CEV

I'm planning on making a lost mold fuselage. It would be for an aerobatic airplane with a .91 motor and 1.8 m of length.
Before staring I have several doubts:
-What is the best material to do the lost mold of?
-What is the ideal fiberglass density and haw many layers are required for an optimal strength/weight relation?
-How can I obtain the best finishing: sanding, primming it, ....?

As you can see I have many things to get settled down before starting.

Mike James

Probably more aptly called "lost plug" method, but I know what you mean...

The finish is the tricky thing with these parts, since you're glassing the outside of your material. Normally, in a female mold, the mold surface gives you a smooth part, but with the lost plug method, you must be careful to somehow get a smooth finish without using excess resin. (so you don't make your part too heavy)

For ease of removing that material, most of us use foam, which can be either dug out through some opening, or dissolved with a solvent such as gasoline or acetone. (Obviously, be careful of fumes, flames, and the general messy "goo" this creates.)

A few years ago, I did a very simple "fuselage" for a flying wing design, using that method. The article is at http://homepage.mac.com/mikejames/rc...ws/wing01.html

I highly recommend digging the foam out as much as possible before using any solvent to remove the final bits. It creates less of a mess, and uses less solvent. If you haven't done this before, I also recommend dissolving a very small piece of foam first, (say 1" square) so you see in advance what happens.

CEV

Thank you for the "lost plug" correction.
I agree with what you have said, but I still don't know what fiberglass desnity is appropriate and how manys layer are required. I know that the lower the density, the more layers are requiered to obtain the same strength. The fuselage I intend to do isn't very complicated but it isn't either a straight box.

Magne

One thing to consider when using "thin" (i.e. not sandwich construction) fibreglass skins is that flat surfaces are very flexible. It is therefore recommended to round off the fuselage sides as much as possible, as your structure becomes much stiffer. To obtain a good surface finish, you should use one or two layers of low weight cloth (1-2 oz) as your finishing layers. Since nobody else have recommended any material thickness, I would guess that a total cloth weight of 400-500 gram/m2 (12-15 oz/yd2) would be a starting point. (e.g. two layers of 5 or 6 oz, followed by two layers of 1-2 oz cloth) Why don't you make a smaller piece of foam, maybe a foot long, with approximately the same cross section as your fuselage, and try it out. Weight/strength is always a compromise.
Magne

Mike James

Magne is right.

For fuselages of the size you mentioned, I typically shoot for a target weight of "8" to "10". (meaning, as Magne said, a layer of 6 oz. cloth and a layer of 4 oz. cloth, as one example)

The shape affects how you should do the layup, and obviously, the stresses you expect the part to take also affects it. Internal formers can help too, but are somewhat more difficult to place, in a lost plug part. In a typical molded part, the formers could be placed in the mold, but if you insert them in the foam, then you have to be careful not to destroy the bond when removing the foam or other material.

If you have any photos or drawings of your planned fuselage shape, some of us here could be of more assistance.
Regardless, good luck, and let us see your results here.

CEV

Thanks for your suggestions. I think I will try to make my mold as round as possible and start with two layers of 3-4 oz and then finish off with a couple of layers of 1-2 oz.
It will take me a while to make my plug mold but as soon as I have it I will post it up to recive your comments. All I can tell is I'm plannig to do a fuse pretty much like the Widebody 60 from CAmodel.

JasonS

Here my take:
If you are going to the trouble of making a nice plug, go ahead and spend the time to use that plug to create the traditional female molds. It really doesn't take all that much longer because the lost foam method will eat up a lot of time trying to acheive a good finish and its lots harder to keep the weight down. Plus, it difficult to get all the foam out. Even with good solvents, its a hell of a mess. Finally, if you make a decent female mold you have the option of producing additional parts relatively easy. I would be interested in one of your fuses myself if the price is right!
I've used the lost foam method a few times, and everytime wished I have made female molds afterwards.

That being said, If you are set on using the lost foam method, cover the plug with low temp film before using it. You will have a much easier time wetting out the cloth and the foam won't capture resin plus its much easier to cleanup that way. Consider using a very lighweight foam in the vert stab area and leaving it uncovered and them leaving it in place for stiffness.
Let us know how it goes!

CEV

What do you mean by "low temp film"?
The thing is that if the fuselage turns out all right I might do several planes, so right now I don't know what to do: a lost plug or female molds.

Mike James

Low-temp film is a heat-shrink film, such as Monokote, but one that works at a lower temperature, so as not to melt your foam. I'm not an expert on the best current low-temp films, since I mostly do composite planes, but I'm sure it won't be hard to get some advice here from others. (I believe the Coverite film, for example is low-temp.)

If you think that there's even a remote chance that you'd want to repropduce these, I would go ahead and make a proper plug. (something hard and durable) Once you've done that, making molds for a 2-meter fuselage isn't that expensive, and will give you a the chance to sell a few to your friends, perhaps recovering your investment. It's also a good learning experience that may help you in your future modeling activities. Warning... If it works well, you may end up addicted to composites.

Have fun!