Labels:book | daily | grass | mountain | newspaper | roof | rotogravure | sky | tree | windowpane OCR: Helicopter This famous drawing shows screw proposed by Leonardo to screw -shaped device made power their flight. starched linen stretched over metal frame The linen screw was designed to rotate rapidly, powered someone pedaling bencath 1f Some consider this invention forerunner to the modern helicopter Ir 1936 the German engineer Heinrich Focke developed the first practical helicopter Today helicopters use rotary propellers rather than the overa beneath
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