madfreestyler

Can anyone recommend a good site for calculating the stregnth of composite structures.

I know it will depend on thickness, resin, cloth etc etc.

The question stems from some ideas that i have to make a few composite planes and hopefully if i can get it right maybe produce a few. What i am trying to figure out is if i need an internal structure to add to the stregnth of the aircraft or if the right resin and thickness is used maybe not required i need to get my head round the trade offs?

Same applies to the wings, if you are going composite like the comp arf stuff do you still need formers in the wings? More to the point how do you mould a wing and get the leading and trailing edges joined right.

I know i had a eurosport a while back and im sure that only had a former at the root. The fus was pretty much the same, there were formers for the turbine and i think there was 1 towards the back where the under faring was fixed.

Any pointers much appreciated.

ByLoudDesign

Although this site is for tubes the properties are the same for sheet or layup!
http://www.carbonfibertubeshop.com/t...roperties.html

seanreit

My Feedback: (60)

It all depends on design. This jet has NO formers! The "formers" were built into the design to be composite. See where the hatches are, there is structural support across the fuse from the original glass lay up. This jet is 3 layers of 6 ounce, doubled up around the intake area for about 3 inches.

Mileage may vary, but this one is a good example of what can be achieved to save some weight and extra work.

madfreestyler

Awesome!!!!!!!!! Dont suppose you would care to share your mould making and moulding technique?

What do you use to design your models?

madfreestyler

Awesome!!!!!!!!! Dont suppose you would care to share your mould making and moulding technique?

What do you use to design your models?

seanreit

My Feedback: (60)

There isn't really a secret. As incredible as it may seem, I have read every single thread in this forum related to composites and mold making. I was out of town on business a few years ago, and spent literally about 40 hours in the evenings reading and absorbing information. I've invested thousands in technology, materials, and equipment to make my dreams a reality. It's a lot of work, but can be very rewarding.

Good luck, and when you get going, if you run into questions, post them, there are many here with more experience than me that can help.

Magne

It is also worth noting that models such as the Comp-ARF (and others) that don't have formers, are all sandwich structures, i.e. fibres (glass/carbon/kevlar) laid up on each side of a light weight core material. The core material could be balsa wood, honeycomb or foam. The core material separates the two skins, and make a stiff, lightweight structure. A single skinned structure would need to be many times heavier to achieve the same stiffness.

Magne

SCALECRAFT

My Feedback: (13)

Fuses have been done for years,lot of info. The wings/stabs are the trick..

I would also like to make a point about stiffness in the glass "skin"construction. For the last 16 years my buddy and I have been making/flying all composite warbirds that have a few layers of glass along with minor wood/composite framework in the fuse and hollow glass wings.

The glass skin in wings mainly, without a sandwich core material may not be stiff to the touch at some points as a core type wing, but the glass/frame wing is just as light and has never failed us in flight. Plus the glass skin coreless wings don't dent as do the sandwich construction type, like the old fiberclassics P-51 for example. Not a very durable surface, the foam core would dent and not bounce back.

ALso glass coreless wings are cheaper and easier to do. No vacuum bagging needed. Well this is what has worked well for us for many years.

Once you start to work with glass , shapes, and angles, you'll see how it can all work together.

Jet remember, its not going to be crash proof.

Steve

Magne

Steve.
I am very interested in what you are saying.
Could you please indicate what lay-up schedule has worked for you.
What material thicknesses are required for adequate handling strength, and what is you recommended distance between the ribs/formers.

Thanks,
Magne

Ed Smith

I am always experimenting. As seanreit stated there are no secrets. Many years ago I incorporated Carbon tow in my fuselage layups. The added stiffnes was quite amazing. I wind the tow into a string and sandwich it between the layers of cloth. The pictures illustrate this, The tow is sandwiched between two layers of six ounce cloth. The extra strips above the front of the wing saddle stiffen the fuselage above the landing gear. The weight of the added carbon was less than one ounce. No additional resin was needed the carbon just suck up what was being used for the layup. I use only epoxy resin. The fuselage in the picture is 48" long. Apart from the firewall and the servo tray there are no other formers in the fuselage.

The strips in the cheek cowls did not do what I wanted. I should have laid them across the cowl not along them. Next time. The wheel pant strips of carbon are self explanatory.

Ed S

seanreit

My Feedback: (60)

Ed, on my fuse, on the jets one of the factors is that we carry a lot of weight in the nose that has to be supported (we figure supported around the intake area). I have never used Tow, and we were thinking just lay in some carbon fiber triangle cut from the intake area up the sides towards the nose.

Does tow have much strength say over a two foot run? I was thinking surface area of carbon fiber, but if this stuff makes a beam over length, might be just the trick??

Note: fuse is three layers of really loose weave 6 ounce. Lots of curves in this baby and the loose makes the turns quite well.

Ed Smith

Sean,

The lengthways strip in that fuselage is 20" long. It is 1" tow twisted into a string. Just laid in flat the tow does not add much stiffness. Twisted into a string the difference is quit amazing. Two or three strands laid side by side would definitely give you a "Beam" like structure add a couple of verticals and the fuselage would be very stiff.

I mold my own Pyon Racing fuselages. I use the same principle with those. For me Carbon Tow strategically placed is the lightest easiest way to stiffen a fuselage.

Ed S

tony-howard

My Feedback: (224)

One of the best books on composite structures the I've seen is:

Composite Basics by Andrew C Marshall
Published by Marshall Consulting
720 Appaloosa Drive
Walnut Creek CA 94596

My copy was purchased in 1994 and finding a copy may not be easy now.

