH rare, engine failure is a stressful thing for a helicopter pilot to have to deal with. The pilot of a fixed-wing aircraft that loses power can at least rely on those wings to provide him with lift until his planes forward velocity falls below its stall speed. A helicopter, by contrast, derives both propulsion and lift from its blades. If they stop rotating, a rapid and terminal encounter with the ground beckons.
To prevent that, the blades of most helicopters have a special clutch that disengages them from the engine if the engine stops. The pilot has then to change the pitch of his craft to allow it to enter a mode called autorotation, in which the rush of air as it descends keeps the blades whirling, thus providing lift that slows the fall. The transition to autorotation is perilous, though, because it involves a reversal of the airflow through the blades. And even if he survives that, the pilot has to perform a second manoeuvre, known as flaring, just before he hits the ground. This involves pitching the machines nose up, in order to reduce its forward velocity and increase the speed of rotation of the bladesand thus the amount of lift they provideto soften the landing.
Both of these maneuvers would be less hazardous if the pilot could call on a second source of power to turn the blades for a few seconds while he was performing them, and Jean-Michel Billig and his team at Euro copter hope to provide just that. They are introducing into helicopters the fashionable concept of hybrid-electric drive.
Electric motors provide lots of torqueexactly the sort of twisting action needed to turn rotor blades. Mr Billings team have therefore fitted one of the firms AS350 light helicopters with such a motor, and some lithium-ion batteries to power it. They are now testing the arrangement to work out how much power is needed to keep the craft aloft during the transition to autorotation, and during flaring. Mr Billig thinks Euro copter will be able to offer the system commercially in about a years time.
That raises the question of whether it might be feasible to build an all-electric helicopter. At the moment, the answer is no. As with cars, the amount of charge a battery can hold is insufficient for robust, everyday use of the vehicle without the security blanket of an internal combustion engine. But batteries are improving, and if they were good enough then an electrically powered helicopter would be a more elegant solution to the problem of locomotion than the serial explosions that keep an internal-combustion engine ticking over.
牛津实用英语语法:278 分词的完成式(主动语态)
牛津实用英语语法:275 go,come,spend,waste,be busy
牛津实用英语语法:316 say,tell及其他可替代使用的引导动词
牛津实用英语语法:255 不定式的完成式
牛津实用英语语法:258 用做主语
牛津实用英语语法:209 一般将来时的用法
牛津实用英语语法:249 可起连词作用的不定式
牛津实用英语语法:214 将来进行时与will +动词原形的比较
牛津实用英语语法:232 would 表示过去的意图
牛津实用英语语法:277 代替从句的现在分词短语
牛津实用英语语法:270 go on,stop,try,used(to)
牛津实用英语语法:287 劝告的形式
牛津实用英语语法:230 用will,would表示习惯
牛津实用英语语法:274 catch,find,leave+宾语+现在分词
牛津实用英语语法:276 代替主句的现在分词短语
牛津实用英语语法:257 形式和用法
牛津实用英语语法:242 动词+ how/what/when/
牛津实用英语语法:262 动词+所有格形容词/宾格代词+动名词
牛津实用英语语法:237 should的其他用法
牛津实用英语语法:294 care和like
牛津实用英语语法:253 位于句首或句末的不定式短语
牛津实用英语语法:248 分裂不定式
牛津实用英语语法:238 不定式形式
牛津实用英语语法:269 agree/agree to,mean,propose
牛津实用英语语法:273 位于表示感觉的动词之后
牛津实用英语语法:282 其他表示命令的方式
牛津实用英语语法:295 care,like,love,hate,prefer
牛津实用英语语法:289 建议
牛津实用英语语法:will/would,shall/
牛津实用英语语法:条件句
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