A Review of Research on the Mechanical Design of Hoverable Flapping Wing Micro-Air Vehicles

Most insects and hummingbirds can generate lift during both upstroke and downstroke with a nearly horizontal flapping stroke plane, and perform precise hovering flight. Further, most birds can utilize tails and muscles in wings to actively control the flight performance, while insects control their flight with muscles based on wing root along with wing’s passive deformation. Based on the above flight principles of birds and insects, Flapping Wing Micro Air Vehicles (FWMAVs) are classified as either bird-inspired or insect-inspired FWMAVs. In this review, the research achievements on mechanisms of insect-inspired, hoverable FWMAVs over the last ten years (2011–2020) are provided. We also provide the definition, function, research status and development prospect of hoverable FWMAVs. Then discuss it from three aspects: bio-inspiration, motor-driving mechanisms and intelligent actuator-driving mechanisms. Following this, research groups involved in insect-inspired, hoverable FWMAV research and their major achievements are summarized and classified in tables. Problems, trends and challenges about the mechanism are compiled and presented. Finally, this paper presents conclusions about research on mechanical structure, and the future is discussed to enable further research interests.

Batch Manufacturing of Split-Actuator Micro Air Vehicle Based on Monolithic Processing Technology

Microrobots have a wide range of applications. The rigid–flexible composite stereoscopic technology based on ultraviolet laser cutting technology is primarily researched for the design and manufacture of microrobots and has been used to fabricate microscale motion mechanisms and robots. This paper introduces a monolithic processing technology based on the rigid–flexible composite stereoscopic process. Based on this process, a split-actuator micro flapping-wing air vehicle with a size of 15 mm × 2.5 mm × 30 mm was designed. We proposed a batch manufacturing method capable of processing multiple micro air vehicles at the same time. The main structure of 22 flapping-wing micro air vehicles can be processed at the same time within the processing range of the composite sheet with an area of 80 mm × 80 mm, and the processing effect is good.

Soap Film Visualization of a 10 cm-Span Flapping Wing

Flapping wing micro-air-vehicles (FWMAVs) animate the small-space dexterous flight, hovering, and energy-saving characteristics of birds and insects, and are believed to have enlightenment for the development of bionic flight in the future. When designing FWMAVs, detailed unsteady aerodynamic information is required. Besides the computational fluid mechanics (CFD) technology study, the flow visualization is also needed to assist this research. This article innovatively used soap film visualization with high-speed photography to record two kinds of the 2D flow fields laterally and longitudinally, respectively, generated by a flapping wing of 10 cm span. Different from the qualitative comparison of soap film imaging with the conventional smoke tracing method, the subsequent processing of the soap film images was demonstrated. This work explains how to quantify the soap film imaging into lift and thrust forces, and the corresponding results are compared with the wind tunnel force measurement data preliminarily.

Role of Active Morphing in the Aerodynamic Performance of Flapping Wings in Formation Flight

Migratory birds have the ability to save energy during flight by arranging themselves in a V-formation. This arrangement enables an increase in the overall efficiency of the group because the wake vortices shed by each of the birds provide additional lift and thrust to every member. Therefore, the aerodynamic advantages of such a flight arrangement can be exploited in the design process of micro air vehicles. One significant difference when comparing the anatomy of birds to the design of most micro air vehicles is that bird wings are not completely rigid. Birds have the ability to actively morph their wings during the flapping cycle. Given these aspects of avian flight, the objective of this work is to incorporate active bending and torsion into multiple pairs of flapping wings arranged in a V-formation and to investigate their aerodynamic behavior using the unsteady vortex lattice method. To do so, the first two bending and torsional mode shapes of a cantilever beam are considered and the aerodynamic characteristics of morphed wings for a range of V-formation angles, while changing the group size in order to determine the optimal configuration that results in maximum propulsive efficiency, are examined. The aerodynamic simulator incorporating the prescribed morphing is qualitatively verified using experimental data taken from trained kestrel flights. The simulation results demonstrate that coupled bending and twisting of the first mode shape yields the highest propulsive efficiency over a range of formation angles. Furthermore, the optimal configuration in terms of propulsive efficiency is found to be a five-body V-formation incorporating coupled bending and twisting of the first mode at a formation angle of 140 degrees. These results indicate the potential improvement in the aerodynamic performance of the formation flight when introducing active morphing and bioinspiration.

Design of Gust Wind Tunnel With Unsteady and Shear Main-Flows

Micro air vehicles (MAVs) have been developed for many fields. The MAVs usually receive strong impact from a velocity change in time or space, and facilities for aerodynamic experiments of MAVs under a gusty environment have been required. The present study has developed a gust wind tunnel to generate unsteady and non-uniform flows. We developed a small wind tunnel with eight multi-fans and a shutter mechanism at the upstream of the test section. We controlled the outputs of the fans independently and obtained a linear shear layer with an error of 5 percent. The velocity gradient of the shear layer was from 5 to 8 s−1. The shutter mechanisms provided a longitudinal gust with the velocity change from 2 m/s to 10 m/s within 0.3 seconds.

Experimental investigation on flow structures of steadily translating low-aspect-ratio wings in low Reynolds number flow

In recent decades, Micro Air Vehicles (MAVs) have been a hot topic for their promising future. But the promotions of MAVs are hindered by their short endurances. To solve this problem, inspirations are brought from migratory butterflies who utilize the ‘flapping-gliding’ skill during long-distance migration to improve the flight efficiency. The butterfly’s gliding flights, which can be simplified by considering the steadily translating fixed wings, have drawn high attentions. Previous studies mainly focus on the aerodynamics of the low-aspect-ratio fixed wings at Re ≈ 105 via force measurements. However, few experimental studies have measured the 3D flow fields. Consequently, the underlying high lift-to-drag ratio mechanisms in the steadily translating butterfly-shaped wings are still not clear. To shed new light on this problem, the 3D flow structures around butterfly-shaped wings were captured and investigated in detail.

On the Aerodynamic Analysis and Conceptual Design of Bioinspired Multi-Flapping-Wing Drones

Many research studies have investigated the characteristics of bird flights as a source of bioinspiration for the design of flapping-wing micro air vehicles. However, to the best of the authors’ knowledge, no drone design targeted the exploitation of the aerodynamic benefits associated with avian group formation flight. Therefore, in this work, a conceptual design of a novel multi-flapping-wing drone that incorporates multiple pairs of wings arranged in a V-shape is proposed in order to simultaneously increase the propulsive efficiency and achieve superior performance. First, a mission plan is established, and a weight estimation is conducted for both 3-member and 5-member configurations of the proposed air vehicle. Several wing shapes and airfoils are considered, and aerodynamic simulations are conducted, to determine the optimal planform, airfoil, formation angle, and angle of attack. The simulation results reveal that the proposed bioinspired design can achieve a propulsive efficiency of 73.8%. A stability analysis and tail sizing procedure are performed for both 3-member and 5-member configurations. In addition, multiple flapping mechanisms are inspected for implementation in the proposed designs. Finally, the completed prototypes’ models of the proposed multi-flapping-wing air vehicles are presented, and their features are discussed. The aim of this research is to provide a framework for the conceptual design of bioinspired multi-flapping-wing drones and to demonstrate the sizing, weight estimation, and design procedures for this new type of air vehicles. This work establishes the first multi-flapping-wing drone design which exploits the aerodynamic features of the V-formation flight observed in birds to achieve superior performance in terms of payload and endurance.