RC Electrics
Bob Kopski, 25 West End Dr., Lansdale PA 19446
Overview
This column discusses a useful propeller technique, recent motor problems, a circuit-board assembly caution, things seen at a recent meet, and a new battery management experiment.
Late-Arriving News
The Silent Electric Flyers of San Diego have announced the Third Annual Mid Winter Electrics Meet for February 11-13, 2000. There's a number of events and other activities planned. You can get full info at the club site: http://sefds.org or from Don Wemple at [email protected], or call (619) 469-5566.
Bent prop / sorta-folder idea
A photo shows the front end of a Revolt! with what appears to be a "bent prop" or "sorta folder." Actually this idea is someone else's (I don't know who) that I saw somewhere in the distant past and recently applied for my own purpose.
The prop is a cut-down Master Airscrew 12 x 8 folder that is further modified by tightening the blade hub screws. The idea is to snug up a bit on the normal floppy blades so that they no longer flop around on their own, but maintain an open position. They also do not fold upon power shutdown, no matter the dive angle! At the same time, the blades will yield upon even mild impact.
But why do this?
I've enjoyed much flying with the Magnetic Mayhem Reverse truck motor. I typically fit this low-cost motor with Leisure gears, and prefer 3:1 for 10-cell airplanes that ground-handle and 3.8:1 in hand-launched designs.
Normally, the former would be flown with a fixed-blade prop and the latter with a folder. However, because the plastic gears I use (chosen for available ratios) are comparatively unforgiving, a simple rotating prop touch to the ground can peel off one or more spur gear teeth. The repair is easy and cheap enough, but annoying.
I've found that this sorta-fixed-blade prop is much easier on the gears, and results in far fewer gear replacements. Just pull any displaced blade forward before the next flight!
The 12 x 8 cut to 11 x 8 and used in the manner described on the Mayhem and 10 cells flies the Revolt! very well. In fact, this combo has become somewhat of a field standard where I fly. The flight results are indistinguishable from those with a Master Airscrew 11 x 7 E-wood prop, another favorite.
I use a Sonic-Tronics 13 x 7½ folder with the Leisure 3.8:1 gears and 10 cells on the E-Motion. Both allow maximum motor currents of about 25-27 amps—a good number for this economy motor.
Magnetic Mayhem / Mega-Monster motor observations
This leads me into some recent observations about the Magnetic Mayhem Reverse—a motor that readers brought to my attention several years ago.
At that time, this product was named Mega-Monster Reverse—nomenclature consistent with car-crowd ways! The name change occurred about two years ago; at the same time, some obvious modifications were made to the motor, including some cooling openings in the motor housing (a good idea).
Initially, the Magnetic Mayhem Reverse seemed to run every bit as good as the Mega-Monster. As time went by and I got more recent Mayhems, motor life seemed to deteriorate dramatically. This has taken the form of the motor throwing windings after 25-50 flights, and has become commonplace on my flightline. I have no way of knowing if this is associated with a design or manufacturing change, or is merely confined to a particular manufacturing lot.
This difficulty notwithstanding, I've recently purchased even more motors and will watch what happens to these. If the problem persists, I'll probably try binding the windings near the commutator connections; the motor is well worth the trouble. (I routinely replace the bushing bearings with ball bearings, which do much better under gear loads; another reader input.)
This motor works great as configured above, and I'm not anxious to give up on it. The same motor also seems to work very well with direct drive, except now the Magnetic Mayhem is used—not the Magnetic Mayhem Reverse. So far I have only limited experience using six cells and a 7 x 6 or 8 x 4 nylon fan on rather simple 40-ounce airplanes, but it works very well.
In this case I did not bother with ball bearings, since there is no side load on the bearings from a gearbox. More as experience accrues.
This is the only "car" motor that I know of that is practical to use for direct-drive applications. It has more turns and a longer housing/magnet/armature stack than the typical car motor, and turns at lower rpm with greater torque.
I do not recommend the typical car motor for use with direct-drive applications. They are designed to turn at much higher rpm. This speed dictates a much-too-small and not-too-effective prop just to hold the current to a reasonable level.
Circuit-board assembly caution
Aeromodeling Electronics is popular with many modelers, and MA has offered several "build-it" articles over the years. Many of these items used ready-made printed circuit board (Radio Shack) as the basic building substrate, with various electronic components soldered thereto. Many other project-in-kind have appeared in magazines over the years, using either printed holoboard or custom PC boards.
Instructions often include "leveling" the cutoff circuit component leads on the bottom of the circuit board before soldering. In my "Sport Speed Control" article (September 1999 Model Aviation), I stated, "Nip off any leads that are not already cut close, and gently block-sand (180- to 220-grit) the bottom of the board assembly."
