Radio Control: Electrics

Radio Control: Electrics

Bob Kopski 25 West End Dr., Lansdale, PA 19446

Meet Announcements

• Harbor Soaring Society — Second Biannual Sportsmen/Novice F3E FAI Contest
• Dates: August 19–20, 1989
• Open to all U.S.-resident AMA members except those who qualified for the U.S. AMA FAI F3E team.
• Cash prizes to fourth place, beginning with $1,000 for first.
• Contact: Felix Vivas, 1800 16th St. #H-3, Newport Beach, CA 92663; tel. 1-714/645-3263. (Tell 'em Bob sent ya.)

• Electrifest — Second Annual Electrifest
• Dates: September 16–17, 1989
• Sponsored by Slo Fliers, Cuesta Community College, San Luis Obispo, CA.
• Contact: CD Cal Drake, Box 1557, Morro Bay, CA 93442.

• Keystone RC Club — Tenth Annual KRC Electric Fly
• Dates: September 16–17, 1989 (field unofficially open to early arrivals on Friday afternoon, September 15)
• Location: Buc-Le Aerosportsmen Field, Quakertown, PA.
• This year's format includes all the usual events plus special activities for the 10th anniversary: Saturday evening dinner/social, at least two complete ready-to-fly electrics to be given away, and fun events including All-Up/Last-Down, Maxi-Flight, Most Loops, and Most Rolls — all with cash prizes to fourth place, plus other judgings and prizes throughout the weekend.
• Contact / Meet CD: Bob Kopski, 25 West End Dr., Lansdale, PA 19446.

Amptique Kit Advisory

Keith Shaw passed along an advisory about some Amptique kits: a batch left the factory with rather soft 3/16-in.-sq. balsa intended for wing spars, and a number of wings folded on the same day in his area.

This is not meant as a negative reflection on Amptique (a fine plane) but as a caution: inspect the spar material in your kit and make sure it is suitable. If in doubt, I suggest substituting 1/16-in.-sq. spruce for the main panel spars and go flying worry-free. The same idea applies to any model: make sure the spar material is suitable. Nuff said — thanks, Keith.

Car Battery and Charging Considerations

Larger power systems, and sometimes simply lots of flying with smaller ones, can surprise you with a dead car battery. Let’s look at some numbers.

• Example charge: charging at 40–45 amps for 15 minutes. With average cell voltage about 1.4 volts, that totals roughly 107 watt‑hours to the pack.
• Most modern booster chargers are only about 70%–80% efficient, so you're drawing more like 140–150 watt‑hours at the wall — a significant demand on the car battery.
• At a nominal 12‑volt level, that corresponds to the car battery delivering about 12 amps for the quarter hour needed to charge this plane.

Many auto batteries (when new) have a nominal capacity around 50 ampere‑hours. Ideally, a 50 Ah battery would provide roughly 50 / 12 / 0.25 ≈ 16 charge cycles of the example pack. In reality, most batteries don't deliver rated capacity and most are not new. On average you should count on about half the expected capacity and leave some margin to start the car — translating to an approximate eight‑flight expectancy in this example.

Other factors (lights left on, running multiple chargers, etc.) can reduce that number further. If you drain your car battery, depending on type, you may damage it. Popular "maintenance‑free" sealed batteries are prone to significant degradation if deep‑cycled; they lose capacity and can rapidly become of little value. That’s what happened to me: I ran the battery down (I later discovered a light left on in the cassette compartment), charged my Exciter a few times, and the battery deteriorated quickly. The car was less than one year old.

I replaced it with a top‑rated, "limited maintenance" battery (water caps not sealed) so I can monitor electrolyte level. The battery I bought (warranted 60 months for automotive use, and half that for deep‑cycle use) can tolerate deep cycling better — exactly what I need for long summer flying sessions. Many brands are warranted only for normal automotive use, not deep‑cycle use, so check warranties if you do a lot of charging from your car.

Routine practice now after a demanding flying session: put the car on charge overnight with an ordinary household auto battery charger. I fly almost every night in summer and my short drives (8–10 minutes) don't recharge the battery. These lessons did not come cheaply — Electric (or any other flying) should be fun, and a car that won't start isn't.

Next time you replace your auto battery, keep this info in mind. If you do a lot of flying and relatively little driving, consider getting an inexpensive household auto‑battery charger. (The battery brand I bought is Mega‑Tron.)

