# Glass Types

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#### pylon500

##### Well-Known Member
Just trying to get some opinions on glass layups with regard to amounts of resin used for different types of weaves versus their overall strength.
I'm planning on using unwoven bi-directional with a well 'stippled' wet layup without vacuum bagging, the idea being to keep down time and cost.
My real question is; will bi-directional suck up more resin than woven?
And what happens when you add extra layers?
I've got enough moulds now to actually start making BITS:ban:
Arthur

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#### orion

##### Well-Known Member
By bi-directional, are you referring to what's commonly called "knitted"? In other words, there are strands in the 0 and 90 deg. directions but they lay flat on top of each other instead of being interwoven. This is a great material since the fibers are not subject to the crimp and shear factors normal woven fabric sees. As such, the material is stronger and stiffer than normal fiberglass fabrics.

However, due to the way the fabric is held together, it tends to be very poor on compound surfaces and does need to be vacuum bagged for the cure otherwise it will maintain a relatively uneven wet-out, creating some air voids. It is also relatively stiff adn does not soften much with the application of the resin and so it will bridge tight radius concave corners without the vacuum bagging.

Since the knitted material also comes in fairly heavy weights, it does soak up quite a bit of resin and it does tend to be a bit harder to wet-out.

I guess the best thing will be if you could post some pictures of the weave.

#### pylon500

##### Well-Known Member
What I have is actually unwoven bi-directional BIAS cloth, 4 foot wide.
All the rovings are at 45 degrees to the roll, held together by light stitching.
I've done a layup in a tail mould using Vinyl Ester, which I stippled out to a matt surface.
I had thought of doing the 'Pro' bit by weighing the glass and only mixing that amount of resin, but I sorta just rushed into it and did it.
I reckon I put down about 8 square feet of glass and probably used about 1/3rd of a pint of resin.
I've got a bit of reinforcement to add, then I'll pull it to see how stiff and heavy it feels.
Will get some photos as well...
Arthur.

#### orion

##### Well-Known Member
OK, I know what you got. The plus and minus 45 deg. orientation does help the material in forming curves but again, due to the construction of the fabric, it tends to be stiffer than woven material. Here it's sometimes refered to as Knytex (also name of one company that produces it). It is also available with a light mat backing that is oriented towards the surface of the tool, thus preventing print-through of the fabric weave onto the finished surface.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
I am using Biax and Triax extensively in my bird. Biax is what you describe. I use Biax for inside of panels and for control surfaces, and Triax for the outside of wing and fuselage skins.

As ORION pointed out, it is rather stiff, but for bulkheads, ribs, wing and tail skins, and some fuselage parts, it is great. I used two Bid for both skins of the turtle deck and forward deck in seperate layups on a male mold. That makes for a lot less in the way of repairs.

I found that for plain layups, Biax and Triax are just sponges. Weigh the glass, weigh the foam, make the part, discover that you have WAY more epoxy in the part than glass. And epoxy will run out of the layup leaving voids. Multiple layers are bad. You really have to vacuum bag this stuff.

Gougeon's booklet on vacuum bagging is great and practical. Once I figured out my vacuum bag routine, I was getting parts with epoxy/glass ratios quite a bit lower than unity. My sample parts proved to be stronger than book values too, so I was happy.

I use peel ply, perf ply, peel ply, Triax, foam, Biax, peel ply, perf ply, heavy weight polyester batting (for quilts, etc), and then bag and pull at least 20" Hg. Works great. Use the good bag film. I use Gougeon Proset 125 resin and their slow hardener - it wets easily, excess wicks into the batting nicely, and will post cure.

Billski

#### Dust

##### Well-Known Member
The cozygirls have come up with a cheap low Vacuum method, might want to look into it

#### wsimpso1

##### Super Moderator
Staff member
Log Member
The Cozy Girls do indeed have a low vacuum method as do some other folks - just do a search on "vacuum" "aircraft parts" and you will get all sorts of good stuff. "Cozy Girls" will get you their website. Anyway, their methods are designed around the many small layups (as well as the big ones) used in the Long-Eze/Cozy/etc type of moldless airplanes, where you are always applying fiberglass over some other shape, usually foam or prior fiberglass.

