Radio Control: Electrics
Bob Kopski
25 West End Dr. Lansdale, PA 19446
This Month's Topics
- The Electric Connection Service
- Motor Noise—Still More
- Motor "Size"—Again
- Direct vs. Geared Drive
- Skyvolt
- A Reminder—The License Plate Contest
- EAA—Where Are You?!
The Electric Connection Service
The Electric Connection Service has a single offering this month. He is Charles Simerka, 11644-150 CTN, Jupiter, Florida 33478. Charles has recently returned to modeling after about 20 years absence and has done so with electric power. He is anxious to find others nearby to share in the joy that quiet power brings, so please look him up and tell Charles "Bob sent ya!"
It's interesting to note that many former modelers who separated from the hobby for a decade or two are returning—to electric. This is a routinely occurring story in my incoming mail.
Motor Noise—Still More
Motor noise has become a column specialty. Reader reaction to recent columns addressing noise has been strong. Many letters, calls, and encounters at the last KRC meet voiced gratitude for the information on capacitors on motors in the October issue. The suggestions given then have worked well for many modelers.
To review: that column discussed and presented photographic "how to" regarding installation of capacitors at the motor brushes for more effective noise reduction and therefore less motor-induced in-flight glitching.
Additional input has refined that advice. At the KRC event, Bob Johnson (author of the "chokes" article in the August '91 RC Report) and George Minnear (Westmoreland Electric Soaring Society, PA) both pointed out that the recommended disc ceramic capacitors can be less effective depending upon self-resonance characteristics. They are correct. Capacitors plus lead length form a series‑tuned circuit that will be especially effective at certain frequencies; when the resonance falls outside the frequency of interest, results may be disappointing.
I investigated the behavior of the recommended Radio Shack parts with a grid‑dip meter. I found the resonance generally between about 50 MHz and 70 MHz depending on capacitor lead length. I hope to explore specifics in greater detail in the future. For now, I recommend making capacitor leads as short as possible for greatest effectiveness.
Two modelers were reluctant to disassemble geared motors to install the recommended capacitors. Both had geared Cobalts and chose to route capacitor leads out through the gearbox area, resulting in relatively long leads. One might assume such long leads would be ineffective—wrong. Both installations worked fine, and both modelers reported flying without motor-induced glitches. I think this is a matter of degree: short leads are best, but in some installations longer leads are still better than what was there before. In other cases, long leads might not work at all. Not all cures work in all cases; often combinations of fixes are needed.
Another useful tip comes from Dennis Vollrath (referenced in the December column). Dennis had problems with speed-control switching transients getting into the receiver and shielded the receiver with foil. He also found that twisting wire pairs (battery wires, motor wires, etc.) greatly reduced offending emissions. He described how twisting increased the range of a particular .40-powered craft by about 100%. A simple and effective trick—thanks, Dennis.
Bob Johnson's RC Report article on chokes described the application of toroidal cores to motor and power‑system wiring and listed a supplier. The supplier is Amidon Associates; their new address is:
Amidon Associates P.O. Box 956 Torrance, CA 90508
Current core prices (subject to change):
- FT-50-43 — $0.75
- FT-82-43 — $1.20
- FT-114-43 — $2.25
Postage: $4.00
For reference, recent Model Aviation "RC Electrics" columns that discussed motor noise problems and solutions include: 12/89, 1/90, 8/90, 9/90, 2/91, 10/91, and 12/91. Earlier columns addressed some of this material as well, but much has been repeated or updated in the issues listed.
Motor "Size"—Again
Motor size and what it means has been a recurring topic. There are no consistent motor size standards within the hobby, which makes comparing products and choosing the right motor difficult. One of the most popular reader questions is still "which motor/which plane."
Electric power has matured rapidly and is no longer just for gliders, but motor descriptions and ratings remain archaic—left over from when electric was mostly glider power.
The most familiar motor size nomenclature was introduced by Astro Flight in the early 1970s (10, 25, "can", 05). Those labels were intended to give rough equivalents to sport glow engines of the time. But today's motors (for example, modern cobalt 05s) are far removed from those early "can" motors in capability, so the old comparisons are misleading.
Comparisons to glow engines may be useful for those familiar with "wet" power, but not for new electric-only modelers who have no wet reference. Fundamentally, I question why any comparison should be needed. Early modelers had no fuel-engine comparison when they adopted glow power—they learned by experience.
