Chapter 14 - Centersection Spar

Making the Jig

I got my 3/4 particle board from Home Depot and had them cut two 12 1/2 inch strips. I made the jig in one day. All the measurements are in 1/100th of an inch. I doubt that anyone can cut particle board to that level of accuracy, but I got pretty close. The jig looks like something used to play toy cars on. I think I'll mount it on the wall as a shelf when I'm done.

Foam and metal

There are a lot of parts to prepare. This took me a whole day. Wicks were short of the 1/4 foam and only sent me one sheet, but I managed to find enough off cuts to make all the parts. I microed the foam in place. Had trouble with the plans recommended "pine sticks". To get these vertical you need to make the shelf supports flush with the edge. I hadn't so I couldn't. I put the top (forward face) in place and made sure I had flush edges all the way around, then I checked that everything was vertical with T squares. When I was done Char did her customary inspection... "Its a little bit off!", she said. I went into my John Cleese "dead parrot" act. "A little bit off!" "A LITTLE bit off!" After I settled down again, I explained that the curve in the side (bottom) was deliberate to allow the fuel to drain toward the fuselage. Not only was it deliberate, but it was also quite hard to get exactly right. I hope my tirade doesn't stop her doing the quality checks. I really value them.

Next came all the aluminium pieces. I noticed that Wicks had sent me T4 instead of T3. Archive research told me that the difference between the two is slight. I had enough 1/8 T3 to make all the 1/8 parts, but had to use the T4 for the 1/4 inch parts. My (borrowed) band saw gave out towards the end. The blade wasn't really designed for cutting 1/4 aluminum. I made all the parts for the wings while I was at it. Wings are a long way off so I labeled each part with its part number before storing them.

The dreaded inside layup

I'd read that this layup can take 10 hours or more, but I didnt want to do it in 90+ heat so I started at just after 5pm. I also dumped a bunch of slow hardner into my epoxy pump. I had all the glass prepared so I started microing all the foam. I considered doing one half, then the other as one builder had done, but valor got the better of discretion and I microed everything. This took about an hour. Next came the hardest part - the one ply BID overall. I put a bead of micro down all the 90 degree angles and layed the glass down in the center. I stopped the glass falling inwards by placing wood strips on the edge to trap it. At one point it seemed that the layup was goinging all wrong. Sticking the glass to the sides is tricky. I changed gloves, took a break and started in again. It took a lot of manouvering to get the glass in place, reaching all the edges with no bubbles in the corners. Mostly I used an epoxy wet brush to wet the ply as I moved along. I had to cut a few darts in the corners. Once it was all done I put wax paper over it and squeeged excess epoxy up to the edges.

I microed all three bulkheads and carefully put them in place. Plans say to glass one side, over the top and down the other. There was no way this was going to work. I tried radiusing the top of one and still got air. I settled for cutting the glass at the top, which is probably what I was meant to do. I'll flox the edges when I put the top (forward face) on. Notice the orientation issue. Top = forward, bottom = aft etc. You really need to get orientated with how the spar is going to go in the plane to follow what the plans say, otherwise it would be real easy put a hardpoint or a reinforcement ply in the wrong place.

Finally the six hardpoints. I had drawn lines at the correct BL points before glassing. I did this by working on the lower edge of the back of the shelf and transferring the lines with a T square. I had spent quite a long time checking and double checking, then drawn the outlines of the hardpoints where they should be. Installing the hardpoints was a snip. I didnt cut the UNI in advance. I just wet out a large 30 inch square, covered it with wax paper and squeeged. As I needed strips I cut them from the square and transferred them to the layup. Finally the metal, flox and the last ply of BID went on. I peel plyed (helps me get rid of air bubbles), added saran wrap and weighted with 5lb diving weights. Haven't done much diving lately, but those dive weights sure come in handy for airplane building. I think they'll become my ballast too.

Next morning I removed the peel ply and did my inspection. Char's son, Jason is an A&P experienced in composite work. He also gave the work a close look and pronounced it good. There are a few very small (less then 1/8 inch) bubbles in the micro beads, but otherwise everything looks great. I was extra picky to ensure there were no bubbles or voids near the hardpoints. They'll be holding my wings on!

The layup took me 5 hours start to finish. The first BID ply was fairly tacky, but not unworkable, by the time I was done. I had to wait another three hours to knife trim. I just love this MGS epoxy!

Attaching the top (forward face)

I still had slow hardner in the pump, so after a few hours I knife trimmed a bit early, then microed the forward face in place while it was still tacky. Double checked for flush all around, weighted and left to cure. 2 days later I removed the spar from the jig and set it in place, just to see how it looked. Jason says it would probably fly like that, but only if I use a VERY big engine.

