Comparison of the Aerodynamic Forces on a Flying Sphingid Moth with Those Predicted by Quasi-Steady Theory

Abstract

A camera and strain-gauge probe were used to record the wing beat and instantaneous vertical and horizontal forces on a sphingid moth flying in a wind of 3.36 m s⁻¹. The data were averaged over several wing beats to remove beat-to-beat variation. The moth was producing suffcient upward force to balance its weight of 14.7 mN and approximately sufficient thrust to balance its drag. The film of the wing beat was used to predict the wing inertial and virtual mass forces. The experimental aerodynamic force was obtained by subtracting the inertial force from the recorded force. The film was also used to predict the quasisteady aerodynamic forces due to wing translation and rotation. The vertical experimental aerodynamic and total predicted (quasi-steady plus virtual mass) forces have a similar shape, but the experimental downstroke peak (70 mN) was larger than predicted (47 mN). The horizontal recorded forces are smaller than the vertical forces, and wing inertia is a large component. However, a small aerodynamic thrust in the downstroke and a larger thrust of about 25 mN in the upstroke are both predicted and recorded. The experimental aerodynamic power output was between 21.6 and 30.0 W kg⁻¹ body mass or between 72 and 100 W kg⁻¹ of flight muscle.