Which electric power system would be best for our six foot (2 m) wing span Piper Cub? As before, not as hard as it looks.
In this series I use a six foot (2 m) scale model of a Piper Cub as an example for performing various simple calculations useful for model airplanes. This is the second article in the series. Here’s a link to the other articles:
Where We Left Off
In a previous article, we figured that our Piper Cub model needed to weigh about 4.8 pounds (2.2 kg) to match the flying behavior of the full size airplane. Based on that, we estimated that the motor needed to weigh about eight ounces (227 g), but a smaller one would probably be fine, too.
Since it’s a Piper Cub scale model, we are looking for an easy flying model that resembles a trainer more than anything else. As an educated guess, let’s say that this model only needs about two-thirds of the power of a typical sport model. That brings down our motor weight to about five and a half ounces (150 g).
We all have our favorite brands of outrunner motors. That’s fine. If you find a motor at a level of quality that you like and a price you can afford, you might as well stick with it. I’m not here to convince you to do otherwise.
For the sake of argument, I’m going to assume that the brand of motor that we like is Model Motors Axi. These are premium motors with matching premium prices. You may prefer a different brand at a lower price point. Again, I’m just using the brand as an example.
Looking at the Axi motor lineup, the 2820 motors weigh 151 grams. Bingo! That’s right on the money and those are the motors that we need to use. What if our target motor weight had been in between the weights of two similar actual motors? Then it becomes a judgment call, but I would bet that either choice would work fine.
The voltage constant of a motor (Kv) works just like the gearing of a bicycle. Do you want to turn a big propeller slowly so that we can fly slowly (low Kv) or a small propeller fast so that we can fly fast (high Kv)? In the case of our Piper Cub, a slow flying model, we want to pick a motor with a low Kv value.
Looking at the Axi 2820 motors, the Kv values range from 860 to 1500. Let’s go with 860.
If you are putting together a fast flying model, like a pylon racer, be careful in choosing your Kv value. This is not the case with Axi, but with some brands the motors with the highest Kv values are intended for ducted fan applications. These motors would not be a good choice for our pylon racer. For example, Axi could have had motors with Kv values from 860 to 1500, and then a motor with a Kv of 3000. The 3000 motor would have been for ducted fans.
My power system rule says to make the battery about 15% of the total airplane weight. My power rule assumes a six minute target flight time, which is how long most of us fly. But because of the reduced power needs in our Piper Cub, 15% gives us enough power for a nine minute flight. That’s fine.
15% of the flying weight comes out to about 11.5 ounces (330 g). A battery pack with three cells in series is standard in this airplane size range, so we will go with that. A quick search online shows that a 3S LiPo battery pack with 4000 mAh cells meets our target weight.
Let’s do a quick calculation to see how much power our model needs to fly. My power rule assumes that a sport model needs about 75 watts of power per pound of flying weight. We cut that back to 50 watts per pound for our Piper Cub. That means we need 240 watts (4.8*50). With a three cell battery pack, the output voltage is about 11.1, which means that we need about 22 amps of current (240/11.1).
As a safety check, let’s look at the maximum power that our motor can produce. Outrunner motors can produce about 75 watts per ounce. That is a maximum that normally we do not want to exceed and preferably, even approach. Our Axi is therefore good for up to 400 watts (5.5*75). With our battery pack, that works out to a maximum of 36 amps (400/11.1).
Battery C Rating
Let’s make sure our battery can handle the maximum current that we might demand from it. The C rating of a battery is just the multiple of its capacity that it can handle in current flow. If our battery pack were to have a C rating of 10 (which would be very low), then it could be discharged at a 4*10=40 amp rate. Since the maximum current that our motor can handle is 36 amps, any battery with a reasonable C rating will meet our needs.
Electronic Speed Control (ESC)
Picking a good ESC for our power system is easy. The main challenge is knowing what’s the maximum current that we will need. We already know that: 36 amps. Therefore a 40 amp speed control will work.
We are on the home stretch! All that’s left is to select the propeller for our Piper Cub. Then we can go fly!
I have good news and bad news here. The bad news is that I don’t have an easy formula for estimating which propeller you need. There are just too many variables that need to be taken into account.
The good news is that propellers are inexpensive, and coming up with a first guess as to what is the right propeller is not hard to do. If the manufacturer doesn’t give you any hints, just look for another motor with about the same size and Kv value. Then buy some different propellers with slightly different diameter and pitch values. Fly them all, and see which one you like better.
In our case, Model Motors recommends a 12×8 propeller when this motor is used in a trainer. I would also buy at least 12×6 and 13×8 propellers for the flight testing.
Before pulling the trigger and spending money to buy all this stuff, let’s step back for a second and do a sanity check. We can cheat a little here since Model Motors gives us a list of recommended configurations for this Axi motor. They say that it can fly a trainer weighing 2.2 kg with a 3S battery pack and a 40 amp ESC. Check, check, check. Okay. Let’s go fly!