#13 Covering The Wings

November 19, 2013

I was not going to let this happen but here I am.  Nearly two months since a new post.  The longer I put this off the more difficult it becomes to keep the info organized and assembled into a coherent and accurate account.  I want to emphasize that I am learning as I go and what I have done up to this point is to merely convey that which some times turns out to be a major blunder or innocent error on my part so beware in what you take away should you decide to repair or build an Easy Riser!  I am using the Stits or now called Poly Fiber chemicals and process vs the Stewart system or the old tautening nitrate dope or Ceconite process.

Upper Left Wing Panel

The first swath of glue working from the center both span wise and chord wise.

I later on decide not to glue down the rib cap strips until last (after shrinking).  The root and tip tubes were glued down first using a couple clamps to hold down the opposite end with 3-4 pounds tension. I will soon find out that there is enough shrink available in this fabric to take out puckers and folds I was concerned would not come out and this was accomplished at only 250 F (actually, the Poly Fiber manual states 225F as the first pass temperature to use). I will also find out that the fabric will stand 400 F as measured on the small iron with no noticeable problems so I have plenty of margin to work with. In retrospect, it seems like the best process would have been to have glued down the entire perimeter of both the top and bottom blankets and iron the entire wing at a low temp (225F). This should allow a nice, even tension of the entire panel without having the ribs involved. After that is done, then the ribs would be glued down and a final shrink done at the lowest you need to use to just get the surface wrinkle-free (no more than 250 should be needed unless you did a really poor job of covering). With the frame I'm working with, I still don't believe I can do much better than about ¼ inch sag between the ribs (measured at the height of the leading edge) mainly because I seem to be so limited on the amount of span wise tension I can apply without putting a dog leg in the frame.

Poly Brush brush-on first coat complete. Notice the bottom panel is done first and left to dry overnight and then the frame is flipped and the top is then finished off so that I didn't have to rest the top panel surface coated with Poly Brush onto the bench surface and risk having the P Brush stick to the bench top.

Notice the paper slit templates laying on the table.

Arrived with the empty trailer ready to take this wing panel back to my shop and probably end up storing it until I get painting weather next spring. I don't think the weather is going to last long enough for me to finish two more wings so that I would be ready to spray paint.

Right Upper Panel.

First drew a glue line onto both top and bottom fabric tangent points for both the TE and LE directly onto the spars using the finger guide technique or FGT (rest your middle and ring finger on the spar as you move down the spar while holding the pencil between your index finger and thumb). This works really well and is fast to do. Applied full strength glue between these two guide lines for both spars and bottom surface rib caps. Cut the bottom blanket to length leaving about 3-4 inches of extra at each end (never needed more than that) and squared up the fabric on the frame. Pulled tension on the center section of the chord and clamped it at the root and tip. Beginning in the center of the span, glued a section of the rib caps working out to the root and tip.

Again, I later decide this is not the best process.  Better to glue down the cap strips after the shrinking step. Since this was an upper panel, I was using very light tension chord-wise tension for the top blanket. Forgot the sequence I used on the left wing and clamped the trailing edge root and tip with moderate span wise tension and burned the bracket slots. LE first is the best order to do this (the reverse of what I ended up doing on this panel).  Used a paddle tip on my soldering gun to burn both the TE mid and TE tip slots. Marked both sets of slits while fabric was tensioned. I left it tensioned and slipped a small piece of 4”X4”,¼ inch plate of plywood under the fabric and used it as a backer plate to burn against.

I then glued the TE starting at the middle bracket, working out to the tip and root ends using a very light touch chord-wise to just get the fabric smooth and no more. It is very easy to pull just a little bit more on each 2-4 inch tack which you just have to resist or you will end up with puckers where the last tack strip stopped.  Be checking as you work to make sure this isn't happening.

Next, I applied 3-4 lbs span wise tension on the LE, marked the bracket slots at the “just snug” chord-wise tension point and burned them. I proceeded to glue down the LE using very little chord-wise tension (just enough to pull the little pucker out right where the fabric hits the LE (fabric tangent point). Next, I finished up the rib cap strips all the way to the spars.  Using my fingers here to hold the fabric down on the cap strip....best to use a couple magazines for this like the plans say.  I did that later and it worked much better.

In tacking down the LE and TE, I stopped about 4 inches away from the tip curve and the root rib tube corner on both the spars and the rib tubes. I finished these four corner areas last.

