CONTROL LINE AEROBATICS
Frank McMillan 12106 Gunter Grove, San Antonio TX 78231
WHY DO THE SUCCESSFUL FLIERS seem to have everything work well on almost every flight? They don't, but it seems as though they do, because they pay attention to details. The details, or technical solutions, evolve over time, are proved by field testing, and if adhered to, provide solid flying packages.
Often these "details" seem insignificant, but can have an impact on how your airplane performs. For example, look at tolerances in the control system. Do the experts pay attention to what's happening here? You bet!
There are several philosophies or approaches about how to set the systems up, which we should discuss before we get further into the control system. For the moment, we'll limit the discussion to leadouts, bellcrank, horns, and pushrods — the heart of any control line model.
Leadouts
There are two options: solid and cable. Each has positives and negatives and the final choice is personal, but the choice should be made knowing the "playing field."
- Solids
- Solids are simple to use, and proponents say they provide a very positive feel.
- No matter which type of bellcrank is used, the leadout holes do not have to be bushed to guard against wear.
- In installation, take care in bending the wire as it goes through the bellcrank to prevent angles that can and will fatigue and fracture. Keep the angles very shallow in the joining loop.
- The joint should be soldered, per the AMA rule book, or it will slip under a heavy pull-test.
- At the wingtips, solids are always susceptible to an accidental bend, with all the associated implications.
- Cables
- Cables are flexible and therefore not prone to being bent and fractured, but they have problems opposite those of solids.
- Over time, cable can saw through materials or resaw through itself. If your bellcrank is Delrin, nylon, carbon composite, or phenolic, you must develop a system that bushes the cable, preventing it from rubbing on the plastic.
- One solution is a semicircle of 1/16" brass tube, with the cable looped inside and held in position with epoxy. This has worked for me, but recognize the problem and find your own solution.
- Old-fashioned aluminum or steel cranks must also be bushed, because metal-on-metal causes fatigue, wear, or both. I've seen crashes where cable in an aluminum bellcrank wore through in just a few weeks of flying. You don't want this to happen on a good airplane for want of another few hours' work.
Bellcranks
The bellcrank is the heart of the system, so take care. Look at the really competitive designs that have been published and focus on how the bellcrank was installed.
A post of 1/8" music wire has proven satisfactory if well supported. Don't use bolts; the actual diameter (affecting strength) is much smaller than the major diameter.
For plastic bellcranks I prefer at least a 1/4" diameter bearing. My thought is that 1/8" wire does not provide sufficient surface area to hold up to heavy tension and vibration over time. This means a turned bushing, preferably phosphor bronze or brass, which has a nice tight fit to the wire and bellcrank (approximately .001"). Dan Winship, in the carbon composite he manufactures, also provides a complementary music wire fit.
During installation, it's important to keep heat away from the plastic parts. Practically, this can be accomplished with inverted eyelets and soldering the joints away from the bellcrank.
Connected to the bellcrank via the pushrod is the flap horn — one of the most critical, high-stress areas in the control system. The fit of the pushrod wire to the bearing in the horn must be correctly aligned to prevent excessive wear. This bearing can be machined or simply the 1/8" I.D. brass tube that has been in common use for decades.
This serves well if the wire is the correct size. In reality, the commonly available "1/8" wire is undersized, measuring .092" rather than .09375". This gives a sloppy fit in a very critical area, so try to find some true 1/8" wire or make a bushing with the correct size. There is a metric drill (2.35 mm) that's .092", but you've got to hunt a bit to find this size.
There is another problem with the current wire available. Bob Gieseke mentioned that wire he recently purchased from several sources broke under one right-angle bend. This indicates too-brittle temper in the wire. I don't know how widespread this is, but it would be wise to run tests before you bet your airplane on defective wire.
Now the flap-to-elevator connection. Here is where we get into some controversy: "To slop or not to slop, that is the question." There are two schools of thought:
- Some believe in allowing some loose movement of the elevator (approximately 1/16" up and down). This is viewed as providing inflight benefits. A pleasant groove to level flight can be achieved, albeit with a forward center of gravity. Tracking is supposedly improved.
- The other approach is to use as tight a fit as you can achieve. This seems more attuned to the current trend of positioning the center of gravity (CG) as far back as the airplane will tolerate. Tight controls permit more precise control.
No matter how tight I get the controls initially, I usually experience some play in the system that stabilizes after the first 100 flights.
My personal recommendation is the latter approach. Over the years, intentionally sloppy control systems did groove well but presented loop-tracking difficulties and imprecise bottoms. Provided the airplane is accurately built, the tight system tracked better and gave more precise control. Since we are aiming at longevity in our system, the "tight" system will last indefinitely. The "sloppy" system will pound itself out, eventually forcing you to rebush the system—usually after one season.
To summarize: Pay attention to alignment. Any errors will tend to compound with flight loads and vibration. Try to get the wire-to-bearing fits as tight as possible. Take precautions to isolate any plastic or epoxy-bonded units from excess heat. For example, when you assemble the carbon pushrod and solder the keeper washer to the wire, consider a heat sink to isolate the epoxy joint. A hemostat or small Vise-Grip pliers will serve nicely.
This is a critical area, and the more time you spend here, the greater will be your reward — with better performance and longer system life.
Tech Tips
Classic Stunt continues to increase in popularity and has drawn many fliers back to their control-line roots. If you strive to make these recreations as close to the way they were, you also try to use the engines from that bygone era. Although they may sometimes be difficult to obtain, they are available, and they often offer the challenge of making them run well.
I recently ran across a problem where I could not get two different-make engines to run without severely choking them. They would start and essentially burn out the prime but not draw fuel — even with the needle valve wide open.
George Aldrich picked up on the solution: Some of the old needle-valve bodies were intended for pressure use or were erroneously drilled with a very small outlet hole (approximately .040"). The solution is simple: drill out the body hole to .055" (use a .054" drill) and the problem goes away.
Transcribed from original scans by AI. Minor OCR errors may remain.
