Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 7
For large scale helicopters, Figure 7.5 predicts figure of merit values between 0.80 and
0.87. This agrees reasonably well with the actual values for many full-scale helicopters.
The results are seen to be relatively insensitive to airfoil performance. For micro-rotors,
Figure 7.6 paints a very different picture. In this regime the figure of merit is highly
sensitive to section performance. The two micro-rotor designs considered here, the four-
blade 2.5cm diameter rotor and the and the five-blade 2.2cm diameter rotor, have
comparatively high solidity values of 24% and 35% respectively, but the upper bound on
figure of merit for these rotors appears to be between 0.60 and 0.65 based on the
predicted optimal section performance of Chapter 3. Under the best case scenario then,
centimeter scale micro-rotors can expect a 20% to 30% reduction in figure of merit
relative to large scale helicopter rotors.
With the upper limits of performance reasonably well defined, there is still room for
improvement and motivation for research and design for centimeter scale rotors. The
rotor design examples described here still fall short of this target. The computed and
measured values of the figure of merit for the four-blade 2.5cm diameter rotor are
provided in Figure 7.7 as a function of the rotor RPM. The rapid analysis method
provides the most optimistic estimate of approximately M=0.55, the measured values for
this single sample are lower, at approximately M=0.39 for the higher RPM of primary
interest.
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