Flying for Fun
D.B. Mathews 909 N. Maize Rd., Townhouse 734, Wichita KS 67212
THE INFORMATION in the February column about micro 3-D Electrics contained an error that requires some attention. Contrary to what I wrote, Depron, fan fold, and blue foam are not the same. I misunderstood what I was told during an interview with a builder and what I read on the Web site. The following is from an e-mail on the subject from Tim Hart, who is responsible for the designs featured in the February column and maintenance of the Web site www.foamyfactory.com.
"Blue foam is an expanded bead polystyrene (similar to white 'wing core' foam) typically used in hot-wire cut foam core wings, then sheeted with balsa and often fiberglassed. BlueCor is a brand name manufactured by Dow and Georgia Pacific, and is primarily used as insulation under vinyl or aluminum siding. This is the stuff referred to as fan fold, BlueCor, FFF, and has a clear plastic coating on both sides of the sheet.
"Fan fold is a generic term and has to do with the manner in which the material is packaged for shipment. The Dow product is blue, the Owens Corning is pink, and the Amocor is green. A few other brands are also available. This foam is sold in 3/16-inch and 3/8-inch sheets that are four feet tall and fifty feet long. It is then 'fan folded' back and forth to create 35 four-foot by two-foot sheets.
"A bundle that size usually sells for about $25.00 and will build 10–20 airplanes, depending on how much foam you waste and how big the models are. The foam used for posters etc. is called Foam Core or Sturdy Board, and while it can be used for models, the paper has to be peeled off and it also gets pricey.
"Depron is a brand name and is only widely available in Europe. All the Depron used in modeling in the US is imported from Europe. We have no source here in the US that manufactures the material. In flat sheets for modeling use, Depron is superior to fan fold in many ways; it is more rigid, stronger, and lighter, and it comes in 2-, 4-, and 6-mm thicknesses for modeling use. It also does not have any coating on it!
"Both types of foam have their advantages and disadvantages but work well for constructing mini 3D Electrics. Unfortunately, Depron is much more expensive."
There is no particular magic associated with the name Depron. It is an expanded bead polystyrene but is manufactured in a closed mold to produce dense spacing and smooth surfaces. This sort of product is used in various fast-food containers that are thermo-molded in the U.S. I recently learned that Depron USA is importing the material; go to www.depronusa.com for information.
A Web page that includes illustrated, step-by-step construction articles for the foamy fliers I showed in the February column is www.rcgroups.com/forums/. This site is also an open discussion forum.
Apples to Apples:
In the fall of 2002 I had the privilege of spending five days as a guest of Fred Marks and his family (of FMA Direct) in the Baltimore, Maryland, area. The principal purpose of that visit was to investigate the latest and greatest in receivers and servos and the revolutionary Li-Poly battery cells that they had just begun to import from Korea. To say I was impressed with these Li-Poly cells is an understatement. They solved the longtime major complaint about electric-powered model airplanes: the fuel is too heavy. If you have been reading the excellent series by Bob Aberle or Bob Kopski’s RC Electrics column in this magazine, you are probably aware that Li-Poly batteries can produce as much power and increased duration as a Ni-Cd or NiMH setup, with one-third the dead weight.
Some problems with my vision following the Baltimore visit prevented my doing any “cut-and-fly” evaluations, but since other writers have done such an excellent job of analyzing the advantages and special care and feeding of Li-Poly batteries, there was no need for me to try to add anything more.
I am not an engineer and admit to never being fully comfortable with mathematical formulas and equations to predict performance of a new idea or product. Consequently, I’ve avoided the subject until now, leaving it to those who enjoy crunching numbers to prove their points.
I’ve been waiting to explore the advantages of the Li-Poly cells in an apples-to-apples-to-apples manner; I wanted to compare the battery choices to each other and to "wet" (glow) power with all the other variables constant. That is, use the same airplane and motor, changing only the battery type, and then do side-by-side flying comparisons. My longtime flying buddy George Sauer has done that, and his findings confirm the others’ theories, but in a completely nuts-and-bolts comparison.
Months ago I wrote about a Sig Astro-Hog that George powered with an AstroFlight Cobalt 15 electric power plant using Ni-Cd packs. That setup flew, but not with any gusto or performance. George then repowered the model with an AstroFlight Cobalt 40 motor and an AstroFlight Superbox gear drive. This version initially used Ni-Cd packs (21 cells of 2400 mAh capacity).
