Trade-off of lifting and propulsion capacity of tandem flapping-fixed airfoils by inclining the stroke plane

A tandem flapping-fixed airfoil configuration with a narrow gap can be found in insects and micro air vehicles (MAVs). Compared to a single flapping airfoil, the interaction between the two airfoils can bring about a thrust enhancement, whereas it is not sure whether this thrust enhancement can be transformed into an improved lifting capacity via some trade-offs. In this paper, the idea of inclining the stroke plane is adopted, and the aerodynamic performance of this tandem configuration at different stroke plane angles is investigated numerically using the lattice Boltzmann method. Results show that most of the extra thrust is converted into the lift increment at the 75° stroke plane, which proves the feasibility of a trade-off between lifting and propulsion capacity. In addition to the contributions of geometric projection and effective incoming flow, the lift increment also originates from an extra transient lift peak when the flapping airfoil and the fixed airfoil are near their closest locations. This lift peak is attributed to the enlarged low-pressure and high-pressure regions at the rear end of the upper and lower surfaces of the flapping airfoil, the underlying flow physics of which is the formation of a reversed horizontal flow and an upwash between two airfoils. These findings introduce a novel interaction mechanism between an inclined flapping airfoil and a fixed airfoil. Further analysis shows that this mechanism is valid at multiple plunging amplitudes and thus it may be utilized to enhance the lift in a three-dimensional (3D) scenario. Our findings reveal a novel lift-enhancing mechanism by tilting the stroke plane and may promote the design of MAV in this configuration.