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This study was inspired by the flight performance of the rhinoceros beetle

This study was inspired by the flight performance of the rhinoceros beetle

This involved designing the flapping mechanism, which combined the 4-bar linkage and pulley–string mechanism, such that it was able to flap the wings with a high flapping amplitude of approximately 192°, as shown in figure 7a. However, only the wing tips approximately touched at this flapping amplitude (figure 6a) exhibiting near-clap-and-fling case due to the limitation of the mechanism design . Moreover, the wings were flexible during the flapping motion creating a chordwise camber and spanwise twist (figure 7b,c). During the clap and fling, the effect of wing flexibility allowed the fling to act like a peel, such that the wings separated along the wing chord from the leading edge to the trailing edge, and then clapped in a manner similar to a reverse peel [12,17,58]. Thus, the clap and fling in the FW-MAV developed in this study could be regarded as a near-clap-and-peel mechanism.

Owing to the presence of the mirror wing during the clap and fling, the airflow beneath the trailing edges was pushed downward throughout the period of clap and fling (figure 12a, t/T = 0

Both experiments and simulations were used to investigate the effect of the near clap and peel on the force enhancement at a relatively high Reynolds number of approximately 15 000. The measurement results showed that the clap and flings enhanced the vertical force by 16.2%, while the simulation indicated that the clap and flings increased the vertical force by 11.5%. This could be due to the slight difference between the fitted and the measured flapping angle at the end of each stroke (figure 7a). Although a fitted function consisting of eight terms was used, the peak-to-peak value of the fitted flapping angle was still approximately 3° smaller than the measured flapping angle. Therefore, the distance between the two wings at the ends of the strokes along the wingspan in the simulation exceeded that in the measurement. This could have reduced the effect of the clap and fling on the force generation in the simulation .

Hence, in this study, the clap-and-fling effect was implemented in a two-winged FW-MAV for the first time

The contribution of each phase to the augmented forces was considered in the developed FW-MAV. Several previous studies on insects and robotic wings at low Reynolds numbers (Re 10 000), the effect of clap was not considered or its contribution to the force enhancement was not significant . However, the study results indicated that the clap as well as the fling contributed significantly to the vertical force enhancement in a relatively high Reynolds number (approx. 15 000) environment. 96–1.12) providing an additional vertical force. The behaviour of this downwash differed from previous studies, in which the downwash was observed when the wings clapped together only [26,28,36,58]. In addition to fling phase, the IoA in the low-pressure region between the wings caused another increase in the vertical force. Therefore, the fling and the clap contributed to approximately 62.6% and 37.4% of the vertical force enhancement, respectively. There were two clap-and-fling mechanisms at the dorsal and ventral stroke reversals in the developed FW-MAV. As shown in table 2, the dorsal clap and fling at the end of the upstroke and beginning of the downstroke made a contribution to the enhanced vertical force by 53.1%, while the ventral clap and fling at the end of the downstroke and the beginning of the upstroke contributed to 46.9% of the vertical force enhancement. This slight asymmetric contribution could be the result of the asymmetry in the flapping motion, where the distance between the left and right wings at the dorsal stroke reversal was less than that at the ventral stroke reversal.

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