FAQ: frequently asked questions
BY BOB ABERLE
The first round of input regarding the "From the Ground Up" series ended with "Advanced RC Systems," which was published in the February 2004 Model Aviation. During the almost one-year period of this series, Bob Hunt and I have received many letters from readers that contain detailed follow-up questions and concerns.
The volume of these letters makes it impossible to answer every one personally. I try to respond to as many as I can, but there is never enough time in the day. The alternative is to sort through and select the more important questions and then, as I am about to do, provide an article addressing the most frequently asked questions (or FAQs, as I'll call them!).
This will be my first time doing this, and as a result the questions have been edited to save a great deal of space. Many of you write pages and pages to ask one question. I won't identify any readers by name or location (my choice).
The subject matter will be random; questions could be about radio control (RC) equipment, electric power, model building, model flying, etc. In this first FAQ article, most of the questions involve batteries, battery testing, and battery charging. It is the major source of concern and causes the most problems for the average RC flier.
I'm going to number each question sequentially. It is hoped to eventually expand on this FAQ concept and provide a search capability on the AMA Web site so you can seek out specific subject matter. Here we go!
Q1: Trickle charging — when should it be used?
A1: In my "Battery Basics" article (October 2003 Model Aviation) I wrote: "The advantage of trickle charging is that the batteries are available the moment you want to fly. However, you can only maintain a charge level at trickle; you can't recharge a battery that has been used at the trickle charge rate." Including the words "that has been used" was an error and changed the sentence's meaning. The sentence should have read: "You can't recharge a battery at the trickle charge rate."
You only use trickle charge (a low level) to maintain the charge in a battery that has already been fully charged at C/10 overnight or at a fast charge rate (2C or 3C). Fast charging is generally used at the flying field between flights. You can't charge a battery at the trickle rate.
Q2: Can old Ni-Cd cells be rejuvenated?
A2: When a pack goes down (shows lower-than-normal rated capacity), it is usually because one or two cells in a multi-cell pack are starting to go bad. The first step is to identify the bad cells. Opening a pack and searching for bad cells is time-consuming. After identifying them, you either replace the cells or try a rejuvenation technique.
There might be tricks to revitalize those cells, but I wonder how long the process will last. After any such process, you may still have a battery pack with a variety of cell characteristics. The bottom line: is it worth the time? When a pack noticeably loses capacity, I prefer to discard it in favor of a new pack. Considering the cost of a new pack versus the time to repair an old one, I'd rather be out flying with a new pack.
Q3: How do you match electric motor size (e.g., Speed 280, Speed 400) to model application?
A3: Motor identification can be frustrating in the electric world, but look back to the beginnings of fueled engines for model use. In the early days it wasn't obvious what a .29-displacement engine could fly. Over time, people learned to relate engine displacement to classes and then to model weight. The same process is happening with electric power.
The key factor is the motor power expressed in watts. Most of us involved with electric flight own a meter that reads motor current (amperes), motor voltage, and the product of the two—motor power (watts). The AstroFlight Digital Meter (Model 101) provides all of these measurements and is a must-own item.
By experience we have learned that dividing motor power (watts) by the model's total weight yields "watts/ounce." On smaller electric models (Speed 400 and down), the average sport model needs roughly 2.0 watts/ounce to fly normally. A lightweight, lightly loaded powered sailplane might need only 1.25 watts/ounce. More energetic models, for aerobatics and heavier wing loading, need 3.0 watts/ounce or more. My Scratch-One design from this series had 5.32 watts/ounce, so the recommended Speed 400 motor had more than enough power; that model can fly at half throttle or lower because of the extra power. For larger electric models, the parameter changes to watts/pound, but the concept is the same.
This may seem complicated at first, but the numbers and the form of identification get easier with experience. Motor manufacturers are being encouraged to provide power ratings (wattage) for every motor. Published reports (product reviews and construction articles) are beginning to provide watts/ounce and watts/pound as references. If you can't find this information, ask the manufacturer or supplier; they should be able to point you to the correct model application for each motor.
