Letters To The Editor
All letters will be carefully considered; those of general interest will be used. Send to Model Aviation, 1810 Samuel Morse Dr., Reston, VA 22090.
FF Vagabond
Enclosed find a picture of my old Free Flight Vagabond. The 6-ft.-span model used an Ohlsson .60 for power. Its last flight took place 40 years ago. It was on a calm, clear, and warm day when it nose-dived from 2,000 feet into concrete. It sure made a spectacular crash.
Up to that time it had 21 absolutely flawless flights. To this day I still wonder what could have caused the crash. Could someone please enlighten me?
Bill Spathiat 2727 Lawtherwood Ct. Dallas, TX 75212
We suspect that something shifted. Perhaps a rubberband gave out, resulting in a changed balance point or incidence setting. Any other ideas?
Frequency Control
In 1991 and beyond it is a major concern. It seems that "1991" has developed into a demon that will swoop down and destroy our miniature airplanes without warning. The facts say this is far from the truth when we use some common-sense guidelines. As modelers we seem to be bombarded with all kinds of gadgets that will enhance our modeling capabilities. Some are good and needed while others may be more of a frill. One such gadget is a beeping frequency-control pin board as seen in the May 1990 issue of Model Aviation. This is available from Custom Tronics in Texas; not to be confused with one of the AMA sticker stations in California, Custom Electronics.
I've been flying R/C since 1967, and, being an electronics technician, have enjoyed the growth in R/C equipment. Today more modelers are flying R/C than ever before, making the acquisition of more R/C channels and 20-kHz spacing very much needed. Not all channels may work well in a given area, but now we have more R/C channels to choose from.
In the last several years we have learned some new terms like 3IM, 2IMD, and IIM. Some interference is beyond our control, but most is not. What can your club do to help prolong a model's life?
One, your club can have a frequency survey done at your flying site using a scanner or receiver. This will let you know what frequencies should be avoided, if any. Second, a club can become radio-wise. Radio-wise is knowing what 3IM, 2IMD, and IIM are—and what to do to minimize the effects. Let's review each of these.
3IM (third-order intermodulation) occurs when two transmitters, each operating on a different frequency, generate an interfering signal on a third frequency which falls on another R/C channel. An important factor is how much difference between control RF power (your transmitter) and 3IM RF power must be present before interference can affect your receiver.
I was in attendance at a major pattern meet where I used an ICOM-7000 receiver and observed 3IM in action. Showing this to a fellow modeler and AMA President Don Lowe, we were both amazed to see 3IM on an R/C channel and then watch a pilot turn his transmitter on that channel and proceed to fly unaffected. Receivers must work as designed, of course, and some are better than others. The pilots had no interference problems during the whole three-day meet—and not all receivers in use were dual-conversion, narrow-band designs. However, all transmitters had gold stickers indicating they were narrow-band units.
Their "secret" was pilot spacing. The four pilot stations were separated, thereby reducing 3IM-generated RF power enough to not affect same-channel receivers. In each case the receivers were able to reject the interfering signal and obey the controlling transmitter. This system (spacing of pilot stations), along with assigned frequencies to each station to take care of adjacent-channel interference, solves the problem.
Next, 2IMD (second-order intermodulation distortion). Many wideband receivers, using a single-conversion superheterodyne design, are susceptible to this type of interference. The interference occurs when two transmitters operate on channels that are frequency separated by approximately the receiver's intermediate frequency. In other words, avoid simultaneous use of R/C channels that are two channel numbers apart. Using the AMA's Interim Alternate Plan I (May Model Aviation, page 122) along with narrow-band transmitters will help solve this.
Now, the last type of interference: IIM. This is the phenomenon in which one's airplane seems to be flying in a very uncontrolled manner due to the controlling influence of the brain — you know: pilot error! We all have experienced this one. Pilot error is more than just dumb thumbs (giving up elevator when making a high‑speed inverted low pass). It includes questions like "Did you fully charge the batteries?" "Did you check for the correct direction of control movement?" and so on. Also included in IIM is the guy who turns on his transmitter on your frequency while you're flying. As R/C equipment becomes more reliable, IIM begins to loom as our biggest problem.
The above systems and frequency control procedures have been tried and proved many times at fly‑ins and contests. They are currently being used by my home R/C club, The Prop Nuts in Highland, TX. We have not had any 3IM or 2IMD problems (although on occasion old IIM gets us).
If your club has any questions or frequency problems, contact your AMA district frequency coordinator (names and addresses appear in the "AMA News" section). I'm a member of the AMA Frequency Committee and will be glad to help, too. For further frequency-control guidelines, see the May 1990 Model Aviation, page 120.
Larry Dungan Box 1254 Pasadena, TX 77501
Continued on page 12
About Her Uncle
I wrote this poem about my uncle and his best friend. Perhaps some of your readers would enjoy it. Maybe some of them are R/C nuts, too.
The RC Nut
You always know his whereabouts At the field, there is no doubt. He flies his plane every day And nothing ever keeps him away. Even when it is cold outside Warm clothes are donned in stride. When the snow is all around Skis are used to get off the ground. If his plane does crash He fixes it up in a flash. With MonoKote, iron, and instant glue He makes it look just like new. He works straight through the night To be ready for the next flight.
Gloria Shaffer Duanesburg, NY
Bell-Anchor
The most intriguing and exquisite piece of writing and artistry I've ever been privileged to be conned by was the "Bell-Anchor" in the April 1990 issue. Scratch Billright is to be commended — a super con artist.
Ole dummy me sat down and thought he'd like to build the Bell-Anchor. Whoa—can't be done, says he. Sorry, buster, but you've been nice. Nice job. I'll remember you! When is your next "con" to be published?
A. G. Ronay Hoffman Estates, IL
Ligeti Stratos
It's easy to understand why Model Aviation is so well liked. You took my pretty good article and made it great—thanks in no small part to Bob Banka of Scale Model Research for some great photos of the full-size Stratos.
Ultimate Solitaire
That 18-ft.-span powered sailplane, the Ultimate Solitaire, mentioned in the article (with a question mark at the time) flies very nicely with the O.S. .61 four-stroker. It's a real floater at 17 lb. The picture, with me on the left and Dennis Reichenbery on the right, was taken at our annual Jumbo event.
Marvin Reese 329 N. Milstead Wichita, KS 67212
Transcribed from original scans by AI. Minor OCR errors may remain.