I used the small iron set at about 350 and slowly worked the fabric around the tip curves and root camber arch. This time it worked really well mainly because I had not yet cut the excess fabric off and had something to pull on as I used the iron to heat form around the curves. I used a pencil to trace a fabric cut line onto the fabric right at the spar fabric tangent point using the FGT. I had already traced the glue boundary line onto the spar which I could see through the fabric as I draped it around the spar. I traced the cut line just shy of the glue line to make sure the glue held down the cut edge. Cut the extra fabric off both the TE and LE to wrap onto the spar AFTER double checking that I had finished heat forming the fabric around the tip curves and both tip and root rib curves past the glue line. I did not allow much more than what I needed to produce an inch and a quarter overlap for the top blanket. It took a fair bit of time to brush on 50/50 glue/MEK mix and glue it down to the spar. If you go at this carefully and only do a short section at a time (like about 4 inches) you will be able to smooth out the fabric and get it to lay down nice with no wrinkles or bubbles. This saved a lot of time later when I used the small iron to go all around the perimeter smoothing out all the air pockets, wrinkles and glue topography for a good, smooth fabric to fabric overlap joint. Noticed that I did not say at what point I burned the LE bracket slits in the #12 report which is a critical item. I will guess that I did it after gluing a center swath of rib caps out to the root and tip. This would give me some chord-wise constraint as I attempted to properly locate the slits and maintain minimum chord-wise tension.

Top Blanket

This is a shot of the upper right panel flipped and the top blanket being applied.

Tacking down the tip end.

The entire root and tip tubes are glued with moderate tension prior to starting on the spars (LE & TE). This shot show the results of detailed snipping and cutting to fit the fabric around the root rib tube LE spar joint.

A shot of typical ripples visible on the finished panel. These can easily be ironed out at around 225F.

Gluing the LE while propped up on a post for easy access worked well.

A look at the bottom blanket in final form prior to sealing.

During the sealing process, I applied the Poly Brush too heavily over the tape area and got these nasty solvent bubbles that I could not get out. I will try to remove them by brushing over these areas later with 65-75 Reducer. This is the only area this happened on and I think what happened was that I did not have the filler mixed in well enough which allowed a lot of solvent to go right through the fabric and get trapped on the tape surface.

Lower Left Panel

Trimming off excess fabric on the bottom blanket.

Leaving the excess fabric on while using the high temp small iron was best to get the fabric to conform around the tip curves and both rib tube curves.

Started the top side blanket on the lower left panel after assembling the upper panel to it with temp rigging wires and all struts in place including diagonals and rudder to check for fold down binding. There was none and the rudder rotated freely so I am calling it good enough. The only issue is that after folding the panels down the leading and trailing edges don't line up and are off about 2 or 3 inches at the tips. Since there is nothing I can do about this short of rebuilding the entire panel frames, I am going to push on (I had made this decision back when I repaired the upper left panel spar and was aware then that the panels were skewed relative to each other when folded down). The rudder control cable was too short to reach the rudder horn so I decided to disassemble the cable from the twist grip.

UFM shipped these assembled. Notice how they had attached the cable to the twist grip tube. The tube was padded with standard duct tape with the cable wrapped around the tube. Filament tape was then used to secure the cable to the tube and then the whole grip area was wrapped with what appeared to be bike handle wrap tape. Brian Porter suggests I fit mine with 90 degree “handles”. His opinion was that the twist grip is OK to learn on but after you spend an hour in the air, those twist grips get really tiring on the forearms.

I am going to put more span-wise tension on the lower panels than I did on the upper panels because they should be able to take more stress since they are shorter/stockier. This time around, I cut out templates from notebook paper to use in locating the bracket slits and it worked fine, no need to use heavier card stock paper and also easier to modify. The trick still remains as to the best way to locate the slits prior to burning them.

Top Blanket Process Steps;

