Bio-Inspiration in  Flapping-Wing UAVs
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Göksel Keskin

Bio-Inspiration in Flapping-Wing UAVs

Issue 8 - 2020
Bio-Inspiration in  Flapping-Wing UAVs

For centuries, humans have been inspired by the flight of birds, bats, and insects. As it’s mentioned in previous articles, early attempts at building aircraft by replicating the shape of bird and bat wings. Flying animals that power and control flight by flapping their wings perform excellent flight stability and manoeuvrability while steering and manoeuvring by rapidly and continuously varying their wing kinematics. Current studies about flapping-wing designs are generally classified in micro air vehicle status. Micro air vehicles (MAVs) are a relatively new and rapidly growing area of aerospace research. They were first defined by the US Defense Advanced Research Projects Agency (DARPA) in 1997 as unmanned aircraft that are less than 15 cm in any dimension. Later in 2005, the DARPA defined aircraft with all dimensions less than 7.5 cm and lighter than 10 g (carrying 2 g payload) as nano air vehicles (NAVs). MAVs (or NAVs) generally fit into one of the three categories: fixed wing, rotorcraft, or biomimetic. Biomimetic MAVs (BMAVs) mimic the flapping wing motion of flying organisms (e.g., insects, birds, bats, etc.).

Comparison between an actual dragonfly wing and the simplified wing frame is invented and used by a research group. Spatial network analysis utilizes geometric objects within a region specified by vertices or edges. Although this method is commonly used in geographical information systems (GIS) to explore geographic spatial patterns, the idea of applying this algorithm to a biological structure was first introduced in this article. It was inspired by observing the compactly arranged geometrical patterns inherent to dragonfly wings. This method allows this complex biological structure to be mimicked by a simplified frame structure that can be fabricated by machining or 3D printing. The results show that the ABS wing has considerable flexibility in the chordwise direction, whereas the PLA and acrylic wings show better conformity to an actual dragonfly wing in the spanwise direction. 

Based on the wing motion of the bee, a multi-DOF (multi-degrees of freedom) mechanism with complex three type motions for the bee-like MAV was proposed. The tips of bee wings can trace a figure-of-eight or banana, and two wings can twist. New structural properties and multi-DOF flappingwings were studied according to the bee flight mechanism, including six links and seven kinematic pairs. With the kinematic chain transform theory, a new pattern flapping-wing mechanism was analyzed. A parameter optimization model of the mechanism was established to realize the motion track of the bee. The flapping-wing with compound motion can produce higher lift and thrust. The motion parameters that have influence on lift and thrust of flapping-wing include plunging amplitude, plunging frequency, sliding amplitude, pitching mean angle and pitching amplitude. The increase of pitching mean angle reduces the thrust. A miniaturized flight control system with high quality was developed in the center of MAV.

More recently, Robo Raven, a highly maneuverable robotic bird due to its independent wing control was developed in the Advanced Manufacturing Lab at the University of Maryland. Each wing driven by a high-speed, high-torque servo. As a result, a desired wing position or velocity can be programmed in order to achieve a wide range of flapping profiles and aerobatic maneuvers by varying flapping frequencies, flapping ranges, and positions. Robo Raven has a wingspan of 114.3 cm, a platform weight of 285 g with wings, a maximum flight weight of 328.8 g, a payload of 43.8 g, and consumes 36 W during flight. Like most FWAVs Robo Raven has a limited flight time due to the small on-board 370 mAh lithium polymer battery used to power the platform. With the limited flight time and large surface area provided by the wings, it was used as the base platform for this research. The goal of this work is to increase vehicle endurance and overall system efficiency though the usage of multifunctional structures, specifically integrated flexible solar cells and batteries.

Another microflyer has a wingspan of 16.5 cm and a total mass of 19 g. It can hover for around 4 min and can fly at a speed of 6.7 m/s. The flapping-wing mechanism is powered by DC motors and the wing-flapping frequency is 30 Hz. The remarkable feature of this microflyer is that it has a fuselage shaped and painted to make it look like a real hummingbird, which makes it ideal for covert operations. The elec¬tronics and control system were developed in-house and are enclosed within the body. All the control inputs are gener¬ated by varying the lift on the wings, and the vehicle does not rely on a tail for stability. The microflyer can fly stably outdoors under the control of a human pilot and transmits live video to a ground station.

Flying animals are the most futuristic and advanced flyers on Earth, and bioinspired flapping flight systems as an integrated system offer an alternative paradigm for MAVs when scaled down to insect and bird size, which, however, normally brings low-speed aerodynamics and flight control challenges in achieving autonomous flight. Replication and inspiration from all living things will be constantly applied to unmanned aerial vehicle systems with the development of techonology.

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