secret to choosing a motor, The

Model Airplane News, Jan 2006 by Beshar, Joe

THOUGHTS ABOUT THRUST

With the popularity and growth of electric flight, much has been published about motor-size requirements. Whether articles explain how to convert a glow-powered model to electric, create a design, or power a new model, they reference voltage, amperage, kilovolts, watts, rpm. etc., but they generally ignore thrust; and when thrust is discussed, many electrics novices are confused by the complex equations and relationships that are usually cited.

I have an easier approach. Since I started designing electric models, thrust has always been my first consideration. The principle is easy to understand and practical to apply, and thrust is easy to lest. I even designed a thrust tester, and my article on it was published in the September 1991 issue of Model Airplane New. I have used that unit from day one to determine the motor requirements of my B-17 models and many others. Back then, brushless motors and Li-poly batteries weren't available; we used ferrite or cobalt motors with Ni-Cd batteries, and micro-electric models were unheard of. Since then, developments have resulted in a wide variety of electronic devices and scales that allow us to measure thrust easily. Let's get started and see how easy it is to choose an electric motor for a particular plane.

No, I'm not a rocket scientist who develops factors; they've been around all along! To use them to choose a motor, we need know only the total weight of the model in grams, ounces, or pounds and the motor thrust in the same units of weight.

Decide which type of performance you're looking for by using the "Basic model performance" factors. Also consider the voltage of the battery pack you'll use and the amp draw it can handle. Remember, the higher a pack's capacity, the longer a model will fly but, of course, it will weigh more.

Now for thrust. This is what's missing! Motor manufacturers usually don't specify a motor's thrust or which propeller and battery you need for maximum efficiency. You'll have to ask its manufacturer whether a particular motor fits your requirements. If enough of us ask, it may serve as a wake-up call for manufacturers to publish thrust ratings. Think how helpful that would be when we compare products!

You say you already have a motor and want to determine its thrust value yourself. I trust you know how to use a scale to weigh things; it's that easy! The photos in the Click Trip show how to use a scale or thrust tester to measure thrust with a given battery pack. First, mount the motor on the test scale and record its static weight; then determine its net thrust by subtracting its static weight from the total thrust reading when the motor is running.

That's it! I hope this helps you to solve the mystery of choosing an electric motor.

SOMETHING ON YOUR MIND? Is there an RC plane topic you'd like to weigh in on? Step up to the "Readers' Soapbox''! Write to us at man@airage.com or 100 East Ridge, Ridgefield, CT 06877 USA, and we'll present your "op-ed" piece here.

JOE BESHAR

Oradell, NJ