RADIO CONTROL SCALE AEROBATICS
Paul Kopp, 1013 S. Sedona Ln., Anaheim CA 92808
I doubt that I would have difficulty finding 25% (1/4-scale) scale aerobatics modelers who agree that lighter airplanes fly better. However, hidden in that consensus is the fact that there is no agreement on what “lighter” actually means.
A few years ago, many builders pursued a single goal: build as light as possible—lighter was better, period. More recently, although building light is still important, a new philosophy of optimum weight has emerged—especially with the larger Giant Scale airplanes. A 25% airplane can’t be built too light because of scaling effects; that doesn’t seem to be true for some Giant Scale designs.
That doesn’t mean weight isn’t an issue; weight is always an issue, and excess should be avoided. As builders, we try to recognize the difference between necessary weight (structural weight) and unnecessary or extra weight.
Lightening takes intuition, engineering skills, and a healthy dose of common sense. A $15 postal scale (available at most office-supply stores) is a great help—especially when making component selections. The common-sense part is identifying weight opportunities without compromising the integrity of the airframe. I watched a YS 1.20 literally fall off the nose of a 1/4-scale Laser because the builder removed too much structure in his quest to build “super light.” There are limits.
Recognize also that a couple of ounces off the tail can be a benefit, whereas the same weight removed near the center of gravity may not. Lightening can also be an exercise in diminishing returns—especially on bigger airplanes. You can pencil a few kit modifications and plan the component group to finish light; after that, removing the next few ounces may require an inordinate amount of effort with questionable benefit.
Some builders have experimented with extreme lightening: foam wings with cavities, lightening holes in plywood formers, replacing ABS with fiberglass, cored-out foam decks, composite landing gear, etc. What emerged were airplanes that some pilots felt were “too light” (and in extreme cases, too weak), proving that not being heavy enough is possible. At this year’s Tournament of Champions, a few airplanes had lightening holes in the rudder—and big strips of lead under the tail.
Three schools of thought on building light
- Not building light (but claiming to): This consists of using a larger engine (sometimes much larger) in an airplane designed for a particular-size engine, reinforcing the airframe, and paying little attention to components or finishing. When the airplane finishes well above the manufacturer’s target weight, the builder concludes the heavier configuration is how the airplane should have been designed. A common example is 72-inch, 1.20-size airplanes mated with 1.80 two-stroke engines that tip the scale at more than 13 pounds. The airplane may fly acceptably by the pilot’s standard, but it certainly isn’t light.
- Building more or less stock: Using the recommended engine and a stock airframe may result in a reasonably light structure, but performance can be compromised by excess weight from poor component choices. Tires, tailwheel, spinner, servos, engine mount, batteries, and especially the engine, all impact finished weight and ultimately flight performance. Kit reviews often show finished weights one to four pounds more than published—significant on a 70- to 80-inch airplane. This generally occurs because builders made poor component or finishing choices, not because the published weight was understated.
- Modifying the kit: This involves making structural changes to reduce weight and is the most controversial. If you are interested in lightening a kit, talk to the kit designer or an experienced builder familiar with the airplane and proven lightening techniques. Many 70- to 80-inch kits offer opportunities for lightening, but you have to know where to start—and more importantly where to stop.
Component selection and weight savings
For many builders, careful component selection yields the best balance of weight and strength. Going through the airplane from the nose aft, here are common opportunities:
- Spinner and prop: Many modelers favor spun-aluminum spinners for rigidity. Manufacturers like Tru-Turn offer lightened backplates for some spinners at minimal extra cost. A 1–2 ounce savings at the very nose can equate to 3–4 ounces change in the moment about the firewall. I swapped a lower-priced 3.5-inch spinner for a high-quality aluminum spinner with a lightened backplate and saved 1.5 ounces; switching a 20-inch carbon prop for a wood Menz saved another two ounces. On a 14-pound airplane, almost three-quarters of a pound was removed from the nose by changing components, improving vertical performance and reducing nose heaviness.
- Engine mount: On larger airplanes with engines that bolt directly to the firewall, the mount is rarely an issue. For smaller airplanes, however, nylon fiberglass-filled mounts offer plenty of strength at roughly half the weight of metal mounts.
- Cowls and wheel pants: Compared to fiberglass, ABS cowls may add more than a quarter pound and substantially more on larger airplanes. Aftermarket fiberglass suppliers offer lighter cowls and wheel pants. Midwest Models, for example, omitted ABS parts from the 80-inch Extra 300XS kit and included coupons and supplier contacts for aftermarket fiberglass parts.
- Tires and tailwheel: Du-Bro and Sullivan offer lightweight tires that are much lighter than their regular lines. Tailwheel designs and weights vary considerably; some manufacturers use dual-spring setups for a scale finish. When selecting a tailwheel, get an idea of whether you will need tailweight.
- Batteries: Battery choice significantly affects weight. Sub-C Ni-Cd packs are discouraged—they are literally dead weight, even in 40% airplanes. Smaller, high-capacity Ni-Cd packs are a better choice. Duralite packs provide higher capacity than Ni-Cds with a significant decrease in weight. However, there are limits: I hear of modelers flying 30% and larger airplanes relying on a single pack. The slightest failure can destroy thousands of dollars and many hours of work. The insurance provided by a redundant or parallel pack is well worth the weight. Two Duralite packs may weigh less than a single Ni-Cd pack.
The bottom line: consider component weight as a group. One reason pilots select larger engines is poor performance from an airplane finished heavier than the designer intended. Often the real fix is to reduce finished weight—not strap on a larger engine. A competition builder pays close attention to minimizing excess weight.
Lightening extremes and cautions
Some builders have taken lightening to extremes—lightened foam wings, extensive formers cutouts, and other aggressive techniques. While these can reduce weight, they also reduce robustness. If you plan structural modifications, consult the kit designer or an experienced builder first. Many kits have obvious opportunities, but removing too much structure can compromise safety and durability.
For the best performance, optimize airplane size, engine choice, and your component group. Don’t focus only on “building light.”
New Product
Dave Patrick—one of the innovators of light 1.20-scale aerobatic models—is introducing a 76-inch-wingspan Extra 330L. The target weight is 11 pounds, so it will be a good candidate for a .40 engine. Early pictures look good!
MA
Transcribed from original scans by AI. Minor OCR errors may remain.
