CONTROL LINE AEROBATICS
Frank McMillan, 12106 Gunter Grove, San Antonio, TX 78231
Technology and regulation
This month I'm going to close out the discussion on future technology by commenting on finishes, props, and structures. At the Vintage Stunt Championships I had long conversations about technology with many fliers, in particular Bob Hunt and Bill Netzeband.
As we talked, it became clear that many of the technical points are already within our grasp. This, of course, begs the question: why hasn't someone gone all-out in a technology demonstration? The answer is equally obvious: most competitive fliers need a competitive model every year, so there is usually no time to make sweeping changes. Therefore, technology infusion has normally followed a methodical, evolutionary path. Hopefully, some of these columns may trigger projects that leap the current paths.
Speaking of changes: the advent of regulations governing organic materials emissions may force us into a new arena. We may be impacted in many ways because of the finishing materials we employ.
Have you noticed the dramatic price increases for butyrate dopes in the last couple of years? Epoxy users may remember a recent halt in K & D availability (everything is now available). Prices were also affected.
Automotive acrylic lacquers we use, single- and multicomponent, may soon be the thing of the past. Water-based paints seem to be the preferred choice because of controlled emissions, but consider how that affects some of our requirements, such as shrinking coverings over open-bay structures. There aren't any definitive answers yet, as the whole picture is not clear. However, it's a good time to start thinking and preparing for the future.
Where to focus: propeller development
A contestant at the Texas State Championships asked me where he should focus his efforts. Given that he had a decent model and his ST .60 engine looked okay, my quick reply was "propeller development."
There have been many references to moderately low, low, and very low pitch setups in magazines and the PAMPA newsletter. The classic non-piped 4-2 cycle engine benefits from increased rpm and a more advantageous spot on the torque curve. And, of course, piped engines are in their element with low-pitch props. But there are some new developments surfacing that are worthy of experiments.
For those running smaller engines (.30–.40), the Top Flite 11x4 Power Point is a must-try. The prop's raked tip is the reason for its name. The airfoil looks more like a high-performance free flight power model instead of the usual Clark Y.
Emerging prop trends
Another trend is severely undercambered props. Brian Eather, working with Dave Fitzgerald and Ted Fancher, has produced props whose distinguishing modification is the airfoil undercamber — only a few thousandths are visible when laying a straightedge across the bottom of the blade. Eather props are airfoiled like a Nordic glider: very thinned-out sections and high undercamber. Only quality handwork or carbon-epoxy makes practical trailing edges paper-thin. The technique of starting backwards from the leading edge must be employed.
Brian recommends depitching the prop because extra thrust is suspect; this is also why Dave and Ted use depitched props to get the best combination of higher horsepower, lower gearing, and controlled thrust. Light weight and high-strength carbon-epoxy moldings limit what prop development has yet reached.
It's difficult to predict the direction the next thrust will take in precision aerobatics applications. We know lighter props would be better because of gyroscopic effect, particularly at higher rpm. New resins and sophisticated fibers being developed may help. Some have tried larger props; some Russians have been helpful in using hollow-blade balsa-core blades—essentially a shell. As one ponders, however, one wonders whether the pitching procedure of heating and twisting would work with a balsa core.
Structures and stress concentration
I'll briefly revisit structures to reinforce the point about stress concentration. Using carbon fiber to strengthen and lighten structures should be done carefully. Free flight competition fliers have discovered that a too-rigid structure fails on high-impact landings. Towline gliders have had wing breaks rather than flex. When integrating composite materials, consider the real purpose for the change.
Because the stunter is subjected to dramatic reversing loads in knife-edge and flat spins, you may create a point of stress concentration by introducing a very stiff local reinforcement such as a strip of carbon fiber or a cap spar. The stiffened area then acts as an effective lever under load, which eventually fractures and breaks off the entire outboard wing. The point should be made that the original wing construction was sufficient to carry any load without the carbon.
Fox .35 update
Enough technology: it's time to share news about a new look to an old friend — the venerable Fox .35. For some time George Aldrich has been thinking about how to reduce the excessive vibration caused by the heavy iron piston of the Fox .35. He has prototypes of ABC cylinder/piston assemblies that are direct dimensional copies of the standard Fox setup. I saw them run, and there is no doubt this significantly reduces the vibration. This is not a hop-up with timing changes, so don't expect that.
The fact that George hand-fits every unit guarantees superior engines. The aluminum piston running on chrome will have a lengthy life, and this, along with the low vibration and fit, makes it a worthwhile change. George is offering two options:
- Purchase the cylinder/piston set as a drop-in exchange for stock parts and install it yourself.
- Purchase a complete engine.
Contact: George Aldrich, 12822 Tarrytown, San Antonio, TX 78233; Tel.: (210) 656-2021.
Fuel tanks: Kevin Smith
If you're like me, you hate to build tanks. This is a vital part of the package, but it is time-consuming and usually requires volume adjustment for each model. I had a conversation with Kevin Smith about his capabilities to fabricate tanks. This is the first occasion I've had to order from Kevin.
He has an extensive line of tanks, but I needed to squeeze as much capacity as possible into a short nose section. Kevin will bend shells or furnish complete tanks to your dimensions. The option I chose was shells that I could plumb the way I wanted and adjust the length to suit the duration I wanted. I was extremely pleased with the workmanship and materials. I recommend Kevin's work highly; you won't be disappointed.
Contact: Smith's Model Products, 521 Jensen Avenue, San Dimas, CA 91773-1617; Tel.: (909) 592-2100.
New profile kit
Kevin also has a new profile kit available: the Pathfinder. It was designed by well-known flier Gordon Delaney. Bart Klapinski recommends this design highly, so give it a try if you need a good-flying profile.
Transcribed from original scans by AI. Minor OCR errors may remain.
