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Jun 25, 2006, 06:38 PM
I get high on flying!

Composite dillema

I am wanting to cover my giant scale airplane in some kind of compositie. My buddy's dad works at a composites manufacturing facility and amazingly has offered to supply the materials for my project. However, I asked for fiberglass and he gives me 3 choises... fiberglass, carbon fiber, or kevlar. I really wanted carbon fiber, but after some deeper investigation I found it might pose a problem with my range. I want to avoid any interference/loss of signal issuses by not using carbon fiber. So I am down to fiberglass and kevlar. What is the weight ratio of the two? I know kevlar is stronger, but I dont want to add another 10lbs to my already pushing it airplane. I only have a few pounds to spare for covering. The airplane is a 120" high wing float plane. AUW should be under 20lbs (the airframe will take much more but I only have a 25cc engine for it right now).

I also have a time crunch for this project. It is comming with me on my trip to Canada on friday. If the airplane isnt ready to cover by wed night then Ill take the airplane and all the supplies and do it while I am on my trip (I drive a truck so I have the space, thankfully Quick responses would be appreciated.

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Jun 25, 2006, 07:03 PM
cody303's Avatar
Doesn't Kevlar have the same interference problems as carbon?

Little Cody
Jun 25, 2006, 08:14 PM
Hutch's Avatar
Is this a fully sheeted structure you are talking about? If you are just glassing as an alternative to film covering, then a layer of .75 oz glass will give you a nice small weave that will be easy to fill and paint.
Jun 26, 2006, 11:12 AM
I get high on flying!
Yes, this is a fully sheeted structure. The airframe is made out of pink foam insulation. The fuselage has 1/8" lite-ply sheeting on the whole inside with basswood reinforcements here and there. The main stress points (where anything bolts to the fuselage - firewall, LG, ect) are 3/8" ply. The wing is a solid foam core (pink here too) with hardwood spars. The tail surfaces are built up from FFF with a 1/4" lite-ply spar, they are also removeable. The elevator spars slip into a reciever in the tail and the rudder removes to provide access to the elevator spars. Pretty nifty design

As for Kevlar having the same interference problems as Carbon Fiber.. Well, I dont know. I know nothing about these materials. Everything I know has come from these forums in the past couple weeks. If there is potentially going to be a problem then PLEASE let me know If there is a problem then Ill just go with fiberglass and be done with it. Since I have been offered these materials, though, I want to use them if it is going to work. Why pass up an offer of high tech kevlar for fiberglass? Now, if theres a problem with interference then theres a reason for going FG.

Another issue I need to adress while were talking about composites is what density should I use? The covering on the fuselage will be for covering - it wont really hold any strength, so that part can be light. The covering on the wing is going to need to be stronger than the fuselage, but since its a solid foam core with hardwood spars it shouldnt need more than 1 layer of a heavier density cloth. Now, the tail surfaces. They are going to be built up with FFF and a 1/4" lite-ply spar. The spars will hold most of the strength but the 1/4" foam is going to be real weak its self. The elevator and rudder, therefore, need the glassing as strength. The foam will be there to give the glass shape. I was thinking about running a couple layers of cloth on these areas. I dont want to add too much though as I want to keep it light. It doesnt have to be bullet proof

Last edited by kc8qvo; Jun 26, 2006 at 11:20 AM.
Jun 26, 2006, 01:45 PM
Registered User
erazz's Avatar
You could have asked in the older thread

Never mind. Here's a table to make sense of everything.

Let's see what we can make of the data...
First, Tensile Strength. Tensile Strength is the ultimate break strength (while pulling) of the material per cross section. That means the force needed to break a filament of a certain diameter.
You can see that the strength is just about the same.

Next, modulus. This is a measurement of how much the filament will strech before it breaks. This translates into how stiff the structure will be. You can see that Kevlar is about 3 times stiffer than glass and that cf is 10 times stiffer.

