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雅思阅读练习题5

雅思阅读练习题5

分类:雅思阅读   更新:2015/1/16   来源:网络

雅思阅读练习题5

    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 plane’s 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 machine’s nose up, in order to reduce its forward velocity and increase the speed of rotation of the blades—and thus the amount of lift they provide—to 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 (part of EADS, an aerospace and defence group) hope to provide just that. They are introducing into helicopters the fashionable concept of hybrid-electric drive.
    Electric motors provide lots of torque—exactly the sort of twisting action needed to turn rotor blades. Mr Billing’s team have therefore fitted one of the firm’s 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 year’s 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 (like an electric car) be a more elegant solution to the problem of locomotion than the serial explosions that keep an internal-combustion engine ticking over.

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