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Selecting an Electric Power System for your Airplane

by Albert Tejera


Most of us once we have experienced the insane power and hassle free cleanliness of electric power we are convinced that this is the way to go. Ideas start popping into our heads about converting our favorite kit to electric or scratch building a foam airplane. Fact is, selecting the proper components will save you money, provide longer life and provide more enjoyment for your hard earned dollar.

It seems that about 75% of power system failures are caused by exceeding specifications rather than manufacturing error or defect. It is not surprising since it is new to many of us and the industry makes it confusing to chose the right motor, prop and battery combination. Motor specifications are sometimes incomplete.

If you are buying an electric airplane kit or heli then often there is a recommended power package. This is great when you are getting started and don't mind letting the kit manufacturer make those choices for you. Usually these recommended systems have been tested to make sure they work well. But what happens if you like doing your own thing, upgrading an old kit, or there is another motor brand on sale, or you already have the batteries so you don't need the power bundle. Bottom line is, what if YOU "want to" or "have to" select the Electric Power System, what then?...


The purpose here is to lay out the principles for selecting or upgrading a motor in a step by step fashion.

Step 1 - Select your Motor - Power is the Key

Those of us that have been in the hobby awhile instinctively want to have glow engine equivalent. I don't necessarily think this is always the best way to go because glow engines vary a lot in power from brand to brand and model to model for the same size displacement. To make it harder, the Max hp rating of engines are at unusable RPM's so what do we do then?

Well one "rule of thumb" (approximation) is that an engine produces about 3HP per cu inch. This estimate is for an average sport motor. Some high performance engines produce more power - and some less. But electric motors are not rated in horse power but rather in watts. Well it conveniently turns out that 1 h.p. = 746 watts. But this is still a little cumbersome to multiply cubic inch times 3 times 746 watts.

Another quick rule of thumb is to simply multiply engine size (without the decimal point) by 20 to get watts. This is not an exact science, but it gets you in the ballpark in a hurry. For example a 40 size engine would be 40X20 or about 800 watts. A 60 size engine will be 60X20 or about 1200 watts.

The above rule of thumb works if you have some idea of the size glow engine you need, but what if you are scratch building or you have an electric kit that came with a motor and you do not like the performance you are getting? A better way to figure out how much power you need is to know your desired weight and the type of flying you intend to do and use the rule of thumb table below:

Powered Glider / Scale - 50 watts/pound
Trainer - Slow Flyer - 70-90 watts/pound
Fast Warbird / Sport Aerobatic - 90-110 Watts/pound
Advanced Aerobatic - High Speed 110-130 Watts/pound
Light loaded 3D / Racer - 130-150 watts/pound
EDF Jet / Unlimited 3D - 150-200 watts/pound

So using the table above, say you want to find a motor for an Advanced Aerobatic Airplane whose flying weight was 10 lbs. The table says 110 to 130 watts per pound so you would multiply that by the 10 lbs to get 1100 to 1300 watt motor.

Now that you know how much power you need you still have to pick out your motor. Knowing how much power you need makes it so all you have to do is find a motor in that power class to get the ball rolling.

Out-runners/ In-runners

Okay the first choice you will run up against is choosing between an out-runner vs an in-runner. Out-runners have magnets on the outside - the outside can spins around - they turn slower and can swing larger props. In-runners have spinning magnets on the inside - the outside is stationary - they turn faster - and require gearing to turn large props. Today's out-runners can turn larger props without gears. This makes them quieter and more maintenance free. In-runners are frequently use for ducted fan applications because of their high RPM. Look at airplanes in a similar class and see what power systems they use to help you decide.

In electric motors the Mass (i.e the Weight) determines power class. More iron provides better cooling and allow higher power ratings. Of course wire size and magnets have a lot to do with it but the more it weights then the more power it can produce. One of the more important parameters in the motor specification is called the KV. It is simply how many RPM per volt the motor wants to turn at under no load. With a prop load your RPM will generally be around 90% of the unloaded RPM. In general a motor with a Low KV turns bigger props and prefers more voltage (and less amps) to produce its power. A motor with a high KV turns a smaller prop faster and may require less voltage (and more amps) to produce its power.

KV is primarily determined by the size and number of turns of wire. There are other motor parameters that I discuss on our podcast. If you want to learn more check out our podcast here.

Bottom line, within that power class look for a motor that will turn the size props you want for your style of flying and note the battery recommendations the motor manufacturer suggests to turn that prop. Battery weight will be a factor on performance just as much as motor power so balance your options. Remember more voltage means more cells which generally mean more weight.