These mesurements are all good and well but we're interested in a lightweight stucture. Hence the Specific Strength column. This is the strength per weight data. That says that a 2oz. per sq. ft. piece of carbon fiber is roughly 1.5 times stronger than a piece of 2oz fiberglass and that kevlar is 1.9 times stonger.

What does it all mean?

Well, if you want a very stiff stucture (thin wing that wouldn't flutter for example) then you should use cf. If you want the best strength to weight ratio then use Kevlar.

Some more factors to consider:
1) Kevlar is much much harder to work with. Some epoxies do not adhere to it too well. You do not want to sand it since it raises "hairs" that are all but impossible to remove on a large structure.

2) Kevlar does not cause any interference problems since it is not conductive.

3) Carbon fiber corrodes. It is usually reccomended to cover it with fiberglass to seal it and protect it.

4) Usually fiberglass is strong enough and light enough. It is easier to work with and comes in some very light weights (try finding cf in 3/4 oz. weight).

Hope this helps!
Jun 26, 2006, 02:32 PM
I get high on flying!
Muchas gracias Erazz!!!! Yes, that is VERY helpful!

Well we have figured out that kevlar is stronger than fiberglass for the same weight cloth. Thats good. My airplane wing is going to be strong as it is and it is not thin, so the stiffness is really a non-issue.

We have figured out that Kevlar is much much harder to work with than fiberglass. If I may dig into this further, can we examine the methods used to work with this material? I have no need to sand down the material (as long as it isnt like sandpaper or has mountain ranges all over it ). The airlplane is going to be painted eventually, but lets forget about that for now. My plan originally was to use fiberglass. I have heard of people using Minwax polycrylic instead of fiberglass resin. How will this work with Kevlar instead? Will it adhere to the material? If not, what kind of resin should I use with it? How easy is it to get? And last but certainly not least, how much will it cost? I will be able to get the cloth for little $$ but I am not sure about the resin.

If I go with Kevlar, what density is going to be the best? 3/4oz fiberglass is way too light. Even 1oz looks like it is going to be on the light side. If I were to use fiberglass I would probably use something in the neighborhood of 3-4oz. Given this, how can we convert that to Kevlar? One more question regarding strength. Since my tail surfaces are going to be pretty weak on their own I was planning on doing two layers of glassing on those. However, since Kevlar is substantially stronger, will it be alright to use 1 layer? Will it be flimsy at all? Just because something is STRONG doesnt mean it wont BEND. I want the best of both worlds on the tail surfaces.

Would it be OK to go with maybe 1 or 2 sizes lighter cloth with Kevlar? The strength would certainly be higher than fiberglass of the same weight plus some weight savings over a heavier FG cloth. I think we should think strength here, not just use Kevlar for the weight savings on the same strength FG but more of a combination of increasing strength while decreasing weight.

Again, Erazz, thanks so much for the information. This is PRECISELY what I needed

Jun 26, 2006, 08:28 PM
Registered User
chlee's Avatar

Hmmmm, 3-4 oz glass actually strikes me as very heavy. Before you commit to using one material or another, you might want to ask yourself what the precise function of the composite reinforcement will be in each part. When you say 0.75 or 1.0 oz glass is too light, where and for what type of load do you think it's insufficient?

From your description, it sounds like the fabric will mainly provide dent resistance and a smooth surface for painting, since the wood structure is meant to take most loads. In the wing, it can provide torsional strength as well as ding resistance (but in my experience, the boinks and doinks coming out of a car trunk is the tougher structural requirement for the wing skin).

- If you don't have vacuum bagging equipment, then I would start by trying two layers of 0.75 oz glass with polyurethane (one layer wherever you can get away with it, three layers only where absolutely necessary). If you feel you really need a third layer on the wing, then you might consider applying any additional layers to just the inboard parts of the wing first. I strongly advise against Kevlar if you will not vacuum bag, because it is difficult to work, will be heavy, and tends to leave prominent seams where it overlaps.

- Kevlar will be heavy? Huh? Well, the densities of the dry fabrics are somewhat misleading. Kevlar generally soaks up more resin than a similar weight of fiberglass, and this effect is significantly magnified if you don't vacuum bag.