1. burn the tip LE bracket slits
2. tension the fabric span-wise on the LE
3. mark position of mid-span bracket slits (after checking that the fabric drape is parallel to the LE...i.e. probably better not to mount the blanket crooked)
4. burn mid LE slits ALL GLUING FROM HERE ON IS ONLY A TACK LINE APPROX ½ INCH WIDE USING 50/50 GLUE/MEK (I did this because it allowed me to correct spots that weren't right later)
5. re-tension fabric and glue LE from root to tip (used very light touch here to avoid inducing puckers along the glue line). I ended up with a wrinkle and no way to pull it out on the tip as I glued down the LE tip curve (do the curve last)
6. tensioned TE span-wise leaving tip bracket outside clamped area, marked and burned the middle bracket slits. I was careful to apply almost no chord-wise tension when locating these slits...just barely enough to take out the chord-wise looseness.
7. Glued fabric from mid bracket out to the root end of the TE spar and then from the mid bracket out to the tip just shy of the tip bracket.
8. Did the best I could to mark the tip bracket slits and burned them. This was hard to do and I didn't get the location very good and ended up with some big, ugly slits to cover later. This bracket is so near the tip curve that it's by far the hardest bracket to locate slits for. Probably a better technique...I didn't find it.
9. Tack glued both the root and tip rib tubes then went over the entire top side blanket looking for loose spots and wrinkles. I used the big iron on low(225F) and did only spot ironing to tighten out the waves and puckers I could see. I then went back over with the small iron on high (350) and went after the smaller, more difficult wrinkles and folds and got nearly everything smooth.
10. Flipped the panel over and pulled the extra fabric over the spars snug so that the glue line I had drawn in as a guide in gluing up the spars was visible through the fabric. Used the finger guide against the spar technique (FGT) to sketch in my cut line onto the extra fabric with a pencil. Marked around the entire panel and then used the soldering knife tip to hot melt the cut line. I used an aluminum straight edge as a backing plate to cut against after finding that using a wood piece kept making the tip wander as it tried to follow the grain vs the cut line. I quickly concluded that using a hot knife to do this was a poor choice. A sharp scissors leaves a far superior (smooth & precise) fabric edge to glue down and takes less setup.
11. Glued the rest of the fabric wrap around each spar plus the root and tip ribs
12. Detailed the bracket slit areas to remove extra fabric and used the small iron to smooth things out at each bracket in prep for the cover patches. Marked and cut out cover patches as described previously except I found I could make the paper templates much easier with notebook paper vs card stock. Glued them in place using Poly Tak.
13. Decided on a 20 inch piece of reinforcement fabric (4 inch wide bias tape from Aircraft Spruce) to protect the TE tip section of the panel. Both the tips come into ground contact when the glider is parked so they get the most wear. Applied a glue line after marking it out in pencil. Glued down a section on the top surface inboard of the tip bracket and let it dry. Came back later and pulled tension on the tape, wrapping it around the TE tip curve and clamped it. Used the small iron starting in the middle of the curve and slowly ironed out all the wrinkles and then glued the top part only. Used the same method on the LE where I decided to add an additional piece of reinforcement tape although not quite as long....approx. 16 inches. I later read the Poly Fiber manual and realized I had done this wrong. Apparently all reinforcement tapes, including gusset patches applied onto the fabric are cemented on using only Poly Brush, not fabric cement (Poly Tak). The reason given is that Poly Tak cement is not flexible enough. Flipped the panel and penciled in the glue line for the reinforcement tape sections and used the small iron to smooth out the tape onto the glue area prior to wetting out the fabric.
14. Went over entire frame top and bottom with the small iron and smoothed out air pockets and patch areas. Used large iron to finish off the top surface after blocking in the washout (making sure the panel is properly blocked up prior to the final ironing allows you to see where you need to iron and where you don't since the frame is so flexible.

I discovered that Poly Brush solids (the ingredient that enables the fabric pores to be plugged) settle out and do not go into solution even after vigorous shaking. I had to stir the stuff off the bottom where it had settled out and gummed up. You can tell you're getting into it since it is white in color against the iron oxide red of the solvent mix. The Poly Fiber manual talks about the function of Poly Brush being to seal the fabric pores and some good discussion about the cause of pinholes and what to do about them. I was supposed to thin the Poly Brush with 65-75 Reducer (according to the Poly Fiber manual) but Ernie never dilutes Poly Brush with the first brush-on coat. I plan to use the Reducer when I brush coat the last wing just to see if I have less or more pinhole/bubble problems. I have now completed the first lower wing panel through Poly Brush sealing and am on to the last remaining wing panel. Here are the other photos of the progression on the first lower panel;

Used the side of my hand to apply even but very light tension chord-wise as I tacked the spars.

View of the tip end after tacking it in place. Loose areas like this one I released the tack and pulled most of it smooth and re-tacked it. What was left the iron took care of easily.

A shot of the TE tip area after tacking. Notice there are some folds that will need to come out with the iron.

I continue to screw up the bracket slit locations. Fortunately, these ugly holes are easily covered with the patches applied after Poly Brush sealing.

A shot of me using the FGT (finger guide technique described earlier) to draw the fabric cut line.

Here I am trying out the hot knife approach to cutting the fabric along the cut line. This did not work well. It required a backer plate to cut against, was hard to hold a straight line and also tended to form melt beads in the fabric since you cannot control the tip temperature. Use a good, sharp scissors instead.

