A selection of aerobatic electric aircraft of various sizes. The Wind Suses a mix of five mid/mini JR digital servos while the Sukhoi uses a pair of digital Spektrum 6040 servos on the ailerons and two mid-size JR digital servos on the tail. With smaller control surfaces and smaller control throws, the Rx and servos of the Wind (above left) are powered by the BEC of the ESC. The Sukhoi (above center), with large control surfaces and 3D throws, exceeds the max current capacity of the ESC BEC, and uses a Castle BEC PRO to feed the Rx
tery chemistry for R/C use, the lithium bat- teries. The lithium iron nano-phosphate (A123) and lithium iron phosphate (Li-Fe) cells have approximately the same capacity (by weight) as Ni-MH with minimal voltage drop (<5% over the usable range) centered at approximately 6.6 volts (for two cells in series). These characteristics make the A123 and Li-Fe cells very viable for use without a Vreg, assuming the Rx and servos can handle 6.6 volts and the small drop in voltage is not objectionable. The highest energy density battery cur-
rently viable for use is the lithium polymer (Li-Po), offering approximately twice the en- ergy density of Ni-XX batteries, allowing for substantial weight savings in larger sizes. The nominal (7.4) and peak (8.4) voltage (for 2 cells in series) requires the use of a Vreg to reduce the input voltage to the servos (and some Rxs) to a safe level. The higher in- put voltage to the Vreg creates extra work for the Vreg (greater drop in voltage re- quired), but also allows a higher output volt- age to the servos (typically 6.0 to 6.5 volts). Up until this point, I’ve not mentioned
battery elimination circuits (BECs). BECs are nothing other than Vregs integrated into electronic speed controls (ESCs) for electric powered aircraft. BECs tap into the input power from the motor battery, and drop the voltage before sending it to the Rx. A very important consideration with BECs and Vregs is that they have a maximum input voltage, and the peak amps they can provide to the Rx decreases as the differential be- tween the output voltage and input voltage increases. Therefore, the BEC output capac- ity is increasingly likely to be exceeded as the cell count of the motor battery increases and the servo current demands increase to drive larger control surfaces. The range of electric powered aircraft that
can be flown using an integrated BEC varies substantially, depending on the ESC/BEC in question. Older ESCs (such as the Castle Creations Phoenix line) used linear BECs with current capacities of approximately 3 amps with a limitation of 3S Li-Po (12.6 volts) for the motor battery. Newer ESCs (such as the Castle Creations Phoenix ICE and ICE Lite) use switching BECs (and some have adjustable voltage output) and can supply as much as 5 amps with input voltages as high as 8S Li-Po (25.2 volts). ESCs intended for high power electrics (more accurately, high voltage electrics) generally do not include a BEC.
FLYING MODELS
and servos. Despite the 78-inch length and a top end speed of over 100 mph, Dave’s Bravo Pattern plane (above right) only uses 75 mAh for a 10-minute flight (approximately .5 amps average draw). Low current draw is realized by low vibration, good mechanical advantage, and moderate control throws. The model uses a pair of Li-Pos and Tech Aero PLR5-E adjustable linear regulators in parallel which together can supply 10 amps. The parallel configuration in this case is purely for redundancy.
Once an electric has outgrown the BEC capacity of the ESC, the BEC is generally disabled, and the challenge of providing power to the Rx and servos is no different than a glow/gas powered plane (save the greatly reduced vibration and corresponding reduced current demand by the servos). Whether electric or glow/gas, properly siz-
ing the power supply system for the Rx and servos is not a simple task (another topic de- serving a column unto itself). A couple of key points in brief: starting with manufacturers recommendations, consider not only the number of servos, but the type (digital vs analog, heli vs aircraft), control surface size, amount of control throw, airspeed, and vi- bration from the engine/motor. Similar sized aircraft can easily see peak and average cur- rent demands vary by a factor of 4 or more. The recent trend in Rxs and servos is the capacity to accept higher input voltage, up to as much as 9 volts. Servos specifically de- signed to be “Li-Po” friendly are commonly designated “HV”, for high voltage. The most extensive line of HV servos are found in the JR servo line, with substantial offerings from Futaba and Hitec; Airtronics has re- cently released a few HV servos. Of late, I have frequently heard it said
that the introduction of HV servos means Vregs are no longer needed. I cringe every time I hear this. While it is true that an HV servo can be safely paired with a Li-Po bat-
tery, it is only a true statement if unregu- lated voltage is acceptable to the end user. The current generation of HV servos do not include voltage regulation, so the speed and torque will decrease as the input voltage from the Li-Po drops (typically 10% over the usable range). So why use an HV servo if a regulator is
still required? Quite simply, depending on the design of the HV servo, the benefit of the HV servo will be greater efficiency and low- er power consumption (for the same torque/speed), or higher performance (at greater power consumption). A side benefit is when using BECs or Vregs capable of high voltage inputs (i.e. 4S to 12S Li-Po, or up to 50 volts), the higher output voltage (i.e., 7.0 or 8.0 volts) reduces the voltage drop (work) required by the BEC or Vreg. Thusly, there is more capacity in the BEC or Vreg to sup- ply more current (or operate with a greater margin of safety). When looking at smaller glow/gas planes,
the use of a regulated NiXX/Li-Po for stan- dard or HV servos may be of questionable value, as the net weight of the Li-Po and regulator may exceed the weight alone of an A123 or Li-Fe battery. However, for small- er electrics, the use of HV servos provide the benefit of easing the workload on the BEC, assuming the BEC output voltage can be adjusted from a standard 5.0 volts to 8.0 volts.
Castle Creations Phoenix ESCs. Left to right, ICE Lite 50, ICE Lite 100, Phoenix 80, Phoenix 10. Newer ESCs such as the Castle Creations ICE line include switching regulators with adjustable voltage, a nice feature to optimize servo performance, and handle a wider range of input voltages. Older ESCs (Phoenix 80 and 10) used fixed voltage linear regulators and often had the BEC disabled in favor of alternate power supply to the Rx and servos on larger planes. Text has details.
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