A few months ago, Super Troll expressed skepticism/called several of us fabulists (in his own inimitable and hopefully unique style) for claiming we built the airplanes with removeable flaps. Since then, I developed a minor problem and have had to remove the flaps again (for the 4th time since June 2020), so here are some pictures of it (below).

    Note that the reason I had to do this was interesting and superficially disastrous.  I noticed after our Stunt Farm session the other day that the airplane was making a lot of rattling noise while I was carrying it down the road. When I got home, I grabbed the leadouts and tried moving the flap, and there was some minor slop. This is a potential catastrophe, because using the original Rocket City ball links I have airplanes 35 years old with at least 1000+ flights that still have nearly no slop at all in any of the control connections.

     This was not a lot, maybe 1/32" at the TE of the flap, but doing the math in my head, that might mean that one or the other of the screws holding the link to the bellcrank or flap horn might be moving 1/16-3/32" in the hole. The hole through the bellcrank faces and insert is a #33, which gives about .002" of clearance, and should have been filled with JB weld, so if the nut was loose, it must also have banged the hole bigger and was probably tilting, which will degenerate quicky and eventually fail. The control horn upright is drilled and tapped 4-40, then has a nyloc jam nut, also staked with JB Weld.

   If either of these was the problem, then the only solution is cutting into the airplane and fixing it. I spent the last few days figuring out how I might do that, it was certainly doable but very, very ugly.

    As it turns out, the slop is where the horn goes into the flap. For removable flaps, this has to be a slip fit so you can get them off again. I have had no end of problems with the horn jamming in the slot, (usually from rust), and once, from the hardwood swelling up from water exposure. That time I had to cut it loose. It was particularly bad because it happened between round at the 2006 NATs, which I ended up winning!  We were in the Signature fixing at 1 AM Thursday morning.

      While I have always managed to get it loose, I spent extra effort making darn sure that the clip was going to have clearance even after paint, and by sealing the wood up extra-well. It was just fine when I started, but it has now pounded itself out. The quick fix for now is to wrap the arm with FASCAL to take up the extra space. I am still working on a longer-term fix, with the leading candidate being cutting off the existing aircraft ply clips, and machining new clips out of 6061, and tapping that for 2-56 set screws on the bottom to bear on the arm.

   At any rate, it is little things like this that people spend their time on - details matter!  1/32" of slop doesn't sound like much, and I can fly it that way with reasonable success (after finally solving a bunch of other trim and engine setup problems at the Golden State contest), but it was definitely triggering a few little mistakes here and there.

      Brett

p.s. Two additional things. While he normal Klett hinge pin is 1/32 (.032) wire, you only use that to set it up and install the hinges. Once you are done and the hinges are installed, get the next size down (K&S the nearest common size is 0.025") and use that. That will remove any possibility of binding.

   Another thing is that, while it used to be great, the Scotch Clear Mylar tape used to pull off with no residue. But the recent rolls I have gotten leave a sticky mess behind, just like FAS-CAL. You can clean it off of *my* finish with Acetone but if you use dope you would have to use something like mineral spirits/paint thinner, which takes much longer. If anyone knows of a good alternative clear plastic mylar tape to use for this, I am open to ideas.

Thank you for that Brett.

Sorry for the stupid question. How do you retain the wire? By the photo it is not retained at the wingtip end.

Craig

   Not stupid - I bend the end of the wire over, and hold it with a bit of clear tape up against the end of the flap. Of course it would be simple to put in a notch and hold it with a screw, but I haven't done that.

     Brett

The "full-pull" hinge wire is also used on full-scale aircraft flaps, for example on the RV-4. (It is also often used as a hidden method of holding together two cowling halves. Yes, you curve the piano hinge. To get the pin in and out, just chuck the end in an electric drill and spin it while pulling. Reverse to reinstall.)

You might try 3M Blenderm tape. This is a type of medical tape that is a bit more stretchy than the clear plastic tapes you most often run across. I think it is the same stuff, repackaged by DuBro(?) and sold in hobby shops as 1/2A and foamy hinge tape.

https://www.3m.com/3M/en_US/p/d/hcbgebm20001/

Dave

On my butyrate dope finishes I use WD-40 and a soft cloth to remove tape residue with no ill effects to the finish. After removing the residue I use Sprayway Glass Cleaner to clean everything up.

Dennis

You can also use lighter fluid for stickum removal with no bad effects.

Huh.  I always hook the wire ends over, jam them into the balsa wingtip, and call it good.  I have had them migrate maybe 1/4" in several days of flying, but you just stick them back in.

