To accommodate a range of Li-Po positions for c.g. adjustments, blindnuts (above left) are located in several positions. Blindnut mounting area is reinforced by a strip of 1⁄8-inch plywood. Here we see (above right) a typical series connection
and possibly the addition of Loctite. I buy the 4–40 stud (18–8 stainless) and T-nut from McMaster-Carr to ensure high strength and precision fit of the threads. As the Li-Pos in Yuri are centered between the landing gear plate, the 4–40 stud was actu- ally mounted in the “floor” of the fuselage, which was reinforced with a piece of ½ × 3 × 1⁄8-inch five-ply birch plywood. To ensure plenty of allowance for fore/aft positioning of the Li-Pos, I installed three blindnuts. With the Li-Po tray removed, the Li-Pos will sit on the remaining balsa support blocks, with the addition of a center sup- port block. I faced the support blocks with shelving liner; the mesh type grippy stuff available at hardware stores. While the Li- Pos need to be well supported, leaving ex- posed surface area will aid in keeping them cool.
Cooling considerations are another reason I prefer to use 5S pairs of Li-Pos versus the 10S “brick” Li-Po. My experience is that sep- arating the Li-Po pairs by approximately ¾ inch results in cooler temperatures at the end of the flight. Thusly, I add some ¾-inch square balsa spacers (to the floor) to main- tain the spacing of the Li-Pos. With the Li- Pos sitting in the plane, on either side of the spacers and 4–40 retention bolt, it is time to prepare the “clamp”.
My preferred material for the Li-Po clamp is composite 1⁄8-inch fiberglass/endgrain bal- sa/fiberglass. The clamp is very simple, 2 inches front to back, slightly wider than the Li-Pos, center balsa spacer (matching the
for two 5S 5000 Li-Pos using conventional wiring to make the series connection (R); unmodified Ultra Deans connectors (C) and finally Ultra Deans connectors with slightly beveled edges allowing series connection without extra wiring (L).
floor spacers), and balsa “edge rails”. The area of the clamp that contacts the Li-Pos is faced with shelving liner.
As the clamp is quite easy to make, differ-
ent clamps can be made to accommodate various form factors of Li-Pos. The clamp is held in place essentially by a 4–40 nylon wingnut. My earlier systems used a combi- nation of a large threaded nylon washer backed up by a nylon wingnut, but this proved to be overkill.
Currently, I make my own retaining nut from a ½-inch thick slice of 5⁄8-inch nylon 6– 6 hex stock (also available from McMaster- Carr). When cutting threads into the nylon, I only tap about 90% of the depth of the nut, leaving the remaining threads to be cut by the 4–40 retention bolt. This ensures suffi- cient friction such that the retention nut will never come loose. My variation of the “Earl” system generally weighs in at about 20 grams for the 4–40 rod, blindnuts, clamp, and retention nut; saving approximately 30 grams compared to a tray and hook and loop system.
Having lost a secure structure to which the ESC could be attached, my plan “B” for
Yuri involved affixing a small stringer (1⁄2 × 1⁄8-inch bass wood) to connect the top and bottom of the “F3” fuselage former (factory installed, located between the belly pan and canopy cutouts). I chose to affix the stringer with 4–40 bolts and blindnuts so it could easily be removed if needed.
With the ESC attached (with hook and loop material) to this stringer, it receives
ample airflow, and wiring paths are short to both the motor and Li-Po. While the Neu F3A-1 (and many other motors) comes with bullet connectors, I chose to direct solder the motor and ESC leads together to save weight and increase efficiency.
For connection between the Li-Pos and ESC, this is an area where I often see exces- sive amounts of wiring and adapters; need- less weight and wasted efficiency. The sim- plest and cleanest method is to build a series “Y” block from a pair of Ultra Deans connectors.
I lightly sand the edges of the connectors such that the positive and negative solder spades will contact each other, and then flow solder to make a solid connection. The remaining positive solder contact goes to the positive lead of the ESC and the remaining negative solder contact goes to the negative lead of the ESC. A small plywood spacer can be used between the housings of the Deans connectors to make it a little easier to plug in each Li-Po.
Plugging in the first Li-Po will do nothing. Plugging in the second Li-Po will complete the series connection with a noticeable “pop”. The pop is nothing more than a small arc between the connectors, prompted by an inrush of current to charge the capacitors on the front end of the ESC. Some ESCs do have spark suppression, and inline spark suppressors can be added, but in practice there is little value or benefit to this, as the arcing does not affect the mated surfaces of the connectors; only the ends.
With the housings glued together with a bit of CA, solder can easily be flowed across the solder tabs (above left) to make very strong, light, and efficient
FLYING MODELS
series connection. The completed series connection (above right) with heatshrink tubing.
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