Electric Powered Flight – Part 2: Sizing the power system

By Scott Rhoades and Mike Wizynajtys –

Welcome to the second article in the series of de-mystifying electric powered flight for the glow pilot. In part 1 we covered Volts, Amps and C rating. If you haven’t read that article yet you can find it here in the “Techinical stuff” section.

In this segment we are going to talk about the area of electric flight that typically trips up those of us accustom to using glow engines and that would be sizing the power system to the plane. There is one simple phrase that will make understanding electric flight very easy… Forget everything you know about glow engines!

Most guys want to mistakenly think of electric motors as having a direct equivalency to glow. To say electric motor X will work in a Y size glow plane would be misleading. Even though some motor manufactures have started advertising such numbers, other factors beyond the motor must be considered. So such a numbering system is at best, very rough! Once you read this article hopefully you will understand why.

Before getting into sizing the power system we need to introduce Watts to the discussion. First a little refresher… Remember the analogy of comparing electricity to water in the first article? Volts (V) = pressure, while Amps (A) = flow. Volts is like pounds per square inch (psi.), however it says nothing about how much water is flowing, only how hard it is being pushed. While amps, on the other hand, is flow; similar to gallons per minute of water going through the pipes, regardless of the pressure. Does all that sound familiar?

Watts is simply Amps times Volts (A x V = W). A single Watt is a unit of electrical power. The number of watts tells you how much electric power is being used. To explain watts, we’ll continue with the water analogy. Picture a waterwheel like the ones that were used to turn grinding stones in watermills. For our example the waterwheel is the motor and the grinding stone is the prop. If we take a hose and direct it onto the waterwheel to get it to turn, the power generated by the waterwheel can be increased two ways. The first way is by boosting the pressure (V) of the water coming out of the hose, as it will hit the waterwheel with greater force. Second way is to increase the flow rate (A). The waterwheel turns faster because of the weight of the extra water hitting it. It’s this combination of the two that equal’s watts (power).

It can also be said that watts serve the same purpose as the horsepower rating of your car’s engine. In fact 746 watts = 1 horsepower. As we will see, the size and efficiency of a motor and the load imposed on it by the propeller affects the Volts and Amps. Keep this in mind, Amps don’t fly a plane and neither do volts. Watts are the measure of electric power that fly’s a plane. You can have a high voltage and low current (amps) and generate as many watts as low voltage and a high current.

The Battery

A battery is more than a fuel source, it’s the fuel tank, fuel pump and a supercharger all rolled into one. It feeds/pushes energy to the motor. So we must look at the battery and the motor as one complete unit when sizing power systems for electric planes. What we’re about to tell you may seem completely backward… In many cases the size of battery is often determined first. The motor can’t deliver the power to the prop if the battery can’t deliver the power to the motor.

Matching up the pieces

The idea is to choose a motor, battery and propeller combination that will fly the model in a desired manner and within the specifications of the components. Ideally we want the motor to be operating near its peak efficiency. Further away from peak that a motor operates, the more a battery’s power will be turned into heat instead of performance.

Just like glow, we want a power plant suitable for the type performance we wish to have. Obviously a trainer plane will need considerably less power than a 3D model of the same weight. So to help figure out what is needed, below is a table providing performance in watts needed per pound of airplane, which includes motor and battery weight.

70-90 watts per lb. Trainer and slow flying aerobatic models.

90-110 watts per lb. Sport aerobatic and fast flying scale models.

110-130 watts per lb. Advanced aerobatic and high speed models

130-150 watts per lb. lightly loaded 3D models and ducted fans.

150-200+ watts per lb. Unlimited performance 3D models.

Note: The general rule of thumb above was first conceived by Mr. Electric himself, Keith Shaw.

