Wings be bagged!

Ok, first of all, some info about the bagging equipment. This specific equipment came from Aerospace Composites http://www.acp-composites.com/ Though several years old, it is essentially their "EZ Vac Bagging Kit"

For those DIY-ers around, there's really no reason not to make your own system, to be honest. Glad ForceFlex garbage bags will work nicely as bags (It IS best to use polyethylene bags and ForceFlex are 100% polyethylene, and flexible enough to withstand the vacuum without tearing or stretching too much.). Some duct-tape can seal the ends, and a simple pipe - NPT adapter from lowes, a few rubber washers, and some 1/8" ID hose will handle the air exchange. Then all that's required is a pump.

The pump CAN be made from quite a few different "household" or "easily sourced" items. Up to you if you want to go to that hassle, however. What you're after 6-7" inches of pressure...definitely no more than 10"! I've seen some pretty amusing stories about VERY thin wings being produced with 127" or more. You'll also need to have some way to regulate it, as you'll want to leave these wings in the bag AT LEAST over night (more on that in a moment).

So, all things considered, it's PROBABLY easier to go ahead and spend $90 on an EZ-VAC from ACP, and know that you'll get just the right pressure, it'll last, and you don't have to muck about building anything.

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Having said all of that, our bagging saga begins.

First things first...we checked the bench from every angle to ensure that it was flat. This is CRITICAL. It doesn't have to be LEVEL, but it does need to be FLAT! And I mean FLAT! Even a slight bow or warp anywhere is going to result in your wings doing an excellent job of molding themselves into that bow/low spot. While 1/8" bow in the table doesn't look like much, a 1/8" bow in a wing is downright annoying. *heh* Given the pressures involved (I read somewhere that bagging wings like this is essentially equivalent to setting a small car on them), these wings WILL squeeze right along any un-even spot on the table.

Next, we laid down a big sheet of plastic. This too, oddly, is "essential". We're going to be working quickly, and spreading epoxy everywhere, and we don't want epoxy all over the bench. Here's everything laid out ready to go...note the plastic.



A quick note here...you may be able to tell in that picture that that is Great Planes Finishing Resin. That lasted for precisely one wing skin, and we changed over to Z-Poxy Finishing Resin. More on that in a minute. Worth noting here, however, finishing resin IS the best choice here...just not Great Planes.

Next, we arranged everything in the shop so we had a smooth "work flow". With two people, we were able to do both wings AND the stabs in one go...one of us spread epoxy, the other laid skins in shucks and taped things up.

Finally ready to go, epoxy was mixed. YES, as silly as it seems, I STRONGLY encourage that you mix the epoxy ON A SCALE. The BIGGEST benefit to this method is SAVING WEIGHT. How much weight can you save? Well, take a look at this:



That's all that's mixed per wing sheet. Yep. 1 oz. And if you spread well, there should be some waste that spreads off the wing skin, and is left in the roller.

Now, let's stop and think about this for a minute. If you simply used TWO ounces of epoxy here, spreading it out with, say, a bit of wood, and left the skin with a wet shiny look...VERY easy to do...you just added 4 ounces to your wings. Repeat that process on the tail, and guess what...we're talking about half a pound of extra weight...for glue that ISN'T DOING ANYTHING. Once epoxy binds to wood, anything left over does NOT increase the strength of the bond...it's COMPLETELY dead weight.

Next step, then, is to spread this stuff onto the wing:




Spread it out with a small paint roller just as THIN as you can get it. I mean, MASH DOWN, and REALLY spread it out. YES, a lousy OUNCE is MORE than enough. The epoxy will be SO thin, that you'll have to pick the sheet up and tilt it to the light to even see if there's any missed areas. it will NOT be shiny, it will NOT look wet. If you touch it with a finger, it will feel BARELY tacky...less so than masking tape even. Roll, Roll, Roll, and Roll some more, getting every last millimeter of wing skin. Tip it up to the light, as mentioned above, to check for missed/dry spots. just remember, you should HAVE to tip it up to the light to do so.

I know, I know...that simply can NOT be enough epoxy, right?

Yeah...it can. I won't drop names here, but let's just say that the fellow you see spreading epoxy in the background has built some awfully expensive sets of wings for some awfully well known guys. As a more personal example, I used this method for my pride and joy 1/3rd scale pits, and it ain't folded a wing DESPITE my best clumsy efforts. *heh*

Yes. It's enough.

Now...bit of a diversion here...one of the reasons for using finishing resin is it usually spreads a bit easier, and it won't go off for quite a while. Well, take a look at that picture of the epoxy being rolled above...see how thick and goopy it is? That's the Great Planes stuff. It spreads for crap, to be honest...makes it MUCH more difficult to get the whole sheet covered, meaning, of course, you'll use more. We used the full 1 oz on the skin we did with this stuff, no waste at all. Everything else, done with ZPoxy finishing resin, had rather large globs of epoxy left on the table around the edges of the skins...meaning, of course, lighter wings.

Once epoxy is spread, just start putting skins on wings and wings into shucks. It's ESSENTIAL to line the shucks/wings up as precisely as you can. Once everything's put together, tape it up (so you don't jostle things around sliding wings into bags), and move on to the next wing.

Finally, we did the stabs. There was another 1 oz of epoxy used for the ENTIRE tail...both horiz stabs, and vert stab. All in all, not bad...we probably introduced, after waste was scraped away, no more than 4 oz of weight to the airplane.

Having now said all of that...and ranted about how little epoxy is needed...here's WHY:



The wings in their bag, the air starting to come out.

As said above, it really does kind of "escape the mind" just how much pressure is on these things. Figure, though, that even just a couple more inches of pressure would, quite literally, COLLAPSE these wings into wafers, and it gives you an idea...you are, effectively, putting just about as MUCH weight on these things as they can take without being destroyed...certainly, a WHOLE BUNCH more weight than you'll manage with a few concrete blocks.

The other advantage is that the weight is evenly distributed....it HAS to be, by definition. Once the pump has pulled the bag down tight (this'll take as much as a half hour or so), the pressure throughout is a constant.

This means that there's no chance you'll have an edge that doesn't quite bond, or whatever. And you don't have to move weights around trying to get this edge to lay flat, that section to stick, etc etc.

Finally, since you'll be leaving the wings bagged at LEAST over night (an extra second night won't hurt), the epoxy will be well and truly cured.

When these guys come out of the bags, I'll show some more pictures, and weigh things so we can see how we came out.

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Last but not least, a couple of tricks that we DIDN'T use last night for one reason or another.

1) You can be even MORE obsessive, and save even MORE weight by spraying the wingskins with a light coat of hairspray before rolling the epoxy out. You'll find the epoxy a bit easier to spread, and thus use slightly less. We tried, but of all the luck, the hairspray can gave up the ghost. *sigh*

2) If you'll REALLY intense, there's another "layer". On larger wings, or if you're just completely anal about weight, you can put a layer of carbon fiber mat from ACP in between skin and foam. The "trick" here is that the SKIN doesn't get the epoxy, the CF MAT does. You lay IT out on the table, roll epoxy through it, and it, essentially, carries enough epoxy in the fibers to do the job. Had I wanted to spend the extra time, effort, and $ to go this route, we quite possibly could have done all the wings AND tail group in 2 oz or so. Probably not worth it for this "weekend flyer" Cap, but ABSOLUTELY worth it for high end FAI stuff, or what have you.

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There you have it. Wing Bagging 101

This obviously isn't the only way to do things, but it's absolutely a known, proven, reliable way to produce strong, light wings. And that can't be a bad thing.