The Iron Butterfly

Aug 27, 2009 No Comments by

Pilot Project of the Month, November 2009

By Peter Donk

(see photos below)

I’m a member of the Genesee Valley Aero Modelers RC club in Honeoye, NY. I joined GVAM and the AMA in 2008 when I got back into the hobby after many years away from it. I got my feet wet again that summer with a Goldberg Protégé ARF (very nice flying plane) and was looking for an interesting winter project because I really enjoy building as well as flying. Due to my renewed interest in all kinds of aviation (I’m an aspiring GA pilot, too—soloed January 18, 2009), I found myself looking at all kinds of different ultralight designs. One day I stumbled across a website (www.affordaplane.com) and discovered a unique little ultralight called the Affordaplane. I joined a Yahoo discussion group dedicated to the design and construction of the Affordaplane and learned all I could about it. After much consideration, I decided I’d like to build a ¼-scale RC version of this remarkable little aircraft.

This project would prove to be full of “firsts” for me. First “plans-built” project (always built from kits before), first giant-scale plane at 82.5 inches, first attempt at anything more than “sport scale,” first gas engine, first engine conversion and first fabric- covered model. I figured as long as I was trying many new things, I’d also try to recreate the design as closely as possible. The full-scale plane is built mostly from aluminum bolted together much like an overgrown Erector set so I decided my model would be too. I wasn’t sure how heavy that would make the final product, but I wanted to stay true to the real bird as much as possible. Part of my reasoning for that was there doesn’t seem to be much evidence of completed Affordaplanes actually flying successfully and there seems to be some argument about how air-worthy the design actually is. So, adding to the challenge of this project, I’d also be trying to prove an apparently unproven design to satisfy my own curiosity.

I started just before Christmas 2008 with a 31cc Ryobi trimmer engine in front of me on the workbench. In keeping with the ultralight theme, I wanted to convert an engine for my purpose rather than buy one because so many ultralights use converted snowmobile or industrial engines or anything they can find that will work. After doing some research about that particular engine, it seemed to be a good candidate for RC conversion. I sent the cylinder and flywheel to my brother Doug in California (he’s a machinist) for some milling work. I had him remove as much of the flywheel fins as possible and machine .025 inches off the base of the cylinder to increase compression to a more reasonable level. I used a band saw and hand grinders to remove any material from the crankcase that wasn’t necessary for my purpose. I installed a Walbro 11mm chainsaw carburetor and tweaked the stop on the reed valve to allow it to open a bit more. I figured those mods would give me an engine with plenty of power to fly the model I was planning to build. I went with the stock magneto ignition also in keeping with the budget-minded, low-tech intent of the Affordaplane. After a bit of R&D and fine-tuning, I ended up with an engine turning a 16×10 prop at 8,100rpm on the test stand. My calculations indicated it should produce around 12 pounds of static thrust. “Good enough,” I decided.

After purchasing the full-scale Affordaplane plans from the designer’s website, I decided it would be a fairly simple task to reduce all the measurements to 25% and simply follow the instruction manual as if I were building a full-scale version. The plans included full-size templates for the gussets and wing ribs and some of the smaller pieces in PDF format. It was no big task to reduce those images to 25% and print them to use in my build. I went through things step-by-step, following the manual and slowly my project started to take shape—just as I’d read about the full-scale plane taking shape. Square aluminum tubing and bolt-through gusseted sandwich joints are used for the single plane fuselage. Round aluminum tubing is used for front and rear wing main spars with foam board for ribs (the full-scale design uses rigid foam insulation board that’s too thick for a model so I substituted Elmer’s foam craft board). The tail group is all tubing bent to shape (I used solid aluminum rod). I did opt for a little extra bracing in the tail because I knew there’d be considerable shrinkage in the covering and I didn’t want any bending of parts.

The plans allow some room for personalization and a certain amount of customizing by the individual, so I built custom aluminum control horns and opted for no engine cowling (as some others have), as well as a custom seat and four-point harness for the pilot. I located the ignition and radio switches on the instrument panel to mimic real controls but there wasn’t really a good way to hide the radio/battery and tail servos to keep the CG where I wanted it (about 1/3 of the wing chord) so they did end up mounted in plain sight (in a black box) toward the rear of the fuselage. I enclosed the wiring in a single shrink tube that follows the underside of the fuselage to be as unobtrusive as possible, but still be serviceable if need be. The radio is a Futaba 6EX 2.4GHz mated with run of the mill S3004 servos (knew she wasn’t gonna be a stunt plane so standard servos would be plenty).

Again, in keeping with the theme, I decided this plane HAD to be fabric covered. Any plane I’ve ever built I finished with MonoKote so I had no experience with fabric of any kind. After reading up on the subject, I decided to give Sig Koverall a try. I used their Stix-It adhesive and sealed it with nitrate dope. For color I used garden variety Valspar spray paint for the yellow and vinyl decal material for the purple trim. I chose to put the name “IRON BUTTERFLY” across the right wing because of the all-metal construction and the fact that’s it’s an ultralight. Added to my affinity for rock music, it seemed a very fitting name. All things considered, I’m very happy with how my first experience fabric turned out.

After six months build time using my spare time evenings and weekends, she was ready to head to the field for the moment of truth. A perfect morning for a debut dawn patrol arrived on Sunday, June 7 with almost no wind and partly cloudy skies. After a few quick taxi tests and some right rudder trim to keep her tracking straight, I pointed her into what little wind there was and advanced the throttle to full. After a fairly long ground roll, the tail came up nicely and very little elevator was required to get off the ground. I immediately realized this would be like no model I’d ever flown. The engine was winding away nicely but air speed was not that high. She climbed out slowly and lethargically and was both sensitive AND slow to respond to control inputs (if that makes any sense). After gaining some altitude and circling the field a couple times, I was able to get things trimmed a bit and the color started to come back to my white knuckles a little. Once I had a few minutes to fly the plane in better trim, it was clear that I in fact had a very scale flying ultralight model. She liked to run wide open and with lots of parasitic drag air speed was pretty low by comparison to any conventional RC model. Gentle turns and slow climbs were all she wanted to know about and wind of any kind was the enemy. In fact, the model definitely closely resembled ultralights I’d seen flying before with their 2-stroke engines winding away and slow air speed and gentle maneuvers on calm days only. Power-off performance was different as well. She didn’t exactly drop like a rock, but the glide slope was steeper than I was used to, to say the least, and I could tell the first landing was going to be a challenge. After about 15 minutes in the air and a couple practice approaches, I decided it was time to land. As expected, a hot approach was necessary and touchdown was at a hi
gher ground speed than I’m used to, but all in all it was mostly smooth and my goofy little Affordaplane was back on the ground in one piece. After three more successful flights that morning, I was very happy with the result of all my work and was as proud as a new father having seen my plane circle overhead for the first time and return to Earth safe and sound.

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