Didn't read the manual and glued on this reinforcement bias tape to the high wear area on the TE tip along with all the gusset patches on this wing. All tapes and patches should be put on AFTER the first brush coat of Poly Brush per the manual since glue is too inflexible.

Finished with initial brush-on coat of Poly Brush.

Lower Right Panel

I will try to describe the best of everything I've learned for this last panel starting with the finished frame.

First off, I wrapped all the rib gussets with chafing tape (athletic tape works fine) as well as anywhere the fabric wraps over rivets or gussets to minimize wear on the fabric. The plans did not call for this but I think it's worth doing.

Used the FGT(see above) to draw glue lines onto the spars at the fabric tangent point. I did not strip the old glue (Super Seam) after the Poly Fiber tech told me these glues are compatible.

Applied full strength glue between these lines with a ½ inch brush with the frame propped up off the table with a support post for easy access rather than flipping the frame and having to make two, separate glue passes to get the glue area covered.

Applied glue to the bottom surface rib cap strips only. Save the top surface rib cap glue for later.

Bottom Blanket

Roll out the fabric and cut to length. Even out the overlap on both LE and TE and clamp the four corners so that the fabric slack is mostly pulled out. Tack down both the root and tip rib tubes with desired tension (3-4 lbs) alternating back and forth from root to tip to keep the fabric even. Stop about 4-5 inches away from the tip curve and the same from the root rib tube corners. Take out the chord-wise slack and apply 3-4 lbs tension to the LE fabric and clamp it in place. Tack down the LE starting in the middle and working out to within 4-5 inches of the root and tip rib tubes. Use a very light touch on chord-wise tension. The idea is to tack down the fabric along the straight line of tension you are applying parallel to the LE and resist pulling on it which only pulls it out of alignment. Repeat on the TE except this time you want to take out the chord-wise slack using enough tension to have a final, smooth fabric surface free of slack or major wrinkles. Continue to tack the root and tip rib tubes moving right on around the tip curve and the root rib tube and finish off both the LE and TE to completely join the tack lines. This is the easiest panel surface you have on the entire airframe (it's short and strong and also has no bracket slits to contend with). so do it first saving the upper panel bottom surfaces for last. You may get some small wrinkles and puckers especially in the root and tip sections which will have to be ironed out later so try to keep them from happening in the first place. Use the FGT and pencil in the cut line onto the fabric tail to be cut off all around the perimeter of the panel. Make sure you draw this slightly short of the glue area on the spars to ensure the fabric edge will hit glue when cut off and wrapped onto the spar. Use the small iron on high (350F) and heat-form the fabric around the tip curve. Best technique I found was to pull tension on the tail and wrap the fabric around the tube while working the iron around the tube. Start in the middle of the curve and work out and take some time. If you do, you will likely get zero pleats or wrinkles. Keep wrapping and forming until you are an 1/8 inch past the cut line. Work the rib camber curve area in the same manner for both the tip and root. There is also some detail cutting and snipping to be done to fit the fabric around the root rib tube where it joins the TE and LE. Using a sharp scissors, cut off the excess fabric and begin the finished gluing of the entire frame perimeter. I used the same order as I did to tack the fabric. If the glue lines you penciled in are not still easy to see, re-trace them now. You will need to be able to see them through the top blanket fabric.

Top Blanket

Set the panel up onto a support post and apply full strength glue between the glue guide lines you drew for the bottom panel. Keep the glue from going past the edge of the fabric which should stop right at the fabric tangent point. If you don't, when you do final gluing of the top blanket, the fabric can stick down past the tangent line and form little puckers just inboard on the spar and they are almost impossible to get out while maintaining very light chord-wise tension.

Lay the panel flat on a table with a washout support block in place and apply glue to the rib cap strips going right on over the filament tape. Go extra heavy over the anti chaffing tape because it will absorb the glue like a sponge.

Rolled out the fabric and cut it with about 4 inches extra on both ends.

Tacked down about 12 inches on the root rib tube just aft of the last LE tape strip using maybe 4-5 pounds of tension (a bit more than the bottom because you need the tension to hold the fabric up between the ribs so support the small amount of chord-wise tension you'll need to apply to get a smooth fabric surface that won't flutter in flight.

Repeat on tip. Ended up with some small span-wise waves and puckers which will come out when I apply a little chord-wise tension later on.

Extended gluing on both root and tip tube ribs stopping short of the tip curve and the spars at the root end.

