Pixel; the world’s smallest R/C helicopter
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Who hasn't dreamt of flying an RC model in the living room, or having one that loops above the kitchen table and drops a sugar cube into your coffee with just a flick of the transmitter switch?
In 1996, European modeling magazines started reporting on a new wave in our hobby: slow flight. Some very inventive people had broken all the rules with planes that weighed no more than 100 grams or so. They called it "slow flight" because the planes flew slower than walking speed. I researched the new technologies and learned about 2.4-gram proportional servos, 10-gram propulsion motors, 0.5-gram speed controllers and 4-gram receivers. Most of these components were marketed by WES-Technik.
As a veteran modeler and RC helicopter freak, I started dreaming about a tiny RC helicopter. I picked up the phone and called Walter Scholl, who heads up WES-Technik. An idea was born, and about a year and a half later, I had managed to design, build and successfully fly four prototype Pixels.
The Pixels are all my own design. All the mechanical pieces, including rotor head, swashplate, etc., are handmade. The tools I used are plain, standard stuff: mainly a Dremel, a hobby knife and some files. Roughly 99 percent of the frames are carbon. The first blades were balsa covered with fiberglass; now, they are fully carbon molded and the tail-rotor blades are carbon, too. Most of my time went into designing the rotor head and transmission system. It was crucial to determine the correct combination of rotor dimensions (span and chord), pitch, rpm, gear reduction ratio, motor and battery selection.
It may surprise you how easily I went from 125 grams for Pixel I down to 59 grams (2.1 ounces) for my latest design, Pixel IV. The biggest challenge was to get maximum lift with minimal power input. The 8cell, 50mAh packs don't perform very well because the internal resistance is quite high, and I draw up to 1 amp out of them. So I needed to limit power consumption. At 59 grams, Pixel IV can fly for about 11/z minutes; with a larger battery and at 70 grams, it can fly for 31 minutes.
Helicopter rotor heads are very complicated units, and just making the big ones smaller does not work. In standard RC helis, much of the stability is accomplished by adding weight-not something you want to do when you're trying to build as small as possible. I froze the design around a dual-flapping, fixedpitch, Bell-bar, Hiller-paddles design. The fixed-pitch technology works great for indoor applications. The helicopter does not yo-yo up and down because there are no wind gusts. And with some tail - rotor mix on my Futaba 9ZAP unit, the tail is locked in. You may be surprised to know that there is even a piezo gyro on board.
The Pixel has a separate motor for the tail rotor. The tail is controlled by changing the rpm of the motor. This reduces enormously the complexity of the tail's mechanics (no gears or transmissions from the front to the back of the helicopter) and, even more important, makes it much lighter.
Mathematical models indicate that fat houseflies cannot fly, yet I go crazy when they buzz around my head. The point is that whatever the mathematics say, you have to give it a try. While testing blades for the Pixel, I learned that wider, shorter blades seem to perform better than narrow, long ones. This goes against all the theories, but I can't help it. Overall, longer blades (given a certain width) always score better. However, a small helicopter needed to have short blades, so I made them wider. The highest efficiency I got was with elliptical carbon blades. I now need no more than 4 watts to produce the hovering lift force.
One of the reasons that Pixels fly so well is that they have only about half the disk loading of standard R/C helicopters. For Pixel IV, that is around 10 grams per square decimeter; for any other helicopter I know, it is in the 20s and more.
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