Sheer web & spar cap prep

I've read that the spar surface can be proud of the wing surface when installed leading to lots of filler. I plan to remove about 1/8 from the top and bottom foam surfaces to ensure that this doesn't happen. I removed a little, about 1/16. When I cut the troughs for the spar caps I noticed that the top is tapered before you get to the trough. If you dont do that taper the spar will be too thick. I bet this is where the extra height is coming from for some builders. If I'm 1/16 low when I come to fit the wings, this will prove the point. Cutting the troughs and the 1 * 1/2 angle took me most of a hot Sunday. When done I double checked the dimentions of the trough with the templates and found that it was about 1/4 inch wide in one area. To fix this I cut a 1/4 sliver of foam, 5 minuted in place and sanded to contour. Next came the six alum hard points. Had to work hard removing micro under the foam to get them down flush. The sheer web went on without too much trouble. I started it during a thunderstorm and the low temperature (76f) was enough to make the epoxy loose viscosity quite quickly. A hair dryer solved the problem. This 4 ply layup took me 3 hours. When the layup was ready for knife trim I found that I had a small problem. My masking tape was along the top of the trough, per plans. When I cut along the line at the base of the trough and pulled the trimmed glass about 1/8 of foam came with it. More in places. Now my troughs are too wide and I'll have to put slivers of urathane down before doing the spar cap. "Next time" I'll mask the edge of the troughs too. After removing the peel ply next morning, the layup looks good. Not an air bubble to be seen, and a nice smooth finish - except where the peel ply bubbled a bit during cure. This always happens when I squeege the peel ply, but I get a lot of excess epoxy out that way. I find that squeegeing the layup is difficult sometimes, especially when there are lots of joins. I would have used wax paper instead of the peel ply for this, but the dogs ran off with my wax paper roll. This is one problem with working on the patio - the dogs go past my bench on their way to the yard. They often sniff around and search for a new treasure to steal. Sometimes it's just a scrap of foam, sometimes an alum angle bracket. Once they took my dremel tool! I think they learned something that time.

Top Spar cap

I built the dam with some 1/2 inch ply off cuts. Problem was that they were only 1 inch wide and it was hard to get the dam dam to stay vertical. In the end I achieved it with clamps and lots of bondo. Next time I'll use 3 inch strips or wider. Larry asked me to report on how the 2 1/2 inch tape worked on the spar cap. Answer is that it worked fine. There are plenty of issues regarding laying the spar cap, but width of the tape and getting the fibers straight round the "bend" are not a problem. Think about it. Simple physics says that the fibers will lay down when wet, just like a bunch of greased pipes will fall down parallel when dropped. If the friction between the pipes (or fibers) is reduced by the grease (or epoxy) they will slide into a parallel state when force is applied.

They call the glass tape we use for spar caps Roving fibers for a reason. Just like in the song "I'm a Rover, seldom sober.... They're sometimes a nuisance when they're in a bunch (or a bundle), but once they get a drink [of epoxy] all they want to do is get layed [in epoxy].

The first ply took me a while to get right. After that everything went fairly smoothly for a while. I got into a routine. See Tips and Tricks for my layup routine. Getting the fibers straight and laying them around the bends happens pretty much by itself. It was taking me about 15 minutes per ply. When I got to the 10th ply and was about to lay the last full span tape I noticed that the layup was a bit hot. Not too hot to touch, but close. Air temp was around 90F and I had been using the hair dryer a lot. Also I'm running low on slow hardner, so I had added a bit more fast than I would have preferred. Probably a 75% ratio. I had a minor exotherm going on. No smoke or anything, but the accumulation of curing epoxy was definately heating itself. I held off on the next ply. When laying the sheer web the daily 4pm thunderstorm had given me problems. This time it saved me. Rain came, temperature dropped into the seventies and things settled down again - but I mixed my epoxy in smaller amounts now to make sure it was water-like when applied. You guys with temperature controlled shops don't know what fun you're missing!

I added ply after ply for hours. A total of 6 hours to be exact. I put the cross threads for each ply in a row so I could see how many plys I'd done. The trough filled up nicely and I had to skip a couple of plys to avoid overfilling. After the 21st ply I added peel ply and collapsed.

