In the second sentence, the writer has simply chosen to use the plural; it could have read ' resembled the later helicopter '-- 'the helicopter, a machine that appeared later in history'. The plural form carries the same meaning. Similarly, the first sentence could equally well be recast as ' looked like early helicopters '-- 'looked like helicopters, as if they had appeared contemporaneously with Da Vinci, earlier than in reality.
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Da Vinci drew plans which looked like an early helicopter; Da Vinci drew plans that resembled later helicopters.The content ('Da Vinci') of both sentences leaves no doubt in the reasonable native reader's mind, and without thinking, that the writer means the generic helicopter, primitive or m