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Analysis , Design , and Testing of Airfoils for Use at Ultra-Low Reynolds Numbers

Advances in technology have begun to make ultra-low Reynolds number flight a real possibility. The growing interest in vehicle development has created a need for an improved understanding of the relevant aerodynamics. A reasonable starting point is the study of airfoil section aerodynamics at ultra-low Reynolds numbers. General effects on performance of several geometry characteristics of two-dimensional airfoils are explored using an incompressible Navier-Stokes solver. Reynolds numbers considered are 10,000 or less and the flow-field is assumed to be fully laminar. Variations in the thickness, camber, and leading/trailing edge shape are considered. Initial results indicate an increase in the maximum lift coefficient with decreasing Reynolds number. The lift to drag ratio decreases over this same range, but for certain applications, the ability to generate lift is a primary constraint on feasibility. The associated drag corresponds to increased power requirements or reduced endurance. Neither option is desirable, but detailed airfoil section design for these Reynolds numbers offers a partial solution. The small body of experimental work has utilized simple plates or existing low-speed airfoil sections. Contrary to the notion that most sections perform similarly at ultra-low Reynolds numbers, the computational results indicate that the detailed section geometry still has a profound effect on airfoil performance. Improved section performance requires that the overall section thickness should be minimized, but the camber distribution also appears to have a strong influence. To explore the design space, the flow-solver has been coupled with an optimizer. The results of the optimization study indicate that unconventional camberlines may offer significant performance gains. Application of a computational tool to this new flight regime dictates the need for some form of validation. The scaling difficulties eliminate most conventional methods of airfoil testing. The experimental method currently being developed involves the manufacture of micro-gliders for flight testing in still air. Performance estimates for these vehicles have been developed using the computed two-dimensional section properties and conventional aircraft performance calculations. The measured flight performance will provide good indication of the accuracy of the computational results.