Converting a Glow Model to Electric
By Cliff Tacie
Glow Engine vs. Electric Motor Basics
Engines: "Size" determined by C.I. displacement (.15, .25, .40, etc.)
Power rated in HP, typically at 15,000 – 17,000 RPM
.10 = .25 HP
.25 = .60 HP
.40 = 1.1 HP
Motors: Power rated in Watts. "Size" determined by Watts or physical size and RPM/Volt (Kv)
Convert HP to Watts: 1 HP = 750 Watts
.10 = .25 HP x 750 = 180 Watts
.25 = .60 HP x 750 = 450 Watts
.40 = 1.1 HP x 750 = 825 Watts
2814-6 Motor: 28 = Diameter of Stator in mm
14 = Length of Magnets in Rotor can
6 = # of wire winds (turns) on Stator
KV or RPM per Volt
High # of wire winds = high Kv = use small props at high RPM
Lower # of wire winds = lower Kv = use large props at low RPM
(Think 2-stroke glow engines vs. 4-stroke glow engines)
What "Power" motor to use?
General Rule of Thumb (Dr. Keith Shaw, 20 years ago):
50-75 Watts per pound of loaded model – "Cub" type, trainer, Scale.
100 Watts per pound of loaded model – Sport aerobatic or Pattern
130 Watts per pound or more of loaded model – 3D, EDF, etc.
Cub/Trainer Sport 3D
.10 = 180 Watts 2.4 lbs 1.8 lbs 1.4 lbs
.25 = 450 Watts 6.0 lbs 4.5 lbs 3.5 lbs
.40 = 825 Watts 11.0 lbs 8.3 lbs 6.0 lbs
What "Size" motor to use?
Use a motor rated to handle the Amps required to provide the Watts you want on the battery Voltage you want to use. Primary variables are the propeller size and the impact it will have on the Amps drawn. Larger diameter and pitch propellers will draw more Amps. Unlike a glow engine that would "sag" with too much prop, an electric motor will keep trying to drive it!
Example: You have a 4.5 lb sport model and have determined that you want a 450 Watt motor. You expect to use a 10" propeller and want to use an economical 3S battery pack of 11.1 Volts. Now do the math…
Amps = Watts divided by Volts. Amps = 450/11.1 or about 40.5 Amps
An easy way to find the right motor is to use manufacturer or distributor web site information to get motor specifics. Most have tables showing specifications such as physical size, Kv (RPM per Volt), Watts, Amps for various battery sizes (Volts) and even suggested model weights.
Another way to select a motor is to go to an E-Flite, O.S. or ElectriFly (Rimfire) motors that are designated with Glow engine sizes. The advantage is that you select the motor size (.25, .40, .60) that is called for in the kit, and it will probably work just fine. The disadvantage is that you may not be able to use the exact battery pack/voltage that you want and the motors have a single Kv rating, so you may be limited in propeller selection. The primary disadvantage is the price. Expect to pay about twice as much as you will on a generic motor that may work just as well.
Easier way to find the right motor is to just find another model of the type and size you have in a review or advertisement and copy that power setup! Don’t try to reinvent the wheel!
Electronic Speed Controls
Once you’ve determined the "Size" motor you’re going to use, select an ESC that is able to handle the current (Amps) draw of your expected setup. If you expect to draw 40 Amps, get AT LEAST a 45 Amp ESC. It’s okay to use an ESC that is larger than needed! If possible, use a Watt Meter to check your setup and know exactly what Voltage, Amperage and Watts you have.
Useful Web Sites:
Simplified Electrical Power Systems For Model Airplanes by Russ Peterson
Greg Covey's "Amp'd"
Everything You Wanted to Know about Electric Powered Flight by Ed Anderson
Tips for Converrting RC Gas Engines to Electric Motors by Carl Baer
Os Electric Motor Guide