CONTROL LINE AEROBATICS
By Frank McMillan
12106 Gunter Grv., San Antonio TX 78231
In the last column I discussed getting ready to build and a philosophy to get the most out of your available time. Hopefully some of the points connected, and you’re on your way to a successful building season and an even more successful flying season.
This time I’ll follow up on construction and take a few steps back to look at some details that will make your model more functional, lighter, and durable. Realize that these points will not affect how your kit, design, or original performs. So why even consider incorporating them in your package? The short answer is that they do help the “big picture.”
Wing ribs: accuracy, wood selection, and cutouts
Let’s discuss wing ribs. Accurate ribs are a primary concern, as is the selection of good wood. Quarter‑grain, lightweight wood is preferred, but here is where a real problem can occur.
Less-experienced builders will typically try to save more weight by taking too much out of the ribs with cutouts. Shape of the cutouts can also be a major problem.
Some years ago I observed a hard landing at a contest. I thought “no problem,” then I noticed that the covering on the wing had shattered and the interior of the wing needed major repair. The ribs had fractured along the grain. This was puzzling; the hard landing didn’t seem severe enough to produce the observed results.
However, the causes became obvious as we all looked more closely. The wood used for the ribs was clearly a well-defined “A” grain that ran from leading edge to trailing edge. The ribs had been cut out to lighten them, and the fractures started at the points of the cutout.
There were actually two problems: the shape and size of the cutouts, and the wood grain. The cutouts consisted of lines that paralleled the outside shape of the ribs (everybody does that), but those lines intersected with vertical lines close to the spar. This created a near-right angle and a stress concentration—big time. This effect was significantly highlighted by the wood-grain selection. When the model impacted, all these problems relieved the stress by fracturing most of the ribs. I doubt this would have happened had there been better ribs on the model. Remember that all the details count together, and the weakest point will fail.
Making radiused cutouts
What I’m going to suggest is not new, but it is worth revisiting. Bob Hunt is a super practitioner of artwork in stunt wings. When cutting rib-lightening holes, he uses large-diameter brass tubing, sharpened, to obtain radiused cuts as trace points on each rib. The reason is that the tubing provides an extremely smooth cut that you can’t obtain with an X-Acto blade. The procedure is to make the circular cutouts, then connect the radii for an elegant-looking cutout.
Attending to making these nice cutouts are location and weight savings. Strangely enough, the primary consideration in this case is strength. Too many builders try to save weight by taking out too much material. If you are using good wood, you can’t take enough additional wood out to matter over what is necessary for reasonable strength. A ballpark figure would be at least a 1/4‑inch margin around the cutouts. By the way, take some extra time and care to line up the cutouts.
As I’ve stated many times, building is just like flying. The more frequently you do it, with plan and purpose, the better you get. Also, the longer you don’t perform an activity, the less proficient you are in that activity. There are numerous examples, such as covering with silkspan or carbon veil or setting up a control system.
However, you can and should learn from each model you construct and fly. Evaluate the design and methods you used, and see where there is room for improvement in your techniques and procedures.
Review of other details
Let's review a few details, just to give you an idea of what to look at. Be aware that I'm including kits, plans, and personal designs in this discussion, because everyone has a favorite way of doing things. The way you develop a design has as much to do with how you do things as how the original designer has his set up.
The more you fly, the more you find out what works. A great case in point was Al Rabe when he was in the "all out." Everything had logic to the end that when he crashed (he would leave no evidence on the field), he would dissect the pieces to find out what was too strong or too weak.
For what was too strong, he would analyze the area to see where he could save weight. For what was too weak, he would see where he could adjust the structure to optimize weight and strength. It's a great lesson!
Many think of model design as the outlines, areas, and moments, but there is much more. Take the cowl‑separation line, for example. It is probably okay if you're going to have a one‑piece cowl. But look at the power package you are using.
Was the model designed for your setup? Where are the items you need to get to, such as the needle valve, glow plug, pipe setup, and disassembly concept? Have you ever gotten to the point in construction where it dawned on you that something was amiss because your package wouldn't fit? This is what I'm getting to. If you want to get the most from your model, you have to be able to "service" or maintain it. That means the parts that can wear or require adjustment or change must be accessible.
Items to consider making accessible:
- Needle valve
- Glow plug
- Muffler/exhaust and pipe connections
- Anything that will require periodic adjustment or removal
Front‑end design for serviceability
My approach to the front‑end design of the fuselage is to remove the power package (engine/muffler, pipe, etc.) as a unit once the cowl is removed. To do this, I establish the cowl‑separation line through the needle‑valve centerline and the exhaust projection, if side‑exhaust/muffler configured.
If pipe‑equipped, the engine lifts out and the engine/pipe assembly slides out. This buys you the ability to work on your engine outside the model without changing any settings, unless you want to.
Remember that simple is better when you consider how to design your model. The more complex you get, the more likely something will fail. If you're just starting to fly more‑complex models, study all the plans you can get for ideas and approaches to problem‑solving.
Design details and evolution
The more you build, the more you learn your personality for building. By that I mean you will like certain methods and approaches to construction. That's really what you're after, because you will invariably compromise your model if you are not comfortable. That doesn't mean you shouldn't try new techniques, but a good stunt program is typically an evolutionary process.
Look at Bill Verwaage and how he has evolved various designs of many models. He made small changes to proven designs and made them better.
Bill's USA‑1 is a World Championships Classic‑legal design that evolved through many changes. Bill flew the later models in many forms, up through the GEO‑XL. What he changed and why make an interesting story. The basic layout was set up for the popular ST .46 engine and the power available. This dictated a thin airfoil to reduce the frontal area/drag.
The next big step was an attempt to make a smaller variant (Bill tried quite a few slightly differing sizes to see if there was an optimum) to fly better. He cut into the leading edge of the wing in the outer approximately 20%, and wedged it open to thicken it.
Even with Bill's ability to control weight, this particular model had some "funnies" with the tips on hard corners. By thickening them — in total thickness, and moving the high point of the airfoil forward a bit — Bill got more lift at the tips. This stabilized them, and the "funnies" were gone. That increase was passed on to later examples.
Part of the moral of this story is, don't be afraid to cut into a model to make it work. A pretty, poor‑programming model does not do you any good!
What's next
In the next column I'll look into a new area of engine development: the four‑stroke.
This type of engine has long been popular overseas, but really hasn't been the subject of competitive application in stunt in the U.S. That's changing on the East Coast; many are flying these engines and seeing advantages.
MA
Transcribed from original scans by AI. Minor OCR errors may remain.
