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Written by Greg Gimlick
Guidance on selecting your aircraft’s propulsion
As seen in the July 2020 issue of Model Aviation.

the authors seagull models bristol m1c from vq warbirds
01. The author’s Seagull Models Bristol M1C from VQ Warbirds had many power options.

he chose a cobra 5330 22 205 kv motor and castle
02. He chose a Cobra 5330/22 205 Kv motor and Castle Creations Edge HV-120 ESC for power.

Whether you fly glow, gas, or electric, most airplanes and kits come with recommended setups that are proven to work. But what happens if you don’t quite agree or want to use something you have on hand instead of buying a new engine or motor?

It’s not that the recommended system isn’t a good one, but it’s nice to know how to stray from that when you want. You might just want a more powerful power system than the stock system. You might be okay with the stock system performance but want more duration. Sometimes there are other considerations that come into play.

I will focus on electric power because that’s my forte. The beauty of electric power is that it’s so flexible. With the right combination of battery (source) and propeller (load), a single motor/ESC combination can power a wide range of aircraft.

My Project Example

To help get my points across, I will use a recent example that I did for a review. The airplane is one of my favorites, a World War I Bristol M.1C. This is a British monoplane that was ahead of its time and calls to me for some reason.

I’ve built one from a kit, but the most recent one is a Seagull Models M.1C 1/5-scale ARF from VQ Warbirds. It impressed me as soon as I opened the box with its incredibly well-done covering scheme. Everything about it screamed quality, but experience with my previous M.1C and other WW I airplanes told me that I would probably need considerable weight in the short nose to get it to balance well. That’s the nature of the beast with WW I aircraft.

The recommended power system for this ARF is either a 20cc gas engine or an electric system providing 2,000 watts using an 8S or 10S LiPo battery. My gas choice would be a DLE-20RA rear-exhaust engine with electronic ignition that would weigh roughly 2 pounds—all of which would be well ahead of the firewall. On the electric side of the house, a 2,000-watt motor and ESC would weigh slightly less than 1.5 pounds. Both setups would have the gas tank/battery right behind the firewall.

If you’re curious, 1 hp is roughly 750 watts, so the 20cc gas engine will put out 2.5 hp with a 17 × 6 propeller, which equals 1,875 watts. Another way of comparing gas engines to electric power is to use Lucien Miller’s (of Innov8tive Designs) rule of thumb of 1,500 watts per cubic-inch displacement. With that method, the 20cc engine (1.22 cubic inches) would equal 1,830 watts (1.22 × 1,500 = 1,830). You can see that both recommended systems generate similar power.

two venom 6s 5000 mah lipo battery packs
03. Two Venom 6S 5,000 mAh LiPo battery packs in series provide 12S power.

Why I Chose Something Different

There were two ways to come up with the 2,000 watts I needed for the airplane. I could increase voltage or increase amps to come to the projected watts required (watts = amps x volts). To get 2,000 watts with 8S LiPo battery packs, I would need to pull approximately 67.5 amps (2,000 watts ÷ by 29.6 volts = 67.57 amps).

I can also get those 2,000 watts by increasing voltage and decreasing the amps. The concept of using higher voltage was introduced to me 25 years ago by Tom Hunt, but it was difficult to get the voltage in the days of NiCd batteries without having significant weight penalties.

The concept was sound, but the timing was off. That’s different today with LiPo battery packs. I prefer high voltage over high current because voltage losses in any power system increase with the square of the current. In this case, let’s see how many amps are required if I use a 12S instead of an 8S battery pack.

Divide 2,000 watts by the 12S pack’s voltage of 44.4, which results in an amp draw of 45.05. That’s a pretty significant drop in amperage and the negative things that come with high current. On the other hand, it’s an increase of weight from the 8S pack (36 ounces) to the 12S pack (54 ounces). An 18-ounce increase isn’t something to sneeze at. So, how did I come to the decision to use 12S instead of 8S?

