Radio Technique
George M. Myers
WHAT'S HAPPENING with those RC frequencies? This month I have received several letters asking about AMA changes in the RC channel assignments for 1986. There aren't any. All of the channels you have been using, you will continue to use in 1986 and 1987; and all forms of modulation (AM, FM, PCM, etc.) are, and will continue to be, legal and proper. They will continue to be legal and proper through 1991 and beyond, so far as anyone knows today.
In 1988 (that's two years away), we will be obliged by the FCC to abandon the old even-frequency channels (72.08, 72.16, 72.24, 72.32, 72.40, 72.96 and 75.64 MHz). Therefore, you should have converted your old transmitters and receivers off those old RC channels and onto one of the new ones before January 1, 1988. We've been urging you to do that since January of 1983.
Frequency Committee meeting
Your AMA Frequency Committee met at Reston, VA on November 15 and 16. Due to peculiarities of air travel, I was over an hour late on a 52-minute flight in clear air, and I missed the Friday night session. Therefore Jack Albrecht got stuck with the task of defining a new RC ribbon for the new AMA rule book. It seems that the flag with the numbered tags has damaged some transmitter antennas and upset some tempers.
I shared a room with George P. Steiner and several meals with Jack Albrecht, Dave Brown, Bill Hershberger, Jim Oddi, and Warren Plohr, where we compared notes and swapped data. That got me up to speed for Saturday morning, where the bulk of the time was spent on the plan for adding a low-price transmitter-testing station to the AMA booth at trade shows like the WRAMS show in NY and the Weak Signals show in Toledo, OH. A subcommittee was charged with working out the details.
Saturday afternoon's session included a long list of representatives from industry, and many problem areas were discussed. Saturday evening's session was devoted to technical topics. Look for the official minutes of the meetings in the AMA News section of this magazine. I will now discuss things which I think will interest you.
- The committee appropriated funds to purchase a frequency scanner for each AMA district. The scanner will be held by the District Frequency Coordinator and issued on request to AMA clubs in the district.
- We anticipate a price under $200 per unit on a bulk purchase, based on the fact that Nats manager Vince Mankowski paid about that much for the units he bought for the 1985 Nats. If enough clubs or individuals show interest in acquiring similar units, a similar bulk buy could probably be worked out by AMA HQ through the AMA Purchasing Department.
- While I was AMA Frequency Committee chairman, I had a pair of scanners and provided that service for District II (NY and NJ) with the simplest of rules:
- A) Two-week loan.
- B) Requesting club pays postage both ways.
- C) A written report of the procedures and findings is required.
We expect to follow the same procedure again. By this procedure, we hope to build a documented database that describes interference problems in the field. To be perfectly honest, we didn't learn much in District II. All we learned with the scanners was that the interferences we could identify were legal and normal. Perhaps we will gather more useful information with more machines in circulation—and more people on the job.
AMA tried to set up a national clearinghouse for interference complaints before, using the Interference Report Cards published in MA, June 1983, but the response was underwhelming. I suspect that we failed because folks lacked the ability to monitor the RC channels while the interference was taking place.
Until the AMA can send out qualified "accident investigators" the way the FAA does, we never will build up a decent database from which to improve anything. Use of scanning monitors may prove to be a first step toward using the talents we have in every district to build that database and help solve AMA problems.
- The wording of the RC section of the 1986–87 AMA rule book was reviewed. Much discussion ensued on what we should do for the 1988 rule book. We, and the manufacturers, have known since 1982 which RC channel numbers we plan to activate in 1988. The question was, "Is there something better we can do?" The answer for now is, "Not that we can find!" The Frequency Committee will continue to study the alternatives and will make a decision which will be published and distributed to the manufacturers.
- Considerable time was spent on the subject of publishing a ROC (Required Operating Characteristics) for RC systems. If the Frequency Committee can come up with a suitable ROC and then present it to RC manufacturers, they (in turn) may be able to design systems that will be free of some of the problems I have been discussing in this column. (You will notice that I said "if.")
- In the session with the manufacturers, we heard some people express the opinion that the AMA should have taken, and did not take, a formal leading position in the control of non-aircraft RC frequencies. All sorts of horror stories and fears were discussed. I don't recall that anything was resolved.
From what we heard, the big problem is that millions of RC cars have been sold—and millions more will be sold this year. The non-aircraft RC frequencies are becoming saturated with them, and there have been many instances where "aircraft only" RC frequencies have been misapplied. At the same time, toy manufacturers have sold millions more RC cars and robots on 27 and 49 MHz frequencies, and there is concern that they may soon move into the 75 MHz frequencies.
The basic problem in working with the toy manufacturers is that they are a secretive lot. They now use 49 MHz frequencies at under 10 mW because that can be easily approved through the FCC Type Acceptance procedure. They also use 27 MHz at under 100 mW for the same reason. Apparently they have stayed away from 75 MHz channels to avoid the expensive and time-consuming Type Acceptance process.
It was suggested that hobby manufacturers could do everyone a favor if they would voluntarily limit 75 MHz transmitters to 100 mW or less, which should be sufficient for all terrestrial (non-flying) models. That restriction would, in time, reduce many of the interference complaints we heard about at the Frequency Committee meeting.
My personal opinion is that such a restriction would be a good thing for all concerned, since cars and boats usually operate closer than 100 yards from their transmitters because it's hard for the operators to see them beyond that distance. Based on our recent test results (see the January 1986 issue of MA) and upon the fact that cars and boats run perpetually in a bad 3M environment without seeing much in the way of glitching, I believe that current receiver performance at 20 kHz spacing is adequate for terrestrial uses. Therefore, it seems reasonable for the car and boat people to start using all 30 channels in the 75 MHz band right now if they want to do something to lessen interference between operating locations. End of opinion.
