A helicopter blade uses a swashplate and a set of connections to change the angle of the blades. Not only does it vary the collective pitch to increase or decrease thrust, but it varies the pitch of each blade throughout each rotation, allowing it to produce more lift on one side of the rotor. This presented a problem [worked in theory] – he builds an RC helicopter from scratch, and this mechanism added both complexity and weight. Instead, he started looking for a way around the problem.
There were two options for eliminating the connections: a linkage-less swashplate design used in Black Hornet drones, and an under-actuated design with angle hinges to control the blade angle. [worked in theory] starting with the Black Hornet design, which has the end of each front run along the swashplate on wheels, while a central spring presses them down. Friction in the hinge and wheels proved to be too high, and it stopped responding to higher speeds. A second, low-friction design seems promising, but it still requires actuators, and thus weight, to control the float. [worked in theory] therefore moved to the under-actuated approach. In this design, the blades are mounted on angled hinges – by varying the engine speed throughout each rotation, the inertia causes the blades to swing backwards or forwards, changing their pitch.
[worked in theory] tested two such rotor designs: one with one hinge per rotor, and one with one hinge for both rotors. The single-hinged design seemed less sensitive to friction, was more responsive in testing, and had a more linear control response, so he used that. To actually make the leaves, [worked in theory] 3D printed them as flat blanks, then thermoformed them with printed molds. In testing, the blades had weights added to pull them back to the standard angle under constant rotation; Fortunately, however, this has the side effect of damping out unwanted vibrations.
There is still a long way to go before the helicopter is ready, but this is a very impressive start. This is the second under-actuated propeller we’ve seen recently, both inspired by [Tom Stanton]the original design.