PECK-POLYMERS J-3 KITTEN
BUILD A SCALE RC SLOW FLYER
The full-scale J-3 Kitten is an ultralight aircraft that closely follows the lines of a J-3 Cub at about ¾ scale. Designed in 1984, the Kitten has won many friends and admirers and is available in several countries as a kit. One look at this machine makes it clear that it just begs to be modeled as an electric slow fiver.
Peck-Polymers has come to the rescue by producing a 24-inch-span (nominally, 1/15-scale) model kit. Its construction is very familiar to those who have built Peck-Polymer kits before: stick and tissue with a nice airy frame that is amply robust for RC flying. If this kit were a term paper and I were a professor, I would give the kit a grade of B+, and the finished model an A! More on these marks as we go along.
Unpacking the kit revealed nicely packed, good-quality materials. There is one sheet of yellow tissue, a bundle of balsa sticks, some sections of balsa of various thicknesses, a laser-cut sheet with the wing ribs and formers and a plastic bag with miscellaneous supplies and parts to build a free-flight model. Two very clearly drawn plan sheets and instructions are also included.
Tail feathers. The instructions recommend that you build the tail first, so I laid out the plan on the building board and covered it with wax paper. I used medium CA for most of the model. The instructions present a unique method of framing the tail: pin the hinge spars down, glue in all of the rib strips oversized and then cut them to length with a straightedge and a razor blade. Then glue the perimeter pieces into place-neat! When the glue had dried, I lifted the parts and block-sanded them to clean off any glue “stickies.”
Wing construction. Moving on, I tackled the wing next. Here is where I ran into my first minor problem. I had four leading-edge strips and no trailing-edge strips. A bit of slicing from a 3/32-inch-thick sheet of balsa solved the problem nicely. After I removed the ribs from the sheet, I found that they had been cut 1/32 inch undersize, so I did not sand off any of the laser scorches. Other than this little glitch, the wing assembly was straightforward with good, cleanly fitting joints. Take care to get good-fitting butt joints when you join the two panels to the center section; all of the dihedral bracing is riding on those butt joints. After a bit of shaping and sanding, I set the wing frame aside.
Fuselage. The fuselage framing is typical of rubber-powered models. Put down-sticks over the plan and glue until each side is complete. I found I had to moisten the front of the lower longeron to make the required curve. I then built the second side over the first to get the best match possible. After a bit of sanding with a block, the sides are ready to be joined. The width of the section under the wing remains constant, so I started by lightly gluing the crosspieces to one side, carefully lining them up, and then added the second side. The key is to use just enough glue to hold things together so that you can shift the two sides until they are perfectly square and match the top view. Add a bit more glue now; then add the 1/16-inch-thick balsa shelf for the RC gear.
When it came time to pull the tail cone together, I moistened the longerons again to help with the sharp bend at the wing trailing edge. Add all of the crosspieces, and you are ready for the bulkheads. When the bulkheads are in place, you can add the top stringers. The instructions became a little confusing at this point: the text told me to leave off the two diagonal braces at the rear of the wing, but a photo of the framework showed them installed. I followed the photo so that I could cover the entire fuselage while the covered wing was clamped to a board to cure.
Final details. Add the landing gear, including the balsa filler between the wire struts. I installed my drive system next-an MPS-1A from DJ Aerotech. This is the closest match I had to the one called for in the instructions, and it worked well in the Kitten. Be sure to check the drive out carefully at this point; from here on, it is built in.
Covering. The single sheet of tissue included in the kit provides enough material for one application with some to spare for repairs. It is important to line up the parts to be covered to get the most out of the sheet. The provided tissue is referred to as “domestic tissue,” which is not very strong when it’s wet. You can easily shred it if you start tugging on an edge after you put on the glue. I used the recommended white glue and applied it sparingly; the covering job went just fine. I was careful to apply the tissue panels so that they had minimal slack after gluing. It helped that I had attached the tail surfaces before I covered the rear turtle deck. The tissue runs out on the top of the stabilizer, and using one piece looks much neater. I used a light mist of rubbing alcohol to shrink the tissue and, to prevent warps, pinned the wing to a board each time I wet it. It came out fine. Two light coats of Krylon Crystal Clear spray provide a very adequate finish.
Decals. I he kit furnished some very attractive water-slide decals for the vertical tail and fuselage side. Unfortunately, when I wet them for release, the artwork melted off before the decal came off the sheet. This will be corrected in future kits. Meanwhile, I settled for a few stripes down each side of the fuselage.
Power and control. While the King was still separate from the fuselage, I installed the RC equipment. I used WES-Technik servos, a lightened GWS R4P receiver and a JMP speed control. One of the battery options specified was a 2-cell, 135mAh Li-poly. I had a pair of 145mAh cells, so I went with them. As I planned the installation, I checked the plan for the target center of gravity (CG). As it turned out, the instructions don’t mention a CG location. Because the front spar is at 30 percent of the chord-a nice, conservative CG location-I shot for that. Everything came out fine when I packed the receiver, ESC and battery into the nose.
The wing went on next, along with the struts, exhaust system and landing-gear detail. The result Is a really cute model.
After the usual range checks, I headed for my grassy flying field. From the start, it was obvious that the Kitten is one sweet-flying model. The recommended control throws felt just right, and the cruising speed was amply gentle. The rate of climb with the 145mAh battery was a bit leisurely, and the run time was fairly short (6 to 7 minutes). For more pep and longer flights, 230 to 250mAh cells would be a better choice. A pair of the new Falcon servos, a JMP servo receiver and a 2-cell circuit board would save some weight and help with the balance issue. These parts are available at Bob Selman Designs.
There are a few small areas in which the kit could be improved, but this really nice-looking little bird flies well and shows no bad habits.
See the Source Guide on page 190 for manufacturers’ contact information.
Copyright Air Age Publishing Jan 2006
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