Author: Greg Gimlick
Edition: Model Aviation - 2014/02
Page Numbers: 29,30,31,32
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A BEGINNER'S GUIDE TO ELECTRICS

DEMYSTIFYING THE TERMS AND TECHNOLOGY USED IN ELECTRIC FLIGHT

by Greg Gimlick

Ienc = ∮ H · dl = ∫ (∇ × H) · dS

It's that simple! All you need to do is be able to rewrite Ampere's Law using Stokes' Theorem and you can get right into electric flight without any problem at all! Get online, grab your credit card, and enjoy the hobby!

That's the way it feels to many people who are new to electrics, and those of us who have been around the technology for a while are partly to blame. In our excitement and zeal for electrics, we tend to dump a ton of minutiae on newcomers to the point of running them off.

There is no way I can teach you all there is to know about electric flight in the next couple of pages, but I can get you started with a little effort and no fear.

Terminology

Don't get bogged down and overwhelmed by the new vocabulary and terms. Here is a basic list of the most important terms and what they mean—in non-engineering terms.

  • Volts: This is a term with which most are familiar. Your automobile battery is 12 volts and your house wall sockets are generally 120 volts. You've used 9-volt batteries in smoke detectors. Think of voltage as water pressure: if there is no pressure, there will be no water. Without voltage, our systems wouldn't have the power to fly. This is abbreviated as "V."
  • Current: Expressed in amps, current is what makes things go. It's the electron movement through the wire and might be compared to water moving through a pipe. It is measured in amperes (amps) and abbreviated as "A." In electrical equations you'll often see it as "I."
  • Power: Expressed in watts, power is what we refer to often. Watts and horsepower are different units for measuring the same thing (1 hp = 746 watts). Electric fliers use watts because input watts are easy to measure with a wattmeter. Wet fliers use horsepower because internal-combustion engine output power is measured with a dynamometer. If you compare input watts with output horsepower, you can determine motor efficiency. We abbreviate watts as "W," but in electrical equations the symbol is "P." Power is calculated by multiplying volts by amps (P = V × I).
  • Resistance: Expressed in ohms and abbreviated with the Greek letter "Ω," but in equations we use "R." Think of resistance as a crimp in a hose causing a restriction. Low resistance is always our goal to get the most out of our systems.
  • Gauge: Refers to wire size, measured in American Wire Gauge (AWG). The bigger the conductor diameter, the lower the AWG number. A 10 AWG wire is thicker than the 22 AWG common on servos. Bigger wire means lower resistance.
  • Power loading: When discussing the power in our electric airplanes, we refer to watts per pound. If an airplane weighs 5 pounds and has 500 watts of input power, it has 100 watts per pound (500 watts / 5 pounds = 100 watts/pound).
  • Kv: A motor constant, specifically the voltage constant, indicating how quickly the motor would turn at a given voltage with no load (no propeller). It's expressed as rpm/volt. A motor listed as Kv 500 will turn 500 revolutions per minute for every volt applied with no load.
  • Efficiency: Nothing is 100% efficient; we aim to reduce losses such as resistance. Efficiency is the ratio of output power to input power. For practical purposes in modeling, basic power requirements are determined using input power.
  • ESC: Electronic Speed Control that connects the battery and motor and interfaces with the receiver throttle channel. It is generally rated by the number of cells and the current the system can handle.

Battery Terminology

If you are using LiPo batteries, which have become the standard in electric flight:

  • 3S, 4S, etc.: Battery packs are made up of a number of cells in series. A 3S pack has three cells in series, each with a nominal voltage of 3.7 volts, giving a total nominal voltage of 11.1 volts. A 4S pack is 14.8 volts (4 × 3.7 V = 14.8 V).
  • Pack capacity: The capacity is given in milliampere-hours (mAh) or ampere-hours (Ah). A typical 3S pack might be listed as 2,200 mAh or 2.2 Ah. Large packs might be 5,000 mAh, etc.
  • Discharge rating: "C" represents the capacity of the LiPo pack. Labels typically show the discharge rating (for example, 25C, 30C). A 2,200 mAh pack rated at 30C could theoretically be discharged at 66 amps (30 × 2.2 = 66) without damage. Discharge ratings are guidelines; manufacturers' claims can be optimistic. Packs with higher discharge ratings generally have lower internal resistance.
  • Charging: Buy a balancing charger to ensure each cell in the pack matches the others. Get a charger that will handle the maximum number of cells you expect to charge and can charge at the rated pack capacity. A 2,200 mAh pack would typically be charged at 2.2 amps; a 5,000 mAh pack at 5 amps.

I Thought It Was Simple!

It is! Electric power can be as complicated as you want to make it, and some pilots like to get into the details. But it can also be simple. You’ll see and hear the terms listed often and it’s important to have a basic understanding of them, but a degree in electrical engineering is unnecessary. You will appreciate knowing the terms if you call for technical support and the manufacturers begin asking questions.

If you want to try an electric airplane or helicopter and learn as you go, call a reputable hobby shop or online vendor and select a system that has been fine-tuned to work. There are plenty of glow-powered Plug-N-Play setups and most work well.

If you have an airplane that is powered by a glow engine, you can convert it to electric power without having to earn a degree. Many current kits on the market have already been converted by some manufacturers. Tell the vendor what airplane you have and get the system they suggest. Knowledgeable vendors can make recommendations if you give them basic information about your conversion.

Figuring It Out Yourself

Most of us eventually want to understand our systems and how to figure out a particular one for a project. This section shows how easy it can be. The bar graph on the next page (in the original article) is a basic listing of requirements for various types of performance using power loading.

These guidelines make it easy to select the right motor, battery, and propeller combination for any project.

