Automatic Flap — Aileron Reflex Trim
The complex yet ingenious control mixing linkages required before affordable computer radios were widely-available.
The first mention of AFART was in №89–1 of Dodgson Designs’ Second Wind newsletter. This article from back in that era explains it’s setup and use. It was a highly effective analogue solution in a simpler time before everything went ‘software and servos’.— Ed.
Dodgson Designs is thrilled to be able to offer our customers a new and most effective control innovation. It can be installed in Lovesongs, Windsongs, Camanos and Pixys and it can easily be retrofitted into ones that are already built. This amazing mixer was designed by Gary Brokaw of Spokane, Washington, an innovative Windsong flyer.
We call this new device our Automatic Flap — Aileron Reflex Trim or simply AFART for short. Now for the first time, anyone can have full-span flap and aileron reflex on a single trim lever and with only three servos in the Pixy, Camano, Lovesong and Windsong. Aileron trim was not practical before, on the Pixy and Camano, due to the added weight of the extra servo and minimal space in the fuselage. Now it is easily accomplished and with nearly zero weight gain and with no increase in aileron slop.
The advantages of using AFART on the Lovesong and Windsong are simply that the weight of a one-and-a-half ounce servo can be saved and that you can reflex the flaps and ailerons simultaneously from a single trim lever, even with any simple transmitter. The disadvantages are that you may need an extra powerful servo on the flaps, such as a Futaba S-31S, to handle the added load and you will lose the spoiler function on the ailerons. The trade-off of the aileron spoiler function to achieve the single lever for trailing edge reflex trim is well worth while as the spoiler function is seldom used.
AFART consists of a simple 1 /4" hardwood dowel that is cut 2-1/4” long for the Camano and Pixy and it is cut 2–3/4" long for the Windsong. It is capped with a brass tube 9/32" OD x 1" long. Secure the cap with Zap or epoxy. A 3/32" diameter hole is drilled through the cap and dowel at 3 /4" from the top, a 1/16” diameter hole is drilled into the center of the top and is about 1/2" deep. A tiny pilot hole is punched into the bottom of the dowel for the 3/8" long screw. This dowel becomes the support for the two bellcranks that are normally used in our kits. The slot in W9 should be cut so that when the dowel is all the way to the rear of the slot, the bellcranks are in the same location that they were on our conventional installation, as shown on the plans. A razor saw can be used for this task in a retrofit installation. The slot should also be cut a hair under sized so that it can be filed to a no slop fit, side-to-side, for the capped dowel. Locate the hole for the bottom screw so that it goes through the center of the fuselage at the point where the dowel is vertical, in the fuselage, when resting against the back of the slot. Make the hole just large enough so that you can install a very small rubber grommet in it.
Install the two bellcranks onto the top of the dowel, as shown on the back of this sheet, with the bearing flanges together in the center. Use a 2–56 flat washer both under the bottom bellcrank and on top of the top bell crank. Bend one end of the 1/4" wide x 5 /8" long metal strap (with holes at each end) to the shape shown for the rubber band retainer and install it on top of the bellcranks before inserting the 1" long bolt. Install the 1" long 2-56 bolt by screwing into the hole in the dowel top. Make it snug. (NOTE: you will probably want to install the aileron push rods onto the bellcranks, as the regular plans show, before securing the bellcranks to the dowel.)
Now put a washer on the 3 /8" screw and put the screw through the grommet in the fuselage bottom and screw it up into the small hole in the bottom of the dowel. Snug the screw up gently leaving enough play in the grommet so that the dowel can be pivoted forward in the slot about 1/4”. Make the AFART pushrod out of 1/16" music wire and crimp one end so that when you put the small washer, with a 1/16" diameter hole in it, the washer will not slide off the end. This type of washer is supplied with sets of Du-Bro E-Z Links, which you may need anyway to secure the aileron pushrods to the aileron servo arm. Cut the AFART pushrod to length so that the end washer is against the dowel (with the dowel against the back of the slot) and the flap servo is set at neutral flap. Solder the threaded coupler onto the servo end of the pushrod and screw on a metal ‘kwik link’. Put rubber bands as needed (or a spring) from the brass wing alignment pin tube extending to the rubber band retainer on top of AFART. Adjust the transmitter flap trim or put a metal or wood stop where needed on the transmitter case, so that when the trim is thrown down the flaps are in perfect neutral and when the trim is thrown full up, the flaps are at the six degree negative position. Adjust the AFART pushrod so that the ailerons are not affected when positive flaps are used and when the flaps are in neutral. Set it so that when you move the trim lever up, to reflex the flaps, AFART is pulled forward just far enough to also reflex the ailerons six degrees like the flaps.
Better yet, install a 0–20k pot in your transmitter connected to a toggle switch between the flap trim pot center wire and one of the trim pot outside wires (which outside wire depends upon which direction on the stick that you have neutral flap at.) With this new switch installed, you can adjust the 0–20k pot to give the perfect throw for the six degree reflex, allowing you to simply flip the switch to toggle back and forth between reflex and neutral flap/ailerons without having to move the flap trim lever.
The tension on the rubber-band or spring should be adjusted so that when you put reasonable pressure on the ailerons, AFART does not shift forward at all. Yet you do not want so much force on the elastic device that the flap servo cannot readily move AFART, forward when told to do so. If the elastic tension is too slight, high speed aileron flutter could possibly occur. This could also occur as the result of a sloppy fit of the dowel in the slot. If the fit is too loose, shim the inside of the slot with 1/64" plywood or coats of glue, Hot Stuff etc . until the fit is snug. With a snug fit, AFART produces no more slop than the bellcranks do all by themselves without AFART.
Properly installed and adjusted, AFART should require no further service and should give precise neutral and reflex flap positions. Since the 90 degree flaps, with the built-in elevator trim compensation, are so unbelievably effective for landing, speed and glide-path control, Lovesong flyers find that they rarely use the aileron spoiler function, provided with the fourth servo. The good flyers do, however, rely heavily on the performance edge that they can get by reflexing the entire trailing edge of the wing. Now, anyone can use even any cheap four or more channel radio and get full trailing edge reflex capability at one fingertip and with only three servos. All that is being given up is an extra servo and an additional trim lever to fool with, along with the aileron spoiler function that is not normally used much anyway.
If you think that you can’t get something for nothing you haven’t tried AFART.
2022 Commentary
Only a handful of Dodgson Designs glider flyers bought the earliest computer radios before the AFART system came out. The new radios had just arrived and they cost about $700 and were quite complex to program.
Being able to get full function on my gliders with just a simple radio was a big deal — especially to be able to switch between minimum sink thermalling mode to higher speed cruising mode by simply moving one trim lever to reflex both the flaps and ailerons.
My gliders could do it all using regular bigger servos in the nose — they helped get the CG in the correct location and, significantly, kept the gliders lightweight which improved overall performance. Most Dodgson Designs glider flyers did not go to computer radios until the costs came down on the radios — there was no urgent need to.
My gliders could do it all using the supplied hardware. Thank you all for reading and happy flying!
©1989, 2022 Bob Dodgson
Resources
- Du-Bro E/Z Links — Yes, you scan still get them! That said, almost everything else mentioned in this article is in permanent backorder status.
- The Dodgson Anthology — The complete works of Bob Dodgson as featured in the New RC Soaring Digest.
Are you a fan of the retro Dodgson Designs logo? Is so, you might want one of these for your flying field attire. Otherwise, now read the next article in this issue, return to the previous article in this issue or go to the table of contents. A PDF version of this article, or the entire issue, is available upon request.
