Clap-and-Fling Mechanism

One of the most complex kinematic maneuvers in flying animals is the wing-wing interaction of the left and right wings during the dorsal stroke reversal, termed the clap-and-fling mechanism (see Fig. 3.17). This unique procedure enhances lift generation and is particularly observed in the flight of tiny insects [68]. It has been further observed in other studies [70] [303] [304]. A modified kinematics termed “clap-and-peel” was found in tethered flying Drosophila [305] and larger insects such as butterflies [26], the bush cricket, mantis [306], and locust [307]. It seems that the clap-and-fling mechanism is not used continuously during flight and is more often

observed in insects during maximum flying performance while carrying loads [308] or performing power-demanding flight turns [307]. Marden’s experiments [308] on various insect species found that insects with the clap-and-fling wing-beat produce about 25 percent more lift per unit flight muscle (79.2 N kg-1mean value) than insects using conventional wing kinematics (such as flies, bugs, the mantis, dragonflies, bees, wasps, beetles, and sphinx moths; 59.4 N kg-1mean value).

The clap-and-fling mechanism is a close apposition of two wings at the dorsal stroke that reverses preceding pronation. By strengthening the circulation during the downstroke it can generate considerably large lift on the wings. The fling phase preceding the downstroke is thought to enhance circulation due to fluid inhalation in the cleft formed by the moving wings, which causes strong vortex generation at the leading edge. Lighthill [209] showed that a circulation proportional to the angular velocity of the fling was generated. Maxworthy [43], by a flow visualization experiment on a pair of wings, reported that during the fling process, an LEV is generated on each wing and its circulation is substantially larger than that calculated by Lighthill [209].

Lehmann et al. [273] used a dynamically scaled mechanical model fruit fly Drosophila melanogaster wing to investigate force enhancement due to contralateral wing interactions during stroke reversal (clap-and-fling; see Fig. 3.18). Their results suggest that lift enhancement during clap-and-fling requires an angular separation between the two wings of no more than 10°-12°. Within the limitations of the robotic apparatus, the clap-and-fling augmented the total lift production by up to 17 percent, but the actual performance depended strongly on stroke kinematics. They measured two transient peaks of both lift and drag enhancement during the fling phase: a prominent peak during the initial phase of the fling motion, which accounts for most

of the benefit in lift production, and a smaller peak of force enhancement at the end of the fling when the wings started to move apart. Their investigation indicates that the effect of clap-and-fling is not restricted to the dorsal part of the stroke cycle but that it extends to the beginning of the upstroke. This suggests that the presence of the image wing distorts the gross wake structure throughout the stroke cycle.

Recently, Kolomenskiu et al. [309] investigated the clap-fling-sweep mechanism of hovering insects using 2D and 3D simulations at specific Reynolds numbers: 1.28 x 102 and 1.4 x 103. The results showed that the 3D flow structures at the beginning of the downstroke are in reasonable agreement with the 2D approximations. After the wings move farther than one chord length apart, the 3D effects can be seen in force history and flow structures for both Reynolds numbers. At Re = 1.28 x 102, the spanwise flow from the wing roots to the wingtips is driven by the centrifugal forces acting on the mass of the fluid trapped in the recirculation bubble behind the wings. The spanwise flow removes the excess of vorticity and delays the periodic vortex shedding. At Re = 1.4 x 103, vortex breakdown occurs past the outer portion of the wings, and multiple vortex filaments are shed into the wake.

This clap-and-fling mechanism is being applied to enhance lift of actual flapping wing MAVs (de Croon et al. [310] and Nakata et al. [311]; see Figure 3.19).