Nothing has captivated my interest in model aeronautics the way electric powered modeling has. After flying glow models for nearly 20 years, and dabbling in electric flight for a small portion of that time, I sold off all of my glow engine and support equipment in the early 1990’s and went “totally electric.” I have not looked back. I have absolutely no interest in cleaning glow fuel residue off the model or the living room floor any longer. I enjoy the quiet of the flying field, with nothing but the sound of the prop beating the air into submission.
I became quite bored flying glow fuel powered aircraft. In that 20 or so years, I flew fixed wing, helicopters, jets, sailplanes, seaplanes, even fixed wing VTOL models. With the exception of the latter, there were few new challenges. Electric modeling can be more “cerebral” than glow and the challenges to make an electric model fly well greater. For that reason, many are intimidated by it—but you need not be.There are basically three types of electric modelers.
The “Experimenters”: those who are generally technically educated, have a thirst for knowledge and are looking for a better mousetrap.
The “Absorbers”: those who are very interested in electric flight, come from all walks of life, and get their “knowledge base” from the experimenters, internet and paper periodicals.
And then there are the “Turn-Keys”: those who buy completely engineered mass-produced airplane systems and who have very little desire to know what makes them tick.
This column is dedicated to the absorbers. I hope, in each installment, to help you, the inquisitive reader, to become the finest electric “flight engineer”/ pilot you can be. The column will also introduce the reader to what’s new and upcoming in this fast-developing arena. Some aspects of electric flight do not change with technology. I will share with you the basic rules and tools that are guaranteed to make the hobby fun and not a chore or a disappointment.
TOOLS OF THE TRADE
Electric model aircraft require many of the same tools as their internal combustion brethren. The three that stand out as essential are: the volt-ammeter, tachometer, and charger.
Commonly referred to as the Whattmeter (by AstroFlight), this device displays the current drawn from the motor battery pack, the voltage of the pack (at rest and under load), the power delivered to the motor (in watts) and the power consumed (in milliamp-hours or “mAh”).This tool, and others like it, will tell you whether you are about to damage your electrical device (battery, electronic speed controller (ESC), or motor), by applying too many amps to the system. It will also tell you (once you learn the basic numbers) whether you’ve made a funny looking car rather than a flying model (too little power to fly)! Experienced users can also use this device to determine the health of the battery, ESC or motor.
Glow modelers can generally tell the health of their “engine” by the sound it makes. Probably less than 10% of the glow modelers at any club field even use a tachometer. This tool is essential to the electric modeler as the sound the prop makes can be deceiving and may not be enough, by itself, to determine the health of the system. A tachometer, combined with the aforementioned volt-ammeter, will tell you nearly everything you need to know about the system installed in the model. It is good practice to write down the amps, volts, and RPM of any new system so that a “benchmark” can be referred to at some point in the future should you suspect something is not right.
Battery technologies and their corresponding charge techniques seem to change as often as the seasons. For the immediate future, Nickel Cadmium (NiCd) and Nickel Metal-hydride (NiMH) are still the predominant battery technologies that drive our electric motors. Lithium cells (3.6 or 3.7 volts per cell) come in a few varieties: metal (Li-M), ion (Li-I) and polymer (Li-Poly). Until recently, Lithiums were only available in very low power ratings and could sustain discharge currents no greater than 2-3 amps. The Li-Po type have been improving rapidly and are now providing higher discharge rates. They soon may supplant the NiCd and NiMH cells as the cell of choice. But for now, I would like to discuss NiMH and NiCd chargers.
These chargers come in two forms—AC/DC and DC only. The AC/DC charger can be plugged into a wall outlet or powered by a 12v battery. AC/DC chargers have a built in power supply to convert house voltage from 110v AC to 12v DC. These chargers generally can be used for up to 8 cells only. The AC/DC charger offers convenience: it can be used in your shop powered by house current or at the field running off the car battery or a separate “deep cycle” marine battery.
The DC-only charger can only be used from a 12v DC source. If not using a lead-acid 12v high capacity battery, one must buy a 12v filtered power supply that can supply the charger the required voltage and amperage from a wall outlet. DC chargers have the capability of charging as many as 36 cells (1.2v/cell nominally).Nickel technology cells like to be slow charged once in a while to “equalize” any imbalance in the cells from age or use. This may require a completely separate charger. Where most “field” DC chargers are designed to charge batteries at a “slow” rate as well as “fast”, some of these chargers do not charge slowly enough for the smallest of the battery packs we use in our smaller electric models. One should look for a “slow” charger that can handle as many as 10 cells and have a milliamp (mA) setting that can be turned down as low as 10-15mA. Always look for a “fast” charger that has the capacity to grow with your involvement in electric flight. Don’t buy a 6-8 cell charger that is inexpensive, when a year down the road, you may desire to grow into models in the 16-24 cell class.
Also, look for chargers that advertise “peak detection” or “delta peak.” These chargers shut off the current to the pack when the battery is fully charged. Slow charging or “overnight charging” is considered a C (capacity/10) rate. A slow charge for a 1000mAh pack would be 100mA and take 10-12 hours to fully charge the pack from a discharged state. Fast charging can be defined as anything from a 1C to a 5C rate or 1 to 5 times the rated capacity. A 2000 mAH (2AH) pack would need to be charged at 10amps to reach a 5C rate. This is unobtainable by many chargers made today. Five amps is the most common factory-set maximum charge rate. Newer, higher capacity cells are driving charger manufacturers to re-think this limit.
Well, now you have got your start! If you don’t have some of these tools, please go out and get them. They are your compass in the woods. They will lead you out of the darkness during the early stages of your electric model flight adventure. Until next issue, work and fly safe.
Ace RC, distributed by Ace Hobby Distributors
www.acehobby.com, (949) 833-0088
www.astroflight.com, (310) 821-6242
GloBee, distributed exclusively by Great Planes Model Distributors
www.bestrc.com, , (217) 398-6300
ElectriFly, distributed exclusively by Great Planes Model Distributors
www.electrifly.com, , (800) 682-8948