Avian whiffling-inspired gaps provide an alternative method for roll control

Abstract

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.

Whiffling-inspired gapped wings alter trim and mitigate gusts

“…span. 4,6 The wings were unswept, untapered, and capped with circular endplates. We considered 43 distinct gapped wings by varying gap width, gap length, and number of gaps.…”

“…The density of the wings was taken to be 1010 kg•m -3 . 4,15 To determine the lift, drag, and pitching moment curves of each wing, we conducted Reynolds-averaged Navier-Stokes (RANS) simulations in Siemens STAR-CCM+, a commercial CFD code. We simulated each wing at 0 • , 2 • , 7 • , and 10 • angle of attack.…”

“…Recently, whiffling has been taken as a novel source of bio-inspiration for aircraft control surfaces. [4][5][6][7] While whiffling, airflow approaches the wing from the back side, causing the primary flight feathers to twist open. 8 This passive twisting causes gaps between the feathers that likely lead to decreased lift.…”

“…Gaps in the trailing edge of a wing−inspired by this feather rotation−have been found to change the aerodynamic forces and moments of a wing. 4,5 The gapped wings were investigated for rapid descent and lateral movement, and showed promise for roll control in certain situations. 4 Here, we evaluated the whiffling-inspired gapped wings with respect to another hypothesized function: gust response.…”

“…4,5 The gapped wings were investigated for rapid descent and lateral movement, and showed promise for roll control in certain situations. 4 Here, we evaluated the whiffling-inspired gapped wings with respect to another hypothesized function: gust response.…”