Tips and Tricks

Builders Log

Download a copy of the nice little freeware builder's log program - 4MB written by Todd Romain. Highly recommended. Wish I had this when I started.

How to do a lay-up

the first ply wetted out I was about to do a medium sized lay-up for the cowling lips one hot day in August '01, when I remembered how strange and foreign this all seemed when I started. Quite a few new builders have asked if they can come and see me do a lay-up. Right, I thought. This is an opportunity to help people who are wondering if they can do this, or are as new to all this as I was. So, if you know how to do a lay-up, skip forward. If you don't, or you'd just like to have a laugh at my expense, read on.

This layup consisted of 10 feet of 2.5 inch strips of 6 ply BID. I cut 6 sheets of BID 30 inches * 10 inches at 45 degrees to the run of the cloth. That is, 3 strips with the main lines running up and down, and 3 strips with the main lines running across. The plan was to lay the cloth up on plastic, cut it into four 30 * 2.5 inch strips and carry it to the cowling. I got my squeege ready, put on latex gloves and stuck a nail up the hardener outlet of my epoxy pump to clear out any gunge. The epoxy pump is in a hot box at 115 degrees F. I layed one strip of the BID on 4 ml plastic, checked it was a little more than the right dimensions then pumped 4 squirts of epoxy. Painting the epoxy on quickly with a 1 inch brush I got most of the first ply to wet up in a couple of minutes. Stippling with an epoxy wet brush made the rest of the glass look completely wet. All the white had gone. I had about 1/2 squirt left. epoxy from the first ply wets out most of the second I layed the second strip of BID (lines going the other way this time) and stippled with a dry brush to help the epoxy seep up from the earlier layer. Most of the second ply became wet. I pumped one squirt and used this to stipple any dry spots. The other 4 ply went on in a similar manner, except that I used a heat gun to help the epoxy wet out during the last 2 plys. The remaining plys took 1 1/2, 2, 1, and 1 squirt of epoxy each. A total of 10 squirts of epoxy in all. I think 1 squirt is about 1 oz, but you could check with the 'sticky stuff' pump manufacturers. just a bit of the third ply to do

Once the last ply was wet I folded the 4ml plastic over to cover and enclose the layup, and squeeged through the plastic using the heat gun. Yes, I almost always use a heat gun, even on a hot day. I cut the edges off the layup using a straight edge and a stanley knife, then I cut it into four 2.5 inch strips. Total time to get the layup ready to go on the plane was 40 minutes. Cut into strips

I carried the four strips, now covered with plastic on both sides, to the plane. The cowling and fuselage had already been prepared with masking tape where I need release and roughed up where I need a bond. Using an additional 2 squirts of epoxy I wet the edges where the layup would go. I stripped the plastic off one side, applied the lay-up, then removed the plastic from the other side. I used wet epoxy brush to stipple the strips of epoxy into place, then added new plastic around the edges and squeeged to get a good transition.

And now..... the rest of the story

Holding the strips in place You've read how the layup was done. Now, with apologies to Paul Harvey, let me tell you the rest of the story. The layup didn't quite go as planned. It was 94 degrees F on my patio and that day. I had a 50 / 50 mix of fast and slow hardener in the pump and the hot box was 115 degrees F. You experienced epoxy people already know whats coming, don't you? I'm right in the middle of exotherm country. No big deal. I'm getting really good at this now. I can handle that. I stopped a few times to take pictures. Once I stopped for a five minute break to cool off inside. Plenty of time. No rush. I'm an expert. Right?

When I folded the plastic over the finished lay-up and put my hand on the plastic to help it lay down, the lay-up was HOT to the touch. uh oh. Getting a bit of an exotherm here, I thought. Six ply isnt that thick. I got my squeege and realized that it wasn't clean. More delay. I ran it against the belt sander for a few seconds to clean it up. Next I squeeged the layup through the plastic using the heat gun to get the air and excess epoxy out. More heat. I cut the layup into strips, quickly pumped and mixed 2 squirts of epoxy, and took the strips over to the plane. They seems a bit stiff as I carried them. The first strip went on without too much trouble, but it was quite stiff. The strips were for the upper cowl lips, so I was working upside down. The first strip didnt want to stay up, so it took a while for me to get around to the second strip. I had to warm it with a heat gun to unstiffen it, but it went on ok. Now I was juggling two upside down strips that both wanted to come off because they were beginning to cure. The third strip was a real bear to get in place. I went to paint the edges with epoxy, but the cup came too when I picked up the brush. Exotherm in the cup. I squirted more epoxy in a new cup, and got a new brush. I painted the strip itself both sides, and warmed it with the heat gun. Eventually, in between heating the strip and brushing the other strips back into place just before they fell off, I got the third strip on. Now came the last strip. It didnt want to soften, even with the heat gun. I got it on, but it wouldnt stay in place. I decided I needed something to hold it in place, so I went for some plastic strips to use. As I was getting the plastic strips from under the bench I heard a clunk. My forth epoxy strip had fallen off. I stuck it back and pressed it in place with two flexible plastic rods. It took 40 minutes to prepare the layup. It should have taken another 20 or 30 minutes to apply the strips, even upside down. It actually took just over an hour.

