Realisation and testing of novel fully articulated bird-inspired flapping wings for efficient and agile UAVs

Abstract

Large birds have evolved an effective wing anatomy and mechanics, enabling airborne mastery of manoeuvres and endurance. For these very reasons, they are difficult to replicate and study. The aim of the present work is to achieve active wing articulations to mimic natural bird flapping towards efficient and agile Unmanned Aerial Vehicles (UAVs). The proposed design, bio-mimicking the black-headed gull, Larus ridibundus, has three active and independent servo-controlled wing joints at the shoulder, elbow and wrist to achieve complex controls. The construction of the wing is realised through a polymeric skin and carbon fibre–epoxy composite spars and ribs. The wing movements (flapping, span reduction and twisting) envelopes of the full-scale robotic gull (Robogull) are examined using the Digital Image Correlation (DIC) technique and laser displacement sensing. Its aerodynamic performance was evaluated in a wind tunnel at various flapping parameters, wind speeds and angles of attack. It is observed that a flapping amplitude of 45 $^\circ$ is more favourable than 90 $^\circ$ for generating higher lift and thrust, while also depending on the presence of span reduction, twist and wind speed. The model performs better at a flying velocity of 4m/s as compared with 8m/s. Both lift and thrust are high at a higher flapping frequency of 2.5Hz. Combined variation of the flapping frequency and stroke ratio should be considered for better aerodynamic performance. The combination of a lower stroke ratio of 0.5 with a flapping frequency of 2.5Hz generates higher lift and thrust than other combinations. Span reduction and wing twist notably and independently enhance lift and thrust, respectively. An increase in the angle-of-attack increases lift but decreases thrust. The model can also generate a significant rolling moment when set at a bank angle of 20 $^\circ$ and operated with independently controlled flapping amplitudes for the wings (45 $^\circ$ for the left wing and 90 $^\circ$ for the right wing). Based on the optimal values for the flapping amplitude (45 $^\circ$ ), flapping frequency (2.5Hz) and flying velocity (4m/s), the Strouhal number (St) of the Robogull model is 0.24, lying in the optimal range ( $0.2 < \mathrm{St} < 0.4$ ) for natural flyers and swimmers.

A Review of Flapping Mechanisms for Avian-Inspired Flapping-Wing Air Vehicles

“…Meanwhile, in [70], a mechanism was developed that can actively implement and control flapping, flexing, and twisting by using servo motors. This FWAV is to be used in wind tunnel tests to analyze aerodynamic characteristics according to various parameters.…”

“…Rigid mechanism [61][62][63][64][65]75,76] Compliant mechanism [57,77] Active Rigid mechanism [5,70,78] Compliant mechanism [66] Wrist flexing…”

“…Rigid mechanism [5,[78][79][80][81][82][83] Compliant mechanism [67,68] Active Rigid mechanism [70] Compliant mechanism -* passive: Different motions occur simultaneously while flapping without increasing the degree of freedom. ** active: As the degree of freedom increases, an additional actuator is required, and individual control is possible.…”