CONTROL LINE AEROBATICS
Frank McMillan, 12106 Gunter Grove, San Antonio TX 78231
Introduction
Without getting too esoteric, I'll continue the discussion on building good-flying Precision Aerobatics models by focusing on building light.
Few Stunt fliers have ever said that their models were too light, and fewer still could consistently create exceptional-looking and exceptional-performing airplanes that drove down the limits. I don't believe Aerobatics models can be built too light; they fly better as you build lighter, assuming that you maintain structural integrity and stiffness.
How many articles have you seen that read, "My airplane weighed 66 ounces, but I believe it can be made at 61 ounces"? I have said those words myself at times, but I have gotten better throughout the years. Al Rabe, Bill Werwage, and Bob Gieseke are modelers who have kept their models on the low end of the scale.
Constructing light Stunters starts with discipline—it may seem strange, but it touches many areas. Consider the components of weight. Light, true, accurate structures take time and must be built without induced or residual stress. Wood selection is important, but not as important as controlling other factors (assuming that you don't get carried away). Associated with accurate structures are precise joints—the structure should be properly aligned without stress, and the minimum amount of adhesive possible should be used to make the structure "airworthy."
Adhesives and bonding
For structures, use the lightest adhesive that will do the job over time. I know I'll get some comments, but I have preferences:
- Quick-setting cyanoacrylate (CyA) for the majority of construction.
- Slow-setting CyA (15–25 seconds) for work that requires positioning—such as laminating fuselage formers (crossgrain 1/16" sheet or 1/32" plywood and 3/32" balsa). These pieces are cut to fit, then reinforced with Kwik-Set around the interior cutout.
- Hobbypoxy Smooth N' Easy (20-minute set) for the nose section, doublers, mounts, and top blocks. When the complete nose section is shaped and hollowed and ready for final touches, coat the inside with Smooth N' Easy to lock everything together and make it fuelproof. Apply heat with a hair dryer to help penetration and flow.
- Clear silicone to hold tanks.
- 5-minute epoxy to hold canopies.
Try them all, but remember that the main point is to use the minimum amount—that's where you save weight.
By "airworthy," I mean the models should be strong enough to sustain airloads and last indefinitely—a fine line to walk when you try to build as light as possible.
A competition Stunter is not supposed to crash, so it shouldn't be built or designed to survive crashes. Al Rabe, one of the most acknowledged lightweight designers, used to dissect his wreckage to investigate where he could reduce structure or change the design to save a few grams.
Structural considerations and stress control
In general, Stunt models are structurally overdesigned. However, avoid reducing structure in the nose section and wing center section, including the bellcrank mounts and landing gear mounts—pay strict attention to stress factors for longevity. Some piece of structure should be tied to the airplane's exterior skin. This may be difficult in some areas, but the model's long-term integrity is at stake.
Because of the single-cylinder engines' nature, it is impossible to fully suppress engine vibration, but good stress management helps dampen it.
Controlling stress in the structure reaps benefits. If vibration is better controlled, you can often reduce interior structure as long as you preserve proper stress transfer. This is especially possible farther away from the forward center of the airplane. Al Rabe used to carve away, with a Dremel tool, as much wing structure inside the skin as possible; Bill Werwage significantly reduced tail structure with his unsheeted Warren truss stab/elevators while maintaining sufficient stiffness.
Wing design and weight-saving ideas
There are design considerations for potential weight advantages.
- The spar cross-section is typically maintained from the center to the tip, but strength requirements diminish toward the tip. Consider tapering the spar and the leading/trailing-edge planking to save weight.
- For built-up wing structures, taper spars and planking toward the tip.
- For foam wings, decreasing the thickness of the foam toward the tip is viable. I don't like planform cutouts that some people suggest.
- If you have wing-mounted gear, laminate the clips and rib reinforcements from Lite Ply. Use two bolts instead of four and substitute aluminum 4-40 cap screws for alloy screws (aluminum is acceptable, except for engine mounting).
Wingtips: For years I've used well-hollowed tips and the best I could do was 0.9–1.0 ounce for both tips with adjustable tip weight box and leadout guide. By going to 1/2-sheet balsa outlines, you can save 0.2–0.4 ounces.
Tail surfaces: I've used 1/32-sheet vertical stabilizers for years and built-up 1/16-sheet-covered surfaces—weightwise these approaches are a toss-up. However, don't be tempted to use 3/8" or 1/2" sheet without cutting it out and inserting ribs.
Fuselage tips
The fuselage is last because you shouldn't skimp on the nose section. The section aft of the CG is where you can make real gains, because every gram saved there affects the moment and requires additional weight ahead of the CG to balance.
Example: save 1 gram at a 1.4 multiplier equals 1.4 grams saved, but you must add 1.4 grams ahead of the CG to balance, so the total effective saving is 2.8 grams. That's significant.
You can disregard convention and use 3/32" sides instead of 1/8". To maintain rigidity and strength, mold the top and bottom sections also from 3/32" balsa. This takes more time because you have to work up the molds, but it can be amortized over several models. Rigidity will be maintained by the accurate formers resulting from the molding process and the curvature of the shells once they are locked down. (Remember: sand the shells only on the male mold.)
Composites and carbon veil
I haven't gone into full detail on integrating composites into structure design. The primary thing is to research new products; tapered spar reinforcements that are thick in the center and taper toward the tip are becoming available. Incorporating these requires structure redesign using the principles described above.
An already popular and increasingly visible material is 0.2-ounce carbon veil as a covering. I've used it on several airplanes and it lives up to its claims. Application methods using dope work well:
- Nitrate method: Apply thinned nitrate dope directly through the veil to well-sanded raw balsa.
- Butyrate method (which I use): Apply the veil to balsa that has 2–3 coats of dope.
With either method, sand the veil after several coats of dope, then add several more coats followed by filler. The primary advantage is that the veil forms a very stable, light, hard surface shell. The ultimate hardness depends on the dope; it will develop over several months—especially with butyrate, which doesn't harden as rapidly as nitrate.
The veil makes a superior covering on solid surfaces, but it will not cover open bays on wings. It is not a woven cloth, so it is fragile to apply and sand until it has some dope to strengthen it. I have used the veil in multiple layers on the nose section to strengthen and shape the surface where there are low spots—just dope on patches and sand them.
Laminating with the veil and epoxy in the center produced no benefit for me. Most gain in strength and potential structure reduction comes when you push the composite to the surface of the structure.
Advanced composite methods
I'm leaving out the more exotic applications I've heard about, such as Paul Walker's carbon spar, which requires an autoclave to properly set the epoxy. Dan Winship has discussed carbon-shelled wings laid up on foam components, autoclaved, then dissolving the foam. These approaches are mature projects that require a significant amount of time to conceive and execute—perhaps someday.
When you mull over the factors of this necessarily brief survey, you should realize that you can't build light quickly; take the time to do things right.
New items
Dick Byron has photographed outstanding Precision Aerobatics flyers and airplanes in action for many years, and he has decided to sell 20 x 30 enlargements, all of which I recommend. They capture the essence of flight and are suitable for framing to hang in the enthusiast's shop or den. Also available are pictures of the World Scale Competitions (in which Dick has participated as a US team member).
Contact Dick Byron: Big "D" AeroArt 1812 Castle Rd. Arlington TX 76014 Tel.: (817) 461-7705
Transcribed from original scans by AI. Minor OCR errors may remain.
