Better Catapult Glider
To improve performance, the author incorporated lighter wood in the two- piece wing as well as the fuselage (above left). Not visible is the mechanism of the DT, which uses an elastic thread to hold the tail boom in
tained from the hobby shop in ¹⁄₁₆-inch square strips to protect the wing from dings and maintain a sharp edge. These details may seem irrelevant but little things add up.
The fuselage incorporates a standard de- thermalizer system seen on many gliders of this scale that uses a button timer, rubber band, and pop-up tail boom. The system works very effectively so I copied it while us- ing a lighter Microlite timer and moving it more toward the nose to save ballast. The fuselage has a lengthened nose and replaces some heavy plywood reinforcements found in the kit in favor of ¹⁄₁₆ C-grain I scavenged from the wing. The tail boom left me with a big decision:
Use the beautiful, but heavy carbon fiber boom in the kit or make my own alternate. The weight savings on the boom were obvi- ous and I used a ¹⁄₈ by ¹⁄₁₆ strip of basswood which is slightly longer than the carbon fiber (to give a longer tail moment) and runs all the way through the nose to provide strength. Although not as obscenely strong and stiff as the carbon, it is adequate for the glider and has suited me just fine. The tail feathers are roughly the same
shape but I like to enlarge the stab for a more stable glide. Make them from ¹⁄₃₂ balsa that is stiff and sand them very thin. It helps
position and simultaneously pull on the tiny button timer (above right). Carving the wing takes patience to do right, but having an accurate and uniform airfoil is critical.
put the same hard leading edges on the tail surfaces to prevent the inevitable warps, chips and dings. A critical step is to glue the stab on the bottom of the boom with stab tilt. Only a few degrees tilt to the right results in a left hand glide and don’t forget this step because the stab tilt is crucial for trimming and the launch. Of course leave maybe ½ inch left of the boom sticking out aft of the stab for a finger grip.
Finishing All parts are sanded smooth with 400-grit sandpaper but the wing is given extra at- tention. I recommend two or three coats of clear gloss lacquer or nitrate dope to seal the wood and a final sanding with 1000-grit pa- per for an extremely smooth surface. Re- member that catapult gliders launch at very high speeds and good craftsmanship is what give you those last few feet of altitude!
Trimming/flying The wing and stab should be set at zero-
zero for the initial trim, meaning no inci- dence at all. Test the glider; it should glide flat and turn left automatically in accor- dance to the stab tilt. Then trim in just enough up elevator (and rebalance accord- ingly) to make the glider recover from a shal- low dive. The pop-up tail boom allows for a
neat feature. The use of a small screw on the bottom of the fuselage where the boom rests makes it easy to accurately adjust the inci- dence in the tail. The catapult is a six-inch loop of ¼-inch rubber. Most gliders like a 45-degree launch with
a similar angle of bank to facilitate an up- ward spiral and roll out on top. This works fine, but if your glider is built true and with almost zero incidence it can be launched just about vertically without pulling a loop or rolling over. When shot, the glider should climb straight and arch over on its back as it nears the top like the space shuttle on launch. When the glider runs out of speed, it transitions to glide with a flip and no alti- tude loss. Perfect. My modified Sting glid- er weighs 20.5 grams finished, 5.5 grams lighter than the 26g original. That equates to a little over a 20% weight savings. It rockets higher and glides much slower than the original Sting 18 and can do 50- or 60-second flights in still air! The DT is mandatory because the first thermal will suck the glider OOS in no time. We’ve al- ready lost two similar gliders that didn’t have DTs!
But wait, there’s more Not to leave well enough alone, we set out
The Sting-18 glider kit comes laser-cut and with every component from the catapult handle to the tiny DT hooks and rubber bands (above). A strip of a ¹⁄₁₆ square hardwood, found at craft stores, protects the leading edge from the inevitable dings and nicks (at right).
40 JANUARY 2012
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