Fiberglass layup

[jaustinmd]

Although I'm quite experienced in aluminum construction (Zenith STOL 750), I have absolutely zero experience with fiberglass! I've decided to do a (hopefully) simple fiberglass project by making some elevator wingtips. I've already made a hot wire cutter and cut foam cores from EPS:



I plan to sand and radius the outboard end (obviously not done yet in the picture!), but still leave the vertical surface mostly flat. I have on-hand a West System kit and some bidirectional cloth.

My question is this:

My assumption is that I'll need to cut a piece of cloth to cover the flat, vertical outboard end of the tip, and cut a separate piece of cloth to wrap around the top and bottom of the tip. IF this is correct, do I trim the vertical piece with a slight overhang and then smooth it down around the radiused edge, and then do the same for the piece wrapping around the top and bottom (in other words, let the overlap occur over the radiused edge)? OR, should the overlap be placed on the vertical or horizontal surface (rather than the radiused edge), OR, should I do something completely different?

John
N750A

[Todd copeland]

John,
Its difficult to fully understand what you are outlining but I have a couple of questions that might help define the way to go. First, how fast is the airplane this is going on? How much strength you need determines the layups of glass, and the type and grade of glass to use. Let's assume you have this part of it determined for safety. I would radius the edges to the tip. This will help get a smooth transition "over the edge". If you have essentially 90* edges it is very difficult to wrap the glass over it. Second, I would cut the glass on a 45* to the weave so that it will wrap over the radius end edge smoothly and I would try to do one piece layups over the whole tip that way. If you need to break up the layers incomplete, I would stagger where the layers end in different locations so as to keep the "level" even and have less bodywork to do.
Are you going to remove the foam after it cures? How will you mount it on the elevator? If the foam will be remaining you also should use micro balloons on the foam to prepare the surface for the glass to bond to the foam. There are probably plenty of you tube videos on the subject and as they say, a picture is worth a thousand words.

[jaustinmd]

John,
Its difficult to fully understand what you are outlining but I have a couple of questions that might help define the way to go. First, how fast is the airplane this is going on? How much strength you need determines the layups of glass, and the type and grade of glass to use. Let's assume you have this part of it determined for safety. I would radius the edges to the tip. This will help get a smooth transition "over the edge". If you have essentially 90* edges it is very difficult to wrap the glass over it. Second, I would cut the glass on a 45* to the weave so that it will wrap over the radius end edge smoothly and I would try to do one piece layups over the whole tip that way. If you need to break up the layers incomplete, I would stagger where the layers end in different locations so as to keep the "level" even and have less bodywork to do.
Are you going to remove the foam after it cures? How will you mount it on the elevator? If the foam will be remaining you also should use micro balloons on the foam to prepare the surface for the glass to bond to the foam. There are probably plenty of you tube videos on the subject and as they say, a picture is worth a thousand words.

This will be an elevator tip on the STOL 750 - designed for "low and slow," it cruises about 75 kts. I intend to leave the foam except I will rout out about 1/2" of it on the inboard end from the hinge point to the aft end to allow it to slip over an elevator end rib with flanges facing outboard. The flanges will have nutplates on top and bottom for securing the tip. I do plan, after radiusing and sanding, to put down a micro slurry and then do the layup. Just talked to a friend with a Vari-eze and he essentially advised the same as you. I may try a one-piece drape-over initially and then if that doesn't work well, try multiple pieces with staggered overlaps as you suggest.
Thanks,
John

[jaustinmd]

I sanded and radiused the edges of the tips. Here's some views:



Given these shapes and edges, would it be reasonable to try to lay the BID down as one piece against the tip end and then smooth out over the top, bottom, and leading edge? I know BID is somewhat conforming, but didn't know if I'll have huge wrinkles that won't smooth out.
Or, perhaps cut an airfoil-shaped piece that overhangs the tip end all the way around, smooth that down, and then lay a strip down that wraps all the way around the top and bottom surfaces of the tip? If I did it this way, guess I could try to stagger the overlaps of the multiple layers to avoid a large "hump."
Finally, I've also been advised the best way to deal with the trailing edge is to just let the top and bottom 'glass trail-off over the trailing edge and then after curing, go back and trim and fill any voids with flox and resin to fuse the top and bottom surfaces and can also be shaped for a nice clean trailing edge.