Since you have molds already, bagging would not be difficult. I presume the parts will have a layer of foam or honeycomb for cores, and they will have to be curved into the parts. Nothing works for that like full vacuum, and then you can get the weight reductions that full vacuum can give too. I would investigate going the whole route. I have commented extensively on the methods, here is the big one I put on:

Billski

#### pylon500

##### Well-Known Member
honeycomb for cores
I'd love to be able to afford honeycomb, I know the little Starlite used it, but it's so **** expensive here in OZ. :wail:
For my prototype I've got a stack of fairly dence white urethane foam in 1/4" and 1/2" sheet, but I'm going to try to cut it down to around 1/8" and see if it will stay in a wet layup.
I have neighbours in close proximity, so leaving a vacuum pump running is not good.
I may be able to get away with 'sand bagging' my flatter shapes, tail, wing etc. :roll:
Arthur.

#### orion

##### Well-Known Member
Honeycomb cores do not tend to work well in wet layup applications, vacuum bagged or not. You generally get poor edge adhesion and if you don't take care in the wetting out process, the core cells may get filled with resin, which results in quite a bit of weight and a possibly damaging exotherm.

Some time back I did see a honeycomb core material that had a light foam filing in the cells - that would work great. But I have never been able to track that material down, nor the name of the manufacturer.

Also, most standard honeycomb cores can mold to only flat or gentle single direction curve shapes. There are honeycomb cell geometries that do have the ability to conform to more dramatic and/or compound curvatures, but they tend to cost more.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Pylon 500,

Your exact problem, how to get the foam core to lay down in the mold, was how I ended up investigating vacuum bagging. Even a weak vacuum was not enough to fully squeeze 1/4" Last-a-Foam cores into the curves of my wing skins. I could not get the air out of the layups. The parts were heavy, and the cores did not conform. I cut up a part that cost me $300 when that happened. It makes good parts to take to EAA chapter talks or for visiting lecturer at the aero engineering composites class. The thicker the wing skin cores are the less ribs you need, so you do not want to go with really thin cores. One method that I considered was rough shaping foam to the mold shape and then, once you have the shape about right, you can try sandbagging, but I think that you will still have the dual problems of big excess epoxy and air bubbles. The Cozy Girls approach might work once you have sanded the cores close, but you would have to prove to yourself that all of the air got out and you got good bonding between glass and foam. My molds were styrene foam hotwired to shape, glued together and an aluminum (roof flashing) skin was vacuum bagged in for the mold surface. On one skin (out of eight) we cracked a mold at a joint during the layup, and the leak resulted in only 4" Hg which is about 2 psi or 300 pounds/ft^2. It was inadequate, but imagine trying to get that with sandbags. And building your tools and tables to stand te sandbag weight. Now with 20" Hg, you have 1400 pound/ft^2 holding the parts against the mold and 20" Hg sucking off all of the bubbles and random volatiles from your parts, and your molds can be almost insubstantial. If your vacuum pump is too noisy to run near the neighbors, you need to investigate other types. My first one was an old lab pump, that operates in its own oil bath and just does a soft pucka-pucka-pucka sound. Paid$20 for it, but it moves air pretty slowly and puts out a bit of an oil mist, so it had to reside outside when it was running. Other options are the many pumps available for various machines that are rather quiet and available on eBay and industrial supply houses. Adding a small engine muffler to the air outlets on my industrial pump helped it out to the point where I run it inside the shop, albeit with a reservoir (old air tank) and vacuum switch.

Good Luck!

Billski

#### orion

##### Well-Known Member
The vacuum pump I use I made from a 5hp horizontal tank air compressor. The air flow is controlled with reed valves so all you have to do is take the head off and flip the reed valve assembly over so that the air movement is reversed. I've now done this with a standard oil type pump as well as one of the new oil-less pumps - both are able to draw in excess of 25". The only problem I keep running into is the reliability of the switch.

On the first pump I made I used a commercial vacuum actuated switch - very expensive and for some reason it lasted only about ten or fifteen cycles before it became unreliable (would not turn the vacuum pump off).

For the next one I used an automotive choke vacuum diaphram. The mechanism is attached to a micro-switch rated for the wall current, and calibrated by a simple spring arrangement. That seems to work but still requries a bit of oversight.

