MA Technical Editor Bob Aberle with sport/aerobatic Acrovolt constructed in 1995. As designed by Tom Hunt, it would have been comfortable with 60-size glow engine but was powered by DeWALT 18-volt cordless-drill motor with belt drive. Text tells how it was greatly improved with new brushless motor and Li-Poly batteries.

FOREWORD: At the 2004 Weak Signals expo in Toledo, Ohio, former AMA District II vice president and current AMA Flying Site Coordinator for the Eastern Region Joe Beshar, who is an experienced electric-power enthusiast, made a suggestion. He mentioned to AMA Director of Publications Rob Kurek that we badly need a standard reference document that will allow any modeler to size the proper electric-motor system to model aircraft of any size and weight. What you are about to read resulted from Joe’s suggestion.

Background: When a person enters model aviation, he or she has a choice of power sources for his or her aircraft. Years ago those options included rubber power, hand launching (as with a glider), towing a glider with a cord or a fueled engine, and gasoline/spark-ignition engines or later glow-fueled engines.

            As time went on and the glow engine became the most popular source of model-aircraft power, modelers quickly became adept at selecting the right-size engine for every size and weight of model. An engine’s cu. in. displacement was related to model weight and model size (usually wing area expressed in square inches).

            Engine classes became known as 1¼2A, A, B, C, and even D. Each category covered a range of engine displacements (such as—if my memory serves me correctly—0.09-.199 cu. in. was considered Class A). Early FF competition rules related engine displacement to weight, so that a .19 cu. in.-engine-powered model had to weight 19 ounces, etc.

            When RC started to come of age in the 1950s, we learned (by experience) that 1¼2A RC models could weigh as much as 18-20 ounces, Class A RC models could weigh as much as 30 ounces, and so on. Although these engines are rated in horsepower (and thrust), to this day most modelers simply relate engine displacement to model weight and size!

Hobbico SuperStar EP is intended for glow power. Bob explains in the text how to convert it to electric power.

Bonnie 20 electric-powered ARF is featured in Sport Aviator review. It could have been powered by .20-.32 glow engine. It also makes a wonderful electric-powered advanced RC trainer.

Enter Electric Power: Although some of the first electric-powered flights were made in the late 1950s and early 1960s, electric power as we know it today didn’t really come into its own until the early 1970s. Then it became obvious that engine displacement, for identification purposes, would not work with electric. To make an early distinction between fuel and electric, it was decided to refer to fueled power as “engines” and electric power as “motors.”

            Bob Boucher—one of the leaders of electric-powered flight in the USA—began referring to his motors by equivalent glow-engine displacements. His “05”  motor was supposed to be equivalent to an .049 cu. in. glow engine, a “40” motor was equivalent to a .40 cu. in. glow engine, etc.

            While this was going on, Germany’s Graupner company was assigning names such as Speed 400, Speed 500, and the like. The numbers were rounded off from the Mabuchi manufacturer’s model numbers; “RS-380” became a Speed 400, “RS-540” became a Speed 500, and so on. Surely that fact couldn’t help you select a motor for a particular size and weight of model.

What to Do? Approximately 30 years have now gone by since that start in electric-powered flight. Although many still have questions, we know much more now and have developed some excellent techniques for “sizing,” or matching the correct motor to any size and weight of model aircraft.

            When you look back at this time frame, you can compare it with the advent of the gasoline engines in the early 1930s, which ultimately led to the cu. in.-displacement sizing of those engines in the 1940s. Identifying fueled and electric engines/motors has taken time.

            The main thrust of this article is to explain how to select, or size, your motors to make them suitable for powering models in flight. I hope to cover such things as motor identification, motor types, direct vs. gear drive, motor power (in watts) as it relates to aircraft weight, and many other details. I hope to make this a standard reference so that no one will have to ask, “What motor should I use?”

You must know aircraft’s exact weight to select motors. Pelouze Model PE-5 scale (L) weighs items as heavy as 5 pounds in 0.1-ounce increments. My Weigh digital scale (R)—from www.goodscale.com/scale—is intended for parking lot and indoor micro RC models.