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( 3 / 30 )I tried breaking the extra "T". Those joints are TOUGH! Supporting the top of the "T" about 1/2" on each side of the gusset, I tried pulling on the stem of the "T". I was expecting a bending or shear failure in the top of the "T", but I couldn't grip the stem hard enough to apply the force needed to break it. I was able to pull with maybe 100-150 lbs of force. In the process, I crushed the end of the stem with the grips.
I cut off the crushed part and tried pushing on the stem in a vise with the top supported similarly to above. The top broke just outside the glue joint. Bending failure in the tension fibers. Just for kicks I pushed sideways on the T until it broke. Same result.
With the next test piece, I intend to devise a better way to apply tension to the stem - perhaps by gusseting another crosspiece to the bottom to form an "I". I also want to try the compression test again by supporting the top closer to the gusset to induce shear failure in the top or compression failure in the stem. If I can get a test that consistently fails the stem in compression or tension (not bending), then I can be reasonably sure that I am testing the joint to its fullest capacity.
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( 3 / 30 )The waxed paper is started to show wear. The epoxy drips are starting to stick, so I am going to have to replace release layer soon. If I were starting over, I might give that plexiglass surfaced jig a shot. As it is, I am just going to replace the waxed paper with transparency film. That should hold up better..
I also made a few extra test pieces. I took 4 strips of plywood and lapped them together to make two strips. One set had sanded gluing surfaces, the other did not. The area of bonded surface is the same for each strip. I am hoping to break these to get a feel for how critical sanding the gusset faces is.
I made an extra gusseted "T" just to play with some test procedures and to get a feel for how strong these things are.
1 hr
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( 3 / 30 )I've noticed a small problem with this rib. The top capstrip was not milled perfectly square. Therefore, not all of the joints clamped up tight. In particular, the large gusset near the aileron cutaway did not lay flat, leaving a 1/32" gap. I know that T-88 can bridge up to 1/16" gaps, but I think I've just made my first test rib.
1 hr
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( 2.9 / 21 )The system is working fairly well.
The images below show a couple of details. The first picture is just a joint showing the epoxy coated gusset and sticks. You can see the mitered ends as well as the typical fillet of epoxy formed from the squeeze-out.
The second picture shows the bond release tape that I applied to the lower capstrip doubler where the aileron is cut away. Once the ribs are assembled on the spars, the lower capstrip and part of the gusset in this area will be cut away with a router. Once exposed, this tape will be removed, and a clean surface will remain for bonding the aileron runners.
1 hr
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( 3 / 30 )Made Rib #3
1 hr
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( 3 / 30 )I removed Rib #1 from the jig. To do this I had to loosen a few of the screws holding the blocks in place, and I had to totally unscrew the dummy block for the rear spar. With some gentle prying the rib was free. I was easily able to push out the dummy block after the rib was off the jig.
The rib is amazingly stiff and looks good. The only squawk I saw was that one small corner of one gusset didn't have squeeze-out. I'll be a little more careful to apply sufficient epoxy in the future. I'll take it to the next EAA meeting for my tech advisor to inspect.
Some gussets and a little epoxy extend past the capstrips. I think a flush cutting router bit will clean them up perfectly.
I weighed the rib on a postal scale - 4.0 ounces. I wish I had something to compare it too.
Since Rib #1 went well, I glued up another - Rib #2
1 hr
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( 3 / 30 )More cutting of rib sticks, a last batch of gussets, and it's finally time to glue up a rib!
Rib #1
I started by lightly sanding each gusset to remove any free fibers at the sawn edge. I also made one pass on each face to break the glaze or remove oils. I also ran a bit of steel wool along the spruce strips to clean up the mill fuzz. This has the added benefit of very effectively uncovering any minute, unopened splits or grain runout. The wool catches in the split and opens it up enough to see. I've discarded several pieces from these splits. There was some mention of concern for the oils on the steel wool as a rust preventative. I am using a well worn piece of wool, and use very little pressure near the glued ends. There is no evidence of oil affecting the joints of any of the test pieces, and the epoxy wets out nicely. However, it is enough of a concern that I am going to get some of the oil-free wool the next time I am at the store.
After dressing up the gussets and sticks, I mixed up the T-88. I have two pet medicine styringes that I use to get a 1:1 volume ratio of the two parts. With this first rib, I discovered that it takes about 7 ml of epoxy for all the joints and the test piece. I prefer to mix only one batch for each rib. That way, the test piece will accurately reflect the epoxy used for that entire rib.
The epoxy was first applied in a thin layer to the inside face of half the gussets, and the gussets were placed glue side up in position in the jig. The entire face was covered with this thin epoxy layer to ensure that the gusset remained sealed from moisture, even in the nooks and crannys that will be hard to coat with varnish later.
Next, epoxy was applied to the mating faces and ends of the vertical and diagonal members before being slipped into the jig. Mating faces of the capstrips were then coated with epoxy and the capstrips were put in position in the jig.
Finally, the remaining gussets were coated with epoxy and placed glue side down into position in the jig. PVC clamps were then applied. I used one large clamp at each joint to hold the capstrips tight horizontally against the HDPE blocks, and two smaller clamps to hold the gussets down (I'll take a picture later). Some places required extra clamps. With the clamps in place, I then inspected each joint to make sure the gussets were in position and the joints were tight.
With the remaining epoxy and some gusset and stick scraps, I made a test joint. The joint was basically a "T" with (unsanded) gussets on both sides. I left an inch or more of the stick ends free to be later attached to the test apparatus. I plan to make one test joint for each rib and then break them before putting the ribs on the spars. The rib and test piece each have a number so they can be matched up later.
The epoxy application and assembly of the rib takes a little over a half hour. My work area is about 65 degrees, so the T-88 is just starting to show signs of setting when I am making the test piece. I figure that the test piece has the most factors working against it, and should therefore fail before my carefully constructed rib.
6 hr
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( 2.6 / 21 )Continued...
2 hr
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( 3 / 15 )Continued...
2 hr
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( 2.9 / 16 )
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