Error analysis and assessment of unsteady forces acting on a flapping wing micro air vehicle: free flight versus wind-tunnel experimental methods

TLDR

Accurate knowledge of unsteady aerodynamic forces on a bio‑inspired flapping‑wing micro air vehicle is essential, and researchers typically use either free‑flight optical tracking or wind‑tunnel force transducer measurements. This study compares the force quality obtained by these two methods on a 17.4‑gram FWMAV. A comprehensive error analysis examines measurement errors, error propagation, numerical differentiation, filtering frequency, and structural eigenmode interference. Free‑flight data acquisition below 200 Hz or position accuracy worse than ±0.2 mm severely limits force determination; while the fuselage‑parallel component agrees between methods, stroke‑plane forces differ due to clamp‑induced restrictions and structural resonance, and clamping position markedly affects eigenmodes and must be considered.

Abstract

An accurate knowledge of the unsteady aerodynamic forces acting on a bio-inspired, flapping-wing micro air vehicle (FWMAV) is crucial in the design development and optimization cycle. Two different types of experimental approaches are often used: determination of forces from position data obtained from external optical tracking during free flight, or direct measurements of forces by attaching the FWMAV to a force transducer in a wind-tunnel. This study compares the quality of the forces obtained from both methods as applied to a 17.4 gram FWMAV capable of controlled flight. A comprehensive analysis of various error sources is performed. The effects of different factors, e.g., measurement errors, error propagation, numerical differentiation, filtering frequency selection, and structural eigenmode interference, are assessed. For the forces obtained from free flight experiments it is shown that a data acquisition frequency below 200 Hz and an accuracy in the position measurements lower than ± 0.2 mm may considerably hinder determination of the unsteady forces. In general, the force component parallel to the fuselage determined by the two methods compares well for identical flight conditions; however, a significant difference was observed for the forces along the stroke plane of the wings. This was found to originate from the restrictions applied by the clamp to the dynamic oscillations observed in free flight and from the structural resonance of the clamped FWMAV structure, which generates loads that cannot be distinguished from the external forces. Furthermore, the clamping position was found to have a pronounced influence on the eigenmodes of the structure, and this effect should be taken into account for accurate force measurements.