Q3: An old-time glow-fuel pilot mentioned that for years he referred to engines by their classes, such as Class 1/2A, Class A, etc. Each class had a specific engine displacement; Class A was understood to be .10-.199 cubic-inch displacement, a .19-powered model was supposed to weigh approximately 19 ounces, and so on.

He went on to write that with Electrics the identification of motor size to model size is difficult, to say the least. He cited examples of Speed 280, Speed 400, etc. How do you decide on application?

A3: The reader is correct in his observation that motor identification can be frustrating when dealing with electric power, but you have to look back to the beginnings of fueled engines for model use.

In the early days it wasn't obvious what a .29-displacement engine could fly. It was an educational process that eventually got everyone tuned into relating engine displacement into classes and then into model weight (which indirectly relates to size). The same is happening in the world of electric power. The power of the motor expressed in watts is the key factor.

Most of us who are involved with electric-powered flight today own a good meter that reads motor current (in amperes), motor voltage, and the product of the two, which is motor power (expressed in watts). The AstroFlight Digital Meter (Model 101) provides all of these parameter measurements and is a must-own item.

By experience we have learned that dividing the motor power in watts by the model's total weight results in the term "watts/ounce." On the smaller electric-powered models such as a Speed 400 (and down), we know from experience that the average sport model needs roughly 2.0 watts/ounce to fly in a normal manner. A lightweight, lightly loaded powered sailplane might need only 1.25 watts/ounce. More energetic models, for aerobatics and heavier wing loadings, need 3.0 watts/ounce and more.

My Scratch-One design from this series had a 5.32 watts/ounce figure which indicated that the recommended Speed 400 motor had more than enough power. That model can fly at half throttle and lower because it has that extra power. As you get into larger electric-powered models, the parameter changes to watts/pound, but the concept is the same.

This may seem complicated at first, but the numbers and the form of identification will get easier with experience. Motor manufacturers are being encouraged to provide power ratings (wattage) for every motor.