It’s hard to believe that we are halfway through 2015 already. 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.
As has been the case in recent years, 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. RC, Control Line (CL), Free Flight (FF), and indoor models are all flown, so there is something for nearly every model aviation interest.
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 anything. Multiple powerplants are allowed as long as they do not individually exceed the .26 glow equivalent in size.
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. Please see the “Sources” listing.
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. I want to emphasize that Ralph’s work was applied to developing a direct solar-powered model. That meant no onboard batteries of any kind.
A hybrid-battery/solar-cell-powered model has an interesting set of challenges. When you eliminate the batteries, challenges become even more interesting. 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 have efficiencies in the 30% to 40% range. Cells available to the general consumer are much less efficient.
When Ralph took on the challenge of developing a workable, direct, solar-powered RC model, he contacted one of the manufacturers of cells used on spacecraft. The company thought his project was 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 cells in hand, he set about building 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 that 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 levels. He found that water vapor in the air dramatically reduced the amount of solar energy delivered to the cells.
Several years after the Sol Mite project, 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 solar cells and was able to get away with using only two.
His new model, called the Micro Sol Mite, uses Plantraco radio gear. Like the larger Sol Mite, it also needed bright days with low humidity to adequately fly. The time window for useable power is in the 10 a.m. to 2 p.m. period. Before and after that time period, the sun does not put out enough solar 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 of use as a power source for RC model airplanes.
A company has come along that offers flexible solar cells that might be a viable RC model power source. That company, PowerFilm Solar, has a number of cells available to the general public. Its OEM solar module model MPT4.8-150 is 5.75 x 3.7 inches, weighs 3.9 grams, and can deliver 110 mA at 4.8 volts.
I had an opportunity to gain some experience with this PowerFilm Solar cell. I built a prototype FF model for ToyLabs, a small, educational toy company. This model is not directly solar, but is a hybrid using super capacitors 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 the PowerFilm solar cells as the primary power source. See the “Sources” listing for PowerFilm Solar and ToyLabs.
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.
