CONTROL LINE AEROBATICS
Frank McMillan 12106 Gunter Grove, San Antonio TX 78231
In fine-tuning our beautiful creations I have discussed many features: engines, props, balance, etc. One major factor I haven't featured in depth is the direct connection between airplane and pilot: the flying lines. This article limits discussion to trimming aspects — not the selection of actual material (solid vs. cable, diameters, etc.) — but rather the relationships and effects of what the lines do.
Line length
First, let's look at line length and how varying the length affects the airplane.
Varying the length changes the lap time. Shortening the line length reduces the distance and therefore the lap time if all other factors are held constant. As a ballpark number, one foot changes about one-tenth of a second. For example, if you have an airplane flying 5.5-second laps and you cut a foot off the lines, you can reasonably expect the new time to be about 5.4 seconds.
Why vary line length? There are several relationships to understand before making decisions:
- Longer lines (up to the rules' maximum of 70 feet) permit larger maneuvers that remain within angle and altitude rules.
- Not all airplanes are capable of carrying maximum line length without compromise; engine/airframe power, drag, and handling preferences matter.
- Line length affects the look and feel of the airplane through the pattern; getting a "locked-in" look may require trading off line length.
Trends in trimming have moved toward a more rearward center of gravity (CG) compared to years past. A rear CG affects stick pressure needed to turn and reduces line tension. You cannot have very heavy line tension if the CG is substantially rearward. If you can accept the reduced tension, carry the longest lines you can reasonably tolerate while maintaining good "up-top" performance.
Lines develop drag that increases with length and roughly with the square of diameter. Realistically, it takes a fairly powerful airplane to carry 70-foot lines — usually a piped airplane on a big .60 engine. Most airplanes currently fly on 66–68 foot lines; a few use 70-foot lines. Smaller airplanes are the same in principle but, because of reduced horsepower, typically cannot pull the extra drag of long (65+ foot) lines. Generally, .35-size airplane/engine combinations settle out at line lengths of about 59–62 feet.
Most airplanes have a "happy zone" of line length where they fly comfortably: speeds are sufficient to generate correct lift in maneuvers, but the airplane does not speed up so much in maneuvers that the shapes become crowded.
Line tension and trimming
Line tension is an overlay on the other factors. An airplane may be properly trimmed aerodynamically but, at certain places in the pattern, it simply won't "hang out" and your shapes will be destroyed. Sometimes a very small change — shortening the lines by as little as six inches — can suppress this tendency.
When trimming a new airplane, I usually start very near the longest length I think the airplane will safely carry and then evaluate how it flies and accepts trimming adjustments. If it has good tension throughout the pattern, I work on other trim elements until I return to reassess the lines. Always ask: have I settled on the correct line length before I complete sets of lines for the contest season?
Final thought on line length, engine and speed
Arriving at an appropriate line length also involves the airplane's and engine's speed characteristics. There may be situations where you need to carry higher aircraft speed (for example, to prevent a heavy airplane from dropping out in the corners). In such cases you may lengthen the lines to fly the same lap time but increase aircraft velocity, which gives more space to maintain speed in cornering shapes. Engine rpm and prop pitch then come into play: you may want to spin the engine up for more power and lower prop pitch to maintain aircraft speed. The interactions can get involved — and fun.
— Larry Shoemake, Springfield, MO
New Products
One of the enjoyable aspects of our hobby is the continual emergence of new products. Here are a few recent items of interest, particularly engines and related products.
- K&B .40 (4050 model): The history of American modeling includes a big chapter under the K&B name, so a new .40 that appeals to the stunt community is welcome. The 4050 is a refinement of the old 4011 and replaces a lightweight front end with a beefed-up version that houses twin ball bearings. K&B also offers high-performance bearings with plastic retainers. The new engine is a true ABC design, with a chromed liner and a flat-top, high-silicon-content aluminum piston.
The porting scheme is somewhat unusual. It is not a pure Perry port design. Typically Perry ports are "Siamese" and angled to direct fuel flow across the piston. The Wiesneeki-style augmentor ports are circular and are plunge-cut square to the liner bore, as opposed to the Perry design which angles severely relative to the bore. The Wiesneeki design builds on earlier porting work that opened up the standard intake ports radially. The purpose of the porting is to facilitate a strong, deep four-cycle effect that hits in the first half of the exhaust. This is a domestically priced engine available through many discount mail-order houses.
- Dub Jett tuned-pipe .50: An established high-performance builder, Dub Jett, is collaborating with George Aldrich to produce a tuned-pipe .50. The engine is a variant of the successful Jetline, timed specifically for a tuned-pipe installation.
Check these products out through your usual suppliers.
Transcribed from original scans by AI. Minor OCR errors may remain.