The electric-powered Astro-Hog flew well and was comparable to the identical Astro-Hog he had constructed that was powered by an O.S. 60 four-stroke engine. Performance was similar even though the illusion of noise caused the glow-powered model to seem to fly faster. Flying the models side by side revealed that they were closely matched. The glow version weighed 7 pounds, 10 ounces fueled, and the Ni-Cd version weighed 8 pounds, 2 ounces.
When NiMH batteries of the same amperage and voltage were substituted for the Ni-Cd batteries, all the performance variables remained nearly identical for the electric version except that duration did increase notably.
George is one of those enquiring minds that are so common in this hobby. He is a retired aeronautical engineer by education and occupation. Perhaps since George’s specialty is aircraft structures, he worships at the altar of mathematics enough to design a part but not be totally comfortable with it until it has been fabricated and fitted to the other structures.
From years of experience, George knows that the part may look good on paper but require adjustment when put into a practical application. Because of that humility, he is a delight to watch while he delves into the new and untried.
After hearing me rave about Li-Poly cells, George read everything he could find on the subject (in print and online), considered the subject thoroughly, and then ordered Kokam Li-Poly high-discharge batteries from FMA Direct.
To obtain the proper voltage (each cell produces about 3.7 volts), George assembled two sets of six 1500 mAh, high-discharge (8C) cells in series and then connected the two in parallel to give 22.2 volts and 3000 mAh. This sort of setup is referred to as 6S (2P). Charging is accomplished using an FMA Direct Li-Poly charger three cells at a time.
The Li-Poly-celled Astro-Hog weighs 6 pounds, 2 ounces, or a full 2 pounds less than the Ni-Cd version and 1 pound, 8 ounces less than the glow-powered model. Using a 16x8 propeller cut to 15.5 inches and drawing 25 amps, the Astro-Hog flies 12–15 minutes and is noticeably more powerful than the Ni-Cd or glow version. That's not much of a surprise when one considers that the model is a full 2 pounds lighter than the other units, and a 16x8 propeller certainly provides more thrust. This is the first time I've ever reported an electric-powered model being superior to an equivalent glow-powered model. Think of the potential!
This setup's performance is different from what is predicted using any of the popular formulas. This is good, old-fashioned, practical, in-the-field information. George believes that the popular formulas apply to small electric-powered models and are inappropriate for larger models.
When one considers the variance in larger wet-powered models' power requirements and flying manners when compared to small models, this all seems more logical. Compare a 40-size model to a Jumbo.
The change to Li-Poly cells in this model altered it from nice to extraordinary. These cells are expensive, but they are certainly worth the investment. As is almost always the case with model airplanes, you get what you pay for. George got the same or slightly more power, increased duration, one-third the weight, and proven results — not mathematically, but in actual use. That is exciting!
If you have one of those climb-and-glide models that were sold in the early days of electric RC and you became totally disillusioned with it, remove the cheap can motor and batteries that came with it, substitute a good, modern geared motor and Li-Poly cells, and you'll have a model that performs well and gives you a great deal of pleasure.
Some confusing stories about charging Li-Poly cells are making the rounds. The chargers used for Ni-Cd or NiMH cells are unsafe for Li-Poly cells because the latter must be charged initially at a constant current, switched to a constant voltage before they are fully charged, and then shut down when a certain voltage is reached. If you understand how a Ni-Cd charger works, you realize that the voltage varies throughout the charge, and the current is constant as the pack charges. A Ni-Cd charger could ignite a Li-Poly pack.
Buy an appropriate Li-Poly charger and read the instructions until you completely understand the principles involved. Proper understanding and management is essential in charging any battery, regardless of type.
Space is short, and there is little reason to present addresses for the suppliers I have mentioned in this text. Most, if not all, of them advertise in this magazine.
Consider converting one of those numerous scale 60-size glow-powered warbird kits to the electric setup George is using. Imagine the project turning out lighter than the engine-powered version and probably flying better. Some modelers are also switching to electric power for very large models. How about that?
Li-Poly batteries may be the missing piece of the puzzle and may bring electric RC to where it is equal to, or even better than, wet power, and may also solve many of the flying-site problems I've mentioned in the last two columns. Now that is exciting! MA
(Editor's note: For the complete story on Li-Poly cells, their use and care, and especially the safety aspects involved with their use, please read Bob Aberle's "Introduction to Lithium-Polymer Batteries" article in the May 2004 MA.)
Transcribed from original scans by AI. Minor OCR errors may remain.
