Backyard Almanac: you mean they're not called "twirling helicopter thingies"?
That's what they're called: samara.
And of course, they're "a winged achene, a type of fruit in which a flattened wing of fibrous, papery tissue develops from the ovary wall. A samara is a simple dry fruit and indehiscent (not opening along a seam). The shape of a samara enables the wind to carry the seed farther away than regular seeds from the parent tree, and is thus a form of anemochory."
And the Wikipedia entry goes on to say "A samara is sometimes called a key and is often referred to as a wingnut, helicopter or whirlybird, whirligig, polynose, or, in the north of England, a spinning jenny. During the autumn months, they are a popular source of amusement for children who enjoy tossing them in the air and watching them spin to the ground."
So. You know what you'll be doing this weekend!
And here's some phenology phenology: what we were talking about last year at this time!