About this Plan
Pixel. Indoor electric-powered RC sport biplane. Wingspan 13 in, wing area 60 sq in. For KP-00 motor or similar.
Direct submission to Outerzone.
Quote: "An ultra-micro 3-channel biplane. Pixel, by Dave Robelen.
The Pixel was conceived as a utility sport model for the current generation of proportional magnetic control systems. I had just recently received the new RFFS-100 control system from Dynamics Unlimited and was looking for a suitable model. My personal preference was for a biplane with all- balsa construction, so I decided to go that way. The wing loading that I had in mind produced a wingspan of only 13 inches, so Pixel is certainly a compact model. The control system comes complete as a package, so I had little to round up in the way of equipment.
I chose to use 50mAh Ni-Cd batteries in my Pixel, but the small NiMH cells would work just as well. My little switch came from Cloud 9 RC along with the small amount of hookup wire I needed. There were a couple of motor options. Quite a few modelers have reported success with the Mabuchi M20 LV motor sold by Toytronix when it is fitted with 4.2:1 gears (Cloud 9 again). In one of my scavenging hunts in the toy department at Wal-Mart, I found a toy plane with an electric motor and gear drive, called an E-Charger. The motor and gears are identical to the KP-00 from Dynamics Unlimited, but it uses a different frame and a different bearing on the prop shaft. The toy plane had a loosely fitted coil spring in a plastic housing for the main bearing. Fortunately, it was a simple matter to remove the spring and install a pair of 1mm-ID ball bearings from David Lewis. I replaced the prop with a U-80 from Cloud 9 and trimmed away a good bit of surplus plastic from the prop shaft housing.
I laid out the Pixel in a design format that has been used in various forms for quite some time. The all-balsa construction requires a nice, light grade of balsa, but you don’t need much: about 1-1/2 sheets of 1/32 x 3 x 36-inch did the trick. There are a few parts from thicker stock, but none of the wood should be of a heavy grade. The wood in my Pixel is about 5 lb/cu ft, or roughly 5 grams per sheet. I used medium thickness CA for all of the bonding.
With the parts all collected, I began looking at my options for a really light but colorful finish. I have had very good success with spray-can automotive touch-up paints on past projects, but this would be too heavy for the little Pixel. My solution was to spray a small amount of the touch-up paint into a bottle and then dilute this 50/50 with thinner from the auto paint store. I then used my inexpensive airbrush to apply this diluted solution much like a stain on bare wood. The effect is very colorful, and the weight gain is minimal.
I followed the installation instructions for my RFFS-100, including those for the rubber-band hinges. I was very pleased with the operation except for the excessive movement around the hinge line. My solution was to cut slices from medium-diameter hypodermic needles (available at large-animal supply stores) about 3/16-inch long and glue them alternately to the stabilizer and the control surface. A short length of 0.015-inch wire through the tubes provides a very smooth and positive hinge, while the rubber band does fine centering it. This whole system must be very smooth and free to work well. Unlike servos, the actuators do not have surplus torque, so aerodynamic balancing of the controls is a must. I also added small weights to the balance tabs on the elevator so it lies level with no load.
With all of the assembly done, Pixel was ready for field trials. With my obsession for lightness, I started with a 2-cell battery. The trim and stability were fine, but Pixel just would not climb out. I added a third cell, and this did the trick. At this point, I was flying with the E-Charger motor and U-80 prop with satisfactory results, but the flights were not as long as I liked. The M20 LV motor uses less current, so I swapped it into the Pixel with the 4.2:1 gears and a 5-inch-diameter prop. This package is even more powerful, and the flight time went up to about 6 minutes with a good bit of maneuvering. The RFFS-100 system is not claimed to be narrowband, but flying in my yard with my Hitec Prism 7X with Spectra transmitter, I have never experienced a glitch. No doubt Pixel would work fine with an infrared system or other ultra-light RC equipment. One note on motors: though the M20 LV is more efficient, it requires that you assemble your own prop shaft and bearings with proper clearances. I recommend the drop-in KP-00 for the ease of installation.
CONSTRUCTION: Pixel is a little different from my previous projects. There is nothing to pin down or build over the plan. As noted on the plan, almost all of the material is 1/32-inch balsa sheet. It is crucial that you use very light wood for such a small plane. Lone Star Balsa is a good source, as is Superior Balsa Products. Wherever you buy the wood, a sheet of 1/32 x 3 x 36 inches should weigh no more than 6 grams.
All of the parts except the fuselage top and bottom planking can be cut out before you start anything else. Let’s do the wings first. Each wing has four ribs, two of 1/16 inch and two of 3/32 inch. Glue the four ribs to the bottom of each wing, being careful not to glue in a twist. I used a razor saw and sliced through the wings at the center of the 3/32-inch ribs. After just a bit of sanding for the dihedral angle, you can glue them together. Touch up with fine sandpaper and set them aside.
Start the fuselage by gluing the reinforcements on the two sides. The landing-gear wire is sandwiched between two layers of 1/16-inch balsa with CA glue. Watch the alignment, and glue in the front and rear bulkheads. When satisfied that things are straight and square, pull the tail together and glue it. I found it useful to score the inside of the fuselage sides behind the rear bulkhead to get a sharp bend. Then I coated these score cuts with a little CA glue. You may now install all of the rest of the bulkheads and the motor plate.
Plank the bottom of the fuselage with the grain running across the width. Plank the top on the sides first and then the top. A little sanding should make everything smooth. Now glue the wing pylon into place, along with the headrest. I used a strip of 1/64-inch plywood for the top wing rest on the pylon.
Now is the time to decide on the finish. Pixel is not the sort of project that you should finish with sealers or clear material. Weight is critical. Leave the wood its natural color, finish with marker pens, or do as I did and spray a very thin coat of color on the bare wood..."
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