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The author selected Tom Hunt’s successful original-design Acrovolt as the test model for this series of articles.
(Editor’s note: In the March issue, Bob explained the background of how power sources for modeling are rated and specifically how electric motors have been related in power output to glow engines. He covered how to go about sizing, or matching, a specific-size motor to a given model’s size, weight, and wing-loading parameters.
Bob provided a list of the various aircraft categories and explained how to measure motor input power, aircraft weight and power-loading considerations, wing loading and how it relates to flying experience and skill, and thrust factors. He included details of the motor-selection process, a comprehensive listing of Web sites containing helpful data, and a listing of computer programs that can help the selection process.
This month Bob gets into the details of the motor/airplane-selection process.)
IT’S TIME TO put all you’ve learned together with several examples of motor selections. Let’s make a glow kit or glow ARF electric-powered.
Many popular glow-engine-powered full kits and ARF “kits” on the market can easily be constructed and/or assembled from scratch with electric power in mind. One such model is Tower Hobbies’ “Perfect Trainer-20,” or “PT-20” as it is generally called. It is intended for .15-.25 cu. in. glow engines. The PT-20 has 515 square inches of wing area, can weigh 3.5-4.5 pounds, and has a wing-loading range of 16-20 ounces/square foot (sq. ft.).
TABLE-THREE “WING LOADING” and Skill Level
|
Wing Loading
(Oz./sq.ft) |
Type of Aircraft and Skill Level |
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5-10 |
Park Flyers, Basic Trainers and Powered Sport Sailplanes |
|
10-15 |
Faster Sport Flyer, Smaller Size Trainers and Sunday Flyers |
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15-20 |
Larger Size Trainers, Sport Scale Models and Sport Aerobatic Models |
|
20-25 |
Fast Sport Models usually with more than adequate power |
|
25-35 |
Scale Models and Larger Multi-Engine Models |
|
35 plus! |
(Not for me!) |
As a start, take into account that the wing area is 515 square inches (3.6 sq. ft.). Pick a wing loading for the intended skill level. Using Table 3, I picked “Larger trainer,” with a range of 15-20 ounces/sq. ft. That is close to the wing loading cited for glow power.
Using an average, I selected 17.5 ounces/sq. ft. as my target wing loading. If I multiply that 17.5 by the 3.6 sq. ft. of wing area, I end up with a target weight of 63 ounces (just less than 4 pounds).
From Table 2 I selected 40-50 watts/pound for the power loading. Wanting a bit more in performance (with some reserve), I specifically selected 50 watts/pound. From that I multiplied 3.93 pounds (63 ounces) by 50 and obtained 197 watts. Since power is amps multiplied by volts, you can work backward using the power (watts) and the estimated motor current.
That motor-current figure will always prove to be the tricky part. It will be an estimate that is a compromise between how long a motor run you desire and how much current your specific motor can tolerate (known as maximum continuous current in motor specifications). This is where the ElectriCalc and MotoCalc motor-selection programs can really help you. For this application I decided on a range of 20.0-25.0 amps and 22.5 as an average. Since power (watts) equals current (amps) multiplied by volts, you can divide 197 watts by 22.5 amps and obtain 8.75 volts.
You can reach a voltage that is close to that figure by using an eight-cell NiMH or Ni-Cd battery pack. It would tend to fall in between two and three Li-Poly cells because the characteristic voltage is 3.7 volts per cell (not 1.2 as with Ni-Cd and NiMH cells). But this gives you a ballpark figure.
From this point you can look up your motor data on one of the six Web sites I listed last month. I generally go for the AXI brushless outrunner motors because they are available in many sizes, and I have found them to be extremely reliable.
Closer look at AXI 4120/18 motor from rear shows accessory radial mounting plate which Bob highly recommends. Collar on shafts moves up flush with rear of motor.
For the average-size AXI motor I use “The Great Electric Motor Test,” at www.flyingmodels.org. I searched through the data looking at motor current, power (watts), and propeller sizes. You have to be patient because this can take some time.
I finally came up with an AXI 2820/10 brushless outrunner motor. On 8.0 volts and with an APC 10 x 7E propeller, it would have a current of 23.0 amps and 176 watts of power. The 10-inch propeller can easily clear the PT-20’s landing gear.
A battery pack consisting of eight Ni-Cd or NiMH cells should work. The Ni-Cd cells will likely produce slightly higher voltage, current, and wattage. A recommendation is an eight-cell Sanyo 1950 mAh NiMH pack, which will get me slightly over my target but allow me to do some throttling back during a flight. That will stretch the run time to longer than eight minutes. Not bad!
You can fly that PT-20 with this motor, propeller, and battery! As you progress, feel free to “what-if” your parameters in either of the two computer motor-selection programs. You can even work up to Li-Poly batteries by carefully selecting a propeller size that will allow you to use a three-cell pack, but not at excessive motor current.
How the Acrovolt’s crowded battery compartment looked when Bob had to employ upward of 18 3000 mAh NiMH cells.
The same battery area today, after switching to Li-Poly!
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