After cure the layup looks good, but I have a problem...
I thought my roll of tape was going down fast. Later I calculated the tape I had used and measured the tape I had left. Total comes to 94 yards. I ordered (and paid for) 125 yards from Wicks per plans. I have been shorted over 20% and I don't have enough left to do the bottom cap. I suspect I've been short a couple of times on my BID, but I didn't take measurements. This time I have the figures. So far I've had nothing but good things to say about Wicks. Lets see what reaction I get to tomorrow's phone call.

I called Wicks, spoke to Sharon and explained the problem. She spoke to her manager and, in less than a minute, agreed to ship another 30 yrds of tape by the fastest possible method. Not bad. People sometimes make mistakes. The trick of good corporate relations is to FIX the mistakes quickly. I wouldn't suggest that we unroll and measure every shipment of glass we get, but I'm going to do some spot checks in future. The missing spar tape came in UPS blue two days later. Great! Now I can go through that 6 hour ordeal again.

Bottom Spar cap

This went much the same as the first with a few minor exceptions. I tried an experiment. I remembered that the plans say to wet out the tape some before removing the cross threads. I tried this once. I found that the bundles were much harder to put back in place if they were lifted by the cross threads. I had to be much more careful removing the threads. On about the 6th ply the cross threads got tangled with the glass as I removed them and pulled the glass into a tight bunch. That took a few minutes of careful unravelling. When I got to the last ply the center span area was already full, but there were slight dips in the sides at about BL 40. One one side the dip was about 3/32 and on the other about a 1/16 inch. I measured the cross sections compared with the plans and found that my spar cap was as close as I can measure to correct thickness all the way along. I must have had a bit of a dip in the foam trough. Hmmm. The next sheer web needs to be level, so I need to fill the dips. The spar cap has all the strength it needs, but I need a glass bond between the cap and the sheer web that comes next, so micro is out. I decided to fill the dips with a bit more S glass spar cap material. I used three pieces on one side reducing from 2 feet to one foot in length. On the other side I used two pieces in the same manner. Tomorrow I'll sand the bumps level and move on. The bottom spar cap took me just over 5 hours.

The layup from hell!

They say a pilot crashes by making three mistakes together. I think its true of building airplanes too. The best thing I can say about the last 4 ply UNI layup on the center spar is that I saved the job and didn't have to scrap the part. The first problem was that I was out of slow hardner and had to use all fast. Should have stopped right there, but "get_there_itus" stepped in and I decided to go anyway. Next problem was the weather. A hot day in South Florida and I decided to start the work at midday. Temperature was to 97 degrees F. Mistake #2. Hey, I'm an experienced builder now. I can cope with any conditions - right? Wrong. I'd underestimated the size of this layup. Laying the plys was like painting the golden gate bridge. By the time you're done at one end, you have to start again at the other. The first ply was tacky by the time I came to lay the second play and so on. I had to use a lot of epoxy and a hair dryer to wet out enough to make the ply mobile. Once I'd got the ply wet I squeeged most of the epoxy back out onto the table. I tried to collect the excess in a cup, but it exothermed on the spot. After a long, hard battle I got to the fourth ply. Rather than start back at the other end I reversed direction and layed up immediately on top of the ply I'd just layed. This helped a bit because the ply underneath was still fully wet.

By now the sun was low on the horizon, shining straight onto my job. Now we come to the third mistake. My wet and squeege technique had used up a lot of epoxy. Much more than I'd expected to use - and now I was VERY low on hardner. I got about halfway down the span before the hardner side of the epoxy pump started sucking air. By holding the pump at an angle I managed to get three good squirts out. I wetted out the next section on wax paper and squeeged until my puny amount of epoxy had wet the whole piece. Got this on the job and squeezed out another squirt of epoxy to lay it down and stipple with. The final small piece went on the same way. Next, I needed some flox for the hard points. One squirt did the job. I reached for my trusty roll of 60 inch peel ply to find that there was about a foot left on the roll. I didn't have enough to peel ply the whole spar. I peel plyed the most important areas and will have to sand the rest dull later. After cure the spar looks ok. Not my best layup by a long way, but it'll do.

I learned a few lessons from this layup...

  • 1. make sure you have ALL the materials needed before starting.
  • 2. Don't try to use all fast hardner on a hot day.
  • 3. Don't get cocky!

    A closer look in daylight revealed that the spar was definately NOT ok. There were dry areas all over much of the last ply and quite a few air bubbles. I attacked it with a wood chisel to test the lamination and, once the edge was lifted, I was able to peel strips of glass off with ease. I delved deeper and decided that most of the last ply and some of the third ply were unacceptible. I called Nat to describe the problem. Basically the rule is "If you can get it off, it needs to come off". Nat advised me to remove the floxed hard points with heat from a blow torch, strip off the last ply and anything else that will come off, then replace the plys I'd removed plus add one extra ply. He added that the minimal additional weight would be over the C of G so it wouldnt have any significant impact. Apparantly the spar is designed to withstand 18g.

