iron eagel

I am using carbon fiber veil applied with dope to balsa to finish an airplane.
Both on my plane, and another a friend built we have seen that the veil balsa combination has a lot of added strength when applied over the balsa.
I was just asked how does something so thin add strength and I replied just as a guess that:

As far as the strength it adds to the structure I am really not sure about how something so thin adds the strength that it does.
As a guess I think a lot of it has to do with the forming of a hard shell on the exterior of the balsa with the veil and dope.
I think it also locks the balsa grain in place and compliments the balsa strength in that way, but I think the "egg shell" skin it forms is the key.

Could any of you explain just how the veil adds strength to the structure?

Ed Smith

There is a difference between Strength and Stiffness. The structure you describe could be broken or punctured quit easily. However bending it would be very difficult. Using dope you have not got the best from the carbon. Use epoxy resin and see the difference. The stiffness is purely a characteristic of carbon in a composite structure

The picture shows a fuselage with carbon tow wound into a string and laid up between the layers of cloth. if you were to squeeze that section you would swear that there was a solid bulkhead at that point.

The carbon props I make are extremely thin at the tips. The will break quite easily but the tips will not bend.

Ed S

iron eagel

Nice looking airplane Ed!!!
And your props are very nice. Nice work on the molds!
How do they work on your models, I would think that they bite the air very well, without loading the engine all that much.

You are right it adds a lot of stiffness, so much so that it is amazing how something so thin does so. But given how I am using it it has made the skin of the airplane very stiff and the overall structure much stronger. One of the things I had done is to try to used the carbon fiber veil to enhance the effects of stressed skin construction and it seems to have done so well with very little weight added. I know I would have gotten much better results with epoxy but I wanted to keep any weight gains to a minimum as this is only a .40 size airplane. And thus far I can see that everything that has been covered with the veil is much stiffer and has had a very positive effect with very little weight gain. The rudder for example was made very thin and shaped as a true airfoil. I was afraid that it would be way to flexible being that thin but was surprised to find that it is very rigid and hard to flex or twist. Granted I had used 1/16 carbon fiber rods on the trailing edge but that alone would not account for how stiff the rudder is.

My major reason for asking the question was that I really did not know how the end result came about.

jvigani

It adds a thin rigid skin to the balsa that helps reduce the deformation under load. The control line guys use it all the time particularly on their stunt planes. Make a great base for finishing as it sands very easily and fills the balsa grain.

iron eagel

Thanks for explaining how it adds strength to the structure.
It was a friend who was building a control line stunt airplane whom had shown me this particular method of finishing, while he was building his plane for this years Nat’s this past winter. (To bad the plane suffered a cruel fate, during trimming, the week before the contest and he ended up having to fly his old practice plane.) I was really impressed with the amount of strength that was added to the overall structure, by the finish. I was even more impressed with how nice the finish came out, given how light it was. The plane that I am using the finish on was originally designed to be covered with plastic film, by adding thin sheathing and the carbon fiber veil over that, the overall structure of the airplane was reinforced a great deal. Thus far all I have got the finish applied to is the cowl, rudder and cooling duct; I have noted a large change in strength of each piece. The cowl I used veil both on part of the interior and all exterior surface and now have two halves that feel much more like plastic of fiberglass than balsa. All of the pieces really take on the look of a gel or plastic finish.

TeamSeacats

Composite structures take advantage of the skin on the outside to transmit and carry the tension loads...pretty close to your egg shell analogy. I've drawn the following diagram to show a simple composite structure. The center shaded area would be the core made from balsa, foam, etc. Proper use of the core can have a dramatic affect on the strength of the structure...but more on that in a minute.

As you apply a bending load on a structure like this, the whole system works together for strength. Separately, the skins and the cores can't resist anywhere near the same loads they can when combined. In this example, the bottom skin does not do much to add to the strength because it is seeing loads in compression - the weakest direction for most fiber based composites. It is trying to be folded. The top skin, however, is in tension and resisting fiercely to the bending load. The core is playing an important role in transmitting the compression loading from the inner core to the tension load on the outer core. While doing so, the core needs to withstand a shear load ...i.e. a load trying to tear it in two lengthwise. The red arrows represent the compression or tension loading on the skins and the green represent the shear loading on the core.

You can build a solid carbon fiber structure of the same dimensions and it will be stronger - but it will be considerably heavier. You can build a solid balsa structure that will be very light but will be weaker (the balsa does not handle tension or compression loading well). You can build a structure utilizing the strengths of both materials that is light AND strong. Balsa, particularly "end grain" balsa is very good at being lightweight but capable of handling significant shear loads...and carbon is one of the best materials in the world for handling tension loads.

That's the macro view of the system. There are a whole lot more loads and forces acting on the system in the micro view.

iron eagel

TeamSeacats

Thanks so much for taking the time to reply plus a nice tutorial.

I’m not a composite or structural engineer and I saw that there was an increase in strength when using the carbon fiber veil, but only had the most basic of understanding of what was happening. All I knew for sure was that by adding this material and building a composite structure I could build something a lot stronger with balsa alone. Using fiber glass cloth and resin was one consideration for this airplane but even using 0.5 oz/yard fiberglass with resin would be far more weight than I wanted to use. The target weight for this build is the major objective. I figured using 1/32 inch balsa and 0.02/oz/yard carbon fiber veil that is omni directional, I should be able to up the fuselage strength and get a nice finish, as well. Normally the plane called for a film finish but to be honest I have yet to do one, that I like how it came out. I work with fiber glass, silk, tissue, and balsa with a painted finishes but really wanted to see what could be done adding the new composite technologies into the materials I could use, and then try my own designs using these materials.

