Paul B. MacCready, an awesomely accomplished inventor who studied circling hawks and vultures to figure out how to realize the loftiest dream of Leonardo da Vinci — inventing a human-powered flying machine — died Tuesday in Pasadena, Calif. He was 81.

His death was announced by AeroVironment Inc., the company Dr. MacCready founded and led. No cause was given.

Dr. MacCready seemed a genuine free spirit — a friend noted his “innocent sense of wonder” in a Time magazine interview — empowered by a quirky, darting intellect. His fascination with aerodynamics came from watching butterflies and moths as a boy, and his dreams for the future included animal-powered flight.

“You can do all kinds of things if you just plunge ahead,” he said in an interview with Science in 1986. “It doesn’t mean you’re any good at them, but you can be good enough.”