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Flapping Flexible Wings
Research into aeroelasticity of flapping wings has recently increased though a full picture of the basic aeroelastic phenomena has not yet been obtained [451]. One important question that needs to be answered to understand the key fundamentals of flapping flexible wings is, Are aerodynamic loads essential to determine the wing deformation during the flights of biological flyers? Daniel and Combes [470] analytically addressed this question and suggested that aerodynamic loads are relatively unimportant in determining the bending patterns in oscillating wings, when the ratio between the wing structure and the surrounding air is high. Subsequently, experimental investigations by Combes [471] and Combes and Daniel [472] found that the overall bending patterns of a Hawkmoth wing when flapped (single degree – of-freedom flap rotation) in air and helium are quite similar, despite a 85 percent reduction in fluid density in the latter, suggesting that the contribution of aerodynamic forces is relatively small compared to that of inertial-elastic forces during flapping motion. However, they also mentioned that realistic wing kinematics may include rapid rotation at the stroke reversals that may lead to increased aerodynamic forces due to unsteady aerodynamic mechanisms (see also Section 3.3). Furthermore, static bending tests done by Combes and Daniel [402] showed anisotropy of wing structures in a variety of insect species. Mountcastle and Daniel [473] investigated the influence of wing compliance on the mean advective flows (indicative of induced flow velocity) using PIV measurement. Their results demonstrate that flexible wings yield mean advective flows with substantially greater magnitudes and orientations that are more beneficial to lift than do stiff wings.
For simpler flapping wing configurations, Zhu [474] numerically investigated the unsteady oscillation of a flexible wing and found that, when the wing is immersed in air, the chordwise flexibility reduces both the thrust and the propulsion efficiency, whereas spanwise flexibility (through equivalent plunge and pitch flexibility) increases the thrust without efficiency reduction within a small range of structural parameters. However, when the wing is immersed in water, the chordwise flexibility increases the efficiency and the spanwise flexibility reduces both the thrust and the efficiency. Shkarayev et al. [475] investigated the aerodynamics of cambered membrane flapping wings. Specifically, they introduced a cambered airfoil into the wing by shaping metal ribs attached to the membrane skin of the 25 cm wingspan model. They found that the thrust force generated by a 9 percent camber wing is 30 percent higher than that of a flat wing of the same size. Adding a dihedral angle to the wings and keeping the flapping amplitude constant improves the cambered wing’s performance even further. The aerodynamic coefficients are defined using a reference velocity as a sum of two components: a free-stream velocity and a stroke – averaged wingtip flapping velocity. The lift, drag, and pitching moment coefficients obtained using this procedure collapse well for studied advance ratios, especially at lower AoAs.
Hui et al. [476] examined various flexible wing structures to evaluate their implications on flapping wing aerodynamics. They showed that the flexible membrane wings have better overall aerodynamic performance (i. e., lift-to-drag ratio) over the rigid wing for soaring flight, especially for high-speed soaring flight or at a relatively high AoA. The rigid wing has better lift production performance for flapping flight in general. The latex wing, which is the most flexible among the three tested wings, has the best thrust generation performance for flapping flight. The less flexible nylon wing, which has the best overall aerodynamic performance for soaring flight, is the worst for flapping flight applications.
Kim et al. [477] developed a bio-mimetic flexible flapping wing using micro-fiber composite actuators and experimentally investigated the aerodynamic performance of the wing under flapping and non-flapping motion in a wind tunnel. Results showed that the camber due to wing flexibility could produce positive effects (i. e. stall delay, drag reduction, and stabilization of the LEV) on flapping wing aerodynamics in quasi-steady and unsteady regions. Mueller et al. [478] presented a versatile experimental test for measuring the thrust and lift of a flapping wing MAV. They showed an increase in average thrust due to increased wing compliance and the detrimental influence of excessive compliance on drag forces during high-frequency operation. Also they observed the useful effect of compliance on the generation of extra thrust at the beginning and end of flapping motions.
Watman and Furukawa [479] investigated the effects of passive pitching motions of flapping wings on aerodynamic performance using robotic wing models. They considered two types of passive flapping wing models. The first model used a rigid connection between all parts of the structure. This design was utilized in several
MAVs [480] and served as a common design used for the analysis of flapping wings. The second model was designed to allow the free rotation of ribs and membrane over a limited angle. This design was used recently in a small MAV prototype [481]. They showed that the former passive flapping wing (constrained) has better performance compared to the latter design because of favorable variation in the passive pitching angle of the wing.
Wu et al. [482] conducted a multidisciplinary exercise correlating flapping wing MAVs’ aeroelasticity and thrust production by quantifying and comparing the elasticity, dynamic responses, and air flow patterns of six different pairs of MAV wings (in each one, the membrane skin was reinforced with different leading-edge and batten configurations) of the Zimmerman planform (two ellipses meeting at the quarter chord) with varying elastic properties. In their experiments, single degree – of-freedom flapping motion was prescribed to the wings in both air and vacuum. Among many conclusions, they found that, within the range of flexibility considered, more flexible wings are more thrust-effective at lower frequencies, whereas stiffer wings are more effective at higher frequencies. They hypothesized that flexible wings may have a certain actuation frequency for peak thrust production and that the performance would degrade once that frequency is passed. A rapidly growing number of studies on flexible flapping wings have been reported recently [351] [ 451]. Because the complicated problems arise from the anisotropy of the wing structure and the interaction among aerodynamic loadings, inertia, and elastic forces, it is worth understanding the aerodynamics and wing deformation of the simplified flexible wing model before tackling the anisotropic wing structure. This approach helps us to make the link between MAVs and biological flyers. In the following section, we highlight the studies focusing on simplified flexible wing models. namely chordwise and spanwise flexibility and isotropy.
