steve90210

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I have a new big Gasser and want to reinforce the firewall corners. Is there a reason not to use carbon or Kevlar with epoxy an inch in on the fire wall and an inch on to the box, instead of aluminum angle?

Will the Kevlar crack over time with the vibration?

Thanks,
Steve

bfd04

Personally I would go with the CF. I have worked with both CF and Kevlar, if you use Kevlar you will need to get special shears to cut it with, which are not cheap. I have cut CF with good sharp sewing scissors. I feel CF to be more user friendly than Kevlar, also there are some Kevlars that will not wick up the epoxy, you can coat the fabric and it will stay but it won't soak into the fibers which is what gives you the strength.

bfd04

I meant to say which gives you the strength and bond[&:]

soarrich

My Feedback: (98)

steve90210

I'd use CF rather than Kevlar for what you're doing.


bfd04

I paid $75 for Kevlar shears, then found out if you buy cheap $1.00 sissors, draw the blade across a sanding wheel, you have great Kevlar shears.

steve90210

My Feedback: (5)

Thanks for the info, So there won't be a problem with the vibration lossening the cf bond to the wood?

thanks

r3tic

as long as you wet out the fabric enough, you won't have bond issues. However, it is very difficult to get CF to do a 90 degree bend. If you happen to get The fibers bent around the corner, chances are that most of the fibers are broken and will give you no increased strength benefit. Using a couple layers of fiberglass should be ample strength.

soarrich

My Feedback: (98)

I haven't found this to be true. I'd put the CF cloth on a piece of waxpaper, put epoxy on it, roll it to get the epoxy in it, and remove any excess. Brush some epoxy on the firewall and sides to wet the wood, then put the waxpaper with the CF on it on the fire wall and sides. r3tic is correct CF is stiffer than glass or Kevlar, so in a corner I put sandwich bags filled 75% full with sand, the sand bags hold it tight in the corners. After it cures the waxpaper pulls of giving a nice smooth surface.

Agreed, but he asked about CF or Kevlar.

steve90210

My Feedback: (5)

I really appreciate all the info, I Just thought CF or Kevlar would be stronger to reinforce the corners. I would like to use something besides aluminum and assumed CF would be stronger than fiberglass and stiffer. I don't have alot of experence with the composites but would like to learn.

If fiberglass is going to get the job done, then i am open to that.

I have to mount a 50cc on the front of a 16lb plane a just want to make sure it stays attached.

Thanks

tschmidt

My Feedback: (1)

Steve,

I've used carbon fiber and Kevlar fabric, but for this application I would suggest using carbon fiber tow (ribbon). You can find this in hobby stores since Dave Brown Products packages it in smaller quantities. Wet it out good, bunch it up like a rope and stick in position. When dry, the stuff is like iron!

Todd Schmidt

Troy-RCU

My Feedback: (1)

Your more likely to see a fail in the glue bond to the wood than a fail in the fiber used (carbon, kevlar, or glass). The main thing to be sure of is that you use a good quality epoxy (30 minute or better) and effectively wet the cloth. I think the point r3tic was trying to make is that general carbon weaves are usually heavy at about 5.7+oz/sq' and they do not take 90* bends well which means there are gaps in the corners. You can fix that with some cabosil to add stickiness and some softness to a hard corner or use multiple layers of a lighter cloth to get into the corner. Also, cut the weave at a 45* bias so the fibers don't make a 90* turn and both directions of the weave criss-cross the joint rather than just one direction. See pic: All white lines cross the center (the "bend" ) and the red lines do not carry any loads. All lines in a 45* bias carry loads across the bend and do it at a less severe angle, the 0-90* only has half the fibers doing work.

Magne

Hello.
I would recommend using fiberglass tape. This is MUCH easier to work with than cutting carbon or kevlar cloth, and trying to lay this in without everything fraying up and making a mess.
However, if using cloth, try to wet this out on a thin sheet of clear poly-ethylene plastic, and when wet, put another sheet of plastic on top. Then cut to shape, with the wet cloth between the layers of plastic. Now it will not fray at all. Remove one layer of plastic before you put it in, and the other layer after the epoxy has cured.
(Poly-ethylene plastic will not stick to the epoxy, and will peel off easily.)

