'duck' means to lower the head or the body quickly to avoid a blow to the head. In your example, trying to avoid the blast of air from the propellors.
It can also be used to refer to a very tall person who ducks down as he enters a low doorway, so as not to hit his head.
Also, it can mean quickly concealing the whole of one's body behind something so as not to be seen eg The cowboy quickly ducked behind a rock to avoid being seen by the approaching Indians.
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