Tacked down the LE starting at the root out to the tip. Very light touch on chord-wise tension. Just barely enough to avoid a pucker at the fabric tangent point on the LE spar.

Clamped LE at tip and root with moderate tension to locate and burn the middle bracket slit.

I made a paper template to pencil trace the slits onto the fabric prior to burning them.

Applied final LE tension and located tip bracket and burned it in.

Starting from the middle bracket, tacked the LE to the root and then to the tip stopping short of the curve at the tip.

Clamped moderate tension on TE and located, then burned the middle bracket slits with just enough chord-wise tension to just take up the slackness in the fabric without any pull down between ribs.

Proceed to tack glue the LE from the middle bracket to the tip and then to the root stopping short of the tip curve.

Applied final tension of TE (guessing 4-5 lbs) and located, then burned the tip bracket slits. Again, with minimal chord-wise tension. There should be almost no detectible pull down between ribs in the area just aft of the last span of filament tape.

Tack glue the TE beginning at the middle bracket to the tip and then to the root stopping short of the tip curve.

Tack glue the tip rib using light span-wise tension. Work from the center of the tip rib tube out to the LE and then repeat out to the TE and continue right on around the curve. Tack a thin strip approx. ¼ wide right at the fabric tangent line.

Tack the root rib tube the same way except pull moderately strong span-wise tension as you glue(4-5 lbs?). If you are careful, you should have almost no wrinkles, folds, puckers or pleats. Even after all the practice I've had doing this, I still get some but they come out with the iron at 225F. Any left after that, I hit with the small iron set at high (350F).

Fix any problem areas if needed by releasing the tack line with MEK and re-gluing. With the panel bottom side down on the table, wrap the excess fabric around the spars and trace a line onto the fabric right over the cut line you drew on the spars.

Heat forming to the curves

Take the small iron and slowly work the fabric down into the glue all along the tip curve while applying some tension on the fabric tail (this is why you didn't cut this off yet, you need it to hang onto and pull to help form the fabric to the curve.....if you cut off the excess and try to form the fabric, you will end up with pleats that you cannot iron out).

The glue acts like hot melt glue when you do this and actually sticks down. Get as much as you can from the bottom side and then flip the frame over and finish it. You should be able to get all the pleats worked out right up to the cut line and a little past it.

Repeat this process all along the root rib tube.

Use a nice, sharp scissors and cut the excess fabric off. There is some detail work around the root rib tube that you'll have to screw with to get it to look good including the slit for the rudder control cable.

Final Gluing

Start gluing the fabric from the middle bracket out on both the LE and TE. The only way this worked for me was using my bare finger and a ½ inch brush doing about a 3 inch section at a time moving as fast as I could. The idea is to smash and work the 50/50 glue/MEK through the fabric into the glue layer on the spar. If it doesn't seem to be wetting out the glue layer underneath, dilute the mix with more MEK. Even though you heat-formed the fabric around the tip curves and root rib tube, it still needs to be glued so just make sure you've gone all the way around the entire frame once you start gluing. Save gluing the rib cap strips for the very last thing you do prior to applying the Poly Brush sealant. Go around the entire frame and iron out any topography over the entire frame glue area you just finished. The entire frame perimeter should be nice and smooth when you are done.

Ironing

Set the big iron to 225F, check that the washout block is in place and hit areas that are the least taut first. The idea here is to get somewhere close to even tension on the entire panel and remove all wrinkles and puckers in the fabric. I watched the inter rib pull down very closely as I did this to make sure I didn't go any further than necessary to remove wrinkles and get a reasonable fabric tension (whatever that is). I never had to go past 225F to get all the wrinkles out. You can go all the way up to 350F with no fabric degradation if you had to. Let the glue dry overnight.

Sealing

Mix the Poly Brush 3:1 with one part 65-75 Reducer. Use a stirring stick to get the filler off the bottom of the can of Poly Brush and get it completely stirred into the solvent. It is a whitish material and settles out really fast as gum on the bottom of the can. I brushed this on in a shop with the temp set to 65F where it was 50F outside and raining in Oregon and had no trouble with blushing. Used a 3 inch disposable natural bristle brush which had nice and soft bristles (the polyester ones were too stiff and not well tapered). Fast and easy was the trick. Avoid slopping it on. Brush out any bubbles that form since they will dry in the coat and only come out if you go back later with solvent which is a pain to do.

What's next

I had some ugly, oversized bracket slits after doing a poor job of properly locating them. I'll cut nice patches later and apply them with Poly Brush sealant over the first brush-on coat applied above