The aerodynamic phenomena that keep insect aloft are drastically different from that of a fixed-wing or rotary-wing based aircraft. While there is a good level of understanding of the principles behind insect flight, the nuances in the aerodynamics are not yet fully understood to the extent where concepts can be translated to practical designs. Insects utilize large unsteady aerodynamic forces to hover in gusty conditions, fly forward, sideways and even perch upside down. The nature of the flow they operate in is highly viscous, which leads to vortex formation and flow separation that is inherently unsteady. High-fidelity simulations can provide precise instantaneous airloads at these length scales, especially where performing experiments can be challenging. Consequently, computational tools can be used to accurately model the unsteady flowfield around flapping wings.
From an engineering perspective, it is useful to compare the performance of different flapping wing configurations (in steady forward flight or coordinated turn) under “trimmed” flight conditions. Without an efficient trim algorithm, trial-and-error based identification of the trimmed wing kinematics is computationally expensive for any flight condition, because the large number of simulations required makes the process practically infeasible. In a global sense the nature of forces produced by flapping wings closely resemble those on a helicopter blade, such that an analogy can be drawn between the two. Therefore, techniques developed for helicopter performance calculation are adapted and applied to the flapping wing platform particularly for analyzing steady flight. The aim of this study is, therefore, to formulate a computationally efficient and robust framework to obtain trim solutions that couples a flight dynamic model using simplified quasi-steady (QS) wing aerodynamics to a high-fidelity CFD analysis. Such an analysis will provide the ability to incorporate the effects of detailed aerodynamic forces from CFD into the trim solution for flapping wing kinematics for the first time.
The coupling process resulted in key insights, both on the numerical and physics fronts. First, it was shown that the coupled trim methodology based on the QS model is capable of driving the CFD towards a stable trim solution. And in doing so, the CFD/QS coupled strategy is much cheaper and faster than an isolated CFD approach, while maintaining the accuracy of the trimmed solution. In steady flight it was observed that the airloads, thrust and power are affected by the trim parameters, and the CFD/QS methodology accurately accounted for these inter-dependencies. The lift-to-power ratio versus average lift was identified as a principal efficiency metric to assess the performance of flapping-wing vehicles for a given geometry and kinematic parameters. All of the relevant nonlinear aerodynamic interactions between the wing and its surrounding environment were first verified experimentally in hover and then explored further in level forward flight and a coordinated turn.
