#12 Covering & Sealing The Wings

My apologies in advance for this “less polished” entry. Since this will be the third time through the covering process, I didn't take photos like I did initially so this will be almost all text in contrast to the scrapbook style I've post previously.  You might notice that I talk about bowing the frame aft in both the bottom blanket and top blanket process summaries.  I am doing this only because my frame tends to bow with even the slightest amount of span wise tension.  Although this counter spring didn't seem to help much, I thought it required an explanation.  My frame may be slightly weak from the impact when I crashed, even though I replaced the spar that broke.  I have not heard anyone else running into this problem and am surprised they haven't given the plan directions that tell you to "pull it as tight as you can" referring to stretching the fabric span wise.

Process summary:

1. block the TE near the bracket positions and at the tip and root. Apply enough pressure to slightly bow the spar aft.
2. clamp 15 inch section on both tip and root making sure to apply some chord-wise tension and snug up the span tension, maybe 1 pound force? And glue it down.
3. Tension the TE with clamps to about 1 pound then locate and cut bracket slits.
4. Glue down 8 inches of the center rib then repeat alternately between the tip and root until you do the last cap strips.
5. Go back to the center rib and work in the same manner toward either the TE or the LE until you reach the rib tape. Repeat on opposite side.
6. Spend some time now experimenting with what tension will do in various locations so you get a good feel on how to proceed. Especially watch what chord-wise tension does.
7. Detach any areas where you have wrinkles that can be removed with a slight amount of either span-wise or chord-wise tension. At this point the surface should be nearly wrinkle-free.
8. Clamp span tension in the spars and glue starting at the center and working out to the tip and root being very careful about chord-wise tension. Finish both spars working right onto the tip and root ribs. Iron these curved areas first with the small iron at 350 + to make the fabric conform prior to gluing.
   

Blanket cut to length and ready to be clamped.

The shot above shows Ernie Moreno (EAA Technical Counselor) looking at my bottom blanket as I attempt to stretch and clamp it prior to gluing.

Apply glue to spars along a strip well clear of the bottom surface tangent. The upper surface side of the spar can be glued well past the tangent since you'll be wrapping the cut edge over into the inside of the panel and you want that edge to lie down onto the spar away from the upper blanket when it's applied. It might be easiest to do the bottom glue line first and then flip the panel to continue the glue boundary around the top side of the spar to the inside of the panel. Use a couple of spring clamps to clamp approximately 15 inch section at the middle of the tip rib (curved aluminum tip piece) while applying a moderate amount of span-wise tension like maybe one pound of force. Wet the glue and smash the cloth into it smoothing out air pockets. At this point you have not yet trimmed off the extra cloth, this will be done after gluing the entire perimeter of the panel. Do the same on the tip after removing the clamps applying maybe a pound of force span-wise. Keep in mind, this is what I found I had to do to avoid dog-legging the spars and NOT what the plans say. The plans say to pull span wise tension as tight as you can. If I did that, I already know I'd dog-leg the spars all to hell. This may be because the spars in my frame were weakened in the crash and are not perfectly straight as they are in a virgin kit. Begin in the center and glue down approximately 8 inches of one of the ribs.  Clamp both ends of the TE with some span-wise tension and cut the bracket slots with a paddle tip soldering iron. I made templates (rectangle cut out of art board) and pencil traced the slots eyeballing their locations on the cloth. Repeat on LE. Some I got OK, others I missed and had to re-cut extra slots in the right location. There is probably a better process to locate the slits exactly. Work out each rib with 8 inch strips alternating between root and tip sides until you have all the ribs glued. Repeat toward either the LE or TE but stop short of gluing to the spars.

Spend a half hour or so and play with the cloth tension at the TE and LE watching what happens when you tension the cloth, especially chord-wise. Detach any really wrinkled areas if you get any and carefully apply slightly more span wise tension. When I finished here I had a fairly wrinkle-free bottom panel with very light span-wise and chord-wise tension. I estimated the bow to be about ¼ to ½ an inch along the entire spar. To finish, I clamped both ends of each spar and began gluing from the center out to the tip and then the root or visa versa...didn't matter. I took a lot of time here and tried to apply as little chord-wise tension as possible and still have most of the wrinkles gone. The problem areas are the tip curves and the chord airfoil curves in both the tip and root tubes. I tried to iron in the curve but had marginal success. I used an hand-held, infra red laser thermometer to monitor the iron temperatures I refer to throughout. Cut some slits and did what I could using the small iron at 345 F to finish off smashing the folds down to a reasonable level. Went around the perimeter and trimmed off the extra cloth ending up with a wrap somewhat past the tangent line (line where the finished cloth surface leaves the spar). Did this free hand with a razor blade. Finished off the panel by gluing down the trimmed edge (wet the glue that I had applied earlier). This was easier after I decided to flip the panel over. Went over the entire perimeter with the small iron at 345 smoothing out any bubbles or wrinkles. Sketched in a glue reference line for the top blanket to get approximately 1 ¼ inch cloth overlap. Applied glue to the area for the entire perimeter and all rib caps if they weren't done prior. Cut the next blanket to length. Clamp the cloth in place with span-wise tension ~0.5 – 1.0 pounds? Trim off the roller perf edge of the cloth along the LE. Set both the root and tip with a short glue tack spot making sure the edge of the cloth is on the pre-marked line to get the 1 ¼ inch of overlap. Note that this is also a deviation from plan directions which show overlapping 1/2” from the cloth tangent to the spar. I did not have to go this far to get the standard 1 inch overlap required on certified aircraft. This also avoids messes in the cloth right where it leaves the spar. I found it best to leave this area completely free of glue prior to doing the overlap of the final blanket. I did this by tracing in straight pencil lines defining a 1 inch wide band along both the TE and LE and applying the glue only in that area which can easily be located entirely on the spar surface and not go past the cloth tangent line.Process summary:

1. block the TE against the brackets and at both ends. Put enough pressure on it to pull the spar straight or slightly bowed aft (opposing what the frame does when it sees span-wise tension from the fabric)
2. tension the LE with clamps and then glue from the middle working outward.
3. Tension the TE span-wise but also applying some chord-wise tension and clamp so that the wrinkles as just barely out. Play with it from tip to root until all wrinkles are out.
4. Tack down the TE from the middle out. Remember to tack, not glue since you might need to release some areas before you're done.
5. Tack down the LE from the middle out.
6. Last, work right on around the tip curves and the root curve (not convinced pre-ironing these curves really helped)

Start at the middle of the LE and tack down the entire LE working outward toward either the root or the tip. Repeat on the opposite end. I used almost no tension here at all since I had already pulled the cloth to tension it along the span. It should lie flat and smooth with no wrinkles at all. Do the same thing on the TE except clamp the fabric using moderate span tension of about 1-2 pounds while at the same time applying slight chord-wise tension to just barely keep the wrinkles out. Play with this going back and forth from the root to the tip to get it nice and smooth. When it looks really good, go ahead and begin tacking down the TE starting at the middle of the spar and working out. Watch carefully as you do this as you will be using almost NO chord-wise tension. Even if you have some waves (these may show up right as the fabric leaves the spar) which I left if I could not get them to disappear without inducing inter-rib sag. I figure the iron will take out some of this and I may also come back and release some of the tack and re-glue later just prior to ironing. I am convinced that this procedure can produce an almost wrinkle-free covering without even ironing if you take your time. It does take some skill and patience though. I did get some inter-rib sag that was noticeable when viewing from the TE but the filament tape area was virtually sag-free prior to ironing. I had applied about 12-15 pounds of force aft at the middle of the TE using blocks screwed to the table about 3-4 ft inboard and then clamped blocks pushing forward toward the LE at the tip and root. I could not really apply enough pressure to move the spars such that I got a concave LE profile....it was almost exactly straight. I left this tension in the frame when I glued down the LE, TE, tip and root. When I released the tension, some very loose pockets formed near the tip at the back half of the chord. I plan to release these areas and re-glue. The relaxed frame looked to still have some sweep curve but I think it will work. Next, I set the iron at 220 F. Ended up watching both my new iron and Ernie's old beater and neither one held very close to a setting. I averaged 245 for the main wing ironing and ended up just barely getting the extra puckers and ripples out of the main panels in between ribs. I did not glue the ribs on the top surface prior to ironing. I think this was best. I plan to do that as the very last step before Poly Brush. I used the small iron set to about 360-380 and got good results ironing out bubbles and wrinkles in the overlapped areas. Also used it to roll the edges on the tip curves and the root rib curve and got them both down with very few wrinkles or folds. I was concerned that I might melt the cloth with the iron this hot but did not see any melting even when it measured 400 F. I was a bit shy on the glue line and long on trimming both the LE and TE overlap (at least in spots) making the glue joint look filled in was hard. It was really nice not to deal with over glued areas past the LE or TE since I left very little tension in the chord to pull these puckers out once they lay down in the excess glue. I did end up releasing one area in the LE and pulled out a fairly major pucker spanning an entire rib bay. It came out looking much better than when I started and after finishing it off with the small iron, it was really wrinkle-free. Rib sag came in to the tune of about ¼ to 3/8 inch on the aft edge of the tape line farthest in from the LE. I cannot envision getting it any better than this and still having a wrinkle-free covering. I had about as little chord-wise tension as absolutely possible and still have no wrinkles or puckers. When I pick up the tip allowing the panel to rest on the root end, there are multiple wavy wrinkles that appear which gives an idea of how loosely I have the sail strung. I applied 50/50 glue/MEK to the rib caps on the top side, holding slight pressure with the edge of my left palm while brushing on the glue with my right. I only had to keep the slight pressure on for 5-10 seconds to get the glue to hold the fabric on the cap strip. The shop was about 65 F when I was doing this. I detailed the surface after blocking in the washout using the small iron at 345 F. It was easy to take out nearly every little wrinkle.

Applying Poly Brush I found that froth formed from shaking the can and did not dissipate until halfway into the panel. I had to really watch for air bubbles as I brushed on the sealant and brush them out as I went. If I didn't do this, they end up being significant topography in the coating and will obviously be seen after painting the final coat. It is best to do the bottom first so that after drying overnight, you can flip the panel and not have the ribs hitting the table. I didn't do this on the first panel and it did not stick but better do the bottom first on the rest. I used a small brush, maybe 1 ½ inch wide? Could have used a wider brush. These were the $1 disposable natural bristle found at Home Depot. I tried a wider, polyester brush I got at Sherwin Williams paint store and found it was way too stiff and just did not work. I definitely prefer a pliable brush to do this. Ernie says I can use the iron after Poly Brush to clean up any wrinkles I miss.

These shots of the first brushed-on coat of sealant (Poly Brush) give you a rough view of the tension I have in the fabric.  There are no wrinkles at all when I've got the washout blocked in but if I lift the tip off the table onto the root spar ends, large span-wise wrinkles appear.  I am counting on final rigging to hold the wing panels flat just like I have it laying on the table here and it should be wrinkle-free.

Finally finished painting the trailer and putting all the door hardware back on.  Here it is at the shop, ready for the first wing panel to come off the table so I can move on to the right, upper wing panel.

Looks nice, doesn't it?  Really having regrets about not spending more time planning out the color scheme on the trailer.....primer white????, what was I thinking.  Now it's going to be a pain in the ass to change!  Probably will just leave it.