With most of the country seeing the first of its spring flying weather, I thought it might be a good time to talk about some of the things relating to electric flight. If you’re a seasoned electric flier, this will be a review, but if you got that first electric airplane for Christmas, this will help you avoid confusion and mistakes—I hope!
Make It Easy on Yourself
Don’t get bogged down and overwhelmed by the new vocabulary and terms you run into. In our excitement and zeal for electrics, we veterans dump a ton of minutiae on newcomers to the point of running them off. Here is a basic list of the most important terms you’ll hear and what they mean.
• Volts: This is a familiar term. Your automobile battery is 12 volts and your house outlets are generally 120 volts. You’ve used 9-volt batteries in smoke detectors. Think of voltage as water pressure and if there is no pressure, there will be no water. Without voltage, our systems wouldn’t have the power to fly. Volts is abbreviated as “V.”
• Current: This is expressed in “amps” (amperes), and is what makes things go. This is the electron movement through the wire and might be compared to water moving through a pipe. Amps is abbreviated as “A,” but in electrical equations you’ll see it as “I.” That confuses many newcomers and veterans as well.
• Power: This is expressed in “watts.” Watts and horsepower are different units for measuring power (1 hp=746 watts). Electric fliers use watts because input watts are easy to measure with a wattmeter. We abbreviate watts as “W,” but in electrical equations, the symbol is “P.” The important thing to remember here is that we determine power in our models by multiplying volts by amps and get the total watts our systems produce (P=IV).
Formula Foolishness
As you’ve seen in the previous definitions, different letters are used to reference the same thing depending on who is doing the writing. Technically, the P=IV formula is the “correct” way of doing things and will keep the electrical engineers from yelling at you.
In reality, we often see the same thing written as W=VA, meaning watts equals volts times amps. This is the layman’s way of writing it and is often seen on websites for hobbyists.
Don’t get hung up on it and don’t be offended if someone corrects you. Just be sure you understand what it means. Make it simple—it’s a hobby!
More Handy Things to Know
• Resistance: This is expressed in ohms and abbreviated as the Greek letter “Ω,” but naturally, in our equations we use “R.” Think of resistance as a crimp in a hose causing a restriction. Low resistance is always our goal, to make sure we get the most out of our systems. Every time we add a connector or switch, we add something that increases resistance.
• Gauge: This refers to the size of the wires in the system. It’s measured in American Wire Gauge (AWG) and the larger the diameter, the lower the AWG number. In other words, a 10 AWG wire is a much fatter wire than the 22 AWG we see on servos. Larger wire means lower resistance.
• Kv: This is known as a “motor constant,” specifically “voltage constant.” It indicates how fast the motor would turn at a given voltage if there were no internal resistance. It’s expressed as rpm/volt. When you see motors listing a Kv of 500, they mean for every volt applied to the motor without a load (no propeller), the motor will turn 500 revolutions per minute.
Disclaimer: Some “experts” will tell you this is all you need to know to figure out what motor to use in an application. Wrong! It’s an important item, but a future column will explain why it’s not the “be all and end all” for determining the appropriate motor.
• ESC: This is the electronic speed control that connects the battery and the motor. It is also connected to the receiver throttle channel and controls the motor. They are generally rated by number of cells and current that the system will handle.
Power Loading
When we power electric airplanes, we target how many watts per pound are being produced to achieve a particular performance level. If our airplane weighs 5 pounds and we have 500 watts of input power, we have 100 watts per pound (500W/5 pounds=100W/pound).
Check out the chart on the Common Sense RC website and you can see a good breakdown of that. I will also cover this more extensively in a future column.
Weak Signals Toledo Show
If you’ve never been to this event, put it on your calendar for April 10-12, 2015! It’s a great place to learn a lot about electric flight and talk with many of the manufacturers and vendors who design and market these products.
It can be an immersion course in all things electric, if you wish, or it can just be a very relaxing weekend among modelers who share a common interest. I hope to see you there.
Wrapping It Up
There is a much more to cover when teaching the various aspects of electric-powered flight, but my goal is to make it easy. Don’t overthink it and don’t believe you have to be an electrical engineer to be successful at it. There are plug-n-play systems that can get you started with hardly any thought at all, but will ensure your early success.
Grow in knowledge at your own pace and enjoy the journey. After all, we do this because it’s fun!
