At just under 3 ounces, the pair of Thunder Power 2S1P 480 mAh Li-Pos combined with a pair of Tech Aero Vregs (above) meets the requirements for robust, redundant, and lightweight power supply to the airborne components. Increasingly difficult to find with modern servos, nylon gears (at right) have benefits in Pattern applications as explained in the text.
age resulting in a power sharing configura- tion which ideally results in the same dis- charge to each battery. Switches are not used (more weight, and another potential failure point), and the Li- Pos are plugged directly into each Vreg at the start of each flight. Considering the av- erage flight of a modern day 2M electric rarely exceeds 75 mAh, this equates to an average load of 0.5 amps (3-amp peaks may occur in snap rolls), well within the capabil- ity of the Vregs (each rated for 5 amps) and Li-Pos (each rated for 10 amps). This airborne power system weighs in a smidgen less than 3 ounces, and can power Yuri for approximately seven flights and still maintain 50% reserve capacity. This system can also be used for glow powered planes; however, larger Li-Pos are recom- mended considering glow can easily con- sume 250 mAh per flight with the higher consumption attributable to the vibration of the engine. The chosen Tech Aero Vregs are also available with an integrated failsafe (PLR5) for those that prefer a switch for the airborne power. Another reasonable option is the use of parallel failsafe switches and A123 or Li-Fe (lithium iron) batteries which have a very narrow operating volt- age range of approximately 6.6 volts and hold voltage extremely well under load. The use of A123 or Li-Fe batteries would require the servos be capable of handling 6.6 volt input. For the more adventurous, a number of
BECs (battery elimination circuits) exist that could be used to power the airborne sys- tem from the 10S motor Li-Po. Examples would include the Castle Creations BEC PRO and the VHVBEC (very high voltage BEC) from Dimension Engineering. While a small amount of weight can be saved via this route, I am personally more comfortable having the airborne power system 100% iso- lated from the motor Li-Pos. It is also worth noting that the selected Tech Aero regula- tors are of the linear type of regulator, and can be run in parallel (valid for all linear regulators) while operation of switching reg- ulators in parallel may not be possible, de- pending on the exact design—in this in- stance, the BEC PRO should not be run in parallel, but the VHVBEC can be run in par- allel.
Having previously specified JR 3517 ele-
vator servos and JR 9411 aileron servos pri- or to the construction of Yuri at RC-Com-
FLYING MODELS
posit, choosing the rudder servo remains. From the JR line, the viable options are the DS8411 (2.0-ounce weight, 0.15 seconds, 188 ounce/inch) and DS8711 (2.4-ounce weight, 0.15 seconds, 403 ounce/inch). I have used both, and find them virtually indistinguish- able in the air. However, my preference is for the 8711 as it is less prone to oscillating at center as the servo ages. Oscillation at center is not uncommon with a number of servos when used on large rudders actuated by pull-pull cables. Should it occur, it can of- ten be eliminated by changing the tension on the rudder cables. While spec’d for operation at 4.8 and 6.0
volts, I have for many years run the listed JR servos at 6.5 volts to gain a little bit more speed and torque and noticed little if any ac- celerated wear attributable to the higher voltage. One other minor “hop-up” tip for the selected JR servos relates to the gear trains. Part number JRPSG911 is the nylon gear set for the 9411, and part number JRPSG3025 is the nylon gear set for the 3517. The trend for servos in recent times is towards metal gear trains for greater dura- bility (rarely a concern with Pattern planes); however, metal gears must have a small amount of play to avoid binding, while nylon
gears can have zero play as the gears can flex the slightest bit without binding. I pre- fer the nylon gears for not only the tighter gear train initially, but for the tighter gear train as the servo ages, as nylon has better wear properties than most metal gear trains. The modern day composite aircraft is not “cheap” per se, but history rarely records high performance at a bargain price. While some cost savings opportunities are avail- able (spinner and prop come to mind), ser- vos are definitely not the place to cut cor- ners. Servos are the heart of a Pattern plane, and lower cost servos in my experi- ence will fall short in terms of initial perfor- mance or durability. Lower cost servos gen- erally have more gear train slop, greater deadband (less accuracy and precision), greater current consumption, and in many instances, do not meet performance specifi- cations. These types of deficiencies are much more apparent in a high precision Pattern aircraft than they might be in a sport plane. The accompanying chart includes a list of servos suitable for Yuri, and includes a “budget” listing of Spektrum servos—which I’ve found to be extremely good in all re- spects, save a slightly greater deadband.
Servo
JR DS9411 ATX 94774 FUT S9551 FUT BLS551 SPK H6040
JR DS3517MG ATX 94761Z FUT S9650 FUT BLS153 SPK A5040 JR DS8711 ATX 94780M FUT S9155 FUT BLS252 FUT BLS157 SPK A6030
Use
Pattern Servo Options Weight
ailerons ailerons ailerons ailerons ailerons elevators elevators elevators elevators elevators rudder rudder rudder rudder rudder rudder
(ounces) 1.4 1.9 1.6 1.6 1.8 0.9 0.8 0.9 0.9 0.7 2.4 2.3 2.2 2.1 2.7 1.8
114 122 128 167 51 66 63 97 66
403 423 192 175 431 278
Torque
(ounce-inches) 95
Speed
(60 degrees) 0.12 0.10 0.11 0.10 0.08 0.09 0.12 0.11 0.12 0.10 0.15 0.15 0.13 0.13 0.14 0.15
37
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