The oil-less pump is very noisy so I'm not sure I would recommend that approach for you, but the older oil type seems to be relatively tolerable. If you enclosed it in some type of insualted box, the noise might be within resonable limits so as not to cause a problem with your neighbors - unless of course you're doing this in an apartment, in which case I would not recommend even the latter.

Keep in mind though that if you do encase it in an insulated box, the pump still needs cooling air.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Once again, Orion and I are talking complementary stories. You would think that we got together on the side and agreed what we would each say, when we live almost a continent apart and have yet to meet.

To my mind, learning how to vacuum bag is a bunch easier than figuring out how to get curved skins with foam cores from plain wet layups.

Billski

#### pylon500

##### Well-Known Member
As I am currently doing the tail surfaces, I'm thinking I can afford to experiment a little, and after recently looking inside a Jabiru and seeing it full of Coremat, I decided to get some and try it.
The local supplier carries a material named 'UPICA' about 1/8th inch thick.
I've resisted using this stuff in the past as I believed it sucked up large amounts of resin, which after laying up some, is sort of true.
However, the bits that I've since pulled from the moulds don't feel to be any heavier than I originally anticipated.
I must upload my latest shots, but for tonight, here's an intiial tail layup...
Arthur.

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#### Falco Rob

##### Well-Known Member
Looks like I'm in the right place to get some expert help on composites, an area in which I'm still on 'L' plates.

I've just purchased some 6mm and 10mm Divinycell PVC foam from which to make my fuselage mounted fuel tanks. The plan is to skin it with 2 layers of carbon on each side.

I've also bought Derekane 411 resin to do the job which I'm assured is fuel resistant, but I'm now a bit worried about my choice of foam.

If all goes according to plan the foam should never be in contact with fuel but it would be comforting to know that it doesn't matter if it is.

The Aircraft Spruce site has a bit of info on foams but doesn't specifically mention if PVC foam is fuel resistant, as it does for Last-A-Foam for example, a polyurehthane foam.

To my knowledge Last-A-Foam is not available in Oz.

So, now that I've rushed out and committed to this stuff (expensive, at about \$80 for a 4' x 4' sheet) can someone tell me if I've screwed up !

#### orion

##### Well-Known Member
One important note: Graphite fabrics tend to develop rather annoying pin holes in the laminate, even in thick layups. Make your tank using glass cloth to make the process easiest.

The Dow resin is perfect - it is fuel proof and easy to work.

If you do a good job sealing the tank, the type of foam should not be an issue.

#### Falco Rob

##### Well-Known Member
Thanks for the help Orion, but . . . umm . . . but did I mention I've already bought the carbon as well?

Maybe a single layer of regular glass on the inside to assist the sealing process?

I'm also advised that the inside of the finished tank sides and floor should have a generous coating of Derekane "painted" on prior to attaching the top.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Pylon 500,

I am a concerned that with a relatively thin skin and high indicated airspeeds, you might need more than just a spar and end ribs to keep this stabilizer in one piece. Have you calculated maximum skin stresses and deflections based upon skin thickness, panel size (between ribs and spars) and worst flight case of pressure difference between inside and outside of the tail??

Most composite wings and tails use foam cored skins to get the skin section stiffness up and spread out the ribs in an attempt at keeping total weight low. For instance, the Lancair 320/360 horizontal stabilizer something similar to 2 BID on each side of 1/4" cores, two spars and several ribs, breaking the skin into several relatively small panels. Without the cores to provide section stiffness, they would need even more ribs or more skin thickness or both. Cores end up being the primary tool for taking weight out.

Billski

#### pylon500

##### Well-Known Member
I am a concerned!
Duely noted, but fear not, that was only the first layer ( but the last shot in my camera at the time)
I have now made both halves of the tail (top and bottom) and both halves of my fin.
Currently I'm reshaping the plug for the rudder.
Meanwhile I'm still adding to the tally of moulds on the fuselarge plug.
I've uploaded most of my construction photos to a YaHoo Site (I thought I had done this somewhere else, but can't find it?)
Here's a sample;

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#### StRaNgEdAyS

##### Well-Known Member
Looking good Artur mate!
I'm still plodding along, I'll be actually making tangible progress in the new year!
:ban:

#### pylon500

##### Well-Known Member
OK, Just an update, I've added more photos to the Yahoo site .
Hard to find the time to handle the pictures, so I've got more in the camera that are still waiting.
Arthur.

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