That is very clever, but the article says that this drone "introduce[s] control surfaces into the airstream to change the direction of the airflow and create a thrust vectoring effect." My guess is tiny spoilers around the inside of the duct to cause separation of the flow in one quadrant of the duct, allowing the Coanda effect to draw the stream in the opposite direction.

For those interested in details, the papers are linked from the project page: http://www.modlabupenn.org/2014/10/23/underactuated-rotor/.

> The main motor directly drives the propeller hub, which is itself connected to the propeller blades by two inclined hinges. The hinge geometry couples blade lead-and-lag oscillations to a change in blade pitch. Instead of only driving the motor with a steady torque, we add a sinusoidal component in phase with the rotation of the rotor to induce a cyclic pitch variation. The amplitude and phase of this control signal determines the magnitude and direction of the vehicle response.

Oh, that's clever. Very nice. Tough to design, cheap to manufacture in quantity.

semanticscholar link: https://pdfs.semanticscholar.org/99f8/063247da7d87de2b9d61c6...