Sweet Fifteen (oz735)

About this Plan

Sweet Fifteen. Free flight power model. For 0.15 engine.

Quote: "Slick new-rule free flight, uses powerful Cox .15 and new-way double covering. Sweet Fiteen, by Frank Ehling.

Every contest free-flight model builder has his eye on maximum performance with the least effort. Yet this combination escapes many - because of varying viewpoints on airfoils, construction and flight technique.

Through the years, with the introduction of new engines, design configurations and construction techniques, the free-flight conception has changed considerably. First the standard cabin type, then the Zipper (oz387) style and now we find essential the high thrust configuration with low underslung stabilizers.

Not only is Sweet Fifteen easy to build, but it is remarkably easy to adjust. This is always a big factor in obtaining top notch consistent performance.

The large flat bottom wing which provides us with ample glide potential eliminates the work of an undercambered job. Despite the fact that it's the size of models in the .29 power range; using the Cox Olympic .15 engine leaves nothing to be desired in the climb. This engine is real competition for the Oliver Tigre in the .15 class.

The high thrust line, permitting a more favorable location of the CG, eliminates the need for excessive downthrust in the engine. The ability to provide the best climb and glide combination without excessive downthrust also means our engine is operating at optimum thrust.

The rudder was placed on top of the fuselage, for handlaunching is all that is permitted. In this manner the rudder would not be subjected to wear. The wing tip ribs are thinned down: this works as if the tips were washed out, again without the labor involved.

To begin cut the parts that form the cabane and cement them together, then set aside to dry. Cut nose sections and fuselage sides, cement these together. The right and left sides differ so the fuselage will not have a sudden weaken ing in strength. Tack cement these to-gether and round off the nose section as shown. This is easier now than later when the sides have been cemented to the pylon.

Cement fuselage sides around pylon; see that the end of the fuselage is even, that one side is not higher than the other. Cut sheet triangle that forms the bottom covering and cement in place; this aligns the fuselage. The bulkheads are cut and cemented in place; then the top sheet cut and cemented. Use slow drying cement and pin edges securely so there will be no air spaces for these could weaken the fuselage structure.

Cement engine bearers in place, add balsa continuation at bottom to form curve; add filler of hard wood to bearers so engine will be centered, Install tank and timer and cover sides - grain of wood runs perpendicular to bearers, making it easier to cement in place.

Add wing platform, together with 1/8 sq runners, cement securely. Add rear tie-down for wing; drill hole, force 3/32 wire for front hold-down through nacelle. Sand pylon to section shown; round edges of nacelle so it protrudes ahead of wing platform. Add stabilizer mount, apply several coats of cement for a firm bond.

Cut rudder, slot fuselage, cement rudder in place. Note that it runs to the bottom of the fuselage and that it cements at that point also. The portion of the rudder which the stabilizer rests on is cut out from hard balsa. This will probably have to be trimmed _until model's glide pattern is established.

Cut wing ribs and assemble wing over plan. Cut only the end rib for each tip, the other tapered ribs are formed by sanding, using the end rib as a guide. Cut wing gussets and cement in place, he sure when panels cement together they are in line for this is where many wing warps occur.

Add the wing tips, these are cut out from soft balsa and serve mainly to ab-sorb shock. The entire wing can now be sanded, round off the trailing edge as shown. Shape the leading edge to con-form with the ribs; this is done with a model plane and some smooth sanding afterwards. Cement all wing joints well; apply at least three or four coats..."

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