I'm building up a pair of Piedmont Temptations from their basic kit (what a deal!). Hopefully the pics and descriptions of what I can offer will be of some use. Starting out a bit in the middle though, mainly because I wanted to get some pictures out of a method of securing the wing tube socket end as a followup to a post on the NSRCA list tonight.

I build up a snug fitting plywood and balsa box that fills the tube socket cutout in the foam core. This is done for two reasons. The most important reason is that it roughly triples the gluing surface area as opposed to the usual method of securing the socket with a single ply plate. This will greatly delay the onset of fatigue damage to the interior of the core. What frequently happens is that the foam beads along the glue joint to the ply plate will crumble and eventually allow the plate to completely separate inside. The tube socket then is floating around and starts to work at high stress against the balsa skin. Failure of the wing panel is imminent and you are lucky if you notice the impending failure in time to repair it.

The second reason for doing this is more of a freebie benefit, but there are sound structural reasons for it. What you get for free is that there is now support for the balsa skin over what would normally be an open hole in the area of highest stress on the skin. However unlikely it may seem, the skin that is in compression during high G maneuvers is always trying to buckle in against the foam core. When there is nothing but a hole underneath, it makes it more likely that a compression failure can occur. This little tube socket box elimiates the hole (obviously!) and adds a bit more structural integrity. Basically, this entire process eliminates a high stress point in a location of the wing where it can normally do the most damage.

Some additional pictures of the tube socket capture box for the Temptation wing. You want to make the balsa pieces that join the two ply plates of sufficient width that they snugly push the ply plates against the interior walls of the foam cutout. With the tube socket inserted in the wnig core (not glued yet!), position the ply plates to slip the socket through. Fit the balsa pieces as described, noting that they too are designed to contact the other two walls of the cutout. I use medium CA to tack glue the balsa to the ply, being careful to avoid using so much that it might leak through and attack the foam. You want to mark and trim these ply plates with a pencil to exactly conform to the surface of the foam core, then remove the box and trim/sand to fit the proper contour top & bottom. This is alot easier than it might sound.

Once you've got a perfectly fitting box, make sure you mark it for top/bottom. You will appreciate this later when you are gluing it altogether during the sheeting process. The final step in making the box is to fit in 1/8" medium balsa sheets between the ply plates and CA them in. This completes the box and assures that the ply plates will press snugly against the interior foam walls for a good glue joint and also gives you support for the balsa skin later on. A little final sanding to match the foam surface contour and the box is done. The whole thing takes maybe 30 minutes per box, start to finish. The finished weight should be about .25oz.

The box gets glued in during the final prep for sheeting the cores. I like using polyurethane glue for this, since it agressively expands and penetrates several layers into the foam. Epoxy will obviously work too. Either way, the box gets glued in as you are laying everything up for sheeting. The glue should not be allowed to set before you wet the skins and place them in the shucks. The entire assembly should be curing together in the shucks. Make sure that you use a sufficient amount of glue to also secure the socket to the box and nearby foam inside the socket hole in the foam core. The tube socket should be an integrated part of the box and core as you prepare to put the skins on. Also, leave the aluminum tube in the socket so that the socket won't collapse slightly during the curing process. This is a one reason why you hear the complaint that tubes don't fit sockets! It is easy to avoid by leaving the tube in during the entire curing process of the sheeted wing.

Ed nice pictures.

In your narrative you say you've increased the gluing area by a factor of 3. Looking at your pictures I don't see that at all, maybe I'm missing something. With the "normal" ply false rib at the end of the tube you glue both sides of the vertical plane to the foam in your pictures it appears to still only have two sides glued to the foam the only extra gluing surface I see is at the ends of the box. Did I miss something?

Very good pics, Ed. Really outlines a most effective way of anchoring that end of tube....very similar to the "Arresti Fix" I had to do after Pawnee Rock last season on Arresti #1. The only thing I did slightly differently was to attach the "box" you fabricated to the chord-wise false ply rib. With lightening holes, it doesn't add but a few grams. I make the false rib full height from sheet to sheet, and usually at least 1/2 chord at that location.
Then, the fiberglass scrim that you show forward of the tube end box, I actually align OVER the wing tube area, and go about 2/3rds span. At the aileron cutout, I make a small rectangle of scrim to tie the hole to the "spar", if it interferes.

You have a very good system designed, looks like it would be easy to do!

My Temptation is the deluxe version (been flying it 6 months now) and I'm a little concerned although I fly Advanced and don't have to contend with the reverse avalanche for now.

Any idea how these 'pre-assembled' Temptations had the tube supported?

I wasn't going to be making a false ply rib, but you're right, if you compare the box to a half rib, the area is probably similar. The increased area claim is over a single ply plate that matches the area of one side of the existing cutout. That's what I had been used to seeing in many large IMAC style model designs and it isn't really adequate. The still works out better than a rib though, because the rib has to transfer the load across a longer moment arm. At the point of highest stress, i.e., around where the tube is, the gluing area has not been increased with the rib approach. So as you progress further out along the length of the rib, the load being transferred along the glue joint is diminishing as a function of that distance. This basically means that the brunt of the load is still along about 1/3 of the false rib. That is definitely better than using a single small plate, I agree. However when you make a box and multiply the area locally, you get an evenly distributed load transferred to core, right where the stress is concentrated. Think of the box as a folded rib I guess!

I would just give Lee a call and ask. Probably the same as what I found in my Focus II wing, which is a false rib. Mine worked loose after 40 or so flights in one wing. Noticed it in time, did surgery and did some preventive maintenance on the other panel and haven't had any trouble. I'm sure that the design is fine, though I prefer my own approach. In the case of my Focus wing socket coming loose, it just wasn't glued enough. You're not going to see any ARF builder do what I am suggesting, because it is too time consuming. There's lots of Focus II's flying and mine and Don Sczur's are the only two I know about that had this happen. Both Don and I got ours in the first batch early last year and we both talked to Lee about it. I would bet they have been on top of getting the socket glue guy to "make with the gluing" the right way.

I'm not trying to get anyone alarmed at all, I'm just putting out a method I use which I feel provides more longevity over many, many hunderds of flights. If you periodically check for anything coming loose, you can catch these fatigue problems and go in and repair them without much problem.

One little correction to what I posted earlier - the load is ultimately being transferred to the skin, but the main point of this exercise is to make a more secure glue joint that evenly distributes the force from the tube to the core interior, i.e., it reduces the shearing force at any point along that glue joint, so it will take more "cycles" to eventually loosen the foam beads along that glue joint. This box also directly transfers more load to the skin, since it produces a multiplied surface area for the skin to contact where there would normally be nothing but foam scented air. Just forgot to point that out earlier.

Ok I think I understand better now. By making the box bigger and increasing the area that the "box" covers it spreads the load better across a bigger area of the sheeting and the foam. Thanks.

Hi
its already at tread that discuss the Temptation on the sit ..why not post all under one?
Rune

http://www.rcuniverse.com/forum/m_19..._1/key_/tm.htm