[Todd copeland]

I would try one piece. Work carefully taking your time. If you end up with a wrinkle, no worries just let it cure and sand out the spot with a wrinkle and on the second layer work to ensure you don't have a wrinkle in the same spot. You will need at least two layers anyway. I think the trailing edge will be your biggest challenge as you have to terminate the cloth and make an edge. I would consider making the edge, a glass to glass. You can do this by not doing your initial layup to the trailing edge top and bottom but rather having it end on one side about one inch short. Then, after curing sand back the foam so that some distance is left with just the cured glass. Then on the next layer take the glass to meet against the glass that you sanded to. This creates a good finished edge. Looks great so far, keep us up to date on the project. I want to see how it comes out.

[jaustinmd]

Thanks for the info, Todd! I think I'll have a go at the one piece, draping it over the tip and working it down. I figure the worst that can happen is that if an unacceptable amount of wrinkles occur, I'll just lift the fabric back up and trim it above the wrinkles and convert it to a two-piece layup! I like your idea that if it there is just a minimal wrinkle, let it cure, sand it smooth and try to relocate the next layer's wrinkle. Although this is just a tip and structural strength is certainly not as critical as a wing, etc., I did plan to do at least a two layer layup, and possibly three.
One thing occurred to me - and I don't know how practical it is to actually do - it seems to me that if there is very much wrinkling, it'll likely be around the leading edge of the tip. If that occurs, would it be possible to simply work the fabric forward (towards the leading edge) to pull the wrinkles out, then trim off the excess (say about 1/2" forward of the apex of the leading edge), then lay one side of the cut down, then overlap the other? In other words, run an overlap along the leading edge? Since I have zero experience, I'm not sure if one can trim fabric that is mostly laid-down or will it fall all apart and make a big mess?
I know the answer to nearly all this is just to try it! I'm often guilty of over-thinking something that sometimes turn out to be not such a big deal! ha!
Thanks,
John

[Todd copeland]

John,
you have the right idea here. Cutting "splits" into the fabric and then overlapping it is a common way to deal with it in these circumstances. It looks to me like you have approached this in a methodical way. I think you will surprise yourself how easy this goes when you do it.

[George Sychrovsky]

Looking at the pictures - You have the best looking composite workshop I have ever seen

[jaustinmd]

Looking at the pictures - You have the best looking composite workshop I have ever seen

Heh, heh, heh! ... believe me, I did NOT sand those forms in the kitchen, but it does have lots of natural light for good photos!

However, since we're well into winter and the garage isn't exactly toasty-warm, I think I'll cover that island with plastic and fiberglass the forms in there so I'll have a controlled temperature environment a little more hospitable to epoxy resin setting-up! My four cats and one dog will definitely be locked in the bedrooms while there is any fiberglassing going-on!
John

[jaustinmd]

John,
you have the right idea here. Cutting "splits" into the fabric and then overlapping it is a common way to deal with it in these circumstances. It looks to me like you have approached this in a methodical way. I think you will surprise yourself how easy this goes when you do it.

Well, Todd, I gave it a shot today and I think it worked out OK!

I coated the foam with micro slurry and then laid down 2 layers of BID. There was indeed a large wrinkle on the leading edge that I just couldn't work down, so I slit it up to the apex of the wrinkle and overlapped the slit edges. On the second layer, I worked the wrinkle into two wrinkles - each lateral to the overlap in the first layer. Then I slit those and overlapped the edges. So, I wound up with 3 overlaps all in different locations, so no big "bump" or build-up. I think a little sanding and filling and it'll be fine!