Some bird species exhibit a flight behavior known as whiffling, in which the bird flies upside-down during landing, predator evasion, or courtship displays. Flying inverted causes the flight feathers to twist, creating gaps in the wing’s trailing edge. It has been suggested that these gaps decrease lift at a potentially lower energy cost, enabling the bird to maneuver and rapidly descend. Thus, avian whiffling has parallels to an uncrewed aerial vehicle (UAV) using spoilers for rapid descent and ailerons for roll control. However, while whiffling has been previously described in the biological literature, it has yet to directly inspire aerodynamic design. In the current research, we investigated if gaps in a wing’s trailing edge, similar to those caused by feather rotation during whiffling, could provide an effective mechanism for UAV control, particularly rapid descent and banking. To address this question, we performed a wind tunnel test of 3D printed wings with a varying amount of trailing edge gaps and compared the lift and rolling moment coefficients generated by the gapped wings to a traditional spoiler and aileron. Next, we used an analytical analysis to estimate the force and work required to actuate gaps, spoiler, and aileron. Our results showed that gapped wings did not reduce lift as much as a spoiler and required more work. However, we found that at high angles of attack, the gapped wings produced rolling moment coefficients equivalent to upwards aileron deflections of up to 32.7° while requiring substantially less actuation force and work. Thus, while the gapped wings did not provide a noticeable benefit over spoilers for rapid descent, a whiffling-inspired control surface could provide an effective alternative to ailerons for roll control. These findings suggest a novel control mechanism that may be advantageous for small fixed-wing UAVs, particularly energy-constrained aircraft.

Several species of birds have been known to invert in flight to lose altitude — a behavior known as whiffling. When the bird flies inverted, the flight feathers twist open to create gaps in the trailing edge of the wing, decreasing the lift produced by the wing. Gaps along the trailing edge of an aircraft wing were inspired by the feather rotation mechanism during whiffling, and asymmetrically applying these gaps on only one side of the wing produces a rolling moment due to the lift differential across the full wing. Previous experimental data and analytical estimates showed that whiffling-inspired gapped wings can produce a larger rolling moment coefficient than a conventional aileron, for a fraction of the actuation work. In the current work, we perform a computational study using Siemens STAR-CCM+ to estimate the work required to actuate nine gaps along the trailing edge of a whiffling-inspired wing and compare it to that of a representative aileron configuration. We show that the results of the simulation agree well with the prior experimental results. The results indicated that the work on the entire gap area may be higher than the work to deflect an aileron, however, the analytically estimated work on a smaller, more realistic, area corresponding to a gap cover was substantially lower than the work to deflect an aileron. These results provide evidence that sliding gaps that open in the plane of a wing require less work than deflecting an aileron into the flow for rolling moment coefficients above 0.0139. This computational validation is the first step in determining if smart materials can be used for this type of wing morphing. In all, the whiffling-inspired gapped wings could provide a far more energy-efficient method of roll control for energy-constrained fixed-wing uncrewed aerial vehicles than conventional ailerons, particularly at higher rolling moment coefficients.

No abstract

Some bird species fly inverted, or whiffle, to lose altitude. Inverted flight twists the primary flight feathers, creating gaps along the wing’s trailing edge and decreasing lift. It is speculated that feather rotation-inspired gaps could be used as control surfaces on uncrewed aerial vehicles (UAVs). When implemented on one semi-span of a UAV wing, the gaps produce roll due to the asymmetric lift distribution. However, the understanding of the fluid mechanics and actuation requirements of this novel gapped wing were rudimentary. Here, we use a commercial computational fluid dynamics solver to model a gapped wing, compare its analytically estimated work requirements to an aileron, and identify the impacts of key aerodynamic mechanisms. An experimental validation shows that the results agree well with previous findings. We also find that the gaps re-energize the boundary layer over the suction side of the trailing edge, delaying stall of the gapped wing. Further, the gaps produce vortices distributed along the wingspan. This vortex behavior creates a beneficial lift distribution that produces comparable roll and less yaw than the aileron. The gap vortices also inform the change in the control surface’s roll effectiveness across angle of attack. Finally, the flow within a gap recirculates and creates negative pressure coefficients on the majority of the gap face. The result is a suction force on the gap face that increases with angle of attack and requires work to hold the gaps open. Overall, the gapped wing requires higher actuation work than the aileron at low rolling moment coefficients. However, above rolling moment coefficients of 0.0182, the gapped wing requires less work and ultimately produces a higher maximum rolling moment coefficient. Despite the variable control effectiveness, the data suggest that the gapped wing could be a useful roll control surface for energy-constrained UAVs at high lift coefficients.