As I assembled the Bristol, I saw that it was going to give me the same weight and balance issues that my earlier version did. I knew I would have to add weight to the nose to attain the proper center of gravity. The question was how much. As I began to mount the power system and check the balance, I kept having to add more lead to the nose. When I got to a pound of weight, I didn’t feel good about it.

When I use large power systems, I always prefer voltage over amperage if I can go that route. Why add lead when I can use that necessary weight for batteries instead? I bolted up a 12S system, put it back on the balancer, and found that my pound of lead in the nose was now replaced with 18 ounces of extra battery weight. It’s slightly behind the firewall, so I actually ended up adding 4 ounces as far forward under the motor as possible.

In the end, my 12S system weighed merely 6 ounces more than my 8S setup with its extra lead for balance. The power required to make up for those 6 ounces is insignificant in an airplane of this size, but the current draw saw a significant drop, and that goes straight to duration! Both systems would give me ample power to fly the model nicely, so that wasn’t a significant factor for me, but duration was.

this is the authors computer projection for the 8s setup
04. This is the author’s computer projection for the 8S setup.

this is the projection for using 12s power
05. This is the projection for using 12S power.

Break Out the Calculator

Next up is determining the flight duration. With the current at full throttle projected to be 45 amps, I can assume cruise power will be approximately 2/3 of that, or 30 amps.

I’m using Venom 5,000 mAh LiPo battery packs, but to prolong service life, I try to use no more than 80% of the rated capacity. This gives me a usable capacity of 4,000 mAh.

To determine the flight duration, I first need to know the battery capacity in amp minutes. I do this by multiplying the capacity in amp hours by 60. In this case, 4,000 mAh is 4.0 amp hours. That multiplied by 60 is 240 amp minutes. I then divide 240 amp minutes by the average flight current (30 amps) and find that the flight duration should be roughly 8 minutes.

For me, that’s plenty. As it turned out, my data logger reported that the average flight current is more like 20 amps, meaning the duration is closer to 12 minutes, which should be enough for anyone.

When Reality Meets Planning

The Bristol project is one where reality worked out even better than my plan. The computer analysis program I used was more optimistic than my pen-and-paper calculations. Reality fell somewhere in between. I can easily fly for 12 minutes on my 12S setup, but I can’t quite get the 18 minutes that the computer projected.

I like that my pen-and-paper calculations are generally more conservative than my real-life results. I’d always rather err on the side of caution and be pleasantly surprised. It also ensures that I’m not pushing the equipment beyond its capabilities.

this avistar is a big airplane that will fly on a variety of power systems
06. This Avistar is a big airplane that will fly on a variety of power systems.

the author chose a 10s system composed of a rimfire
07. The author chose a 10S system composed of a RimFire 1.60 (63-62-250) motor, two FlightPower LiPo FP-50 5,000 mAh 5S battery packs, and a Great Planes Silver Series 80-amp ESC.

the bristol m1c flies even longer than calculations indicated success
08. The Bristol M.1C flies even longer than calculations indicated. Success!

Sometimes There Aren’t Electric Recommendations

I try to keep it simple, but there are times when you need to know how to do the math. Following the rules of thumb for power systems makes it easier, and people such as Lucien help by publishing them online.

If you’re converting a glow/gas airplane to electric power, think about using Lucien’s rule of 2,000 watts per cubic inch for two-stroke engines and 1,500 watts per cubic inch for gas power. These will give you the total wattage required for the airplane, based on the recommended wet power system. These rules will get you quite close, and you can tweak as necessary.

The Big Takeaway

There is no definite right or wrong answer when you stray from recommended setups, but knowing how to figure them out helps and is actually fun. Here are some simple rules:

  • Don’t be afraid to look at alternatives to suggested power systems.
  • Do a little homework so that you’re comfortable doing some simple math.
  • Decide what makes sense for your way of flying.
  • Experiment on a test airplane, preferably a trainer.
  • Look at the benefits of higher voltage and lower current.
  • Read all that you can from various reputable manufacturers’ websites.
  • You can work it in reverse if you want to put wet power on an electric airplane.
  • Have fun!

That’s the end of it—simple!


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