What can you learn from this story? I should have used slow hardener on such a hot day. Failing that, I should have opened the hot box and let the epoxy cool down a bit. Failing THAT, I shouldn't have wasted time taking pictures and standing there taking a break, proudly thinking I was getting SO good at this that I really ought to share my expertise with the new guys. Think it through, make sure you're well prepared, and don't get cocky!

Did I recover from my mistakes? Yes. With composite construction it's usually fairly EZ to fix what you do wrong. Was the lay-up a success. Partly. The wing root lips were fine. I had to dremel the entire lip off the cowling and replace it. Was I embarrased about how I screwed it up. Definately. Would I have wanted an audience of novices watching (and probably cheering) as I ran around trying to recover the situation? No.

SO? Why am I telling you all this? Why not just keep the mistake to myself and move on? Because one of the most dangerous traits in both flying and building is the almighty EGO. It's good to be proud of what you've built, but not to the extend that you hide mistakes. It's much easier to ignore a problem than admit you made an error. When someone visits you're project, do you want them to say how good you're workmanship is, or do you want detailed constructive critisism? The latter is hard to get, because people don't want to offend you. Push them to be honest. Even then, you're best critic is yourself. There are too many details for a visitor to examine everything you do. You may notice that much of this web site is about mistakes. I make a point of concentrating on the mistakes I make. Perhaps this will help others avoid the same errors, but every person who ever builds a plane will have their own, personal, original screw-ups. The important thing is to recognise them (reread Chapter 3 and know what's acceptible), admit them (not necessarilly to the world, but at least to yourself), and fix them. Admitting the mistakes in public is just my way of keeping my own ego at bay and maintaining an honest balance in my quality control.

So! Now you've heard the REST of the story.

How to remove bondo

Plans frequently call for bondo to be used to stick something to something else. A heat gun can be used to get the bondo off later. Once the temp is high enough, the stuff scrapes off easily. Just be sure you dont heat the part to melting point in the process.

General thoughts on hot wire cutting

Hot Wire cutting is fun, but learn all you can about it before doing the real thing with your only piece of foam. I would highly recommend cutting the elevator cores first. They are small and fairly complex. If you get them wrong you can throw them out and do it again without too much trouble. I made the mistake of doing my initial screw up on the canard cores - much more expensive, and I had to wait for replacement foam.

Follow the procedures exactly. Like starting on the nail. Experiment with the voltage to get it as high as you can without spoiling the cut. As soon as the wire appears out of the foam, stop moving and wait for the rest of the wire to cut its way out by itself. Always have your straight edge extending past the end of the foam so you have somewhere to pause after exiting the foam. If you can see big curved lines on the surface after your straight cut your voltage (or tension) is too low. Mistakes cost me $34 in scrapped foam - but - I think we have it down now, and will be able to do the other foam cuts ok. My backup plan was precut canard cores from Aerocad costing $195, so I figure I'm still in profit provided I dont screw up 3 more times! Besides. Hot wire cutting is really fun to do. The plans say this will take about 2 hours. So far I have about 15 hours and 5 days elapsed time into it... but I learned something.

Wayne Hicks sent the following tip:

 try leaning the hotwire saw about 45 degrees into the direction of
 travel and drag it along the templates.  This allows you to keep firm grip
 on the saw without exerting overbearing pressure on the templates.  What's
 best about leaning is when the hotwire hangs on an imperfection on the
 template, the wire doesn't stop dead in its tracks, rather it glides right
 over the imperfection.
That reminds me. Be REAL picky about getting totally smooth templates. Even a small nick will hang up on the wire and give you a bump in the core.