As an example

Using the above formulas let’s calculate how many watts will be needed to fly a 5 pound plane. For this example we’ll assume our plane is a slow flyer designed for medium aerobatics. According to the chart we need 70 watts per pound. Doing the math we need 350 watts to fly this 5 pound plane. (70 x 5 = 350)

If we wish to bump the performance, we could figure 100 watts per pound for our 5 pound plane, now we’re looking at 500 watts (5 x 100 = 500). (Note: increasing watts may lead to the need of a bigger motor and battery thus increasing the overall weight)

Sizing the Battery

Deeming 100 watts per pound to be suitable, let’s figure out what our amp draw will be. First we need to determine how many cells the LiPo pack will be flying our plane? For our example we can safely assume a 2S pack is too small so we’re looking at either a 3S or 4S pack. Let’s run the numbers on what a 3S will give us.

Since each cell of a LiPo is 3.7 volts a 3S pack will be 11.1 volts. (3 x 3.7 = 11.1) Taking our target 500 watts and dividing it by 11.1 volts tells us our target amp draw will be 45 amps (500 / 11.1 = 45).

Using a battery that will supply 45 amps or slightly above will fly our plane and probably do so very well. With the numbers we have so far, we can do more calculations to determine what size mAh pack we will need to provide a desired duration.

Even though it’s unlikely a plane will be flown at Wide Open Throttle (WOT) for an entire flight, we use the duration (how long it takes to drain a full battery at WOT) as our starting point for getting the right battery capacity.

For a good balance between battery size and function, many modelers calculate for 4 minutes of WOT. Obviously this totally depends on the type of flying you intend to do. If you’re a guy that just likes to putt around the sky at half throttle, you may want to calculate for fewer minutes of WOT. With this you very likely can get away with less expensive and lighter weight batteries by going to a lower mAh and/or lower C rated batteries.

Here’s how duration calculations work. Take 60 (minutes) divided by the target amp draw. (For our example that is 45) Now divide the sum by the minutes of WOT looking to achieve. (We’ll use 4 minutes) The result gives us a total of 3 amp/hours.
(4 (60 /45) = 3)

The 3 amp/hour number is based on total capacity of the battery. Since we don’t want to completely drain battery, we’ll take those 3 amp/hours and add in a safety factor so during our 4 minutes of flight we will use only 80% of the total battery capacity. To do this we’ll use a multiplier of 1.25.

The final calculation will look like this:3 amp/hours, multiplied by 1.25 equals 3.75 amps or 3,750 mAh (3 x 1.25 = 3.75)
(3.75 x 1000 = 3,750)

There you have it; if the motor/prop combo does actually draw 45 amps at WOT. A 3,750 mAh battery pack is needed to get 4 minutes of WOT within the 80% safety factor.

Using a 3,750 mAh pack with a 30 C rating shows that we can safely draw 112.5 amps from this pack (3750 / 1000 x 30 C = 112.5). There should be no battery overheating issues here and the battery will live a long and happy life in that job. Now it needs to be noted that just because a battery can supply over 100 amps doesn’t mean you should be using a motor/prop combo that will draw that much amperage because that is just crazy.

Maybe a 3,750 mAh battery is a little heavier or physically bigger than we want and a little investigating show’s that a 3,200 30C pack is just right from a physical stand point. If we can accept a little less WOT time, then that’s probably the right battery to pick.

A more feasible option for this plane just might be going up to a 4 cell pack (14.8v) which will lower the amperage. (500 /14.8 = 33 amps) As opposed to the previous 45 amps.

Same 500 watts but lower current which equals longer flights or you can buy smaller capacity battery and have the same length flights but in either case the equipment will run cooler at this lower current. You can see how there are a lot of options here with no single solution.

Motors

Once our battery needs have been calculated we can now think about a motor that will work in the parameters we’ve established i.e. amperage and battery cell count.

Various motors are available however outrunners have become the most common in recent years due to their low cost and simplicity. Geared inrunner motors still rule the day at the top levels of competition but for simplicity we’ll keep the discussion to just
outrunners.

What is an outrunner? It’s a motor constructed with the copper windings on the inside. The shaft is attached to a “bell”, or casing that contains the magnets, which spin around the copper windings. Because the extra weight of the bell and magnets are further out from the shaft it acts like a flywheel. Generally outrunners produce lower RPM and higher torque than other styles which enables them to spin a larger prop without the need of a gearbox.

