Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 7
efficiency is devastating to the concept’s feasibility, but it also appears to suffer an
additional 2% loss, most likely due to the addition of the four rotor-ducts without
consideration being given to the rotor design. As was seen in Chapter 6, this amount of
variation could also be attributable to geometric variations in the four epoxy SDM rotors.
In both cases, the motors and controllers are representative of the state-of-the-art in
small, high speed electric motors.
For the 5mm Smoovy motors, a significant portion, perhaps as much as half, of the
losses are due to the high speed switching controller required for brushless motors. It
might be possible to improve the overall efficiency and achieve a significant mass
reduction by developing a suitable brushed or coreless direct-current motor, but this is
only speculative and beyond the focus of this work.
It is clear that significant gains must be made in the supporting technologies of
electronics, electric motors, and energy storage before even optimal aerodynamic design
would permit success at the scale of the 15g prototype. Storage-based electric power has
been the focus of this effort, but there are other approaches that have not been explored.
The absence of concepts such as beamed energy, combustion, and other novel concepts
is not meant to imply that they should be discounted; the current focus has been chosen
because the existing technology was thought to be scalable without significant
development effort.
7.5.3 Rotor Aerodynamic Efficiency
Given the large non-aerodynamic handicaps placed on micro-rotorcraft, maximizing
rotor performance should be considered an even more critical issue than in larger
applications. Inefficiencies in the rotor design are effectively multiplied through the
inefficiencies of the motors, controllers, and energy storage system. As an example, any
increase in rotor power is seen by the batteries as doubled when passed through a 50%
efficient system such as the 150g prototype. Unfortunately, rotors at small scales face a
fundamental reduction in performance relative to their large scale brethren.
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