So here's a cheap and lazy guy's version of Brett's and the others system. Still a work in progress.
The 7/32" brass tube clears the boss on the blind nut perfectly.
The bellcrank is threaded for the 4-40 screw and the opposite side is spot faced so the blind nut is screwed into it.
The 'crank pivot hole will get bushed.
May use epoxy to fill in and create a 'teardrop'.
Ara
I think you are greatly overestimating the effort and "tech" level of my bellcrank. The only exotic material involved is the pivot rod. I happened to use the teflon-impregnanted delrin, but conventional delrin or nylon would be fine. In fact, you have decent supply of a perfectly good material - your nylon bellcrank. The rest of it is 20 minutes of work with completely conventional materials and techniques, if you put aside the glue-drying time.
The only reason for the titanium is the large span from one support to the other. My pivot is supported at the ends in a *very* thick wing, and from support to support is ~2.5". Under load, a 1/8" rod (of any reasonable material) would deflect substantially. Still no problem, except for the pivot bearing. The pivot bearing is a full 1" long to ensure the bellcrank doesn't build up slop and tilt on the shelf. So the bearing would bind up on the deflected shaft. If your supports are only 1 3/4-2" apart, 1/8 music wire would work fine.
You need the long pivot bearing because the ball link is on top, and any load on the pushrod causes the crank to want to tilt. A conventional pivot bearing either binds up, or wears out in short order, and the crank tilts. This has proven a fatal condition in previous systems, particularly the original circular bellcrank that Ted had made by John Schwickrath. In that case the bearing wore out, causing lots of apparent slop as the crank tilted, and caused wild hunting. Ted had just won the NATs (86) with the airplane, and he was complaining about it hunting, and I couldn't really tell from the outside. So he said, OK, you fly it. Sure enough, it hunted crazily, not just in level flight, but even in the maneuvers with about a swing and a half on the sides of a square loop! I learned two important things that day - Ted Fancher has piloting skills most of us cannot hope to equal, and don't let your bellcrank tilt on the shaft.
To overcome the tilting problem, you can make very large fender washers to force the bellcrank to stay in plane, or make a pivot with a very long bearing surface. I did the former a few times, but always felt it tended to bind up and got very complicated once I realized it needed to be teflon. But you just can't put on a 1" diameter 1/8" thick teflon washer because it's not stiff enough, so you have to back it up with something like an aluminum washer that looks like a ST prop washer. After prototyping that, I rejected it and used the long pivot method. I If the crank is delrin, you also have to make it very thick to keep it from deflecting up and down. That's really heavy so you need to put in a bunch of lightening holes (see below).
The other problem was there was no good way to connect the pivot tube to the delrin bellcrank. I force-fit it and then used soldered washers but it quickly broke loose, and then pivoted between the bearing and the bellcrank, defeating the effect. So I needed something I could glue or solder to reliably. Hence the aluminum facing sheets. I sat down with this experience, and knew I wanted to get rid of all this absurdly heavy hardware. I weighed a bunch of stuff and found that the hardware in the wing - bellcrank, leadouts, leadout protection, flap horn, etc, was about the same weight as the rest of the structure. I had long realize that the "leadout tube" was heavy and also highly prone to failure, depending on how you did it. Two of Ted's airplanes had either failed, or was found to be down to a few strands, using the "partial teardrop" method. The "full teardrop" was better but apallingly heavy. Of course solid leadouts were completely out of the question, both due to the difficulty handling the airplane and because those, too, had to be grossly oversized to (maybe) stay together at the end fitting. They are notorious for abrupt failures.
So I finally figured out a way to do mine, and I assure you, it was just with stuff I had lying around, aside from the titanium rod (which I got from Jim Rhodes initially, and later got more from McMaster-Carr (which you can get in a few days and $20 for 18" = 8 or so conventional airplanes)). I had planned to use 3/16 music wire until Jim offered me the titanium at the NWR one year. The facing of the bellcrank was also originally supposed to be titanium, too, but all I could lay my hands on in the 20 minutes of scrounging I did was too thick. 6061 T6 is fine for that, and much easier to cut (on a jigsaw).
I am a bit concerned with yours. It's a clever use of conventional materials, but with the leadouts below the plane of the bellcrank, the line tension is cause the bellcrank to tilt and probably wear out the bearing, and for almost certain, bind up under load until it wears sufficiently. Same with the pushrod loads on the ball link. For sure, test it with about 10 lbs of load on the leadouts and see what happens to the arms, and see if it binds.
I like the crankshaft version in the parent post because is solves the tilt problem even better than mine, and potentially, with less friction at the pivot, since roller bearings or some small-surface-area plastic sleeve bearing can be used. I was planning on changing my sleeve bearing to two full-complement ball-bearings for electric (since the control friction is so much more critical), but this seems lighter and less tilt load on the bearings. I don't know if I would put the crank at the end of the shaft because now there is another piece of wire in torsion, adding flex. I would be inclined to let the entire shaft move but still put the pushrod on the bellcrank.
Brett
Topic: Bellcrank and end fittings (Read 16018 times)