The aerodynamics of feather asymmetry and implications for paravian flight (13.4)

Abstract

The presence of asymmetric feathers has been regularly used as an indicator of aerodynamic function and flight capacity in fossil birds and non‐avian dinosaurs. However, most of the flight feathers in living flying birds are not highly asymmetric. Only the most lateral primaries are typically highly asymmetric, and secondaries are typically symmetrical. The effects of feather asymmetry are more complex than simple presence or absence metrics indicate. Complex twisting occurs in the lateral primaries of living birds, which delays stall, improves aeroelastic stability, and promotes dynamic maneuvers and rigorous flapping flight at low speeds. Modeling feathers as simple, straight, flat plates suggests that they will only twist in the stabilizing “wash out” manner if vane depth ratios exceed 4:1. Because the center of lift on a flat plate sits near 1/4 chord, feathers with vane depth ratios of 4:1 or less are loaded primarily in bending. I demonstrate that the lateral primaries in most species are asymmetric enough to twist in wash out, as anticipated under the straight plate model. However, a substantial number of species do not meet expectations, which demonstrates that some combination of feather overlap, curvature, and orientation works to help stabilize these feathers under aerodynamic load. These patterns can be applied to fossil taxa to draw more precise conclusions about wing, hindwing, and tail functions.