Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 1
Major problems have included a technological inability to accurately manufacture very
small-scale machines, an inability to power such devices, and the challenge of
controlling them. Without an expectation of realization, there has been only marginal
interest in the study of aerodynamics at tiny physical scales, but advances in technology
have recently begun to make ultra-low Reynolds number flight a real possibility. The
growing interest in micro-air-vehicle development has finally created a need for
improved understanding of the relevant aerodynamics.
The flight regime of micro-aircraft poses numerous challenges for aerodynamic analysis
and design, but little experimental or computational work exists for aerodynamic
surfaces operating at ultra-low Reynolds numbers (Re) below 10,000. Interest in very
small aircraft, operating in the Re=100,000 to Re=150,000 range, has rapidly grown and
many such vehicles are currently under development, but research and development at
much smaller scales is still in its infancy. Technological advances in micro-fabrication
techniques and in the miniaturization of electronics are beginning to make mechanical
micro-flight vehicles feasible from a systems and manufacturing standpoint. There are
numerous potential applications for these vehicles, but first they must be capable of
flight.
Much of the work presented here has been motivated by the Mesicopter Micro-
Rotorcraft Development Program at Stanford University. The goal of this program, and
a major focus of this thesis, is the design, development, and testing of micro-rotors and
micro-rotorcraft. The broader goal of this work is to contribute substantially to the
foundations of this essentially unexplored segment of applied aerodynamics and flight
vehicle design.
Aerodynamic research at relevant Reynolds numbers has been severely limited.
Experimental airfoil data has been published by Schmitz[2] and Althaus [3], among
others, at chord Reynolds numbers as low as 20,000 to 30,000, but in terms of the nature
of the viscous effects, this is considerably different from the range of interest below
Re=10,000. This data also represents the extreme lower range of operation for many
2