The Thermics flew so beautifully I decided that someday I was going to build an R/C model of one. After some forty years here it is. For quite a few years I kept debating which one to build. They are both the same airplane except the Thermic 50 has a conven- tional fuselage and the 50X has a pod and boom fuselage. I finally decided on the Ther- mic 50X due to its unique fuselage shape which gives it a streamline, graceful look. This model is not an exact copy of the orig-
inal 50X. I had made drawings of the original plans, so I laid out the basic dimensions on paper then drew up the outline freehand, roughly to 50%. The fuselage and wing is close to the original, but the tail shape was changed a little. Refinements were also made to the structure as needed for R/C operation. With polyhedral it is a very stable model and turns tight, making it ideal for small fields. All in all, it still retains all the charms and beautiful flight characteristics of my first model of so long ago. This is not a large sailplane, but with to- day’s miniature radio equipment it’s no prob- lem fitting it all in place. I used micro servos (servos with at least 25 ounce-inches of torque will work) and a 300 mAh battery pack. As for power, engines from .049 up to .10 will work. An .049 engine gives good small field performance with gentle flight charac- teristics. The engine I used was a Cox .049 Medallion, which I have had for years. Cox engines are once again available through Cox International. They can be reached at 877-769-1779 or on the web at http://cox
engines.ca/.
Since this is only a fun fly model I didn’t go to extreme lengths in trying to keep the weight down. Most of the balsa I used was off the shelf wood which was medium weight.
Construction was started by building the tail and wing first. This way they can be fit in place as the fuselage construction pro- gresses along, assuring that everything will fit properly and be aligned when the fuse- lage is finished.
Into the workshop Tail surfaces: These are fairly simple and don’t need much explanation. The verti-
cal fin, rudder and elevator are built from 3⁄16-inch balsa. When building the rudder a 1⁄16-inch deep slot is cut in the leading edge
for the tiller arm with a 1⁄16-inch hole drilled for the end to fit into. The stabilizer is built
from ¼-inch balsa. Ribs for the stabilizer are 1⁄16×3⁄8-inch balsa. After the stabilizer is fin- ished the ribs are sanded to an airfoil shape as shown on the plan view. The stabilizer should taper to 3⁄16 inch at the hinge line to match the elevator thickness.
While not shown on the plans, I covered the stabilizer bottom with soft 1⁄16-inch bal- sa. I occasionally fly from several fields that have weeds and stubble and this keeps the bottom from having tears and holes punched in the covering, so this is optional. The elevators are cut from 3⁄16-inch balsa and joined together with a 3⁄16-inch dowel. Power pod: The actual size and shape of the power pod is not critical, it is basically determined by the engine and tank size. The nacelle is there to provide a stream- line housing for the tank. For aesthetics I wanted to keep the engine nacelle as small as possible, so I used a Perfect Products met- al control line tank. I doubt that you can still find these tanks, but Brodak makes metal
FLYING MODELS
tanks for small engines. They can be reached at Brodak, 100 Park Avenue, Carmichaels, PA 15320;
www.brodak.com. A metal tank was used because once it’s in there, it’s per- manent and they are leak-proof. The tank I had on hand was larger than I wanted so I cut out part of the pylon core where the tank would fit in. The tank was epoxied in place and boxed in with balsa. Be- ing a wedge shaped tank, it was installed with the fuel pick-up on the bottom with the vent and fill lines going out the top with brass tube extensions soldered on. I’ve used this setup on several sailplanes and it works well. The best size tank for .049 engines is ½ to 2⁄3 ounce and 2⁄3 to ¾ ounce for .10 engines.
Start on the power pod by cutting the py- lon core from 1⁄8-inch plywood (not lite ply). The core is cut so it fits over the main and rear spars. The firewall, P2 is also cut from regular 1⁄8-inch plywood. When gluing the firewall in place it should be offset 3⁄32 inch to the right to provide right engine thrust. Once the tank is in place add the nacelle bot- tom, sides and top. A small balsa block is glued in at the rear. Sand the nacelle to a streamline shape.
The pylon core sides have 3⁄16-inch balsa glued on, but this is not done until the power pod is glued onto the wing.
The cowl is optional, but it does improve the looks and cuts down a little on drag. The
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In the radio installation, micro servos were used (above), about 35 oz-in torque. A 300 mAh battery pack is in the nose. Receiver is in foam behind the wing L.E. In front of the battery is a hole for a 3–48 blind nut that holds the hatch in place. Here the plastic pushrods are in place (below). These pushrods are .047 music wire which runs through plastic sleeves. The sleeves fit into a channel in the boom. 8–32 hold-down bolt holes are shown. 8–32 blind nuts or threaded blocks can be used for the bolts.
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