Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 6
efficiencies are only a part of the total electo-mechanical efficiency of the power and
propulsion system, battery, wiring harness, and controller efficiencies further reduced the
total system efficiency.
For their small size, these motors provide very good performance both in terms of torque
production and high rotational speeds. One limitation of this three-pole stepper motor is
the need for additional circuitry to cycle power through the motor coils. This adds mass
to the system; in the final configuration of the 15g vehicle, the controllers represent a
mass fraction of 20% to 30%. Simple circuitry is required to run the motor without
feedback, but the motor efficiency is greatly reduced and the maximum RPM under load
drops considerably, in this case from well over 50,000 RPM to below 25,000 RPM.
Feedback is required to account for phase lags in the coil/shaft position induced by the
rotational inertia of the shaft and rotor, friction, and aerodynamic drag. The feedback
circuitry does not add considerably to the mass, but the complexity of the circuity
increases. Small traditional or coreless DC motors, such a pager motors, are not a viable
alternative. Although they require no additional circuity, these motors provide
insufficient torque and rotational speed at his scale. Their efficiencies are also typically
much lower, often as low as 5% to 10%. It may be possible that a design-specific
traditional DC motor could be built with performance closer to the level of the 5mm
Smoovy, but this is beyond the scope of this work.
6.2.2 Astroflight Firefly Motor
The ten inch diameter rotor is powered by the Astroflight Firefly coreless DC motor with
an integrated 16:1 gearbox, Model #800 [39]. This motor and gearbox have a combined
mass of 12g and a manufacturer’s indicated efficiency of 83%. The Firefly and gearbox
are pictured in Figure 6.2. This, once again, is not the total system electro-mechanical
efficiency, simply that of the motor and gear box. This factor of two to four increase in
efficiency relative to the 5mm Smoovy illustrates some of the practical difficulties
associated with developing very small flight vehicles. In each case these motors
represent the upper levels of performance at their respective scales.
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