RC Pylon Racing
Duane Gall, 1267 S. Beeler Ct., Denver CO 80231; E-mail: [email protected]
Tech Tips
It's the time of year when final assembly and test flying of racing airplanes begins in earnest. This is a time of high risk for your new model. The urge to finish up those last, seemingly insignificant, and not-very-visible details sometimes overwhelms our better judgment. So here are a few miscellaneous hints relating to the last stages of construction, and those all-important test flights.
(Some of these may be in the category of "everybody knows that." But of course, if you weren't one of those "everybodies" before the crash, it will be of little comfort to join them afterward.)
Routing the receiver antenna
It's just a piece of wire, right? The customary sport-airplane procedure is to poke the antenna out a hole in the side of the fuselage and hang the end of it from the tail, using a rubber band or a piece of tape. Some folks just jam it through the elevator hinge gap. This may pass inspection in a world where high-frequency vibration doesn't exist, and aerodynamic drag doesn't matter, but racing is a different world.
When it's humming at 20,000 rpm, the edge of that little hole in the balsa (or worse, fiberglass!) will nip that wire neater than a pair of side cutters. And the drag from that little piece of wire is more than you might think.
The best approach is to route some plastic tubing down the tail, exiting near the tail post, and feed the antenna wire down it until the end comes out the back. Use tubing with at least 1/8 inch inner diameter, or it will fight you. (If it does, add some graphite powder or WD-40.)
Be sure to include a strain-relief at the inboard end of the tube, so that when your caller launches with a mighty heave while the aft end of the antenna is securely pinned to the runway by a size-12 shoe, it doesn't disembowel your receiver.
An ordinary shirt button makes a decent strain-relief. Just thread the antenna one way through one of the holes in the button, then the other way through another. But before you do this, bevel the edges of each hole by sticking the point of a #11 X-acto blade in the center and giving the knife a spin.
Do not allow the antenna to wander through a forest of servo arms and pushrods on its way to the exit tube. Murphy's Law was surely written by an RC racing pilot, and he surely wrote it after his receiver antenna fouled on the elevator pushrod clevis and abraded through in flight.
Another unsuspected source of abrasion is the inner surface of a fiberglass fuselage. This can eat your receiver antenna and the servo and battery leads. Dr. Scholl's Moleskin is a thin, soft, self-adhesive pad you can line the radio compartment with to protect those vital connections. (And if you don't use it in your airplane, you can put it in your shoes to relieve the pain of that long walk to the crash site.)
Short fuselage / long antenna trick (combination strain-relief and spiral "dummy load")
This next trick may come in handy if your airplane has a very short fuselage, or the receiver has a very long antenna. You can reduce the length of the dangling antenna end by making a combination strain-relief and spiral "dummy load" from a section of ice cream stick. A two- or three-inch piece will usually do it.
- Drill a 1/16 inch hole in each end and bevel the edges of each hole as described above.
- Use a file or fine saw blade to make a series of serrations or notches along both sides of the stick.
- Feed the antenna wire through the hole at one end; then, starting a couple of inches from the receiver, wrap the antenna five to 15 times around the stick (depending on how much length it needs to lose; each turn will account for about 3/4 inch).
- Press the wire into one of the serrations with each turn, so it doesn't slip and cause the coils to cross.
- Thread the remainder of the antenna through the hole in the other end of the stick, then wrap the whole affair with 3M clear vinyl tape.
- Feed the free end down the plastic tube and out the back of the fuselage. There should be no more than a couple of inches protruding. Tape this to the rudder so it will stay out of harm's way.
Radio technicians cringe at that suggestion: it does slightly reduce the effectiveness of the antenna. However, as long as you don't allow the antenna wire to cross over itself and if your radio system is tuned up as it should be, you will still have more range than you will ever need for racing. Do a ground range test to confirm this. Compared to the risks presented by your caller's Size 12s and your competitors' propellers, a 10% reduction in radio range is not a bad tradeoff.
Servo tray dos and don'ts
The best servo trays are custom-cut from 1/8 inch aircraft plywood. Before installing the tray in the airplane, do as much of the installation work as you can ahead of time. After all, you'll have to do it one way or the other, and it's easier than when you aren't fumbling around inside the fuselage.
Plan where your servos will go, and cut the outside edges of the tray so that they just contact the fuselage sides in that particular spot. Align the main grain of the plywood crosswise, so the tray will end up looking and acting like two or three "servo rails" flying in formation. It's stronger that way.
Sand the edges of the tray smooth, and also round the edges a little so that when it is mounted in the fuselage, the adhesive will make a nice bead or fillet to "trap" the tray mechanically, even if the adhesive quality of the joint begins to fail.
Cut the openings for the servos. Make each servo opening big enough to allow 1/16 inch clearance on all sides—only the rubber grommets should touch the plywood—not the plastic servo cases.
Drill the holes for the servos. I make a small template from 1/16 inch plywood for each type of servo that I plan to use, complete with dummy mounting holes to drill through for the proper pattern. Chuck up a 1/16 inch drill bit (for #2 mounting screws) in a battery-powered Dremel Moto-Tool to make short work of this process. When the holes are drilled, put a drop of thin cyanoacrylate (CYA) glue in each one to strengthen the plywood where the screw threads will bite into it.
Install the servos in the tray using #2 x 1/2 inch sheet metal screws. Treat yourself to the socket-head kind; the aggravation you save will be worth it if you ever have to change a servo at the field. A little beeswax on the end of the Allen wrench allows you to stick the screw to the end of the wrench, aim it where it needs to go, and start it without any forceps or fumbling.
Make sure that each servo grommet has a metal eyelet in it. The flanged (non-sharp) end of the eyelet should face down, resting against the plywood. The flange is there to keep it from making like a cookie cutter on the tray. The sharp, nonflanged end of the eyelet faces up, and meets either the wide, flat underside of the head of the mounting screw (if it has such a head) or a wide, flat #2 washer (if the mounting screw has an ordinary, narrow Allen head).
Tighten each screw until it presses down firmly on the eyelet but doesn't crush the rubber grommet.
Gather up all the servo pigtails in a twist tie and direct them toward where the receiver will be. Cover each servo connector with a patch of masking tape to keep glue off it.
Mount the servo tray in the fuselage. If the fuselage is wood, use slow-curing epoxy mixed with microballoons or chopped glass fibers (to make it thicker and less prone to cracking after it hardens) and you're done.
If the fuselage is fiberglass, a little more preparation is in order. Fiberglass fuselages transmit vibration more than wood ones, and vibration is the enemy of servos and glue joints.
More gluing area will soak up more vibration, and will help the thin plywood tray stick to the fiberglass. To get more gluing area, add some rails of 1/4 inch hard balsa triangle stock to the bottom of the tray where it contacts the fuselage sides. Medium-thick CYA will do the job here. Triad-fit the tray one more time, and sand the rails if necessary.
Clean the inside of the fiberglass with acetone and alcohol, rough it up with sandpaper, then mount the servo tray to it using silicone adhesive. Epoxy and glass cloth or chopped fiber will also work, but unlike silicone, epoxy is brittle (it may crack) and hard (it will transmit engine vibration to the tray).
Look for the word "adhesive" on the label. Silicone "sealant" is similar, but not as strong. Permatex #66B adhesive, available at NAPA and Bumper to Bumper auto-parts stores, is one of the better brands. I have also heard good things about Household GOOP, which stays flexible and sticks to just about anything.
I hope you've found at least one new tidbit here that will let you say, "Hey, everybody knows that!" MA
Transcribed from original scans by AI. Minor OCR errors may remain.