    I thought the layup was hard work. Getting it off is even harder. Uggh. Hours of hacking, peeling, sanding and scraping have reinforced the lessons learned. I won't be making those mistakes again anytime soon. I'm grateful I made this mistake on the spar rather than on the wings. My next epoxy order will be for a 75% / 25% ratio of slow and fast hardner.

    Access holes, reinforcements and spar front layup

    I cut the access holes and removed foam to allow a glass to glass bond. The dremel wire brush did a nice job of getting the micro off the glass once the foam was removed.

    [Later note: The access holes look pretty cool when you cut them, don't they? Take it from one who has the teeshirt - later, when fitting and removing the wings, you'll be up to you're armpits in these holes while squeezed in the back of the plane which may or may not be upside down. You will do this lots more times than you think, and it isnt fun. Take the time now to clean up any ragged fiberglass around the edges of the holes and the inside of the spar. Your hands, wrists and forearms will thank you later.]

    Finishing Off

    I replaced the damaged ply and added one extra ply for good measure. Temperature was 78 and my hardner was now back to a 50/50 mix. Everything went smoothly this time. The 2 ply UNI on the forward face was no problem. I drilled the two access holes, made flox corners and added a ply of BID to the end bulkheads. After a long month elapsed time my spar is finally complete and ready for fitting... once I build the wings.

    Installation of the spar

    Three months later, Dec 15, '99, after building the wings and winglets, I finally floxed the spar in place. I jigged the plane level on two saw horses to get the wheels off the ground. I must have checked every measurement 10 times, then took the spar out again to add the flox. Plans say mark the shims. Forget it. I might as well have put the flox in first and then set the measurements once instead of twice. I suppose it helps to know there are no final adjustments needed to the hole. I installed the canard and used a long stick to measure the distance from spar to elevator on both sides. For the airplane front to end of spar measurement I used some piano wire hooked to a tape measure. This way the wire would run along the nose and give me an accurate distance. I rechecked level both ways on the fuselage and vertical on the firewall, then checked for vertical between the outboard hardpoints. For leveling the spar I got some clear 3/4 plastic tube and fittings from home depot and converted my hose pipe into a long water level. Hooked over the spar with piano wire it was easy to see the meniscus was level with the center of the mounting hole on both sides. I tied up the dogs and locked the door (per plans). Plans didn't say anything about General George Patton, the cat. Two days later I checked that everything was still level and centered. Checked again to be sure, then 2 BID taped the spar in place. In another week or so I'll cut George's paws out of the flox. That'll teach him!

    A day out

    About a year later I took a day off to help Bulent install his spar. There were five of us there measuring, jiggling and, eventually, floxing. Gary Hall (of teflon hinge fame) produced a laser level which made the job much easier and more accurate. This device puts out a level beam and makes a red dot on you're winglet. Measure the height of the dot from a known water line like the trailing edge, then swing the beam around to the other winglet. Bingo. You now know if the wingelts are level. The laser level makes the water level I used look like it came from the stone age. Buly was about to mix the flox to put on the opening when Vince suggested that we do a dry run. It was a hot day and the flox would begin to cure in 30 minutes. The dry run took 45 minutes, but we got the procedure down. Finally, after a couple of hours of measuring and double checking, Bulent mixed up some chocolate cake mix (at least that's what it looked like to me) and four people started in floxing the edges of the fuselage sides. The wings and spar were level at zero incidence and strapped tightly to the saw horses. Once we had everything floxed we lifted the fuselage into place on the spar and started measuring again.

    We used the laser level to check that the wings and the canard were level. We measured from the center of the IP to the aileron cutouts and from the canard TE to the wings. While this was going on four people were floxing like crazy. Each time we moved something, they adjusted the flox. Eventually everything was neatly floxed and all the measurements checked out, so we closed the hanger doors and went for food.

    The risk business?

    One interesting observation. Five people were involved in the floxing operation. Not one of them wore gloves, and one even used bare fingers to smooth out the flox edges. I was the only one who asked for gloves. Bulent didnt have any. We chatted about the issue. All agreed that epoxy toxicity eventually gets many builders. A couple of the guys even had examples of people who had to stop building because the of the rash they'd get when using epoxy - yet everyone (except me) cheerfully floxed away unprotected. I wonder if this explains why experimental airplane builders tend to have a lot of kids.......
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