some "newbie" questions

[combatflier]

I keep thinking of them, by them i mean newbie questions, but they never got into a thread. I know I have a few, but, i can only remember one main questions at the moment, so, i'll start tonite with this...

Whats the difference between the composite materials and which is suited best in which location? I'm talking mainly about the main 3.

Carbon Fiber
Fiberglass
kevlar

I guess some are harder, some strech and bend better (better elasticity) some are heavier, some are lighter, but, which is lighter, heavier, stronger, etc is beyond me. I dont even know how to compare them since they come in different weights. Also, how are the weights decided? 1oz = 1oz per sq. foot? I assume its dry weight (no epoxy/ca).

I'm asking because I will be doing some modifications to my Nike 2 wing (not quite as in depth as soho, I'm not quite ready to relocate holes in a wing yet). I figured kevlar would be the best bet, its not as brittle as fiberglass (i imagine, could be totally wrong, thats why im asking), stronger, harder, seems like it would hold the wing in one piece the best. I dont plan on using much of any material so i doubt weight will be a huge issue.

I'm guessing a lot of these questions may be answered by checking properties, but, I am not sure i could take those properties and translate those to uses in planes, and i figure, who better to ask than you guys? I also figure phil barnes dvd might help a bit with these questions, but, as I have about 20$ to last the rest of the month (including eating out, and animal food) I cant afford that this month :). Hey, hopefully these questions will help others as well, we all have to start somewhere :).

[combatflier]

somebody has to have some idea :)

[davidfee]

It's just that your questions are rather general. Maybe we need a FAQ already? ;)

Properties:

Carbon (graphite, really): Stiff and low density. Tensile strength varies inversely with stiffness (several different grades). Good when stiffness and low weight are the primary goals. It's expensive in woven form.

Glass: Medium strength and medium stiffness. More dense than graphite. Comes in different grades. It's cheap and very easy to work with.

Kevlar (aramid): Excellent tensile strength, but poor compressive strength. Not as stiff as graphite, but more stiff than glass. Difficult to cut and trim. It's the least dense of the common fabrics. (yes, it's lighter than carbon)

Here is a decent overview article: http://composite.about.com/library/weekly/aa980323.htm

Fabric weights are given in ounces per square yard (or grams per square meter in the rest of the world). These are "dry" without any resin. The yarn size is crucial to the overall weight (and strength) of the material.

Glass is plenty good for a wing center section. Kevlar is more trouble than it's worth in that situation, IMHO. Kevlar does make great fuselages, however, when they will see repeated abuse.

I think the best way to learn where the materials should be used is to look at the commercial molded airplanes. Fuselages are glass or Kevlar, with carbon reinforcements. Spars are carbon. Wing skins are glass or carbon, with balsa (or other) sheet cores for the sandwich panels.

The answers could go on and on... which is why I was so slow to respond.

Welcome and good luck,
-David

[combatflier]

I wrote a nice big ol answer, but, because of maintainance its gone :(.

Anyways, to sum it up, i agree, they are general, and i hope that a faq does pop up.

another general faq question is, when you double up from .5oz 1oz for example, does strength and stiffness also double up, or is it about double, or more or less? If that makes sence.

I want to strengthen the wood "washers" that are in the wing so they dont pop out of the bottom, im thinking i will reinforce them with kevlar washers (about 1" dia with an apropriat sized hole for the screw in the center) so they dont bust out of the bottom (just the 2 for the LE, one per wing) and then using glass over the center.

Would heavier FG be a better easy solution? What weights would you recomend (for the cloth) and resins? I was going to use the finishing epoxy as its smoother once dry (as i understand it).

[davidfee]

If you go from .5oz to 1oz material, you are doubling the number of fibers... so the strength and stiffness (of the laminate piece) will approximately double. There are a lot of variables specific to each application and the math can get very complicated (engineers use computers).

You can use Kevlar washers if you want, but cutting them to perfect circles will be a challenge. I feel that ~4oz glass would be much easier to work with and would be more than strong enough.

You should use a laminating epoxy or "finishing resin," and a good rule of thumb is that your resin/hardener should be the same weight as your reinforcements. In other words, weigh your glass and then mix up that much epoxy. It's difficult to mix small batches in the correct ratio, so it is usually more convenient to add too much epoxy and then blot up the excess.

Finishing resin cures harder and is easier to sand than the 5 and 30-minute rubber junk. Laminating epoxy is generally stronger and harder than any of the above.

-David

[combatflier]

Thanks David. Im playing around with some vacuum pumps in the shop... this could get fun, but, bagging will come later down the road.