I have long advocated using input power expressed in watts as a meaningful standard. Input power (watts) is simply motor voltage (volts) times input current (amps). Both quantities are readily measurable with simple equipment.
Manufacturers could describe motor capability in watts, with the proviso that the motor be used properly (appropriate propellers and recommended rpm range). After a little exposure to what watts do in a given model, a modeler could rapidly learn to match motors and airframes. Of course, manufacturers would need to adopt more detailed standards (for example, motor life) to make such ratings useful and comparable.
For example, a motor might legitimately be rated for 100 watts, but increasing voltage, pitch, or diameter can push it above 100 watts and affect motor life. A possible starting standard might be something like "100 hours continuous duty at 70°F free air" to define a rated condition. Any other ideas from readers would be welcome.
Direct vs. Geared Drive
The direct vs. geared motor debate continues. Personally, I prefer to think in terms of geared propellers rather than geared motors: gearing effectively changes the prop size, not the motor. The motor's electro-mechanical make-up is unaffected by what you put on the armature shaft. The real concern is the prop/plane combination; gearing is a mechanical way to better match prop and plane.
Some models are best flown with large, slow props—examples include some sailplanes, Old Timers, and certain scale jobs. These models are more likely to benefit from geared systems. Small, fast models (pylon racers, for example) are best matched with small, fast-turning props and often benefit from direct drive.
Imagine a small, screaming direct-drive prop on a very large, lumbering Old Timer—it might look and act like a motor test stand. A larger, slower fan moving more air slowly is a better match for that airframe. Conversely, a big low-rpm prop on a sleek pylon plane looks and performs wrong.
There are many middle-range models that will do well either way; they have a wide acceptable range of prop/rpm. My Skyvolt design (Jan. '90) seems to be one of those. I've flown it both ways and found performance very close: direct-drive 15 on twelve 900s with an 8 x 4, and a geared 05 on eight 1200s turning a 9 x 8. Weight and flying feel are close either way. I hope that perspective helps with the "to gear or not to gear" question.
Skyvolt
Several readers ask about the Skyvolt. Plans are available from M4 Plans Service, and the model is also kitted by Today's Hobbies (a company that also produces stick-and-tissue jet fighter designs). While I'm familiar with the folks involved, I am not personally associated with the company.
Other frequent questions concern my Senior Skyvolt. It is simply a scaled-up version of the original: the original Skyvolt chord as published is eight inches; the Senior version is 10 inches, and all other dimensions are scaled accordingly. The Senior version is not planned for publication or kitting, even though I think it's my best electric design.
I do not have reproducible plans—just some outline lines drawn on cardboard. Thanks to everyone who has inquired.
A Reminder—The License Plate Contest
The RC Electrics License Plate Contest was announced in the July and August '91 issues. This is a reminder that a Skyvolt kit is the prize for the reader's favorite. You have until December 1992 to send me a photo of your favorite electric promoting electric flight.
So far, two readers have entered; one example was shown in the July column. I'll publish all entries at once after the closing date and readers can vote on which is their favorite. I'll review the particulars of this unusual contest again in several months—dig out those back issues to see what it's all about.
EAA—Where Are You?!
As best as I can tell, there's no one home anymore at the Electric Aeromodeling Association (EAA). I've always supported electric endeavors such as our SIG within the AMA. Before the EAA there was the short-lived SEAM.
Last January—membership renewal time—I simply forgot to send in my '91 EAA dues. Around June, when I wondered why I hadn't seen any EAA newsletters, I realized I was no longer a member. I immediately mailed three years' worth of dues plus some additional money and requested the newsletter reprints I had missed. Weeks, then months, went by. I heard nothing. My check never cleared my bank.
I wrote again, enclosing a SASE to make a reply easier. No response. No check cashed. Nothing. This was late October.
You may draw your own conclusions. I'm not really worried—electric will do just fine even "with no one home at the EAA"—especially since readers of this column seem to show little interest anyway (as discussed in the December '91 issue). However, if anyone can shed light on this apparent "lights out" situation, I'd be happy to let all readers know the real story. I'd like to support our SIG, but I'm sure most modelers would like to know more before sending dues money.
Closing
Please include a SASE with any letters you'd like me to answer. Have a safe, happy, and quiet Electri-flyin' winter.
Transcribed from original scans by AI. Minor OCR errors may remain.
