January 2014

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

Issue link: http://experimenter.epubxp.com/i/247918

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Page 14 of 38

guide the cables down each wing. Once the flaps were designed, it became apparent that the same action could also be used to droop the ailerons. With a little trial and error, an overhead arm was used to get the proper leverage and travel for both flaps and ailerons. For the endless loop to function and allow the stick to still move the ailerons, a pair of pulleys were added on the bottom rear of the stick torque tube." And speaking of the torque tube, it's just another example of how every single component was analyzed with the thought of how to make it work but weigh less. Everything that will structurally tolerate lightening holes was attacked with a step drill. The edges of each of the holes were carefully deburred and rounded to prevent stress risers, which must have been a tedious, time-consuming job, considering the hundreds of lightening holes everywhere you look. One change Frank said he would make, if he had to do it over again, is the outline of the top of the rear fuselage. He said, "The Cub aft longerons are actually low, just about level with the bottom of the windows, and the upper fuselage has a system of standofs and stringers that give the familiar upper fuselage outline. However, instead of using the top deck gingerbread, we opted to leave all of that out but keep the original layout otherwise. We would raise the upper longerons next time. Would be the same weight but more aerodynamic. Initially, the aircraft was flown with no fabric from the back of the cabin to the tail, like the well-known Super Cub Got Rocks?. We flew this way for more than 200 hours, then added the fabric. "Incidentally, we were going to name our Cub Got Pebbles?, but there's a controversial mine in Alaska, the Pebble Mine. I told Kris we could name it Got Stoned?, but she overruled me; so the politically correct name Lil' Cub evolved." The airplane is a tour de force of outstanding, utilitarian details with the tailskid being one of them. Rather than sufering the weight and expense of a tail wheel, Frank went with an Alaskan version of a tailskid. It features a shoe composed of multiple layers of conveyor belt material. Frank said they replace the outside layer about once a month if they do much flying on pavement. The main gear is shod with 26-inch Alaska bush wheels with single-puck Grove brakes doing the stopping. However, check out the brake rotors; they are mostly holes. When asked why, Frank just grinned and said, "weight." He's pretty predictable. The basic fuel system is as simple as the rest of the airplane with an 11-gallon tank in the left wing that gravity feeds directly to the engine with no header tank between. However, there's a unique, carbon-fiber, 5-gallon tank from Randy Apling at Carbon Concepts LLC nestled in the rear fuselage under the elevator screw jack. Besides giving a solid hour of extra fuel, it functions as a variable weight that can be used to put the center of gravity (CG) exactly where Frank wants it with various loads. The entire aft part of the cabin behind the seat is a cargo area, so his CG constantly varies, depending on the mission at hand. When it came to building the wings, Frank again opted for traditional structure but with his own twist. "We used aluminum angle ribs, and all support braces have lightening holes," ORATEX COVERING: THE NEW KID ON THE FABRIC-COVERING BLOCK Frank Knapp used a new, nontraditional covering method on his Lil' Cub that has been developed by Lanitz-Prena Folien Factory Oratex (Leipzig, Germany). Bridging the gap between iron-on model aircraft covering and traditional aircraft fabrics, Oratex polyester fabric is already sealed and painted, and once glued in place, is shrunk to taut by heat. It is certified in some countries for some certified airplanes, and Frank and Kris can't say enough good things about it. We asked Frank to lead us through the process, and he quickly told us that the advantages to it are that it is light, is super easy to apply, and has no smell or toxic properties. Because the heat-activated adhesive is water-based, there is no painting involved unless you want to. Frank said, "Oratex saved at least 25 pounds on the Lil' Cub. Our experience with the airplane shows that it takes between $500 and $1,000 to lose one pound when replacing something with titanium or carbon, assuming you can find something to replace it with. Even at the light end of that assumption, Oratex saved $12,500 in weight! I think it paid for itself several times right there. "My wife, Kris, took ownership of the fabric work. She had zero experience working with aircraft or fabric. Although I have hacked and cut doing changes and different tests, I think her work still looks great. I helped Kris with riveting the fabric to the ribs, but with two grandparents working on the fabric, we were completely done in four long days! Most processes would take longer just to do a single wing! Here's the process we used: 1. First, we cleaned and lightly primed all metal to be covered, then scuffed again lightly. 2. We brushed on a light coating of glue to the frame parts, wrapping the last tubes to be covered and let dry. 3. Next, we hung the fabric over the area, then marked the back side of the fabric (with pencil) where it would attach to the framework. 4. We applied a light coat of glue to the back of the fabric and let it dry. 5. We hung each fabric section in place and tacked it in place using a small electric iron. No clips were required because the glue attaches with heat. 6. Once the fabric was attached, we started with the perimeter, then shrunk the overall panel using a heat gun. 7. After the previous section was properly attached, we moved to the next, applying glue to any overlapping areas that we would then bond with heat. 8. The fabric was attached to the ribs either with rivets or by sewing. The seams were covered using colored tape that already has glue applied to the back surface. These easily attach using heat. "Anyone taking basic care and having no previous experience can produce a good, safe covering job while not requiring the investment in breathing or painting equipment. My past projects included a lot of painting, and as a result, my body has built up a toxicity level, which requires a lot of caution around any paint products. My wife and I covered the entire aircraft (after preparation) in four (long) days using no special care or protection. We enjoyed the process and look forward to the next. "We found the fabric to be much stronger, I believe, because the paint/UV protection is already applied at the optimum thickness. This makes fabric much tougher to normal flying abuse. While in Arizona, we regularly used a pressure washer to clean the fabric. The 2,500-psi water at 2 to 3 inches from the fabric did not affect the fabric, paint, or even the tapes! When flying the rivers in Alaska, the fabric does not chip or damage like other (thicker) processes. It costs about $75 per square yard. "The fabric doesn't gloss like a new Chrysler, but it also doesn't weigh as much as high-gloss processes either. The shine is a soft gloss and very pleasing to the eye. We enjoyed that the product is nontoxic (doesn't smell) and is so easy to apply. With more than 470 hours in Arizona sun and Alaskan grime, the fabric is holding up wonderfully. I will use Oratex on my next project. We don't sell the stuff; we're just sold on the product!" Oratex is distributed in the United States by Lars Gleitsmann, USA Oratex/ Lanitz-Prena-Aviation Products, 4621 Caravelle Drive, Anchorage, AK 99502. Learn more about Oratex at www.BetterAircraftFabric.com. Contact Lars by calling 907-229-6792 or e-mailing Lars@betteraircraftfabric.com. EAA Experimenter 15

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