If you you like to get a better understanding of electrical terms like watts, amps and volts, please check out our free eBook Basics of Electrics (without math) ( Everything you wanted to know about electrics but were afraid to ask!)

Step 2 - Select your prop

First start by looking at the selected motor specs see what props they recommend. If you like to fly fast and don't care about acceleration then go with motors that prefer the smaller diameter props. If you like 3D aerobatics or fly slow biplanes then go for the larger prop. See what kinds of props other kits like yours using that motor usually use.

When you think you have the perfect motor and prop then search the internet or the manufacturers specs for an estimate of how much current you will draw and how many cells you need for turning that prop on that motor. The end of this article has a bunch of online calculators that will help you estimate current and select a prop that will pull all the available power out of your motor without exceeding the max specifications of the motor.

Step 3 - Select your Batteries

Your motor and prop choice will dictate the battery voltage and max current you need. Your weight limits and flying style will limit your run time. So the goal is to get the most mah (capacity) you can within your weight budget.

Make sure that the C rating multiplied times the battery capacity yields a number that exceeds the max current for the prop you have selected. You will usually not be running at full throttle unless you are a racer. The mah divided by the max current is your minimum run time expressed in hours. Again the calculators at the end will help you estimate and make choices.

If you would like to learn rare and little known tips on the care and feeding of Lithium batteries download our free eBook "The TRUTH about Rechargeable Lithium Batteries". Just enter your email address on the left column to get this Free eBook.


Step 4 - Select Speed Controller

Picking a speed controller is the easy part now. Simply pick a speed controller with a high enough current rating to handle your max current rating of your motor. In this case as well as all the other cases, bigger is better until big gets too heavy. If you are a competitor you will watch the weight like a hawk and push your equipment to the limits. Otherwise buy a controller with a little overhead and enjoy reliable operation even from less expensive controllers.

Many speed controllers have a built in BEC (battery Eliminator Circuit) This circuit allows you to eliminate the need for an extra battery to power the receiver. It regulates down the power pack battery to provide 5 to 6 volts for your receiver. Make sure it can handle the size and number of servos you are using. On bigger airplanes you may want to or have to use a separate BEC. Usually the weight savings of the BEC is significant so it is always a good idea to have one.

The price of the speed controller is usually determined by the quality, reliability, features and warranty. Some feature you may never need unless you fly helicopters or control line electrics. Do your homework. A low resistance spec of the speed controller is key to running cool and efficient. You will usually pay more for a speed controller with a low resistance. If you support American engineered and assembled products like the Xtrema then you may want to look at Castle Creations speed controllers.

Step 5 - Test your System

Once you have acquired and assembled your power system together and have it mounted on your model, the first thing you will want to do is to test and verify that your system is working properly and within the limits of the components. The absolute best way to test your system is with a simple watt-meter (like the one built into our Xtrema charger. Even better is to have the Xtrema charger connected to a PC with the Xtremalog analyzer software. In this way you are free to run your system up and down while the software records and plots all the numbers for you for later analysis.

All you have to do is connect your watt-meter or analyzer system in between battery and speed controller. Run up your engine and make sure you do not exceed the max current specification, max speed controller current and power capacity of motor.

So if your motor says that 80 amps is the most sustained current you should draw then check to make sure that when you run the motor to full throttle you do not exceed that amount. If you do, you will overheat the limited component and cause it to fail. The battery has a maximum too. Remember that is when the capacity is multiplied by the C rating.

Occasionally you will here the terms burst or continuous being used to describe maximum. If you are squeezing every last drop of performance for a competitive application and you have a strongly disciplined sense of throttle management then you may push the battery specs to get the smallest size/weight possible. (generally heavier batteries handle heavier currents) Pushing the electronics is risky business so proceed with caution. Here the more expensive speed controllers will have current limiting and self protection circuitry to save the day and let you squeeze the last drop of power from a super light setup.

Using Computers to Help you Pick Power System

As I mentioned above, there are calculators that help you select prop and batteries for your motor and your performance goals. I have split some of the more popular ones into two groups. Those calculators that run online and those that run installed on your PC. One program in particular "Motocalc" Is the most amazing of all. It basically asks you a few questions about the airplane type , required flying time, wing type, altitude and weight and it will pick out everything from its large database of motors, batteries and props. If you already have some of the components it will work around to suggest complimentary components to complete your system. Do yourself a big favor and download the free 30 day trial to get a feel for what it does. You will be amazed. (No, I do not get a commission if you buy a license.)

Online Calculators

Computer Programs Scorpion Calc is FREE and very usefull! MotoCalc does everything for you except buying and assembling the components. 30 day FREE trial.

Let us know if any of the links have changed by contacting us at this link.