The pylon is made from a 1⁄8-inch plywood core (at left) with ³⁄₁₆-inch balsa glued on the sides. The fiberglass cowl (above) is held in place with #2 screws screwed into small hardwood blocks.
For balance, the servos are mounted in front of the wing leading edge. I used 1⁄8×3⁄8- inch plywood strips glued across the fuse- lage side, with a 3⁄16 × 3⁄16-inch balsa strip under the plywood. A 300 mAh battery pack fits in front of the servos.
With the servos in, pin or tack glue the tail surfaces in place. Hook up the controls and make sure the rudder and elevator work smoothly. The control rods are then epoxied in place and the tail glued on. Glue on 1⁄8- inch top sheeting with the grain going cross ways.
The final item to make is the hatch cover. The base is 1⁄16-inch plywood, with balsa on top. The front is held in place with a 3–48 screw and blind nut. The rear is held down
with a 1⁄16-inch plywood tongue glued on a piece of plywood, forming a tongue and groove joint.
Finishing and flying
Most any lightweight covering or plastic film will do. I covered the original Thermics with tissue and dope. To keep as close to the original ones as possible, I covered this one with medium weight silkspan and painted it with dope. The fuselage however was cov- ered with SIG’s Koverall. This material is very tough and almost puncture proof. Once painted, it looks like the rest of the model’s covered structure.
Before flying, check the balance point carefully. The balance point is shown on the
plans. With the stabilizer having a lifting airfoil, the balance point is farther back than normal. This proved to be a bonus. Originally I forgot about this and balanced the model at the usual 30% point, which took about four ounces of lead. Through test glides I began removing weight until a slow, floating glide was obtained. In the end only one ounce of lead was needed for balance. Pick a calm day and do some test glides. If something is not right, now is the time to find out. If you are flying without throttle control, limit the engine run to one or two minutes on the first flights. I use a ½-ounce bottle to fuel up with. This gives about a 3- minute engine run which is ideal for small fields. Happy flying.
Thermic 50Xis ready for its first flight. David covered the model in an attractive red and yellow trim scheme that is easily seen. The polyhedral wing on the Thermic 50X(above, at right) allows tight turns, perfect for flying from small fields. While there is no mistaking the model’s roots, David elected to change the profile of the tail (at right). Not seen are the other refinements to allow it to be built as an R/C model.
FLYING MODELS 23
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