Author: Paul Bradley
Edition: Model Aviation - 2015/06
Page Numbers: 87,88
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If it’s June, it’s time for SMALL 2015

It's hard to believe that we are already halfway through 2015. Time seems to slip by far too fast. This also means that it is time for the annual gathering of the Small Model Aircraft Lovers League (SMALL) near Little Rock, Arkansas. This all-for-fun event will take place June 4–7, 2015.

Event details

SMALL 2015 will be hosted by the Fraternal Association of Radio Control Modelers (FARM) club. The members do a great job of hosting this event and have a terrific flying site.

What’s flown

  • Radio Control (RC)
  • Control Line (CL)
  • Free Flight (FF)
  • Indoor models

There is something for nearly every model aviation interest.

Size and power restrictions

Although the event's name is SMALL, you will find model aircraft ranging in size from tiny to fairly large. The only restriction is the size of the powerplant—basically an upper limit of a glow .26 or the electric-power equivalent. Rubber power can be any size. Multiple powerplants are allowed as long as they do not individually exceed the .26 glow-equivalent size.

Location and contacts

If you have an opportunity to travel to the greater Little Rock area in early June, consider attending SMALL. Information about the site location and contacts can be found on the FARM club website (see Sources).

(Editor’s note: Look for full coverage of SMALL in an upcoming issue of Model Aviation.)

Summer Is Upon Us

Summer means the sun is out and we have warm flying weather. What about the possibility of harnessing those sunrays to power our models?

In 2004 my brother, Ralph, worked on solar-powered RC model development. Although he was successful, there were some lessons learned. Ralph's work focused on developing a direct solar-powered model—that is, no onboard batteries of any kind.

A hybrid (battery/solar-cell) powered model has an interesting set of challenges. When you eliminate the batteries, the challenges become even more demanding. The baseline problem is the power available from solar cells: they are relatively inefficient at converting solar energy to electric power. Cells used on spacecraft can have efficiencies in the 30%–40% range; cells available to the general public are less efficient.

Ralph’s Sol Mite projects

When Ralph took on the challenge of a workable direct solar-powered RC model, he contacted a manufacturer of cells used on spacecraft. The company found the project interesting and offered to sell him a batch of rejected cells. The company no longer does that, but Ralph managed to acquire a small batch of relatively high-efficiency cells.

With those cells, he built a solar model called the Sol Mite. The outlines are derived from the 1958 Bob Coon–designed Guided Mite. Sixteen solar cells were embedded in the 39-inch wing beneath the transparent film covering. Tests showed the covering had little impact on the cells' output. The Sol Mite was a success in terms of being able to fly using direct solar power. The drivetrain and radio gear were optimized to minimize current draw.

To fly the Sol Mite, Ralph had to choose calm days with low humidity. He found that water vapor in the air dramatically reduced the amount of solar energy delivered to the cells. Several years later, with radio gear continuing to decrease in size, weight, and power consumption, Ralph built a direct solar-powered RC model with an 8-inch wingspan. He still had a few of the high-end cells and was able to get away with using only two. His new model, called the Micro Sol Mite, used Plantraco radio gear. Like the larger Sol Mite, it needed bright days with low humidity to fly adequately. The usable time window for power was roughly 10 a.m. to 2 p.m.; before and after that, the sun did not put out enough energy.

One limiting factor of the cells Ralph used is their brittleness—they are prone to shattering. Until recently, flexible solar cells were too heavy and inefficient to be viable as an RC model power source.

PowerFilm Solar and newer cells

A company called PowerFilm Solar offers flexible solar cells that might be a viable RC model power source. Its OEM solar module model MPT4.8-150 measures 5.75 x 3.7 inches, weighs 3.9 grams, and can deliver about 110 mA at 4.8 volts.

I had an opportunity to gain some experience with this PowerFilm Solar cell. I built a prototype free-flight model for ToyLabs, a small educational toy company. This model is not directly solar but is a hybrid using supercapacitors and a single solar cell. It has proven to be a fun model to fly, and I am encouraged by the solar cell’s performance. Perhaps, with Ralph’s help, I can build an RC model that uses PowerFilm solar cells as the primary power source.

Here we are at the end of the column once again. As always, please let me know what you are up to in our world of small-field flying.

Sources

Author / Contact

Paul Bradley 10201 Scarlette Oak Dr. Independence, KY 41051 [email protected] www.parmodels.com

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