Plastic Peel Ply

You can see that all the air is removed from the side which has been squeeged through plastic. Also notice that the right side is wider. This is becasue the excess epoxy has spread into the glass at the edges You've just gotta try this technique. See Chapter 20. One thought I would add for a new builder... I think it would be good for you to experience doing at least one layup without the plastic. The seatback, for example, does not need to be smooth. It will be covered with the seats anyway. I'd recommend that you do the forward side of the seatback without plastic, then do the aft side (more visible) with plastic to get a feel for the difference. Be careful not to overdo it and squeege too hard when using the plastic. It is possible to remove too much epoxy by pressing very hard. If you do air will start to be sucked through the foam. If you do this you will see a slight whiteness which returns after each pass of the squeege. If this happens you must add more epoxy. Also, be careful at the edges where air can be sucked back in from the side. Make sure you have no white areas at the edges and where the plastic has to cover a complex curve. By the way, I still use peel ply where called out in the plans, then plastic over the whole thing. Don't take the plastic and regular peel ply off until the part is fully cured, otherwise the part can "sweat" and you'll need to sand before any bonding.

Laying up a spar cap

Here is my list of steps for laying up a spar cap:
  • lay the tape
  • cut the red thread in the middle
  • pull it out from each end
  • pull the cross threads keeping it in line with the layup as it moved around the bend
  • finish pulling the cross threads from the other end to avoid lifting the tape bundles
  • pull the bundles straight starting in the middle and pushing with both [latex gloved] hands in opposite directions
  • move to each side and push with one hand towards the bend and towards the end with the other
  • work from the middle pushing the thread bundles straight with one hand and flattening at the same time while holding threads still with other hand in opposite direction
  • Now hair dryer and stipple. This brings epoxy up from the previous ply while helping break the bundles into individual strands.
  • Mix about 3oz epoxy (4 or 5 squirts on my pump) and brush this on lightly.
  • allow a few seconds to soak (MGS epoxy out of hot box 90 degrees air temp)
  • Use brush and firm pressure plus hair dryer to push strands from center towards ends
  • Use Squeege and firm pressure to flatten.
  • Look for white areas - usually bundles which have not come undone.
  • Rub these areas side to side with latex covered fingers to break up bundles
  • Add a little more epoxy in these areas.
  • Repeat brushing, rubbing and squeegeing plus hair dryer as needed until all white areas are gone.
  • Finish off the ply with a final squeege spilling excess epoxy off the the ends.
  • lay the next tape.

    Finishing with Dry Micro

    I'd read about Superfill so I bought some and used it. Later I bought some more, compared to the results I'd been getting with dry micro, the Superfil wasnt worth the expense. I saved a lump of Superfil and a similar sized lump of dry micro. The superfill is much heavier. I began wondering why my micro was lighter than everyone elses apparantly was. I think the answer is the MGS epoxy I use. It's much less viscous and so it can hold more micro before becoming impossible to work. I use it at about 115 degrees out of a hot box which makes it wetter still. To mix dry micro, I start with an 8 oz mixing cup (per plans :) I squirt two squirts of epoxy. Now I fill the cup to about 1/2 inch from the top with micro ballons. I mix gently until I end up with a flakey paste. For relatively flat surfaces I apply this paste in a skim about 1/16 thick using a putty knife and a lot of pressure. Sometimes I need to heat the micro and the part with a heat gun to help the micro stick. The best way I've found for curved surfaces is to slap the stuff on using my [gloved] hands. The paste is sufficiently dry that I use a heat gun to help me spread it. (be careful not to burn your hands or melt the gloves]. The result isnt pretty initially. It looks a bit like British christmas cake icing - there are some example pictures at the end of chaper 11. I'll add some here when I get around to it. The objective is to get some micro on the entire surface. It doesnt matter that the surface is not at all smooth, so long as there arent any areas that didnt get ANY micro at all. When this stuff cures I use a 36 grit belt sander belt over a plywood board, or on a cheap jitter sander, to get rid of the rough bits. Then work through the grits to about 180 and before you know it you'll have a baby bottom finish. When made VERY dry the micro is very easy to sand. Make it just a little less dry and it becomes quite hard to sand. As an example, I microed the upper cowl in an hour or so one evening. Next morning I had it almost ready for primer in about 2 hours.

    I once needed to do some micro work on a hot day when I only had fast hardener. I turned off the light in the hot box and mixed the micro at about 80 degrees F. What a difference! Now I had all the problems Jeff Russell and others talk about regarding micro. There was a hard shell of epoxy on top of the cured micro which made nasty edges, and the stuff was a bit.. to sand. Mix you're micro hot using MGS and you'll love the stuff.