It is intended for full scale homebuilt design, but it contains a wealth of information for any composite structure designer. It's well organized, easy to understand and has one of the better sections on cloth weaves. Edition 4 includes carbon fiber.

HTH

HO-229

Ed,
I like what you have done with the "tow" material, I find it to be a very innovative way of incorporation stiffeners/ribs within the fuselage structure without the fuss of making stand alone rib structures & bonding them to the inner skin structure


Thanks for the tip

Regards
Dave

Spike

My Feedback: (1)

I found a copy here: http://www.actechbooks.com/composite_basics.htm

tony-howard

My Feedback: (224)

Well, I'll be darned. Available for downloading too. Maybe I should upgrade my earlier version.

Thanks for posting it Spike.

SCALECRAFT

My Feedback: (13)

Hi

For years, heres what has worked for our models. Total of 2 layers of plain flat weave "E" glass with 3.75oz or 5.75oz. Combination depends on size of model and shape of surfaces.

Also take into consideration if you have a model that tend to be tail heavy. Plan to use lighter glass in the aft section. Resin is West Systems with slow cure hardner. Fast hardner is thicker, difficult to remove excess. Of course fast cures faster.

Take a look at the Chinese models with the composite construction. Although the Chinese models are using excessive wooden internal support methods we abandoned years ago they are still a good look into the most basic of composite construction. Some even use polyester resin, a much lesser quality resin for our use. Brittle, less impact resistent, and difficult to bond to.

Carbon fiber, a nice material, looks cool, but I have used fiber glass tape to yield at around 3 times stiffer support with the same weight gain as carbon, carbon/kevlar hybrid, "S" glass or kevlar alone. Again, it the configuration of the support and not the addition of layers.

See the site for Spruce Aircraft and Supply, it has a composite section, it shows the stiffness and weights of various composites

Steve

SCALECRAFT

My Feedback: (13)

Note that the carbon and kevlar in the 109 was of minimal help. It mostly makes the fuse feel stronger for the people who need to squeeze the fuse sides. All our models never had carbon, and have never failed in flight. All overpowered of course. Servo trays and one buklhead normally does it.

Again this is what has worked for us. Minimal cost, no vac bags, free atmosphere layups.

Steve

dreadnaut

My Feedback: (5)

I traveled to China last year on buisness not related to R/C, but it involved the use of resin. I found it interesting that my translator was calling it ''Epoxy'' even though I could clearly smell that it was polyester. I am sure that some of the higher end stuff, like CARF, and OAXI use epoxy, but I suspect that nearly all of the other stuff from over there is polyester.

SCALECRAFT

My Feedback: (13)

I think China has come along way in marketing! As long as the price is right. I don't think they really care about materials. Lead paints, air and water pollution, ect. Not yet anyway.

They are more Green($) than Red these days.

Its double edged sword. Helps us, hurts us.

Steve,,

dreadnaut

My Feedback: (5)

Exactly. There seems to be a ''they'll never know the difference'' attitude . The first time I experienced this was in '94 when I worked for a guy that was importing a fan belt tension guage from Taiwan. We wound up having a spring for the device manufactured here, and exported to Taiwan because they could not provide springs that were all the same.

In regards to wheel pants, I like to put a small patch of woven CF where the axle penetrates, CF tow around the wheel opening to prevent splitting. and .5oz CF matt doubling the large flat area opposite the axle, for stiffness. I use some 3.6 oz FG cloth from CST that they call 4HS. This is a twill-like weave, and it has a higher thread count than regular e-glass. I find that since there is less volume of space in thre weave, it requires less resin to wet completely, and it is stronger, so you can use it where you would normaly use 6oz. The trade off is that even though it has roughly the tensile strength of 6oz, it is less stiff, therefore the need of CF matt on large flat areas. Also, the CF matt needs to be vacuum baged, ot it uses too much resin.

madfreestyler

Hey all, some awesome info here, its amazing the info you can pick up in these forums.

Just out of interest what do you epoxy guys use as a gelcoat? And what do you use to make the mould, one of the companies over here suggested putting thixotropic agent into the epoxy to use for gelcoat and first layer on the mould, anyone tried it? I thought it was just for silicone moulds?

With the foam sandwich technique is it just polystyrene you use or is there a special type for this purpose?

SCALECRAFT

My Feedback: (13)

mad

For a gel coat/surface coat of the mold we use cabosil (fluffed silica) and a bit of black pigment to show voids easily in the layup of the parts. Should be like pudding that self levels, but can stick to the sides.

You can also add milled aluminum ($$) or premixed, for durability of the surface coat. Many other additives are available. Expieriment with small parts first. If you are doing this for yourself for a few parts, care in waxing/pva will give long life to your molds.

Make sure you always use a high quality respirator and skin protection and in good ventilation before playing with any of this stuff. Keep you filler material "dust' contained as much as possible. Treat it all as a hazardous material. Who needs extra allergies/ toxins in their life.

Steve

Steve

SCALECRAFT

My Feedback: (13)

See Spruce Aircraft Supply for types of foams. Some cannot be cut with hot wire due to fumes.

Steve

dreadnaut

My Feedback: (5)

For black pigment, I like to use the graphite powder from West System. Not only does this stuff blaken the resin pretty good, it seems to act as a surfactant. This means it reduces the surface tension of the liquid, allowing it to flow better. It also seems to reduse the friction coefficient of the cured surface, which makes parts easier to release.

Like Steve said, pudding consistency, or thinner. Any thicker than this and you risk getting bubbles and voids in your surface.