What is Kv?

Kv is simply the revolutions per minute (rpm) that an electric motor will spin per volt, when under no load. What does the Kv tell us? Well it is related to the power output from a motor, or more usefully the torque level of a motor. This is all determined by the number of winds on the armature and the strength of the magnets. There are so many variables with electric motors and Kv allows us to get a handle on the torque we can expect from a particular motor.

A low Kv motor has more winds of thinner wire – it will carry more volts at less Amps, thus producing higher torque and able to swing a bigger prop.

A high Kv motor has fewer winds using a thicker wire and will carry more amps at fewer volts, thus spinning a smaller prop at high revolutions.

You would not use a low Kv motor in a ducted fan because you need high rpm with the small impeller. You would not use a high Kv motor in a 3D performer because you want to swing a large prop more slowly and obtain a high output of torque.

Prop selection

Prop selection is the third and equally important piece of the power system sizing puzzle. The propeller is the component that puts a load on the power system. The larger the diameter or the steeper pitch of the prop, the more energy or more watts will be required to turn it. Therefore we need to balance the diameter and pitch with the power or wattage of the motor/battery system.

Knowledge of how prop pitch and diameter affect performance is necessary to understanding how a power system is sized, however since this article is directed to the experience glow modeler looking to get into electrics, we will skip those explanations. If you want more info on prop dynamics, here is a good link to start with.
http://www.airfieldmodels.com/information_source/model_aircraft_engines/propellers.htm
To explain how a prop affects the motor and battery let’s revisit the waterwheel example. Remember we said earlier that the grinding stone is equivalent to the prop? Consider what would happen if the stone was too big in relationship to the wheel or it was overloaded with grain to grind? A much greater volume or force of water would be required to turn the wheel and soon the extra force/volume will be too great thus damaging the whole mill. If you want a bigger grinding stone or want to grind more grain, you need a bigger waterwheel.

Fortunately motor manufacturers often publish suggested propellers to use with a given motor/battery combination, therefore making it easy for a modeler to know which prop to use. In the beginning, stay within these manufacturer guidelines. Generally figure your electric power system is going to be swinging a prop, two to three sizes larger than a glow power system would for the same sized plane. Future articles will discuss experimenting with different props.

The wrap up

Keep in mind all motors are made to work within a range of watts and volts. A motor might be fine taking 45 amps at 11.1 volts (500 watts). However going beyond its range and ramming higher voltage down its throat or forcing it to draw more amps than it is rated, will burn it out. The goal is a balanced power system.

As you may have surmised from this article, changing one aspect of the motor/battery/prop configuration will likely trigger a chain reaction requiring a change of the other pieces too. This can easily result in a lot of calculating and recalculating. Fortunately computer programs are available that make all of this combination calculating quick and easy. Once such program is MotoCalc. http://www.motocalc.com

You may be wondering if fancy computer programs exist to figure all this stuff out why even know this stuff? If you can wrap your brain around the calculations above and take a few minutes to work through a couple examples yourself, you will have a much clearer view of the entire electric flight picture. Playing with a computer program and switching the different variables then watching what it does to the final outcome can be a valuable teacher too.

In this article we’ve provided the basic knowledge of how electric power systems are sized. Of course there is more to know and time and experience will teach you plenty, but with this basic understanding we’re hoping electric flight has become less of a mystery for many.

Of course there is always the easy way of matching a suitable power system to a particular plane. Search R/C message boards for a plane that is similar to yours in size and performance and simply copy the set up.

Quoted Sources:

Peter Pine, The Australian Electric Flight Handbook: flyelectric.com

Chris Findlay, Choosing an electric power system for your model: wattflyer.com

Ed Anderson, Everything you wanted to know about electric powered flight: wattflyer.com

HowStuffWorks.com. What are amps, watts, volts and ohms?

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