Improvement of stability for vertical take-off of an insect-mimicking flapping-wing system

In recent years, there has been a lot of progress in developing Flapping-Wing Micro Air Vehicles (FW-MAVs) [1-2]. Most of them were designed for low Reynolds numbers environment (in the range of 5,000 to 10,000 [3]). Therefore, many researchers have been paying attention to principle of insect flight for potential application to improve FW-MAVs or to develop even smaller system, called Nano Air Vehicles (NAVs) [4]. Among many types of insects, Rhinoceros beetle, Allomyrina Dichotoma, has been chosen for our study. Its weight is about 6 gram to 10 gram and wingspan is about 130 mm. It flaps its hind-wings at 30 Hz to 40 Hz with flapping angles of 165° to 180° during forward flight and hovering flight, respectively, and rotates its hind-wings around 140° [5]. It controls attitude by changing stroke plane and differential flapping angles without any control surface at tail. In an effort to mimic flight motion of this beetle, we have been developing a beetle-mimicking flapping-wing system, which has flapping angle of 145° [5-6] and flaps at 39 Hz for 12 V application. At this flapping frequency, the flapping-wing system can produce a large enough vertical force of about 7 grams to lift its own weight [7]. The wings in the system can passively rotate 105° about the flexible hinges. This paper describes how we modified the flapping-wing system for stable take-off in the vertical direction. For the modified system, the hind wings were directly connected to the output links of the flapping mechanism without flexible hinges. Instead, the trailing edges of the hind wings near wing root connected to the body, so that the hind wings can create semi-active wing rotation. Consequently, the wing rotation angle becomes variable from wing root to wing tip during flapping. By replacing the motor and modifying the linkage, the weight of flapping-wing is reduced to 4.6 grams and flapping frequency reduced to 26 Hz at 12 V. The modified system showed stable vertical take-off capacity. It could also stably take off at 23.5 Hz for 10 V application.