Ventilated acoustic metasurface with low-frequency sound insulation.

A ventilated acoustic metasurface consisting of a membrane covered with a combination of different depth sub-chambers is proposed. It can achieve at least a 5 dB sound insulation acoustic performance in the wide frequency range from 100 to 1700 Hz, in particular a 10 dB noise reduction in the range from 100 to 200 Hz and from 437.4 to 1700 Hz, which can therefore cover the low-frequency range of the environmental noise. The physical mechanism of membrane-acoustic coupling for noise reduction in the low-frequency range is further explored.

Efficient flapping wing drone arrests high-speed flight using post-stall soaring.

The aerobatic maneuvers of swifts could be very useful for micro aerial vehicle missions. Rapid arrests and turns would allow flight in cluttered and unstructured spaces. However, these decelerating aerobatic maneuvers have been difficult to demonstrate in flapping wing craft to date because of limited thrust and control authority. Here, we report a 26-gram X-wing ornithopter of 200-millimeter fuselage length capable of multimodal flight. Using tail elevation and high thrust, the ornithopter was piloted to hover, fly fast forward (dart), turn aerobatically, and dive with smooth transitions. The aerobatic turn was achieved within a 32-millimeter radius by stopping a dart with a maximum deceleration of 31.4 meters per second squared. In this soaring maneuver, braking was possible by rapid body pitch and dynamic stall of wings at relatively high air speed. This ornithopter can recover to glide stability without tumbling after a 90-degree body flip. We showed that the tail presented a strong stabilizing moment under high thrust, whereas the wing membrane flexibility alleviated the destabilizing effect of the forewings. To achieve these demands for high thrust, we developed a low-loss anti-whirl transmission that maximized thrust output by the flapping wings to 40 grams in excess of body weight. By reducing the reactive load and whirl, this indirect drive consumed 40% less maximum electrical power for the same thrust generation than direct drive of a propeller. The triple roles of flapping wings for propulsion, lift, and drag enable the performance of aggressive flight by simple tail control.