Q4: Can I test a transmitter battery without removing it, or does a blocking diode prevent testing at the charge jack?
A4: Yes, you will have to remove the battery. I have to do this with almost every brand of RC transmitter in my inventory. Most transmitters have an easy-access hatch at the rear of the case; you can open the cover and usually remove the battery pack with no difficulty.
Most transmitter battery packs also have a connector that allows you to disconnect the battery pack from the circuit. The trick is to obtain a mating connector that will let you attach the battery pack to your discharge testing device (or cycler). Many RC manufacturers will sell this necessary mating cable. Battery suppliers such as Batteries America and Radical RC also sell a variety of transmitter battery cables and connectors. Construct these adapters once, and you are set for the future.
Q5: What does "channel number" mean — frequency channel or control channel?
A5: The term "channel" or "channel number" has two meanings in the RC hobby.
- Frequency channel: In 1982 the Federal Communications Commission granted a series of frequencies on 72 and 75 MHz for RC use. Each frequency was identified by a number (e.g., 72.010 MHz). Memorizing frequencies would be difficult, so the AMA suggested assigning channel numbers to each authorized RC frequency. For RC-aircraft purposes, channel numbers were assigned from 11 up to 60 (a total of 50). On 75 MHz, additional frequencies (and channel numbers) were assigned for surface vehicles like cars and boats. At the flying field you might be asked, "What channel are you on?" and you would answer, "I'm on 32," "I'm on 56," etc.
- Control-channel functions: Separate from frequency channeling is the term "channel functions" or "number of channel functions" in transmitters and receivers. Early radio control in the 1950s was usually limited to rudder—called "single channel." As technology provided more functions, systems became "multichannel." Each control function—rudder, elevator, aileron, throttle—was called a "channel function." A "four-channel receiver" provides four controls. So when you see a decal that reads "FM 7 Channel Receiver," it means the receiver can control seven functions. This has nothing to do with the frequency channel number shown on a transmitter antenna flag.
Q6: Can I charge an 1800 mAh receiver pack with a 125 mA charger instead of C/10 (180 mA)?
A6: You probably could get away with it occasionally, but I would be uncomfortable with that long-term. Charging at 125 mA instead of the recommended C/10 (180 mA) means the battery won't reach full rated capacity. I would prefer that you buy a variable output charger, such as the ACE DDVC, set it for 180 mA, and let it charge overnight (or at least for 10 hours). That way you have maximum battery capacity and longer pack life. I receive many questions about making do with chargers on hand; many alternatives will probably work, but I recommend the technically correct approach.
Q7: If a Ni-Cd cell has a nominal voltage of 1.2 V, why do I need to recharge a four-cell pack at around 4.8 V?
A7: Good question. Through the years the Ni-Cd cell's nominal voltage has been identified as 1.2 volts, so we refer to a four-cell pack as 4.8 volts, an eight-cell pack as 9.6 volts, and so on. However, a fully charged Ni-Cd cell can reach almost 1.4 volts, so a fully charged four-cell Ni-Cd pack might be as high as 5.6 volts. Depending on the pack's rated capacity and usage, it might take one or two hours to drop to 4.8 volts, at which point I recommend recharging before continuing to fly. The nominal voltage is the minimum voltage before the need to recharge. This is terminology carried over from years ago and can be confusing, but it is the established practice.
This has been my first attempt at providing FAQs for readers' benefit. This concept will become a regular monthly column in Model Aviation. It will also be available on the AMA Web site along with reprints of the entire "From the Ground Up" series.
Although this first set of questions and answers was almost entirely about electric power and battery-related items, I welcome your questions on any aspect of our hobby. If I can't answer your inquiries, I'll find an expert who can. I also value your suggestions and criticism of what we are attempting to do in this series. Please share your thoughts!
Direct your questions and suggestions to Bob Hunt at [email protected] or Box 68, Stockertown PA 18083. MA
Transcribed from original scans by AI. Minor OCR errors may remain.
