CONTROL LINE AEROBATICS
Frank McMillan 12106 Gunter Grove, San Antonio TX 78231
ACCURATELY BUILT MODELS FLY BETTER!
No one who has built flying models will dispute this, but it's difficult to quantify and focus this broad statement.
We all know that there is no such thing as absolute accuracy, and that accuracy takes time and can increase costs. So how far does one go in trying to achieve the ultimate—and what is the ultimate?
Paul Walker once wrote that he could tell if an airplane was misaligned by 1/64 inch. Now that's not a lot in any dimension, but bigger problems are often the result of an accumulation of these small errors.
From a practicality standpoint, we should try to construct all of the subassemblies (the wing, tail, and fuselage) in fixtures or jigs, with reference to flat, true surfaces. The better the fixture, the more automatic the accuracy of the piece.
Let's look at fuselage jigs. Generally, they provide a flat base (with an inscribed centerline) with sliders that hold the sides and formers in alignment.
Wing jigs have evolved into several different forms:
- Tube- (or rod-) suspension types
- The Hunt lost-foam-cradle method
The tail surfaces are constructed on a flat surface using traditional methods.
The resulting pieces must be assembled into an airplane. Most versions of final assembly involve a large flat platform (usually with a leveled hard surface) with center and hinge lines inscribed. The platform is used as a reference to measure perpendicularity and incidence. With enough care, these proper methods will result in an accurate airframe.
Constructing a unit in a jig, however, does not necessarily generate an accurate unit. The subtleties of construction can cause major problems. Once the potential trouble spots are understood, they're relatively easy to avoid.
This discussion was prompted by a conversation where a friend said that he had built wings on jigs but hadn't been successful in building one that was straight. The key point is to avoid putting any stress into the structure as you assemble it in the jig. The underlying principle is to make each part fit accurately.
Several years ago I was constructing a built-up wing using the tubes-through-the-ribs method. The tubes were 3/32-inch aircraft tubing; by sighting through the tubes they could be shimmed to a few thousandths. Construction proceeded routinely—the ribs were trued up using a long sanding block, and the leading- and trailing-edge planking was installed.
As an aside, the ribs for this wing were made using the time-honored sandwich method. Although this did result in usable ribs, I found the severely beveled leading edge to be a problem when adhering the leading-edge planking. I had to reinforce the ribs to provide enough gluing area. My recommendation would be to use the sandwich ribs as patterns to make usable ribs, with the outside edge as a reference. This will use extra wood, but it will result in a satisfactory wing. For the most accurate ribs use the lost-foam method!
Up to this point, the wing on the jig was "dead on": a 48-inch straightedge placed on the surface proved this. I planked the center section, then proceeded to the capstrips. When I finished, I found that I had induced a warp in the trailing edge.
This had happened to me before, so I gave the wing a shot of ammonia and figured that it would settle back to true when it dried. It didn't work.
Next I cut off the capstrips on the affected panel; I remembered that I'd been aggressive when fitting the first set of capstrips—I fitted them a bit long, then pushed them down to adhere. This makes for a very tight joint at the leading/trailing edge. However, it puts pressure on the wing away from the most rigid part of the structure—in this case, the leading edge. This warped the trailing edge.
If it were possible to cap the wing, top and bottom, one rib at a time, there wouldn't be a problem, but this usually isn't possible. The technique here is to very carefully fit all of the capstrips so that when they are glued in position there is virtually no contact with the leading or trailing edges.
The ideal situation for any joint is for the joining surfaces to be in smooth contact, without any pressure. Not only does this practice prevent warps, but it also results in the best glue joints. This method has many benefits: a close-fitting joint is stronger, and less glue is necessary—resulting in weight savings. One of the best ways to reduce weight is by taking out materials.
Remember that fitting every joint is important, and areas such as the spar slots are very important for alignment and weight reduction.
One method I recommend is to create special-purpose sanding tools for specific openings. For spar openings I took a section of 220 garnet sandpaper material and glued it to one edge of a 3/16 x 3/8 piece of spar material. After the material was set and trimmed with a knife, I sanded the long (3/8) edge smooth and perpendicular to the sandpaper edge. Now I had a tool to precision-fit each slot to the spar as I assembled the wing. Not much of an investment in time, but a vast increase in precision.
To summarize: strive for the highest degree of accuracy possible. Use jigs and fixtures to assemble your airplane. Pay particular attention to the fit of all of the joints—this will minimize stress and warps, and provide higher strength with reduced weight. Easy-to-make special-purpose sanding tools can make precision fits much easier!
TECH TIPS
This month I have a couple of tips from noted modelers Bill Werwage and Bob Hunt. These have been mentioned before, but were part of a much larger body of information—they work so well they need to surface again.
Have you ever wondered how complex, open-bay structures get buffed out to a beautiful finish? It takes plenty of time to sand the surface to a matte finish.
The difficult part comes in at the edges of the open bays. If you get careless, you'll go through the paint and/or paper, necessitating repair. It's hard to keep going when things are difficult and time-consuming.
Enter Bill Werwage's suggestion: use high-quality 0000 steel wool. Furniture makers use it on fine wood furniture.
When you first use steel wool, you'll think you have ruined the surface with scratches! Keep going! The scratches will even out into a nice smooth surface; it will buff out with fine compound. Turn the pad frequently, to keep a fresh work surface.
With a little practice, you will become more comfortable, and you'll wonder how you ever achieved a good finish without it. Thanks, Bill, for sharing.
In some of his later plans, Bob Hunt shows a trick for shaping blocks to precisely fit attached surfaces, such as vertical stabilizers, scoops, and cowl lines. When he first mentioned it to me it sounded too easy, but I later tried it and wished I had used it sooner!
- Trace the footprint of the part onto 1/32 plywood and cut it out. Make sure you have reference lines to help position the plywood on the block.
- The block should be almost ready for its final rough sanding. Tack-glue the plywood to the block in the exact location.
- Rough-in the block to the surrounding surface with 80-grit sandpaper on rounded blocks. Work with progressively finer sandpaper until you're satisfied.
Remember: you're trying to achieve a smooth blend of the line to the surface. The idea is to minimize any filleting material to reduce the weight of the airplane.
This trick really works well—give it a try. Thanks, Bob.
Transcribed from original scans by AI. Minor OCR errors may remain.
