CONTROL LINE AEROBATICS
Frank McMillan, 12106 Gunter Grove, San Antonio, TX 78231
There are many times I wish I could deliver a new airplane to someone who is really good at sorting out problems, and pick it up later — all trimmed out. For most of us that simply is not possible, so we have to struggle and endure. That means it behooves us to understand the trimming process. There have been some excellent articles in Flying Models, PAMPA, and Model Aviation; seek them out and read them thoroughly.
A very narrow area of trimming I want to discuss is tip-weight adjustment. Until the advent of tip-weight boxes in the early 1970s, most plans called for approximately 3/4 ounce (or, more precisely, one rolled-up empty Ambroid tube). As adjustable tip weights became popular, many experts advised loading the tip until the wing hinges or throws in certain spots — like the top of the hourglass or outside squares. That is good advice when you observe well, build straight, and are in the ballpark with the weight. However, many interrelated factors affect the correct amount of tip weight, and small changes can move you far from the best setting.
Factors that affect tip weight
- Line drag: Tip weight balances the drag of the lines. Line diameter and length directly influence required tip weight.
- Wing asymmetry: For many years equal-panel wings were fashionable. Because the outboard wing moves faster than the inboard wing, equal-panel wings typically require considerably more tip weight (sometimes over two ounces).
- Inboard/outboard panel length: Classic stunt designs often used a 1-1/2 inch longer inboard wing, reducing required tip weight because the extra inboard lift balances the faster outboard panel.
- Aspect ratio: Classic wings had aspect ratios around 5:1 to 5.5:1. As ratios increased in later designs, tip-weight needs shifted.
- Flap-area balance: Newer designs sometimes have longer inboard flap area. The common fix is to add a little chord to the outboard flap tip (about 5/32 inch), which also changes tip-weight requirements.
Common problems and trimming procedure
I’ve chased my tail trimming new airplanes. One situation: a perfectly straight wing with too much tip weight. The wing seemed to roll when changing upright to inverted, and the outboard wing flew low. Tweaking flaps induced roll and started a chain of competing trim problems. Eventually I removed tip weight and retweaked the flaps; as the weight came off, the flaps returned toward neutral and the airplane flew cleaner. It cost a lot of aggravation to get there, so learn from that experience.
Recommended trimming approach:
- Start with the designer’s suggested tip weight. Some airplanes end up with no tip weight at all; no two built wings weigh the same.
- Work tip weight before you start tweaking flaps. Assume the true wing condition:
- If the outboard wing hangs down (low), remove tip weight to try to get the wing level.
- If the outboard wing is high, add tip weight.
- After you have the wings level, then evaluate the airplane and make smaller adjustments with flaps or other controls as needed.
- Aim for the least amount of weight at the wingtip that still gives good handling. Less tip weight is generally more predictable and less likely to interact poorly with other trim adjustments.
New Items
The Precision Aero series of engines by Randy Smith launched successfully with the .40 model three years ago. Competition results demonstrated high standards of performance. Early last summer Randy released the .51, a bored-and-stroked version that retained the same externals and is a drop-in for .40 mounts.
I have been running the .51 for approximately a year and can unequivocally state that it is extraordinary: turbine-like smoothness, exceptional power, and very linear adjustment. Over many years of competition, this is the best engine I’ve had; they are very consistent from example to example.
The next development is a .61. At the time of writing there are prototypes flying and production examples are imminent. Early reports echo the same characteristics seen with the .51 vs. the .40: more power, smoother operation, and greater consistency. The .61 easily turns 13 x 4 two-blade and 12½ x 4 three-blade carbon props.
Why upgrade? The real differentiation shows up in heavy wind and turbulence: the .61 holds the model well, resisting upset while still powering through maneuvers. As with other PA engines, these can be run with or without exhaust tuning because the tuning is mild. Another characteristic of the PAs is that they are exceptionally docile for their power output.
I will report more as I get further flight experience. If you are interested, consider reserving one early — production runs are limited (the first run will be 100). A new pipe design is being developed specifically to extract the most power from the PA .61.
There are other new releases from Aero Products.
Transcribed from original scans by AI. Minor OCR errors may remain.