Motor Sizing Voodoo

If you've used glow- or gas-powered engines, or if you are new to RC, the motor-sizing nomenclature can be a nightmare. It's also a nightmare for many experienced aeromodellers, so don't despair. It is getting better, but there is no standard for naming motors in regard to capability.

In the glow-fuel world you may be accustomed to .40 or 1.20, etc.; in the gas world, 50cc, etc. Some manufacturers try to name their motors with similar numbers to the engines they replace. This is unpredictable, but most come close.

Another common naming method is to use the dimension of the outer case of the motor or the stator dimensions. This is a system I like, but I wish companies would agree to use either case or stator size as a standard and not mix them.

Motor specs tell us that X motor on Y volts will turn Z rpm. The motor must be rated for the proposed power, and size matters—take two 1,500 Kv motors: the bigger motor (length times diameter) will comfortably turn a larger propeller, assuming both are of similar quality and efficiency. Some helpful vendors list motors with both dimensions.

In a perfect world, all motors would be listed as Innov8tive Designs lists its Cobra motors. Here is an example of a Cobra C-4120/12 motor:

Cobra C-4120/12 Motor Specifications

  • Stator diameter: 41.0 mm (1.614 inches)
  • Stator thickness: 20.0 mm (0.787 inches)
  • Number of stator arms: 12
  • Number of magnet poles: 14
  • Motor wind: 12 Turn Delta
  • Motor Kv: 850 rpm/volt
  • No-load current (Io): 2.77 amps @ 14 volts
  • Motor resistance (Rm) per phase: 0.021 ohms
  • Motor resistance (Rm) phase-to-phase: 0.014 ohms
  • Maximum continuous current: 75 amps
  • Maximum continuous power on 3S LiPo: 830 watts
  • Maximum continuous power on 4S LiPo: 1,110 watts
  • Maximum continuous power on 5S LiPo: 1,390 watts
  • Weight: 293 grams (10.34 ounces)
  • Outside diameter: 49.8 mm (1.961 inches)
  • Shaft diameter: 6.00 mm (0.236 inches)
  • Body length: 51.8 mm (2.039 inches)
  • Overall shaft length: 74.5 mm (2.933 inches)

This may appear to be more information than anyone needs, but it is perfect for someone wanting to know what the motor is and what it will do. The numbers in the name represent the dimensions of the stator and the motor. As a beginner, let the vendor help you with the winding if you want more information.

The motor (stator) size is 41 mm diameter and 20 mm long, and the Kv is 850 rpm/volt. Another manufacturer might list this motor as "5052-850" because it uses the outside diameter of the motor instead of the stator size and lists the Kv rather than the wind. That is why it's important to know if a manufacturer's numbers represent the outside dimensions or the stator.

Guidelines for projecting power systems for electric airplanes are courtesy of Common Sense RC.

Selecting Your Power System

This is why you read this far, isn't it? Let's use an example of a standard .40-size trainer-style airplane that weighs 5 1/4 pounds, uses a glow .40 engine, and spins an 11 × 6 propeller at approximately 11,000 rpm. Experienced glow pilots often look for a motor combination that will spin the same propeller at the same rpm and that works, but that's not always the best way to do it. Electric motors are often more efficient when spinning larger-diameter or deeper-pitch propellers or a combination.

I created an example using an old trainer we have in the club from Hangar 9 called the Easy Fly 40. I removed the .40 glow engine and replaced it with a Cobra 4020/12 motor. Using a 4S 4,000 mAh pack, this will spin an 11 × 6 propeller at 10,700 rpm, similar to the old glow engine. The airplane will fly much as it did with the old glow engine, but by tweaking the propeller, I was able to make it fly much better.

The old glow engine was listed as producing 1 hp, which is the equivalent of 746 watts. With loss of efficiency, it's likely approximately 3/4 hp or roughly 560 watts, which works out to 106 watts per pound.

The Cobra motor, with the same propeller, produces 488 watts or 93 watts per pound—barely a noticeable difference—and it flew well. The beauty of the electric was being able to experiment with propellers and come up with a 12 × 8 that produced 659 watts or 126 watts per pound and more thrust from the larger propeller. Thrust increased from 72 to 92 ounces, and that's a huge jump in performance! Pitch speed also increased.

Keep It Simple

Keeping it simple is always the best way to go. In the previous example, I got into some efficiency factors, but I was working on the computer and letting it figure those. As a beginner, you can work with only the input numbers and still be satisfied with the results. Multiply your voltage (14.8 volts for our 4S example) by the projected amps.

If the propeller will draw 56 amps (data available from many manufacturers' sites), you'll come up with 828.8 watts (14.8 V × 56 A) or 158 watts per pound for the 5 1/4-pound airplane. It's higher than my computer-generated projection because the computer factored in an 80% efficiency estimate, which is closer to reality.

Tools

I primarily use two computer programs when I'm working up a new system that hasn't been done before. ElectriCalc and MotoCalc will allow you to get as deep into the planning phase as you wish.

These programs do all of the math and have huge databases of motors, batteries, propellers, etc. The help files alone are worth the purchase price and are constantly updated. Many vendors and manufacturers also have online drive calculators.

The Bottom Line

Select your airplane, decide what performance you want, and choose a motor system. Don't like math? No problem! If your airplane is a 700-square-inch sport aircraft that will weigh 6 pounds and you want to do great aerobatics, try to find a system that the vendor says will produce 900 watts. That's 150 watts per pound and will make you happy.

Try not to select a system that requires you to constantly run it at its maximum capacity, and you'll have a system that will last for years.

—Greg Gimlick [email protected]

SOURCES

Transcribed from original scans by AI. Minor OCR errors may remain.