Beginning Scratchbuilding and Flying

Understanding Power Systems

Understanding how your plane produces thrust - and whether that thrust will be sufficient for your plane to fly - is an essential aspect of scratchbuilding. To begin, let's look at how a motor generates power from a battery.

When an electric current is applied to a standard brushed "can" motor (these account for the vast majority of electric motors in the world today), the motor spins in one direction or the other, depending on which way the motor leads are connected to the positive + (red) and negative - (black) terminals of a given battery. If more volts that are applied to a motor, the faster it spins (up to a point anyway). We understand voltage as the "V" designation on batteries, and is typically around 1.5 volts per cell for many NiCd or NiMh batteris (or your standard alkaline batteries), and 3.7 volts per cell for Lithium Polymer batteries, which are used more and more for R/C electric planes.

With no load on the motor (for instance, without a propellor attached), the motor draws very little current, or amperage, designated as "A" in electrical terminology. When a greater load is placed on the motor - such as adding a propellor - the motor can do more work, or create more kinetic energy. As it does so though, it will also wear down the batteries faster, as the motor is now using more current. The current that the motor is using is referred to as amps - or in the case of many small airplane electric motors, milli-amps, or "mA."

A related, but slightly different point to know as modellers, is how long a given battery pack will last with a given motor, as it gives us an idea of how soon to land to avoid crashing. This is called the batteries capacity, and battery packs are rated in milli-amp hours (or mAh) to designate the capacity of the battery pack at a standard amp draw of 1 amp. Milli-amp hours is different than amps, though. Amps shows how much a device draws in power; milli-amp hours shows how much electricity a battery can effectively "give" at 1 amp. We can then determine how long a pack will last at higher or lower amperage draws.

As a baseline, a 1000 mAh battery will last 1 hours (or 60 minutes) at a 1 amp current draw. But the greater the current draw, the less amount of time the battery will provide electrical current. At 2 amps, that same 1000 mAh battery will last 0.5 hours (or 30 minutes). At 4 amps, it will last 15 minutes, and so on. Let's look at how to determine the length of time some different battery packs should approximately last for a given motor amperage draw.

Let's try an example, but first changing parameters slightly, and assume a 750 mAh battery pack connected to a motor drawing 2 amps. To start, be sure to convert milliamp hours into amp hours, so that the common denominator (amps) is the same. (We can't divide milli-amp hours into amps, only milli-amp hours into milli-amps! Remember 7th grade fractions? I know, I know, it's a pain, but necessary.) Just remember that 1000 milliamps equals 1 amp (milli is a thousandth of an amp). So here's how to convert the milli-amp hours to amp hours:

Now that the Amp hours is know, divide that by the amps that the motor draws:

So, at a 2 amp draw, a 750 mAh (or 0.75 Ah) battery will last about 22.5 minutes. Piece of cake, right? How about a harder one. What about a 1500 mAh battery that is being used on a motor drawing 12 amps?

First, convert milliamp hours into amp hours:

Then, perform the calculation of amp hours divided by amps:

That's a fairly typical example, but keep in mind that most planes rarely run at wide-open throttle for the entire flight too, so the calculation will generally provide the minimum amount of flying time. Furthermore, most battery packs don't tolerate being completely run down, so it's best to assume a time that is 75-80% of the calculated flying time at wide open throttle. "But wait," you ask, "how will I know the amps my motor is using with a given propellor? And how will I know if my battery can supply the necessary amperage?"

Excellent questions! Some motor manufacturers will kindly provide some guidance as to what propellors should be used on their motors. And a few will go even farther and list data to show the maximum amp draw of a given motor at various voltages. A good example is at the GWS products section, where motor data is for their 350 sized brushed motors as well as other motor sizes. Note how they provide the propellor size (a 6030 prop would be a six inch diameter prop with a pitch of 3), volts, amps, thrust and power in watts.

"Thrust in ounces or grams and power in watts? What's that?" you ask. Thrust measures the force (in ounces and/or grams) that a motor/propellor combination will produce at a given voltage. Think of it as the maximum force that the motor/prop will "pull" straight up. Fortunately, R/C planes usually don't need thrust that equals the weight of the plane, since the wings produce lift with sufficient airspeed. Static thrust is measured on a thrust stand, which has the motor on an arm, and another 90 degree arm on a scale. When the current is applied to the motor, the arm pushes down on the scale, and details how much force the motor produces with that propellor. Actual thrust varies slightly on a moving airplane, since the motor is moving through the air instead of sucking air towards it.

Watts is the energy produced by the battery and motor as work, instead of force terms. Calculating the watts produced by a given motor / propellor / battery combination is done with a simple formula:

Watts is a useful comparison tool for determining what motor to use on a given plane, if we set it up on a thrust stand, and measure the current that the motor draws using a "Wattmeter." By placing a Wattmeter between the battery and the motor, we can see with digital accuracy how many volts the battery is producing, the amps the motor is drawing from the battery, and the watts produced by the combination of battery / motor / propellor. So let's take our second example of a 1500 mAh battery and assume it is a 7.2 volt battery pack. Our motor is still drawing 12 amps of current. We can calculate watts like this:

How is knowing the watts produced helpful to the scratchbuilder? Read on!