Author: Bob Aberle
Edition: Model Aviation - 2011/12
Page Numbers: 76,77,78
,
,

Frequently Asked Questions — 2011/12

How to submit questions

Please write in with your questions; that is the only way we can keep this column format going. When referring to previously published questions and answers (for follow-ups), always provide the number as a reference.

Also note that references to addresses and websites are placed at the end of this column under "Sources." A "Tips" feature is provided in addition to frequently asked questions; these hints are numbered in the same sequence as the questions and answers.

Q530

Q530: "Many fliers would like to combine the high-energy density of LiPo batteries for use with conventional RC receivers and servos. The average airborne RC system needs 4.8 to 6.0 volts. But LiPo battery packs come in multiples of 3.7 volts per cell. One cell is not enough, while two cells in series might be too much. Can you comment on what these receivers and servos can take (voltage-wise)? Also, is there some kind of a voltage reducer that I might purchase?"

A530: RC receivers and servos actually have different voltage tolerances. If you asked the manufacturer of your particular system, you would find that a typical RC receiver can work upward of 9 volts and even much more.

Servos are a different story. The electric motors inside every servo are wound for specific voltage ranges to provide speed and torque at maximum efficiency within that range. A servo designed for 4.8 to 6.0 volts will be overdriven if 7.4 volts is applied. At the higher voltage the servo will have greater speed and power, but it will lose efficiency, significantly decreasing motor life and possibly stressing other components. Older servos will not last long on a two-cell LiPo pack.

Many manufacturers now offer servos with motor windings designed to accept 6.0 to 7.4 volts in normal operation. Hitec initially did this with high-end servos and now offers high-voltage-capable sport servos at average price levels. Any of these servos can accept the voltage from two LiPo cells without problem. Other manufacturers are following suit.

If you have conventional or older servos, it is possible to purchase a voltage reducer (a Battery Eliminator Circuit, or BEC) that reduces 7.4 volts (and higher) to a regulated ~5.0 volts. A particularly good example is the Castle Creations Battery Eliminator Circuit (CC BEC). Originally developed for electric-power enthusiasts who required more current capability to operate heavy-duty servos, the CC BEC can accept voltages as high as 25 volts and produce a regulated 5.1-volt output at up to 10 amps. It is well suited for a fueled pilot who wants to use a two-cell (or larger) LiPo pack to power an airborne RC system (receiver and servos). Install the CC BEC between your battery pack and your receiver switch harness. The output voltage is selectable in 0.1‑volt increments between 4.9 and 9.0 volts. You also have the option of setting the output at 5.5 or 5.8 volts to get your servos working a little faster.

The CC BEC weighs only 0.4 ounce (11 grams) and measures 1½ inches long × 5/8 inch wide × 3/8 inch thick. It costs roughly $25. This lets you use lightweight LiPo batteries to power your airborne RC system without the risk of burning anything out. Also note that these LiPo batteries can be fully recharged at the field in one hour or less if the battery already has some charge!

Also included in this column:

  • Digital servos
  • Solid sheet vs. open-structure tail surfaces
  • Cutting O-rings — follow-up
  • Suggestion for Deans Ultra connectors
  • Flight Journal magazine

Q531

Q531: "Bob, please share your knowledge of digital servos and their value to the average sport flier."

A531: This could be a long answer; it might be appropriate for a dedicated article. Here are the basics and a good reference for more detail.

I asked Shawn Spiker of Hitec RCD for his thoughts. His summary:

  • Whether you should use a digital servo depends on the application and level of performance required. General sport flying, trainers, and park flyers up to .40-size aircraft typically do not need digital servos.
  • As performance requirements increase (Pattern, 3‑D, higher speeds, larger control surfaces, heavier aircraft), digital servos offer better control.
  • Why better? An analog servo sends a proportional current to the motor at roughly 30 updates per second. A digital servo does the same at about 200 times per second.
  • Digital servos offer increased resolution, travel speed, more precise centering, and can handle larger control-surface loads. Downsides: they cost more and draw more current than traditional analog servos.

For an excellent write-up on RC servos (specifications, analog vs. digital, selecting a servo), see Paul K. Johnson’s RC servos material on the Airfield Models website. I’ve listed the site in the Sources section.

Q532

Q532: "I've noted in many of your Old-Timer and vintage aircraft designs that appear in the RC Micro World [online magazine] that you substitute solid sheet balsa for the stabilizer and vertical fin instead of the original open/covered structures. What is your reasoning for doing that? Which is really better?"

A532: I choose sheet balsa tail surfaces because they are simple to construct and speed up the building process — I can build sheet-balsa surfaces in minutes. For the small-size models in question, the difference between an airfoil-shaped tail and a flat surface seems to have little effect.

Solid sheet balsa surfaces do weigh more than open structures. To illustrate, I built a sheet-balsa stabilizer and elevator for my new 100-square-inch Bootstrap vintage RC aircraft; that assembly weighed 7.4 grams. I then built the same stabilizer using stick-built open structure; it weighed 5.2 grams — about 30% less.

If your aircraft tends toward tail-heaviness you should use the lightest possible tail structure. In my case I did not want to extend the nose length, so I opted for the open-structure stabilizer and fin. That is a better choice than adding dead weight to the nose to balance the airplane. It will take longer to build, but not that much longer.

Follow-up to Q489: In the March 2011 issue of MA I showed a technique for cutting O-rings from surgical rubber tubing. AMA member Ron Ogren had a better idea. He slips the tubing over a short length of wood dowel, holds the X-Acto knife to the tubing, and simply rolls the dowel. The roll produces a perfect cut.

T533

I bought a high-capacity LiPo battery pack that claimed a 30–40C discharge rating. The pack wiring included No. 10‑gauge heavy-duty wire — correct for high-current use.

I ran into a problem installing my favorite APP (aka Sermos) connectors: the heavy wire would not fit into the 30‑amp pins, and the 45‑amp pins wouldn't fit inside the plastic housing. I initially removed some wire strands so the wire would fit into the APP pin — a bad idea. I even needed clear silicone seal at the wire/connector pin termination.

The proper solution was to switch to the popular Deans Ultra connector and solder the No. 10 wire to those pins; soldering proved easy. Don't forget to use heat-shrink tubing at the pin solder joint.

I've offered several suggestions about separating Deans Ultra connectors. Using a heavy-gauge No. 10 wire gives you a much better grip on the connector; separating the connector halves is then easier and you run little risk of pulling the wire off the pin.

T534

Flight Journal magazine is produced by Air Age Media. It has been out for roughly 10 years and covers full-scale aircraft — sport, military, and experimental types. The Flight Journal website has a surprising amount of information available to the public.

You can even download PDF files of William Wylam or Willis Nye plans with each issue. To find these plans, scroll down to "From the Magazine," then to "Online Free Artwork." New plans are added each month. See the Sources section for the main website.

Sources:

Transcribed from original scans by AI. Minor OCR errors may remain.