Thermic 50X
The hatch base is 1⁄16-inch plywood (above left), the rest is balsa. The bottom view of the hatch (above center) shows the plywood tongue that acts as the
best way to make the cowl is from fiberglass, using a Styrofoam™ plug. Blue Styrofoam™ works best (available from builder’s supply stores). It’s harder than white Styrofoam™ and sands a lot like balsa. Cut a piece of Sty- rofoam™ oversize and tack glue it to the firewall and sand it to shape. Don’t use CA here, it will melt the Styrofoam™. Sand the Styrofoam™ to shape then care- fully sand it 1⁄32 inch undersize to allow for the fiberglass cloth. Remove the plug from the firewall. Two layers of heavyweight fiberglass cloth are applied over the Styro- foam™ plug using epoxy. Fiberglass resin isn’t used since it will partially dissolve the Styrofoam™. The fiberglass cloth can be cut into sections and overlap. After the first lay- er has dried, rough sand it with coarse sand- paper then apply the second layer. Once dry, trim the excess cloth away and remove the Styrofoam™ plug. Imperfections in the cowl are filled in with a mixture of epoxy and mi- cro balloons. Small hardwood blocks are epoxied inside the cowl with #2 screws hold- ing the cowl on. Wing: This is built in four sections due to the polyhedral. I made the outer panels re- movable which makes transporting and storing it easier. If you prefer the outer pan- els can be glued in place which will save on some building time and weight. The wing spars should be hard balsa. Try to select sticks that are straight to reduce the chance of warps. The spars are inside
rear hold-down. The hole at the front of the hatch is for a 3–48 hold-down bolt. The side view of the hatch (above right).
the wing which gives the wing a smoother appearance when covered. Trailing edges were redi-formed, also listed as aileron stock, or you can shape them by using a sanding block. This should be done before gluing them in place.
Construction is started on the center pan- els first. Pin the leading and trailing edge down over the plan. The bottom center sec- tion sheeting is also pinned in place, then W1 and W2 ribs are glued in. A 1⁄16-inch space is left in front of W1 for the wing brace B1. The center 3⁄16-inch rib is glued in when the two center panels are joined together and the power pod core P1 is added.
If you are going to make the outer panels removable, drill a 5⁄32-inch and 1⁄8-inch hole in the end ribs W3 for brass tube joiners. For a one-piece wing, cut slots for the dihedral braces. Glue W3 in place. The main spar can now be slid in. If the spar doesn’t want to slide through a rib, trim around the spar slot. Don’t try to force the spar through or you may break a rib. Once the main spar is glued in, slide the rear spar in and glue. The outer panels are built like the center section. If you make the wing one piece, lay the center panel down flat and raise the out- er panel up 11⁄2 inches. Rib W3 and braces B4 and 5 are glued in as the panels are glued together. Repeat for the other side. To make the outer panels removable cut pieces of 5⁄32 and 1⁄8-inch brass tubes to length and crimp the ends so the wire join-
ers won’t slide out into the wing. The brass tubes are roughened up with coarse sandpa- per for better glue adhesion. Cut 1⁄8 and 3⁄32- inch music wire to length. Slip the music wire joiners in both sections of the brass tubes then fit them into the wing panels as they are joined together. Align the brass tubes in the wing so the wire joiners slide in and out easily, check to make sure the dihe- dral is correct. The brass tubes are secured to the wing spars with heavy thread and epoxy or CA.
To join the center panels together slide di- hedral braces B1, 2 and 3 in place, then raise the panels up to the proper dihedral angle. This should be 11⁄2 inches at the outer panel joint and 4 inches at the wing tip. Glue braces B1, 2 and 3 and the center panels to- gether. The 3⁄16-inch center ribs, W1, can now be slipped in place.
The spar slots are cut all the way to the bottom on these ribs so they can be in- stalled from the top. With these ribs in place, add the power pod and check fit. It should be straight when viewed from the front, that is it’s not leaning to one side. Ribs W1 and the power pod are glued in; don’t skimp on the glue here. Glue balsa in where the hold-down bolts go, and behind brace B1.
Sheet the center section top and drill holes from the wing bolts. Glue 3⁄16-inch balsa to the pylon core sides. A ¼-inch hardwood dowel is epoxied on the bottom of the center
The installation of the control horns shown here (above left) is a little different from what is shown on the plan. You can do it either way. For the rudder, a tiller
20
arm (above right) is used and a 2–56 aileron coupler. The stabilizer is glued to 1⁄16-inch plywood mounts as seen here.
SEPTEMBER 2013
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