Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Using the 150g prototype as an example, conversion to lithium polymer technology
Chapter 7
would permit a reduction from four to three cells resulting in roughly a 50% increase in
endurance at the same total battery mass. There is a one volt drop in the no-load voltage,
but the operational voltages of the two cell types would be much closer due to the
additional headroom in discharge rate and associated reduction in internal losses
provided by the lithium polymer cells. The higher discharge rate also permits
application of these cells to higher current draw situations such as the 65g prototype,
where previously NiCd and NiMh chemistries, with significantly lower energy densities,
provided the best solution.
7.5.2 Electro-mechanical Efficiency
While the primary focus of this work has been the aerodynamics of ultra-low Reynolds
number flight, the design and development of a complete vehicle cannot be undertaken
without consideration of the electro-mechanical efficiencies of the supporting systems.
This issue, in conjunction with energy storage issues, poses a significant impediment to
the development of extreme micro-rotorcraft such as the 15g prototype. This area
encompasses not only the motors and speed controllers, but additional equipment
required for voltage conversion and regulation, communications, and control. As
discussed here, this does not include the battery system, which is considered separately,
or the power required for mission-based payload such as cameras or other sensors.
The 150g vehicle exhibits an overall power efficiency of 52%, this is considerably lower
than the experimentally determined motor efficiency of approximately 65% to 70%, but
in addition to the motors, this system includes a radio receiver, four motor controllers,
and four solid-state piezo gyros. Although the 15g prototype’s rotor system requires less
than half of the power of the 150g vehicle’s rotors, the total system power required for
hover increases by 60%. This results in a total system efficiency of only 15%. This is
once again lower than the experimentally determined electro-mechanical efficiency for
the motor, in this case combined with the closed-loop controller, of approximately 17%,
but there are no additional system components in this case. The 40% drop in power plant
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