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Old May 04, 2013, 12:23 PM
Jim in the Desert
United States, NM, Las Cruces
Joined Aug 2007
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How are hollow wings made?

I've searched last night and this morning, and haven't really found an explanation. I understand the difference between bagged and molded foam core wings, but I am referring to wings with no foam core. If I am not mistaken, Stobels and Blasters are like this. But all the wing threads I have found use foam even in molded wings. Also I see mention of plugs a lot, but don't understand what they are or how they are used. If anybody has pointers to a web site or thread on this I'd appreciate it....

Thanks.
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Old May 04, 2013, 12:41 PM
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United States, CA, Tehachapi
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Yes, Stobels, Blasters, Concepts, Sirius, Steigeisen, and the original Fr3aK are all hollow molded, just to name a few. The process involves laying the fabric into the open molds, laying a thin (~1mm) layer of foam or balsa down, and then a final layer of light fiberglass. This makes a "sandwich" that forms the skin of the wing. Once it's cured, a foam or balsa strip is glued on the spar caps (to bridge between them and form a complete spar, glue is applied to the LE and TE and the two halves of the mold are closed. Tom Siler recorded a few layups of his Fr3aK HERE. The B2 page has some pics showing the construction, too HERE.
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Old May 04, 2013, 05:14 PM
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plugs are typically whats used to make a mold from a shape you want without having to get something cnc cut or routed. its a more at-home way of making molded planes, and saves quite a bit of money over having large expensive metal molds like some have made.
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Old May 05, 2013, 08:36 AM
Jim in the Desert
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Thanks much guys. Bwill6, when the sandwich is put into the mold, I would guess epoxy is laid down on the mold under it, and then on top of the sandwich, and maybe in between the layers. I watched the video (great! thanks!), but it wasn't clear if he was rolling epoxy onto the mold in the beginning (he was rolling onto white, so maybe I was looking at cloth) or on cloth. He says it's PVA, don't know if the mold is PVA or that is part of the wing he has there. Not clear on how the interface between mold and sandwich is, but it must contain epoxy to get the surface of the skin to be smooth. He must use something on the mold to get the epoxy not to stick.

Then I presume you bag this sandwich into the mold while it cures? Then bag the two mold halves together til the glue around the edges cures?

Thanks, Jim
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Old May 05, 2013, 08:37 AM
Jim in the Desert
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Are the hollow wings lighter than bagged or molded foam core?
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Old May 05, 2013, 11:12 AM
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Generally it's more difficult to make a light hollow wing than a light solid core wing, from what I've seen. The PVA is just a release agent that gets sprayed onto the mold before layup. It's a very very thin film that provides a barrier between the epoxy and the mold surface. In the video he first rolls down epoxy to wet the mold, lays in the outer fabric, then rolls more epoxy in to fully saturate the fabric. Then, IIRC, he puts some paper towels over that and puts it in the vacuum bag for a few minutes in order to pull out any excess epoxy (to save weight). After that, and while the epoxy is still wet, he lays in the thin foam sheet followed by the inner layer of glass fabric. Epoxy is rolled into the inner fabric and the whole thing is again put into the vacuum bag, this time until the epoxy is fully cured. That's all that's covered in the video I linked to. On the side bar you can scroll down and see other videos he's made. One of them (THIS ONE) is where he closes the mold after the skins are cured.
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Old May 05, 2013, 01:03 PM
Jim in the Desert
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Quote:
Originally Posted by bwill6 View Post
Generally it's more difficult to make a light hollow wing than a light solid core wing, from what I've seen. The PVA is just a release agent that gets sprayed onto the mold before layup. It's a very very thin film that provides a barrier between the epoxy and the mold surface. In the video he first rolls down epoxy to wet the mold, lays in the outer fabric, then rolls more epoxy in to fully saturate the fabric. Then, IIRC, he puts some paper towels over that and puts it in the vacuum bag for a few minutes in order to pull out any excess epoxy (to save weight). After that, and while the epoxy is still wet, he lays in the thin foam sheet followed by the inner layer of glass fabric. Epoxy is rolled into the inner fabric and the whole thing is again put into the vacuum bag, this time until the epoxy is fully cured. That's all that's covered in the video I linked to. On the side bar you can scroll down and see other videos he's made. One of them (THIS ONE) is where he closes the mold after the skins are cured.

Thanks very much again. Yes I plan to watch those other vids. I'm fascinated by how these are made, whether or not I ever try myself.

Odd that a hollow is harder to make light, since there is no foam in the middle, but I guess there is more "skin" from the sandwich, and more epoxy in it too.
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Old May 05, 2013, 08:52 PM
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I usually roll epoxy on the molds first, then add skins, then more epoxy, debulk (roll or vacuum bag with paper towels) then add the rohacell and inner skin.

The process can start with dry molds, just rolling epoxy through the cloth but it's easier to get a good finish and keep the cloth from scooting around by rolling down some epoxy first.

Foam cores for a DLG wing are around 24g per side. That weight is normally a bit lighter than the combination of two 1mm thick rohacell sheets plus the shear web plus the inner skins and epoxy... and carbon sock drag spars... You get my point. My best hollow molded wings were around 130g. Most of my solid cored wings are around or under that... with some as low as 115g. I've made a few heavier solid core wings but they were using higher density foams (highly durable) and heavier skins (very stiff) than the hollow stuff.

