The aerodynamics of revolving wings I. Model hawkmoth wings

TLDR

Flapping hawkmoth models show that a spiral leading‑edge vortex, sustained by spanwise flow, is essential for generating lift during flight. The study investigates the forces on model hawkmoth wings during propeller‑like rotation using propeller models. Propeller models were employed to analyze the forces on model hawkmoth wings during revolving motion. Revolving model hawkmoth wings produce high vertical and horizontal force coefficients because of a leading‑edge vortex; the forces are dominated by pressure differences, with separation preventing leading‑edge suction, allowing a simple geometric relation between forces and angle of attack; force coefficients are insensitive to leading‑edge detail, twist, and camber, contradicting traditional adaptive function claims, and steady propeller coefficients can approximate conventional lift and drag up to 50° incidence.

Abstract

Recent work on flapping hawkmoth models has demonstrated the importance of a spiral 'leading-edge vortex' created by dynamic stall, and maintained by some aspect of spanwise flow, for creating the lift required during flight. This study uses propeller models to investigate further the forces acting on model hawkmoth wings in 'propeller-like' rotation ('revolution'). Steadily revolving model hawkmoth wings produce high vertical ( approximately lift) and horizontal ( approximately profile drag) force coefficients because of the presence of a leading-edge vortex. Both horizontal and vertical forces, at relevant angles of attack, are dominated by the pressure difference between the upper and lower surfaces; separation at the leading edge prevents 'leading-edge suction'. This allows a simple geometric relationship between vertical and horizontal forces and the geometric angle of attack to be derived for thin, flat wings. Force coefficients are remarkably unaffected by considerable variations in leading-edge detail, twist and camber. Traditional accounts of the adaptive functions of twist and camber are based on conventional attached-flow aerodynamics and are not supported. Attempts to derive conventional profile drag and lift coefficients from 'steady' propeller coefficients are relatively successful for angles of incidence up to 50 degrees and, hence, for the angles normally applicable to insect flight.