Aerodynamics and Design for Ultra-Low Reynolds Number Flight

5.4.1 Motor Power Supply
All tests have utilized a Global Specialties Model 1335 DC power supply for motor
Chapter 5
power. Typical voltages range between 6V and 13V for all tests and current ranges from
100mA to 250mA for all micro-rotor tests and 100mA to 400mA for the larger carbon-
fiber rotors. The integrated digital displays of the power supply have been used for
current and voltage measurement. The significant error incurred is the round-off error of
the least significant digit of the display. The readability limits are 0.1V and 0.01A
resulting in a possible error of (+/-)0.05V and (+/-)5mA. The manufacturer’s stated
display accuracy is much worse, 0.5% of reading + two counts, but voltage and current
have periodically been monitored using a Protek 506 DMM in addition to the supply’s
displays, this DMM is accurate beyond the round-off error of the supply’s displays and
has agreed with the supply’s values within this error bound.
Errors in recorded current and voltage will only manifest themselves as errors in the
calculated electro-mechanical efficiencies derived from experimental power required for
two micro-rotors to be discussed in Chapter 6. These efficiencies are then used to
estimate the power required for several other micro-rotors. The typical input power for
this system ranges from 0.5W to 3W, rendering any effect of voltage and current
measurement error negligible.
5.4.2 Determination of Rotor RPM
Rotor RPM is measured using a TNC Electronics Model PT10S optical tachometer. The
manufacturer’s stated accuracy is (+/-) one RPM up to 100,000 RPM, but in practice the
total error is somewhat higher since this is a handheld device and positioning and
stability during reading can effect the output depending on the application. The user is
left to interpolate across mildly fluctuating readings. For the large, 10” diameter, 2-blade
carbon-fibre rotors, the observed fluctuations are roughly (+/-) 10 RPM, with RPM
ranging from 1,000 to 2,500, representing a maximum error of (+/-) 1%. The micro-
rotors are roughly one inch in diameter spinning at 20,000 to 50,000 RPM. Positioning
87