CF Rigger Build

[Simon.O.]

This may help.
When I built my rigger I built the tub and sponsons to the approximate dimensions of the predator.
Then I took the tub and loaded it with the running gear, I then set the sponsons on the build table in a position that gave me the desired CoB or CoG and AoA.
From here I then plotted onto the hull and sponsons the mount points or more accurately the drill holes for the cf booms.

I can appreciate that you are keen make it adjustable as much as I am with my next design / build. my feeling is towards the fixed rear boom and adjustable front boom that is through 2 small booms / supports off the front of the main tub.

It is my belief that the design should be very close to optimum and the limit of adjustment then only needs to be minimal.

For me, I would go with an external front boom adjustment as it is easier.

Always keen to help.

[Meniscus]

Yes, I did see how you set yours up in the build thread. And I have been leaning towards the front boom adjustment, mainly for ease of execution and less risk of shifting (stronger platform).

So far as the build, I have to get the foam to start shaping the tub. Then I'll lay my ballistic material over, layer by layer with 24 hour set times in between.

[j.m.]

Do NOT use ballistic fabric for composite construction. It is not the same.

Make a set of templates to cut the foam with. They will help you make sure it is symetrical.

[Meniscus]

???

OK, I'm confused. I have some aramid-type fabric (special composite material) that I'm planning on skinning over foam for the main tub in layers.

j.m., I guess I'm not understanding your comment. Please provide clarification. Thanks!

[j.m.]

There are two basic "types" of kevlar fabric.

That which can be used for, and is certified for, making ballistic vests.

There is another which is primarily used for composites construction.

If what you have was not specifically marketed as ballistic grade fabric, don't worry.

As for making the tub, what you are proposing to do will work, but you will NOT get the results you are looking for.

If you are going to shape a plug out of foam, go ahead and do so. Once it is PERFECTLY symetrical, slice it in half. It doesn't really matter how, but cut it along it's length. Most people prefer for the seam to be on the side of a hull. It helps to already have lines on the foam to help shape it and cut it. If you use templates to cut the foam, you can cut the template in half to make it easier.

Now. Once you have your two pieces, you will want to seal them. Begin by giving them a coat of thin epoxy. Once it has dried, sand it smooth. Feel free to add some light glass in there if you want.

Now do it again.

Make sure that all the edges are sharp, not rounded. Any imperfections should be filled in.

Then you can begin to coat the two plugs with thick auto primer. Sand and coat many times until the surface is free of blemishes. It may not look pretty, but it should be smooth. Use progressively higher grit sand paper with each coat.

Once the two plugs look nice, you need to get some MDF. HDF would be great. Thickness doesn't matter that much, but you want at least 1/2". Cut two pieces, one for each plug, so that there is quite a bit of room around the plugs. Say, oh, around 4"-5" extra.

These two pieces need to be flat. I mean FLAT. it is easier to make sure they are flat than to try to match a curve between the two.

You can auto prime and sand the MDF if you wish, but be sure to use a sanding bar bigger than the board.

Measure and align the two plugs on their respective boards so that they are as perfectly aligned as possible.

Check the corners between the plugs and the board. You want a nice sharp edge here.

After they are both muonted and pretty, you need some mold release wax. Coat the board and the plugs in a few thin coats.

Then you can begin to make a mold! Yay!

Start with a thin coat of epoxy, microballoons, and gellcoat (black works best) over both boards. Once that has tacked up a bit, (NOT dried) lay down some layers of light glass. (.75oz/sq yd.) 5 or 6 should do. Use a lot of epoxy here, and don't bother removing the excess too much. Once you have some light glass on there, another liberal oat of epoxy and more layers of heavier glass (~2.5 oz/sq yd.) should follow. By now, the corners (on top) should be nice and round. Laying the glass down and getting it to stick in the corners should not be a problem.

Once these layers are on, you can begin to use even heavier glass to build up the thickness of the mold. 5 oz should do, and after that you an use 10 if you wish. Get the mold at least 1/4" thick.

Once all of your glass is layed up, let everything set for 36-48 hours. You want to be SURE that it is fully cured.

Oncec it's all dry, you can try to slide a flat blad screwdriver between the mold and the MDF (trimming should be required to cut the mold to the edge of the board) and with a little bit of work, the mold should pop right off. You will now see how all your work payed off. The inside surface of the black gelcoat should be glass smooth. Don't worry if it isn't, you can clean it up.

Now you need to take your plugs off the boards and glue them together so they are aligned.

Plan on using a glue that isn't too strong to hold the plugs to the boards, because they don't take a lot of stress.

One your plugs are glued together and the edge is cleaned up you need to insert them into the mold halves, and put the two molds together. They should not wobble or move at all.

Trim the edges of the molds so they match and drill holes through both sides so that the molds will line up.

You should drill the holes to fit some bolts that you wish to use to hold the two halves together pretty tightly. There should be no play present.

Once you are happy with the fit of the mold halves to each other, go ahead and separate them and pull out the plug.

Once you add release wax to the mold, it's surface should be nice and shiny.

NOW you re ready to make some parts.

The mold should last years, and if you take care of it, last through many hundreds of parts.

The outside surface of the finished hulls will be perfectly smooth and shiny. This would NOT happen if you just put kevlar over a plug and then ate away the foam. The surface would look terrible, and if you sanded it smooth (which is VERY hard with kevlar) the strength of the part would be greatly compromised.