However, for a 50cc engine, I would just use glass tape. Takes only 10 minutes to do, PROVIDED that you use laminating resin.
Don't use epoxy GLUE. (Stuff referred to as 5 min. epoxy, 30 min. epoxy etc. are glues, and are intended for a different application.)
Cut the tape to length, place it in the model, and apply the laminating epoxy with a paint brush, wetting the glass and the underlaying balsa. The fiberglass is wetted when it becomes transparent. (With carbon and kevlar, it is much more difficult to judge when the material is wetted, as there is no color change.)

On the other hand, why don't you just use some triangular balsa wood sticks in the corners? Trailing edge stock, with the "long" side along the fuselage side, and the "short" side towards the firewall, should give you all the strength you require.

Regards,
Magne

steve90210

My Feedback: (5)

Magne

The directions for the plane say to reinforce the front with angle aluminum on the outside with through bolts, I didn't want to have to do that, I thought I could do the same thing with glass or cf or Kevlar.

Thanks

bfd04

How I get around the fraying of the CF is I use Johnson&Johnson paper tape(used in medical field) and available at most local drug store chains. I use 1" wide tape lay it down on the fabric where I want to cut, and cut through the tape and the fabric. You get a very clean edge and the paper tape wicks up the epoxy too so you can just leave the tape on the CF when you lay it up.

daven

My Feedback: (1)

Personally, I'd just cut some 1" wide x 4" long strips of 6 oz glass and lightly spray with 3m77 glue and lay them into the corners. Wet them out with epoxy all around the firewall, each overlapping the other by about a 1/4" and you should have a pretty darn stiff nose. Carbon and Kevlar have great applications, but in this case I would think Glass is plenty strong.

rnuo2011

Fiberglass (or fibreglass) is a type of fiber reinforced plastic where the reinforcement fiber is specifically glass fiber. The glass fiber may be randomly arranged, flattened into a sheet (called a chopped strand mat), or woven into a fabric. The plastic matrix may be a thermosetting plastic- most often epoxy, polyester resin- or vinylester, or a thermoplastic.
The glass fibers are made of various types of glass depending upon the fiberglass use. These glasses all contain silica or silicate, with varying amounts of oxides of calcium, magnesium, and sometimes boron. To be used in fiberglass, glass fibers have to be made with very low levels of defects.
Fiberglass is a strong lightweight material and is used for many products. Although it is not as strong and stiff as composites based on carbon fiber, it is less brittle, and its raw materials are much cheaper. Its bulk strength and weight are also better than many metals, and it can be more readily molded into complex shapes. Applications of fiberglass include aircraft, boats, automobiles, bath tubs and enclosures, swimming pools, hot tubs, septic tanks, water tanks, roofing, pipes, cladding, casts, surfboards, and external door skins.

MTK

Oh, that's all you are trying to reinforce? A 50cc is no big deal. Two layers of 4-6 oz glass cloth is plenty. The firewall should be at least 1/4" thick a/c ply (preferred), but 3/8 inch lite ply works too. I use 1/4" cross grained balsa sandwiched between two layers of carbon cloth (3K, or 3000, which refers to # of fibrils per strand of carbon in the fabric). Super light but stiff and plenty strong enough for a 50cc engine.

FYI.... Kevlar is a very tough fabric, think leather tough. It doesn't crack; it's not brittle. Fairly strong in tension but not terribly strong in compression and may buckle in large lay-ups (not in your application tho, just too small an app). Fibers do not cut easily so sanding Kevlar is a real pain.

CF fabric and tow (single ends) are incredibly strong and stiff in tension and compression. Pound for pound stronger than steel, it is the strongest of the readily available reinforcing materials. Sands and feathers beautifully. The fibers are fairly stiff and can have a hard time going around corners and staying put. Someone described a good technique for wetting the fibers properly by rolling the epoxy into the fibers. Well wetted carbon is easier to work with. If you are using chop to form fillets, wet the fibers to wet cat hair consistency. Don't blot out the epoxy but if it runs, remove any excess. If it runs, it doesn't have enough carbon fibers

Glass is possibly the weaker of these reinforcing materials (except for certain high end grades (think Military) which we don't need or use in toy airplane world). Similar rules apply as to carbon

Hope this very very short lesson helps you get started with a basic understanding about these three available and very useful materials

boater

My Feedback: (2)

Steve90210, I hope you will come back to this forum and tell us how it went, whether kevlar, graphite or glass.