Now I may start taping them, though.

..or like in the original Impact plans (if I remember right); a small eyelet pressed & glued into wing t.e where the L-end of piano wire goes. No tape needed. L

Mine are not full span and the wire passes through an aluminum tube to the first hinge.  I bend the end over to match the tip curve then bow the last inch so that it is tight in the tube. I used to use the sealing tape but this plane, for some reason, flies worse with sealed hinges.

Ken

Did I read that correct? You JB weld the lock nuts when they get permanently installed?

Thanks for the pictures. I think these are the first detailed shots I’ve seen of this feature. A question on hinge placement: it appears from your picture the front half of the hinge is mounted flush on the trailing edge of the wing and not recessed at all. It looks like the rear half of the hinge is recessed just under half its depth into the leading edge of the flap. Does the hinge wire rest on the leading edge of the flap or is it recessed into the leading edge of the flap?

     About all I do it inset it so that I have a tiny bit of clearance between the surface and the hinge pin, on the order of .020. Any more, and it binds - you don't want the pin to touch the airplane.  Of course, you could put a groove down the center of TE and LE to give the required clearance, and allow the gap to appear smaller. But, that makes applying the (mandatory) seal much more difficult. The way I do it, you run the seal from the, say, flap, down into the hinge line, stick it to the pin, then across the gap to the TE, right down the middle, then up on to the wing. If you inset the hinge pin, you still need to get the tape stuck to it, otherwise you get an "accordion fold" effect, which binds it up.

    Brett

  I would like to point out again - *I* certainly didn't not come up with this idea. I very shamelessly ripped it off from someone with some actual imagination - Paul Walker. A good idea is a good idea!

    Brett

JB welding the nuts is one of those good ideas I never once thought of. I just might do it on the next airplane for piece of mind!

     The plans for most all kits I ever built used nuts and bolts for the bell crank mounts, and the instructions (if you read them) called for gluing the threads to retain the nuts after you were satisfied with how everything moved.

  Also before his time - since most of those told you to use Ambroid to do it!  JB-Weld is a bit more secure.

Push rods were still generally "Z" bends to attach to the bell cranks and I don't ever remember seeing people having problems with stuff coming apart.

  "Z" bends are OK, but most of the time people used soldered washers for clearance or other reasons (because that's what it showed on the Nobler plans). Soft-soldering to music wire is an iffy proposition at best, because you didn't want to use acid flux, since it corroded the wire, which made it rough, which then sawed through the bushing. Rosin core was safe but unless you knew *exactly* what you were doing, it wasn't a good flowed-out joint. At the end, I and a lot of other people were also putting grooves in the free end of the wire, and wrapping it with copper and then either epoxying or soldering the copper in addition to the washer as a backup.

   Even worse, some kits showed a two-piece pushrod, which you were supposed to wrap and solder as a lap joint, after sliding the wires to adjust the flap/elevator neutral. Same problem, but now you are using the very weakest joint configuration.

    Brett

JB welding the nuts is one of those good ideas I never once thought of. I just might do it on the next airplane for piece of mind!

   On a second read, it's more than just the nuts - I also put some glue in the hole through the bellcrank and on the threadss, to hopefully take up any slop between the bolt and the bellcrank hole, so once it cures, even if the nut gets loose, it doesn't want to move around, and then pound out the hole. That's no much use in the horn upright, because it is already threaded 4-40 and I tighten it down and use the nylok nut as a jam nut (although I put some glue on that, too, to stake the nut in addition to the nylon insert).

    You really can't be too paranoid about stunt control systems, particularly on one-peice airplanes where you cannot inspect them and servicing involves cutting up the airplane and refinishing.

         Brett

p.s. Two additional things. While he normal Klett hinge pin is 1/32 (.032) wire, you only use that to set it up and install the hinges. Once you are done and the hinges are installed, get the next size down (K&S the nearest common size is 0.025") and use that. That will remove any possibility of binding.

Does anybody know off hand what the pin size is on the Du-Bro standard hinge.  I just got a shipment of wire from K&S - .025 and it is SMALL!  I am concerned that there will be too much slop.

Ken

Ken, the std Dubros in my stash measure about 0.030".

Thanks.  I got some in today and pulled the pin on one.  Replaced the wire with .025 and it really has a lot of slop.  Maybe that is OK if you seal the hinges, that will pretty much stop the movement.  I don't like slop but given the source of the .025 recommendation It is most likely OK.

Ken