Smoothing off the projecting component lead stubs from a PC board is usually to get rid of the "sharpness" that is so typical around soldered, cut-off component leads. It's often easy to get scratched from these stubs, or worse, they can cut through wiring or whatever may be placed underneath such a board.
(The practice of sanding or filing off such protrusions dates to the '50s and '60s when there were lots of build-it-yourself electronic articles and kits catering to the early RC hobbyist.)
But there is a downside to this procedure. The operative word in the above quote is "gently." It is actually easy to damage soldered PC board joints with too much "enthusiasm" associated with this smoothing process!
The damage can actually be nearly invisible—taking the form of tiny fractures in the solder/wire interface, resulting in an unreliable connection that may take considerable time to show up. Remember, the idea is to gently remove (level off) sharp projections, and this can be done adequately with light pressure and fine sandpaper for a short time. Copper wire and solder yield nicely to this treatment, so there is no need to overdo it.
I use a well-worn 220-grit block on my circuits—after nipping off component leads with flush-cutting wire cutters. A solvent cleaning with an "acid" brush is needed following this process, as well as a reinspection with a lens to make sure all is well.
Remember: just gently ease that fine-paper block over the wire stubs!
Burlington NJ E-meet
The annual Burlington NJ E-meet was held several days ago as I write this. This is a low-key one-day meet that has been running for several years and always draws a good turnout. It would be great if it was held in May and October—one way of making it a two-day affair!
This meet drew about 30 participants to the BCRC club field in near-perfect weather, and it drew something else: what seemed like a preponderance of small airplanes. I did not do an actual count, but it seemed clear enough for the first time that Speed 400 and other small craft were the meet favorite.
One observer suggested that everyone's other E-craft were no longer around, given that it's late in the season! But I don't think that's the reason, since this was not the case at other times with late-season meets. Rather, I think everyone just got the "small airplane" bug, and it showed up this way.
The other impression I had was that the predominant design of the day was ModelAir-Tech's Elipstick design. Check it out!
Battery management experiment (timed charging)
Last month's column described a possible breakthrough in battery management—a technique that seems to extend good battery behavior longer. I described a long-term experiment beginning with several new packs used in conjunction with the motor cutoff function of the Sport Speed Control, and how the normally occurring "false peak syndrome" has been overcome, or at least delayed in showing up. Results to date are very encouraging.
I'm now undertaking an additional long-term experiment—an old idea revisited—where I've dedicated a new pack to timed charging. Yes, timed charging. Here's my rationale:
From numerous experiments I know that pack heating during charging happens mainly in the first two to three minutes of charge time when the pack terminal voltage rises very rapidly, as does temperature.
Temperature "wins out," and as the pack temperature gets sufficiently high, the terminal voltage begins to drop. This drop triggers peak-detector chargers into shutting down. By this time, however, the pack temperature is noticeably higher than at the beginning of charging, and it's in this condition that most flying begins.
Discharge during flight further increases pack temperature. If the packs are very hot, more heat needs to be cooled down before recharging, but there is an irony to this: the last few minutes of charge does not significantly add to the pack heat. For all practical purposes, charging is pretty much complete at just about the time the pack voltage begins to rise and pack temperature begins to soar.
The experiment evaluates a pack for what it can hold charge-wise. Subsequent charging is done by limiting the duration of a known charge current so that the pack "just fills up." This is only a safe procedure if the pack is known to be "empty" at the outset—assured by the motor cutoff circuit in the SSC (assuming the airplane is flown to that condition, which I always do anyway).
So what's to be accomplished? I'm not quite sure, but I'm hoping for even greater battery life and sustained good performance over that life.
I'm not using any mechanical wind-up timer from days gone by, but a new electronic configuration that is used in-line with my usual charger output cable; when the time is up, the circuit "tricks" the charger with a virtual peak. The charger then shuts down as though it were a real end-of-charge, peak detection.
The other component in all this is the known, precision charge current. I calibrated both of my Astro 110Ds and know the current within one percent: a precise current for a precise amount of time.
I fill the bottle to the top, and no more. No "spillover" in the form of pack heat. Flights then begin (and end) with cooler packs. Better motor runs, and less cool-down time.
We'll see what comes of this as time (and the experiment) continue on.
Closing
Why go through all this? For fun. The hobby is for fun, and this is one way for me to get even more enjoyment from electric flight. And hopefully there's something in all this to boost your fun too!
Please enclose a SASE with any correspondence for which you'd like a reply. And don't forget: Electric flies just fine in winter months too! MA
Transcribed from original scans by AI. Minor OCR errors may remain.