Read through this, it should answer all of your questions, as well as cover other mold making techniques:

http://www.rcgroups.com/forums/showthread.php?t=864161

[j.m.]

Sorry. Double post.

It's my school's internet...

[Meniscus]

OK, just reading the info j.m...

1st, the material I'm using is ballistic type, but a blend that isn't on the market except for military sales. PM me and I can provide the specs.

2nd, my mention of molds was related to the sponsons and was mainly a joke. I don't have the workspace or experience yet to give the proper attention to making a mold at the present time. I'd love to, but I think I would be better served to assist someone else as an apprentice prior to attempting myself.

3rd, so my thought is, shape the foam the way I want, paying very close attention to the angles to ensure symmetry. Then, slowly wrap the foam with my material (thin fabric layers) one layer at a time, allowing for 24 hr cure between applications. Then address any anomalies when applying final coat. I'm guessing that there will have to be 7 or so layers before I have the rigidity that I desire. However, I'm going to have to see how it goes and play it by ear. I will not be dissolving the foam, merely cutting out for electronics. I probably will coat the inside so that I can prevent some water from contaminating the foam. I'm sure that some would say that it can't dry out, but at that point, I'm sure I'll be putting the sponsons on the next version, LOL!


Thanks for all the great info! I didn't see your post until just a minute ago, so I was unaware of the information provided. :)

[j.m.]

You wont get good results with the ballistic fabric like I already said twice. It is not good for composite construction. That fabric is not meant to be bonded together using epoxy. It is not meant to be bonded to form a fiber/epoxy matrix at all.

And I understand completely what you were talking about.

And I am saying that wraping fabric over foam will not give you a good part.

Try it with some glass so you can see what I am talking about.

[Stealth Assassin]

Ah, the lost mold technique...

P.S. The resin needs to breathe to cure.

[QUOTE=Meniscus;66065] I will not be dissolving the foam, merely cutting out for electronics. I probably will coat the inside so that I can prevent some water from contaminating the foam. I'm sure that some would say that it can't dry out, QUOTE]



JM,
Very good explanation! What state are you In? Do you have any pics of some of your work? I am Interested...

[j.m.]

Dang!

I can't believe I haven't updated my profile...

I'm in Raleigh, NC.

I'm 16, and a junior in HS.

This summer, I will be making (and selling) composite sailplanes. I'll try to remember to give you some pics then.

Meniscus, how much did those sponsons cost?

They really are pretty...

[Meniscus]

What I paid for them isn't nearly what they sell for now. Wow! Etti is selling them on Ebay now for $95 shipped! Ouch!


They are nice. I like them a lot. I'm glad I got them when I did. Maybe Steven can work you a deal.

[Meniscus]

Meanwhile, I'm thinking of trying the fabric over foam technique. I appreciate you're input j.m., but I guess I'm stubborn.

If that doesn't work, I may have to talk someone else out of a tub they aren't using. But we'll see.

As a final thought, I am seriously considering taking Fluid's advise and making this rigger a little longer. Thanks again to everyone for sharing your personal experiences.

I'm looking forward to additional input! Keep it coming, good or bad, LOL.

[ghostofpf1]

The problem with the ballistic kevlar is that it may not be "Sized" or treated to bond with ester resins.
I'd make a small test piece first to check bond...where did you buy "ballistic" grade kevlar and is it marked as such ??
Forgive me J.M. if you said any of this in your post:
wrapping the fabric over a foam plug is called the "lost Foam " technique and usually gasoline is used to dissolve the foam out of the center after the lay up is done for doublers,equipment placement and lightness .This usually requires post finishing of the piece by filling any open weave with appropriate fillers and sanding much like prepping to paint a car.

If you use kevlar you'll have to add layers of glass (e-glass or s-glass doesn't matter) on the outside as there's no way to sand kevlar without creating a fuzzy mess that can't be painted but you can sand and fill glass layers very easily.Use low viscosity laminating/finishing resins ONLY as 5 or even 30 minute epoxies don't leave enough working time and are not sanded as easily as a good laminating resin.You can use Saran wrap to wrap the whole thing while wet if you're careful to avoid folds and creases and you'll get away with less filling/sanding

Epoxy Resin DOES NOT need air to cure...I've vacuum bagged many wings for my aiplanes and there's very little air in a vacuum
I'd suggest making a heated curing box if it's cold when you do this...a couple 60 watt bulbs in a box insulated with foam (don't set things on fire) works well as Epoxy resin does need heat to cure properly.

Hope this helps..
Ghost

[Meniscus]

This material came from my work (extra). It is a blend of many different things and isn't available for purchase for general public. Let's put it this way, I seriously doubt that anyone has any experience with this particular material for various reasons. I have tested sections of it and it lays up well, without any issues.

I am familiar with the lost foam technique and this is where my thought process comes from.

Now we're saying that epoxy resin doesn't need air? I didn't think it needed air for the chemical reaction to take place, but I don't have the experience to make that determination myself.

I'd love to get into vacuum bagging, but as I said previously, I don't have the workspace or experience.

Thanks for the input Ghost.

[j.m.]

I use a food saver for my wings. The bag has a built in breather cloth, and works great.