The main problem in the non-aircraft RC frequency world seems to be that far more users have appeared than were ever anticipated. Aside from boats and cars, we are seeing robots, advertising displays, remotely controlled tools like traveling cranes, and other users appearing in the spectrum.
It was agreed to call a meeting at the upcoming Toledo show for industry and special-interest groups (e.g., cars and boats) to discuss frequency problems.
- In the technical session, the discussion centered on third-order intermodulation interference (3IM) and considered test data presented by George Steiner (receivers), Bill Hershberger (transmitters), and the team of Myers and Aberle (field tests of both transmitters and receivers). You have already seen most of the Aberle/Myers data in "Radio Technique" for January and February 1986 and in Flying Models magazine (if you take it).
Subsequent to the Frequency Committee meeting, Bob Aberle and I went back into the field to examine the second-order product interference (2OP) question more closely. There were some who thought we passed it off too easily. Well, I can now report that, based on a much more detailed and elaborate set of tests, we find that the current crop of receivers (both AM and FM, single- and dual-conversion) have been designed very nicely to resist 2OP. It is less of a problem than adjacent-channel interference on a 20 kHz raster, and far less important than 3IM.
Frequency control at flying fields
There was also discussion of on-field frequency control. One method suggested is the system shown in the AMA rule book (page 128 of the 1986–87 book). This requires that you insert your AMA membership card into a pocket on the frequency control board when you remove the frequency control clip that must be in your possession before you turn on your transmitter. For those concerned about losing their AMA cards, this system could be amended to permit using a copy of the card in lieu of the original.
If the prerequisite for flying at a particular field is membership in a club, the system used by the Northern Virginia RC Club (NVRC) is worth considering. Upon payment of annual dues, NVRC issues colored stickers for attachment to members' transmitters. The color changes each year, and the year of issue is printed on the stickers. Proof of AMA membership has to be provided when NVRC members renew their club memberships. So, presence of the appropriate sticker on a member's transmitter signifies both current club and AMA membership. A quick check of the transmitter impound rack at the flying field readily identifies those who are delinquent in dues payment.
Servo-screw integrity
A few issues ago I ran a letter from Peter Young who alleged that use of self-tapping screws to hold together the two halves of servo cases was not a good practice. These may work loose because of engine vibration during flight. Another reader, Jay Stevens, informed me that he, too, experienced this problem and, like Peter, now uses a dab of epoxy on the screws to prevent them from loosening.
As an alternative to epoxy, Glen Grady (Port St. Lucie, FL) suggests using fingernail polish. Glen has successfully used nail polish to secure screws in cameras that are used to record data from jet engines on full-scale aircraft. He thinks that nail polish will be just as effective as epoxy in securing servo screws and will permit easier screw removal should the servo need to be disassembled for servicing.
Electric starters — a safety note
Before signing off this month, a word of caution to those of you who use electric starters on engines not equipped with a spinner. This comes from Glen Grady, who witnessed an accident in which a fellow club member had to visit the hospital to have a piece of propeller removed from his chest. The modeler was attempting to start a .40-size four-stroke equipped with a wooden prop and no spinner. Glen did not state whether the prop was secured with an AMA prop nut. His letter described what happened as follows:
"When the starter was used, it constantly would spin off-center, throwing the rubber cone in the starter out. It just so happened that when he did it one more time, the cone flew out, and the engine fired up with the throttle wide open. The plane lurched forward into the starter because the cone had come off and the vibration of the starter on the spinner prop loosened his grip on the airplane. The prop hit the starter and shattered, sending a piece of wood into his chest about half an inch."
Fortunately, the modeler in this incident was not seriously injured. Nevertheless, Glen suggests that starters should not be used on an engine unless a spinner is used, and never to start an engine with the throttle wide open. (This tip applies to throttle-equipped engines. Free Flight Power and Control Line modelers should be careful as well.)
Have a safe month.
John Preston 12235 Tildenwood Dr. Rockville, MD 20852
George M. Myers 70 Froehlich Farm Rd. Hicksville, NY 11801
Now on to some gadgets
Leon Kincaid, an excellent designer and builder (see his Scooter plans in the October 1983 issue of Model Aviation), has a most clever way of getting an airtight fit between wing roots and the fuselage. It's also good for securing the wing halves. Forget about tape. The idea does take a bit of building finesse, but any job worth doing is worth doing well.
During wing construction, besides the main wing joiners and alignment tubes, Leon installs a second set of tubes just behind the main wing joiner. Location is not critical and tubing diameters aren't, either. However, the larger the diameters of the tubing, the better. Notice we're saying diameters because the key to this system is that one wing tube telescopes into the other. Which wing has the larger tube isn't critical.
Small holes are carefully drilled into these wing tubes and then elongated slightly in the direction of the wingspan. After the wings are joined to the fuselage, a flat steel tapered pin, the narrow end of which matches the drilled holes, is inserted and forced into the holes. As the pin is forced in, the wing halves are snugged up to the fuselage. Nice.
Soaring Products' Thermal Sniffler, which has been in production for the last 15 years or so, is now being manufactured and distributed by Ace R/C, Inc. There are no changes in this fine product; however, the receiver that is offered uses an ear plug to monitor the Sniffler's signal, which isn't a bad arrangement on a tranquil summer afternoon. There are times when a loudspeaker would serve better so that others could listen in—such as during LSF long-distance tasks or cross-country flying, where teamwork is important. Jack Hiner found a need for such a modification, and the circuit and parts list are presented elsewhere in this column.
Gregg Seydel has come up with an idea for stabilizing stabilizers—well, for those that are driven by a bellcrank up in the vertical fin, anyway. The usual technique is to use a bearing made from brass tubing.
Transcribed from original scans by AI. Minor OCR errors may remain.