- For the wing, if you already have enough structure for bending loads and if you are using cloths with bidirectional weave (same number of fibers in both directions), then orient the cloth in a +/- 45 degree bias to maximize torsional strength. On the tail, if you feel you need more bending strength or stiffness, then orient it 0/90 degrees. If there's little else to keep the tail surfaces from fluttering or twisting, then +/- 45 may be useful, though.

- When applying glass on a 45 degree bias, remember that bidirectional fabric is usually stronger in one direction (i.e. the horizontal fibers are stronger than the vertical, or vice-a-versa). Keep track of the two directions when you apply the fabric to the wing, so that strong fibers (whichever they are) on the top and bottom of the wing are parallel. If you get a single large piece of glass cut on a 45 degree bias and wrap it like a taco over your wing, then the "strong" fiber direction (whichever it is) on the top and bottom will be perpendicular. This imbalance can then cause warping under a hot afternoon sun.

- Remember to especially go easy on the reinforcement on the extremities. For example, you will pay for every extra gram on the wingtips with more sluggish turns due to yaw inertia.

Hope this is helpful,
Last edited by chlee; Jun 26, 2006 at 08:47 PM.
Jun 26, 2006, 11:13 PM
I get high on flying!
I appreciate your information, Chung. The covering on the fuselage will just be for ding resistance and a shield between the foam and fuel/exhaust residue. The other parts will be more for strength.

How much weight will I be adding in the resin between fiberglass and kevlar? I will be using minwax polycrylic as a resin (if I even can with kevlar??). Also, how much weight total will I have in the covering? From what I have found is that the weight of the cloth is per cubic yard? So if we add in how many oz of resin? to each cubic yard and multiply that by how much material I will have in total then I should be able to find the weight. Once I install my radio gear and hardware I should be at 16lbs. I am trying to keep it under 20lbs, so do you think 4lbs in covering would be doable? The wingloading should be fine up to 30-35lbs even, its just that I dont have much power to play with

Jun 27, 2006, 11:51 AM
I get high on flying!
I think I may just go with fiberglass. Since there is more information on it and it is easier to work with than Kevlar I might be better off. Ill see if I can get ahold of some 1oz bi-directional bias fiberglass cloth. I can do one layer of that on the fuselage, one layer on the wing (if I need 2 I can add another) and at least 2 layers on the tail surfaces. If I can get more than 1 weight of the material then I will go 3/4oz fuselage, 2oz (or about that) wing and floats, and 4oz tail surfaces.

Jun 27, 2006, 01:33 PM
Registered User
You have not considered fabric in epoxy with compression strength vs. tensile strength. Kevlar in epoxy as compression strength is lousy. Kevlar is tough like bullet proof vests. Kevlar is best with vibration like cowls. Carbon is best for spar caps. Glass is for cheaper price. Thin any fabrics in epoxy have lousy compression strength (buckling) when not supported.
Jun 27, 2006, 02:54 PM
I get high on flying!
Ollie, you are thinking of compression as in taking a cylinder and pushing on the two ends. The way I was using "compression" in my previous post was the force pushing on the surface of the tail surfaces - as in a suitcase laying on top of it (which I am going to try to prevent ). I guess that would be more of a squeezing or crushing force than compression.

Jun 28, 2006, 12:56 AM
I get high on flying!
Just to make things easier on my part I am going to do solid foam cores for the tail surfaces now instead of the built up ones. I will have much more strength that way as well as much less assembly time. Now the glassing wont be as necessary for strength, although it should be stronger than the fuselage glassing.

I talked to my friend today and got some more info on the materials that his dad works with. They are an undisclosed military manufacturing facility and their specialty is composites manufacturing - fiberglass, carbon fiber, and kevlar. I gave my friend the list of dimensions I needed and also specified bidirectional weave. For the cloth sizes I specified .75-1 oz for the fuselage and 2-3oz for everything else. How does that sound? Should I go a little heavier on the wing and tail surfaces or should that be strong enough?


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