Experimental investigation of flexible wings

Various flexible wing models with different size, structure and profiles are investigated. The wing shapes of these models are based on a literature study taking into account aspects resulting from insect flight. The study was performed in a water tunnel and a low-speed wind tunnel to provide qualitative flow visualization to understand the flow physics and quantitative data to evaluate the performance of these various shapes. A four-component force balance was designed to measure aerodynamic forces on the wings in the wind tunnel tests. The wind tunnel free-stream velocity was in the range of 7.62 m/s to 10.7 m/s and the Reynolds number based on the root chord was 5.7 • 10⁴ to 8 • 10⁴.

The water tunnel tests gave incomplete results, whereas the force balance provided reasonably accurate measurements of the lift and drag values in the wind tunnel. The smoke visualization yielded flow patterns around the wing and in the wake. The flexible wing models showed a significant delay in stall at higher angles of attack. It is concluded that a pleated structure leads to improvements in the aerodynamic performance, and that the presence of unsteady effects leading to lift enhancement is a possibility. Finally, a comparison of the performance of the various models with each other and with data available in the literature is presented.