Author: Joe Malinchak
Edition: Model Aviation - 2010/11
Page Numbers: 97,98
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Micro-Flying - 2010/11

Ultramicro brushless motors

Joe Malinchak | [email protected]

One of the biggest challenges we face in constructing and designing a micromodel is keeping the weight to a minimum. With advances in battery technology and ultralightweight receivers and construction materials, we can build models weighing only a few grams or less.

One of the last obstacles is a lightweight power source. Fortunately, several modelers are making great progress in designing single-phase micro brushless motors that weigh mere milligrams and consume less current than standard brushed motors.

These single-phase brushless motors are relatively simple in design. They consist of a coil, magnet, motor shaft, bearings, and a brushless DC motor driver such as the Allegro A1442.

The A1442 uses a single Hall element sensor to detect the rotational position of the magnet. This small chip measures only 1.5 x 2 mm, weighs 3.8 milligrams, and can handle up to 400 mA. Wire size, coil ohms, magnet size, air gap, and chip location are variables that affect the power the motor can produce.

The first model I witnessed flying with a single-phase brushless motor was my friend Robert Guillot’s incredible 3.8-inch-span Ord Hume 7. I was blown away by the scale performance of the little aircraft. He made every part on the 1.1-gram airplane except for the receiver and battery.

The tiny motor was the key to the aircraft’s success. It weighs only 175 milligrams with a 3 x 2 mm magnet and a coil wound to 38 ohms. It can fly the model for 2–3 minutes on an 8 mAh Li-Poly cell.

Robert used Martin Newell’s Rabbit receiver with Hip Hop programming, which is also a major contributing factor to the success of this model. Robert has since built several brushless motors with improved performance.

The author and Stefan Gasparin collaborated on a 1/72-scale MiG-15. I designed the model and Stefan designed the amazing EDF 20. It might be the world’s first 1/72-scale EDF-powered jet.

With 4 grams of thrust, I had high hopes for the MiG; however, I had to fly it without the nose section because of the small intake area. A few changes in the design should make it work well.

Another modeler doing an excellent job on the design and construction of these new brushless motors is Dan Baird. He has been working hard to build a motor with power equal to or greater than the 4 mm, 13-ohm pager motors. That type of motor can power models such as my 1/72-scale designs.

Dan designed a motor that uses a 2.5 mm magnet and an 18-ohm coil. The power plant is close to the 4 mm pager’s performance, with much less current draw and one-third of the weight.

For fun, Dan wanted to see how small a motor he could build. He ended up building an incredibly tiny 0.5 mm motor with an 8 mm propeller.

I think his best achievement to date is his work on a 6-inch-span, four-channel Ultimate biplane. Dan built everything on it except the four-channel Rabbit receiver. The model is constructed from Durobatics foam and weighs 2.25 grams with a 10 mAh Li-Poly cell. For power, Dan designed a geared, brushless motor that weighs only 0.42 gram.

For those interested in building motors, Dan has a wonderful thread regarding his motors and information about his Ultimate biplane. See the Sources listing for the website address.

If you are interested in trying a motor of this type but do not want to make your own, Nick Leichty of Micro Flier Radio has added them to his site. Nick also sells the Rabbit receiver, coils, and many other great products for your next micro project.

1/72-Scale MiG-15 EDF

In the last column I mentioned the progress I had made on my 1909 military Wright Flyer and included photos. I had planned to have it flying by now, but I got sidetracked.

My friend Stefan Gasparin started working on some great new electric-ducted-fan (EDF) motor designs. He sent me a beautiful prototype EDF 30 that I plan to use on a 1/32-scale F-86.

After discussing plans for my F-86 design, I mentioned that it would be great if we could go smaller to power a 1/72-scale jet. He wrote back that it was a crazy idea. I reported that his little 400-milligram brushless outrunner that I used in my 1/72-scale Corsair has plenty of power and suggested that he design a version for a little EDF.

So this joint project began. I knew we needed a model with a wide fuselage for an efficient EDF unit. I chose the MiG-15 because the fuselage is 20 mm at its widest part in 1/72 scale.

I started designing the MiG, and Stefan went to work on the 20 mm EDF. He ended up designing a beautiful unit that used the G09UL 1000 motor.

The complete EDF 20 weighs 1.65 grams. It was slightly heavier than I had hoped, but it showed great potential. Static thrust is 4 grams on the Fullriver 30 mAh cell. My MiG-15 is constructed from Durobatics foam, has a 5½-inch span, and weighs 0.94 gram empty.

I had to keep the weight of everything to a minimum. I used a four-channel Rabbit receiver that Martin Newell made for me, which has pulse-width-modulation output for a brushless ESC. I used only one coil for rudder control.

The completed MiG-15 weighed 4.84 grams, so I knew it had a good chance of flying; however, the initial attempt was unsuccessful. My first thought was that the intake was too small, and I was correct.

I tried the model without the nose cone and it flew. That was only a few hours before I completed this column on August 10, 2010.

The flight was short—approximately a minute—but it was an exhilarating minute. It might be the world’s first 1/72-scale jet model to fly. I plan to redesign it with a larger intake duct and maybe a little more wing area.

Sources

Transcribed from original scans by AI. Minor OCR errors may remain.