CONTROL LINE AEROBATICS
Frank McMillan, 12106 Gunter Grove, San Antonio, TX 78231
In my previous column I discussed the importance of trimming the wings level as it relates to getting an airplane to fly well. Each adjustment in a model has an impact, and all of them are interrelated. Each also has a range or zone, similar to what sports people use as a reference. When one adjustment is out of the zone, it's difficult to really tune the others. When you're close to the zone, the general trimming process is reasonably linear.
One reason why experts have less trouble initially trimming a new model is that they preflight—pretrim—the airplane with known or reasonable settings on all adjustable features.
Center of Gravity (CG)
The adjustment I want to discuss is the center of gravity (CG). I've written about the wing CG (tip weight/leveling the wings) in a previous column; now I'll focus on the fore/aft CG.
The first point is how much the fore/aft CG can affect the airplane's performance. There is also a range of adjustment where all other adjustments work. But you need to be able to recognize the symptoms of an "out of range" CG. In some airplanes, the range can be extremely small—sometimes a few grams can show an effect.
With my Aldrich Magnum, I was experiencing a slight tracking problem with inverted flight. I tried a three-gram increase in nose weight and the tracking improved, but my corner was seriously reduced. I pulled the weight and the corner returned.
A forward CG (nose-heavy) isn't too bad except in the extreme. For many years, a forward CG was used to gain line tension and soften the turn; earlier .35-size models didn't have the horsepower or recent technology. Some designs with minimum wing area and large flaps benefitted from a forward CG; it allowed or required more elevator deflection to turn the airplane with corresponding large flap deflection, hence lift.
Following are some traits to look for if you have the CG too far forward:
- A rock-stable groove in level flight.
- Nice tracking with positive rounds and bottoms in the cornering maneuvers. Some fliers (e.g., Jim Casale and Bob Baron) have favored a nose-heavy trim and done well.
- Tendency to push or stay in track in wind conditions, which requires increasing control pressures to turn the airplane properly—this stresses the pilot and can lead to loss of the model if not handled.
There are ways to moderate the increased control pressures:
- Increase mechanical advantage by using larger bellcranks.
- Use wider handle spacing.
- Use longer horns.
Bob Baron does well using very wide handle spacing; Al Rabe used very wide spacing with his semiscale Mustangs. I measured the handle used on my Martin Baker at 5.5 inches.
I recommend avoiding the dead nose-heavy trim. The general flier is looking for an overall setup that serves in all conditions a pilot can experience. The nose-heavy setup can make a flier apprehensive about wind and control force increases. Don't discount it if you haven't felt it; it's real, and many have lost models by not being able to handle the situation. Worse yet, you may avoid flying in the wind, which inhibits the competitive process.
The other extreme is an out-of-range rearward CG. This can be insidious because current designs often have long tail moments and large tail volumes (stabilizer/elevator areas in the 24%-plus range). These accurately built models track well, corner well, and generally seem to fly well, and they also exhibit light control pressures.
The problem arises in trying to trim this condition. If the CG is too far rearward, the model will tend to roll inward when it is turned. The danger is that you may not notice this as a CG problem and start to correct it with other changes, such as tip weight or flap tweaks. I've done this and seen it in other fliers' models. You start chasing a good setting and no combination works to your satisfaction—nothing seems linear. This is your cue to check the CG. Start moving it forward (add nose weight), and you should see the rolling tendency diminish when you get in range. At that point you'll probably have to retrim, but the process will be linear.
Remember, you won't run into these problems if you follow the designer's instructions—but many of us don't. (It's not macho!)
Products
It doesn't seem possible that it's been two years since Bob Hunt first issued Stunt Flyer Video Magazine. The subjects are of interest, usually visits with "the famous and even not so famous," as Bob puts it. Of even more interest are the "how-to" features that have become the foundation for the magazine.
The last issue includes a visit with one of the most famous personalities in Stunt—Bob Gildalin. The accounts of his early modeling days are fascinating. This piece, and many others, make up wonderful documentation of the history of modeling. Bob Hunt has said that he will donate a set to the AMA museum for preservation.
As a technical subject in this issue, Bob continues with the construction of a sample Stunt machine to demonstrate certain techniques. The subject is proper carving of fuselage blocks; it's very useful to builders of all types of models.
If you have ever noticed a particularly nice model, one of the things that distinguishes it is the flowing lines and shapes. Conversely, merely rounding the edges confirms a less-than-knowledgeable craftsman. Bob does an excellent job of transferring plan shapes to blocks and establishing a successful sequence. One significant portion describes in detail how to use various sanding tools to achieve the desired shapes.
The format of the how-to in Stunt Flyer Video Magazine is a valuable ongoing reference source. I recommend the videos to all model builders. Contact Robin's View Production, Box 68, Stockertown, PA 18083; Tel.: (610) 746-9638.
Tips
I have trouble with stencil residue when I'm working on a new model; the adhesive softens and remains on the surface and is difficult to remove. I've tried Sikrens M-600, Reprol, etc., with mixed results, but nothing is effective. However, Bill Verriage mentioned a 3M product: General Purpose Adhesive Cleaner, part no. 051135-08984. It works extremely well. Also try it on hinge-sealing tape to remove both the tape and the residue.
A useful technique is to apply the cleaner with a Q-Tip at the point where the tape is separating from the hinge line. Slowly work along the hinge line, and the tape will come up as the cleaner contacts the adhesive. Be careful with foam wings, however; the cleaner will melt the foam. Check for the cleaner at your local auto body store.
Transcribed from original scans by AI. Minor OCR errors may remain.