Astro Model 112 DC/DC Constant Current Charger — Bench and Field Notes

Flash: Bob Boucher of Astro reports that the latest Model 112s off the production line are upgraded so that even more output than I reported for the largest packs is available. My unit was one of the first manufactured, so newer units may outperform mine until I get mine upgraded.

I tested the Astro Model 112 on the bench (using a high‑power variable lab supply to simulate car battery voltages) and in field use. Test subjects were an 8‑cell, 1.2‑Ah pack and a 20‑cell, 1.2‑Ah pack.

• Vin variation test: I varied the input voltage (Vin) during charge and checked stability of the 112’s output current (Iload) and measured input current (Iin). Conclusion: output current varies in proportion to input voltage. The Vin range tested covered about 11.0 V (very low auto battery), 12.0 V (nominal), and ~14.0 V (engine running). In practice auto battery voltage changes little during a given charge cycle, so this proportionality is mainly an observed behavior.

• Constant 12.0 V test: with the 112 powered from a constant 12.0‑volt source and charge current initially set at 4.0 A, Iload remained nearly the same during the 15‑minute charge period as the pack terminal voltage rose. This means the 112 holds the set charge current nearly constant during the charge interval — hence the name "constant current." This was an improvement over the (unmodified) Model 102.

Field notes and observations:

• I have been using the 112 in the field for a few weeks and it works nicely for charging along the way while driving to the field. The "charge‑while‑I‑drive" technique is my preferred method: load planes in the car, set up the charger on the table, and charge packs en route.
• The timer switch runs for 13½ minutes, not 15, so I compensate with a slightly higher current. Time your unit to confirm.
• With 12.0‑volt input, the maximum output current available into a 24‑cell pack is 3.6 A. Larger packs cannot be charged at preferred higher currents from 12.0 V and must either be charged for more than one timed cycle or charged with the car engine running. This limitation was evident with the modified 102 as well; the 112 is an improvement but still subject to input voltage limits.

Propeller Size Effects on Power Demand

Last month included data on a number of .05 motors; this month I’m sharing data on the effects of propeller size on power demand. Tests were run on an Astro 15 cobalt with a constant input voltage of 10.0 volts.

Part 1 — Effect of pitch at constant diameter (Rev‑Up props, 8" diameter):

• 8 x 3 — Amps: 12.4, RPM: 1,183, Power: 124 W
• 8 x 4 — Amps: 14.5, RPM: 1,151, Power: 145 W
• 8 x 5 — Amps: 17.2, RPM: 1,110, Power: 172 W
• 8 x 6 — Amps: 19.1, RPM: 1,070, Power: 191 W

Part 2 — Effect of diameter at constant pitch (Top Flite props, 4" pitch):

• 6 x 4 — Amps: 7.62, RPM: 1,252, Power: 76 W
• 7 x 4 — Amps: 9.0, RPM: 1,225, Power: 98 W
• 8 x 4 — Amps: 14.0, RPM: 1,150, Power: 140 W
• 9 x 4 — Amps: 20.7, RPM: 1,050, Power: 207 W

Part 3 — Various 8 x 4 props (popular size for .05–.15):

• Top Flite (wood) — Amps: 14.0, RPM: 1,150, Power: 140 W
• Tornado (plastic) — Amps: 16.0, RPM: 1,115, Power: 160 W
• Rev‑Up — Amps: 14.2, RPM: 1,145, Power: 142 W
• Top Flite (nylon) — Amps: 16.7, RPM: 1,107, Power: 167 W
• Cox Grey (plastic) — Amps: 13.5, RPM: 1,153, Power: 135 W
• Zinger — Amps: 15.0, RPM: 1,129, Power: 150 W
• Master AS — Amps: 16.7, RPM: 1,102, Power: 167 W
• K & W (short hub) — Amps: 15.7, RPM: 1,120, Power: 157 W
• K & W (long hub) — Amps: 18.7, RPM: 1,075, Power: 187 W

Conclusions: motor input power increases rapidly with increases in either diameter or pitch. Also, all 8 x 4 props are not created equal — in my experience, a prop that loads heavier on the bench (demands higher input power) tends to make the plane fly "better" but for shorter duration.

Closing / Contact

Coming up in future columns: "To break in or not to break in," commutator‑brush‑angle information, more motor performance data, and lots of other Electric topics.

Please forward any comments or questions (with SASE, please) to: Bob Kopski 25 West End Dr. Lansdale, PA 19446

Happy, quiet Electric landings, everyone!

Transcribed from original scans by AI. Minor OCR errors may remain.