As a guy who has made both, I'm pretty sure I won't ever mass produce anything hollow again... at least in a DLG size. Solid core can be made in a single step where hollow has to be made in multiple layups. Solid core wings are FAR stiffer as well.
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Old May 05, 2013, 10:39 PM
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Interestingly, the 1.6oz glass over 40psi foam HM51 wing I just bagged weighs 121g and feels about the same stiffness as the hollow Dbox Fr3aK wings I've felt. It's not just the core, though. The extra thickness of that wing makes a significant difference.
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Old May 05, 2013, 11:32 PM
Jim in the Desert
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Thanks Tom for sharing that info, that explained what I wondered about.

Now I wonder why anybody goes to the trouble of making the hollow wing if it's heavier, less stiff, and more fragile...I presume one can get just as accurate an airfoil from molded foam core wings...

Jim
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Old May 05, 2013, 11:56 PM
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Its because the machine time milling foam is terrible. The only people making molded wings who can make them solid to boot are the ones who have their own gantry / router mill to produce the cores. If you had to pay somebody for machine time in a shop to mill cores, you wouldn't be able to sell a glider for any reasonable price.
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Old May 06, 2013, 12:48 AM
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^^^^ What he said. I've got a goal to start building molded wings by the end of the year. I'd absolutely love to go solid core because of the advantages of it. However, in order for that to be even remotely feasible I'd have to have my own CNC mill due to the reasons iwokeupwithgills mentioned. If I went to my local machine shop they'd probably charge me more for each core than I'd pay for a new Helios unless I ordered in bulk - not really an option unless you're doing production (and even then it probably would be too expensive). Since I just want to mold wings for myself and maybe some friends, it's not an option. If I wanted to spend 2 years building a CNC I could, but I'd rather be building and flying airplanes for those 2 years.

Even most serious DLG manufacturers have day jobs and build in their garages/shops. Most are just like me and would rather invest their time and money into a good set of molds than into a CNC. Hollow molded requires less equipment and therefore less capital and less space. I can make hollow molded DLGs in my one bedroom apartment. Can't do that with solid core.

So why do I want to build hollow moldies when I can build stiffer, more durable bagged wings for the same weight? Well, even hollow molded wings offer some pretty distinct advantages. If they didn't then we wouldn't see planes like the Concepts, Steigeisens, Stobels, and Twisters winning contests.

The first and most important is airfoil accuracy. Unless you pour a lot of hours into a bagged wing, it won't turn out nearly as accurate as a wing pulled from a well-made mold. I've checked the accuracy of my bagged wings. Even when I hand shape the cores with LE templates after hotwiring them they still distort some during the bagging process.

The second reason is you can design a better wing for molded. If you value your time (and I do) you don't want to bag a wing with more than 4 panels. It's just a lot of work to hotwire a lot of panels. With molded wings you can have as many panels as you please with no penalty in how long it takes to manufacture a wing. You can get a bit more performance out of such a wing.

The third reason is repeatability. No two bagged wings will be exactly alike, but moldies from the same mold will be almost perfectly identical in shape.

And the fourth reason (for me) is that it's a new skill to learn. The biggest part of what keeps me in a hobby is the ability to learn new things and progress. As long as I'm having fun learning and improving, I'll stick with the hobby. Once I feel I'm becoming stagnant I tend to get bored and move on to something else.

Probably a lot more information than you wanted, but there ya go.

Brandon
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Old May 06, 2013, 12:55 AM
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To add to Brandon's sentiment, I think that the biggest thing about molding is that its EASY. Bagging wings requires artisinal mastery and craftsmanship. Everything is soft, and if you don't work carefully, you can end up with crap; its a very temperamental process.

Molds, on the other hand, require a lot of upfront effort and cost - which is prohibitive for the home builder who doesn't have access to CNC. However, once the mold is created its a relatively straightforward process to make parts - whether they be hollow or solid.

I am also working on a set of molds for myself this summer too - have just put vertical stab #4 in the mold tonight, and hope to be milling my horizontal stab molds by the end of the week, with wings coming in march. For the time being, I have access to an enormous and underutilized machine shop through my university - so I plan to mill solid cores. Once I leave and dont have access any more, the molds are still useful to make hollow molded wings! With people like Tom Siler and Gerald Taylor making so many helpful and informative posts, I feel like anybody can do it!
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Old May 06, 2013, 10:38 AM
Jim in the Desert
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Thanks guys, I feel like I am getting a Master's in DLG making.

So, it's not surprising that bagging will distort the foam, and shaping it can not produce identical wings. I gather you can not put roughly shaped foam inside the molded wings you're making and let the mold take care of the shape of the foam. It must not work that way. And while using CNC'd cores in a bagged wing gets the foam's shape right, the bagging process may then distort it.

Are you guys using Drela designs for your wings, designing your own, or using some other person's? Seems it would take a PhD to design them, but maybe not. Others must be designing new wings but I dont' know who. I think there are the Edge and other post-Drela designs.

Thanks for all your explanations and examples, it is not only interesting but helps me understand the different kinds of DLGs out there.

Jim
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Old May 06, 2013, 10:40 AM
Jim in the Desert
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Quote:
Originally Posted by iwokeupwithgills View Post
Once I leave and dont have access any more, the molds are still useful to make hollow molded wings! With people like Tom Siler and Gerald Taylor making so many helpful and informative posts, I feel like anybody can do it!
If you can use the university's shop to make your molds, you are a lucky guy indeed. That must not be a trivial undertaking though. What wing design are you going to use?

Jim
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