R/CAerobatics A
great number of systems have been employed in R/C planes over the years to supply power to the receiver (Rx) and servos. Aerobat-
ic aircraft are among the more demanding on power systems, given the priorities for light weight and capability to handle brief periods of high power demands (i.e., snap rolls and tumbling maneuvers). Historically, in modern times, the 4.8 volt
(4.8 volts nominal, 5.6 peak volts fresh off the charger, for four cells in series) Nickel Cadmium (Ni-Cd) battery has been the most often used power source for the Rx and ser- vos. As the majority of Rxs and servos were capable of handling 6-7 volt input, aerobat- ic enthusiasts often used 5-cell (6.0/7.0 volt) Ni-Cd packs to increase the speed and torque of the servos. Still, the native voltage discharge curve of
a Ni-Cd leads to a gradual decrease in servo speed and torque with each successive flight (approximately 10% over the usable range). The voltage drop can be minimized by in- creasing the size of the pack to effectively re- duce the load (and corresponding voltage drop) and concentrates the flight time in a smaller segment of the discharge curve (for an equal number of flights). Further im- provements can be had using the newer Ni- XX based cells such as nickel metal hydride (Ni-MH, including eneloops) which have in- creased energy density, reduced voltage drop under load, and additionally lower self- discharge rates and lack memory forming behavior. “Bump” charging, or top-off charg- ing between flights will keep the flights within a smaller range of the discharge curve. For the greatest consistency in servo per- formance, a voltage regulator (Vreg) can be added between the battery and the Rx. Vregs come in two basic forms; linear Vregs, and switching Vregs. Linear Vregs are basi- cally variable resistors that apply resistance (generating heat in the Vreg) to achieve the desired output voltage. Switching Vregs are
by dave lockhart You can reach Dave Lockhart via e-mail at
davel322@comcast.net
PHOTOGRAPHY: DAVE LOCKHART
Two similar size aircraft with substantially different servo power requirements. The Wind S50 is 6 pounds, 11 ounces with a 62-inch wingspan and the Sukhoi 50 is 6 pounds, 7 ounces with a 61-inch wingspan. Both planes use Castle Creations ICE 100 Lite ESCs, E-flite Power 46 motors, and 5S Li-Pos.
essentially high speed switches that turn on/off to yield the desired output voltage. Switchers are more complex, but generate
less heat, operate more efficiently, and are better suited for large voltage drops. Both types can be found with either fixed or ad- justable output voltage. While the introduc- tion of the Vreg does introduce another fail- ure point into the power system (in addition to the switch, wiring, connectors, and bat- tery), Vregs are very reliable when properly sized. A related topic (deserving of a column unto itself) is the wide variety of options for
adding redundancy and backups in the pow- er system to eliminate single point failure possibilities. The key point I’ll make here about Vregs is that the linear type can be used in parallel (for added current capacity or for redundancy) while the switching reg- ulators cannot (absent the addition of addi- tional electronics). With any power systems, it is also critical to note that the standard radio connectors (servo plugs and battery plugs) are rated for 5 amps, and even at 5 amps, some efficiency is being lost. The sim- plest cure is the addition of parallel leads, with the second lead being plugged into a spare port on the Rx. Back to the Vreg. The job of the Vreg is to
Left to right: Castle Creations switching BEC (0.5 ounce, 5-amp capacity at 25 volts input, 10 amps at 8.4 volts); Castle Creations switching BEC PRO (1.5 ounces, 8 amps at 50 volts, 20 amps at 8.4 volts); Tech Aero prototype HV linear regulator (0.5 ounce, 5 amps at 12.6 volts); and linear Flex Reg PLR5-E (0.4 ounce, 5 amps at 8.4 volts). Recommended for larger aerobatic electrics.
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output a constant voltage to the Rx and ser- vos regardless of the input voltage. The typ- ical output voltage (with 5-cell Ni-XX input) range is from 5.1 to 5.7 volts, which gives “headroom” for the drop in battery voltage under load, and drop out voltage for the Vreg. Drop out voltage is essentially the Vreg absorbing some energy to do its work. The use of the Vreg has the added benefit of reducing the peak voltage the servo would see with an unregulated 5-cell Ni-XX. While many servos are indeed “happy” in the main part of a Ni-XX discharge curve, they can be jittery/buzzy/nervous at the peak 7 volts produced by a fresh Ni-XX 5-cell. While the voltage supplied to the servos is consistent with a Vreg, it is also lower (5.4 volts as an example) than the unregulated voltage from the first couple of flights or two with a Ni-XX battery alone (which might av- erage 6.3 volts). Enter the newest viable bat-
DECEMBER 2011
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