Goodbye, glow: Convert to electric

Model Airplane News, Oct 2003 by Gimlick, Greg

Welcome to the inaugural edition of a series of columns dedicated to big electric models. Of course, the adjective "big" may not always refer to the size of a model; it could also refer to the cell count, quality, motor size and speed. We'll look at both scale and sport planes; they can be built from kits, come almost ready to fly (ARF) or be built from scratch. I hope the lessons, conversion techniques and how to's in these columns will add to your body of knowledge and help you make decisions about future conversions.

Of course, before we get to any of that, it's important to have a basic understanding of electrics. This month, we'll take a look at a simple conversion of the Ford Flivver ARF kit from Dymond Modelsports. To fully understand the conversion process and decide how well a model will lend itself to the project, we must first take several factors into consideration-all of which will help determine the model's exact power requirements. Because it's a bit heavier than some other ARFs and the moments make it less forgiving of error, the Flivver isn't the most suitable candidate for conversion, but that's why I chose it. This will help show that almost any plane can be successfully converted to electric. To make things easy, we won't change anything except the motor mount.

When you plan a conversion, the weight and the wing loading should be the first things taken into consideration. (Wing loading is the total weight in ounces divided by the wing area in square feet.) Because the wing loading on the glow-powered version of the Flivver is minimal, it's a good candidate for a successful conversion. It's safe to assume that an electric motor with a speed control will weigh approximately the same as the glow engine, so our battery load will be our only significant added weight.

When it comes to wing loading, lighter is always better. Higher wing loadings will negatively affect takeoff speed, stall and the power the model needs to perform to our expectations. For aerobatic sport planes and warbirds, a wing loading of 20 to 25 ounces is acceptable, and in the case of some scale planes, it can run even higher. If I can keep the Flivver's weight between 6 and 7 pounds, it will have a wing loading of 24.8 ounces per square foot (112 ounces divided by 4.5 square feet)-perfect!

Various formulas exist to determine how much power a model requires; the most popular indicates a model needs 30 watts per pound for level flight, 50 to 60 watts per pound for mild aerobatics and 70 to 100 watts per pound for demanding aerobatic maneuvers. (See the "Watt's Up?" sidebar to learn how to determine how many watts a particular setup will produce.) Because I want the Flivver to be at least mildly aerobatic, the motor system I choose must supply a minimum of 50 watts per pound.

When I decide that a model is a good prospect for conversion, I compile a list of possible power setups. When choosing a power system for a conversion, I rely mainly on my experience. For example, because I know that models comparable to the Flivver have been successfully flown with an AstroFlight 15G motor on 14 cells, I can assume that it is suitable for this project. In addition to the AstroFlight combination, I also decided to experiment with two other power systems: a Kyosho Endoplasma motor with Inner Demon gear drive and a MaxCim Max15-13Y motor-each on 20 cells. The Kyosho Endoplasma setup is a dual-motor setup wired in a modified series circuit developed by Model Electronics Corp. (MEC) that drives a single prop through an Inner Demon gearbox manufactured by Model Machining Services. The most important thing to remember about this setup is that the current, as listed, is about 54 amps, but each motor receives only half that.

The selection of motor systems is not an exact science; years of experience and a substantial knowledge of the subject contributed to my selection of the latter two systems. Much of this process will be trial and error for those who have less experience.

After I choose an appropriate model for conversion and some possible power systems, I enter the data in a simulation program such as ElectriCalc or MotoCalc. In this case, I entered the battery type, the number of cells I chose, the weight of the airplane, the wing area and the selected motors. The computer program does the math for you and provides all the numbers necessary for you to decide which system will prove most successful. (See the "By the Numbers" sidebar.) Such programs also feature databases full of possible motor/cell combinations for beginners to choose from that will help eliminate some of the trial and error process. Do yourself a favor and invest the $30 or so in one of these programs. It's well worth it!

I've flown the Flivver with all three motor setups, and although each one works well, my favorite is the MaxCim motor on 20 cells. It is the most efficient of the three and produces a better power-to-weight ratio. The MaxCim's versatility is perfect for larger airplanes.