Micro Bristol Scout

Model Airplane News, Dec 2002 by Keennon, Matt

FINAL APPROACH

I wanted to build a micro RC plane that would be maneuverable enough to be flown in a 20-foot-square workshop at AeroVironment Inc. The shop has old aircraft, drop cords and air hoses hanging from the ceiling; it resembles a micro indoor obstacle course. A biplane was a good choice because of its high maneuverability and compact size.

Rather than sketch out a non-scale model, I pored over my aviation books for a suitable subject, and I found one in the Bristol Scout Type D-an old favorite of modelers, probably because of its nice lines, good proportions and simple shapes.

I quickly drew up a plan for this semi-scale Bristol Scout D from a 3-view photo in a book. I decided on a flattened fuselage for less weight, and the wing uses polyhedral to enhance maneuverability.

To build the model quickly and keep weight low, I used mainly 1-pound white-bead foam for construction with balsa reinforcement in critical places. I carved bamboo skewers flat to make the wing struts. Since I knew I would occasionally bounce this plane off the walls and other obstacles in the shop, I decided to make the wings detachable. Small super magnets from RadioShack hold the wings on, and a strip of reinforcing carbon fiber on the fuselage prevents it from breaking into two. I made the wing out of thin hot-wire-cut sheets, which I curved over the edge of my workbench and locked into shape with a few balsa ribs. I made the rudder and elevator out of the same thin foam and hinged them using nylon thread that gives precise, low-friction movement.

The airplane uses two custom-made servos that are 3mm micro motors set up with fine, monofilament nylon thread in a pull/pull arrangement. The wires that stick out of the control surfaces are the control horns; the threads are so fine that in photographs they are invisible. The servos are fast and accurate, and they consume little power; each has about 60 discrete positions. The servos control rudder and elevator, and with the receiver, they weigh only 2 grams.

At first glance, the propulsion system seems straightforward, but I used a couple of hightech tricks. The model uses a little Mabuchi motor and a gearbox I made using gears from an old camcorder lens, and I paired those with a CNC-milled maple prop that has an airfoil specifically shaped for the low Reynolds number of 10,000 (it was designed by my friend and coworker John Asplund). With two 4.5-gram, 180mAh, lithium-polymer cells, the model has an amazing 38-minute flight duration!

Running the prop on this gearbox is wonderful; both are very quiet. When I fly the model in the shop, it often takes a few moments for people to realize that there is a small model airplane flying up in the rafters over their heads.

All in all, this little foam biplane is a real joy to fly. It has all the quiet, smooth, maneuverable performance anybody could ever want! Now I just need to find time to put that paint finish on. 4