But it remains hard to shrink engines, batteries, mechanical actuators and control electronics to the point where they are small and light enough to fly without impairing their performance.
不过，要缩小这些飞行引擎、电池、机械促动器、中控电子装置，以使这些飞行器够轻够小地完美飞行，还是相当困难。
Left in natural light for long periods, the solar panel generates enough standby power to enable the phone to be used without any further charging.
只要长时间留在自然光下，太阳能电池就能储备足够多的电力以供手机使用，就不需要再次充电了。
Before they are filled with concrete they are light enough to be installed quickly, without the need for large cranes or other heavy-duty equipment.
在填充混凝土之前，它们相当轻，能够很快组装而不需要大型起重机或其他重型设备。
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