Control Line Aerobatics
By Joe Daly | [email protected]
As seen in the October 2023 issue of Model Aviation.
THERE HAS BEEN a recent surge of interest in vacuum-bagging and composite-building methods for Control Line (CL) Aerobatics (Stunt) airplanes. This growing fascination can be attributed to the scarcity of contest-grade balsa wood and the rising costs of materials. As a result, there is a constant search for superior techniques that allow for the construction of lighter, stronger, and straighter airplanes. In this month’s column, I will delve into the method of composite-building a fuselage by exclusively using a male mold.
The fuselage is ready for assembly with a total weight of 3.2 ounces, which is pretty good for an airplane of that size.
There are numerous approaches and material choices for creating a male mold. I have had great success using blue insulation foam or 2-pound Styrofoam because of their ease of shaping and cost-effectiveness.
To fashion the fuselage mold, I layed out the formers and cut the foam around them to form sections of the fuselage. Subsequently, I glued these sections together and sanded them to their final shape. To enhance the mold’s strength, I finished it with fiberglass cloth. Additionally, I applied a finish because I later employed the male mold to create a female mold, which will be the subject of a how-to article for the Precision Aerobatics Model Pilot Association’s (PAMPA) Stunt News!
After the male molds are complete, the next steps involve molding the filler material. Balsa wood and foam are the most commonly used types of material for this purpose. In my case, I utilized 8-pound, 1/16-inch balsa wood. To conform the material to the mold, I moistened it with Windex or water, and then secured it to the mold using an Ace bandage. After the wood dried, I removed it from the mold.
This is the beginning of the male mold. The blue foam works well for these types of molds.
The male mold is finished, primed, and ready for use!
Let’s move on to the molding process. For the fuselage displayed in the accompanying pictures, I employed .002 carbon-fiber veil as the outer layer. Once again, there are several material options available, including fiberglass cloth and carbon-fiber cloth, both of which have proven successful in my experience.
I cut the carbon-fiber veil to size and placed it on a sheet of waxed paper. I then mixed approximately 1 ounce of West System 105 Epoxy Resin and applied it to the carbon-fiber veil, ensuring complete coverage. It is important to minimize weight, so I used a heat gun on a low setting to warm the resin, and then pressed paper towels onto the carbon-fiber veil to soak up the excess.
After the excess resin has been absorbed, I placed the balsa skin back on top of the mold, followed by the carbon-fiber veil, and shaped it accordingly. I then placed it in a vacuum bag and proceeded to evacuate the air to achieve a vacuum pressure of 4 to 6 inches of mercury. While pulling the vacuum, I shaped the vacuum bag around the corners to minimize wrinkles. After leaving it under the vacuum for 24 hours, I removed it from the bag, which revealed a prefinished, lightweight, and rigid fuselage skin.
The balsa wood skin is shown after being formed over the mold. The author put plastic wrap between the mold and balsa skin. The carbon-fiber veil is ready for resin.
The carbon-fiber veil and balsa skin are in a vacuum bag for curing.
The fuselage side was removed after 24 hours in the vacuum bag. The weight of the fuselage half was 1.5 ounces.
This shows the fuselage halves with the formers installed. This structure becomes extremely stiff after the formers are installed.
It is worth noting that this is just one method for creating composite parts, and there are numerous modelers who have employed this technology for years. As the cost of equipment and materials continues to become more affordable, while the cost and availability of traditional building materials rises, this method of construction is becoming more prevalent.
One thing that characterizes CL Precision Aerobatics modelers is their innovative nature and their commitment to finding new ways to build better, lighter, and more rigid Stunt airplanes!