Again thanks to all who have replied, it really helped my understanding of carbon fiber and some of it uses.


edit to add:
TeamSeacats
You wouldn't happen to have a drawing of the "micro view would you?

Thanks
Paul

TeamSeacats

That gets complicated quickly as the loads start affecting each other throughout the structure. It's been about 15 years since I had any exposure to it and not only have my calculus skills rotted, I'm not sure I remember much more about how the forces fit together!

Jake

Atlanta 60

My Feedback: (18)

TeamSeacats:
Very Good Tutorial!!

iron eagel

Jake,

While it has been 15 years for you it has been 35 years for me, and that was back in the 70's.

I went to school for electrical engineering not structural or mechanical, so needless to say I did not focus that much on any of my mechanical classes, and it was a long time ago.

Your description of how the forces act together did bring back long lost memories...

Thanks again!

Paul

Trinut

My Feedback: (26)

You could think of carbon veil as millions of lillte ropes. They just pile up or buckle if you push on them (compression), but when you pull on them (tension) they can carry huge loads, but only in the direction they are pointing. So they weave them into somehting like a cargo net so they can take loads in many directions.

Now, how to hold the cargo net in alignment so it can do its job? This is the olny purpose for the epoxy. It doesn't add much stiffness by itself.

I agree that epoxy is better, but dope is lighter and IF the carbon doesn't delaminte it is a good choice for that particular application and would be just as stiff (at least untill it delaminates).

An example of this is a model covered in silk and dope. Silk is about as light an flexible as you can get, but when it is doped on to a structure and the silk fibers are put in tension by shrinking and held in alignment by the dope,
it adds a geat deal of stiffness. So much so that when you crash the model, you often still have a nice silk "bag" containing all the pieces!

iron eagel

Thanks,
I have seen the effect of carbon fiber veil, I wanted an accurate description of how it works, to reply to someone who asked the question in the kit build forum I was posting my current build in. I have wondered about the delaminte
I have carried home some silk bags in my day...

I have wondered about the delaminating issue myself but have chosen to try it and see how it will work. Apparently, it is a chosen method of finish for the control line stunt guys, and it does seem to work quite well. One of the things I find impressive about it is that it really gives you a superior looking finish.
But beyond that I really want to learn how to use the new (at least to me) composite technologies in some of my own designs.

Now one thing I have found out is that while carbon fiber is not all that strong in compression it is much stronger than many other similar materials in compression. Or have I misunderstood the properties of it.

ardrhi

(You mean Tension.)

The tensile strength of carbon fiber (even hobby grade) is rediulous.

iron eagel

Yes it is strongest in tension, to the point of absurd given how strong for a given weight of the material. But I have also learned that it is also superior in strength than certain other materials (fiberglass and kevlar as two off of the top of my head) in compression also, not that I would design something with that one aspect in mind I just found it to be an interesting aside...
The fact that this material is close to the same strength of steel given it weight is really wild. Up until recently I did not even know the process of how it is manufactured. It is still something that I look at as being: "really neat stuff" but I am starting to realize just how neat it is.
It is my understanding it has essentially twice the strength of fiberglass. And given that you can obtain the material in many different forms it gives you many options of how to incorporate it into a design. I am really intrigued with some of the material I have seen cast and machined and how these processes could be utilized in modeling. Let alone the way it is now being utilized in full scale aircraft and vehicles. The only real drawback I have seen this far is the price.

ergocentric

(did you mean ridiculous?)

don't forget the other fibres, they each have special properties
Kevlar, Nomex, Twaron, Spectra ...

iron eagel

A few questions if you would be so kind...
Could you explain some of the properties if each of the particular fibers you listed.
I assume that all of the above can replace fiberglass (in some applications) but are they lighter/stronger than fiberglass? How are they as far as price as compared to fiberglass and carbon fiber? What type of form can they be obtained in? (Mat, cloth, tissue veil, or thread) I have also read that they are micro-laminated structural sheets that have some interesting properties, along with another variation of carbon fiber called nano tubes???
Do any have of those have similar properties to carbon fiber, non conductive when used with epoxy or resin; or perhaps a light weight binder such as dope or varnish?

MTK

As mentioned earlier, carbon should be the first choice regarding composite structures for our application. Kevlar and other aramids (nomex, twaron etc) are tough like leather is tough and work nicely as skids on the bottoms of gliders for example. As far as glass goes, structural glass has the best strength to weight but still only around 70,000 psi tensile where carbon is around 200,000 for general purpose stuff. The general purpose glass we usually see and buy has lower strength than the S-glass.

Carbon adds great stiffness, as has been mentioned earlier. It's flex modulus is measured in the millions and is much higher than other reinforcing materials in that regard. Compare to kevlar and glass at around 200.000 or so.

Carbon composites sand easily and finish super smooth as glass can. Kevlar and the other aramids don't sand worth a damn.

I use 0.2oz carbon mat over balsa to produce the desired stiffness and strength. Care must be used when finishing because weight can build up fast. I apply it with nitrate dope which has great adhesive tenacity and is extremely lightweight. It seals the wood and will not allow absorption of the final finishing materials I use which are usually epoxy and polyU paints. The fill coat is usually epoxy clear mixed with large doses of stearate and microbaloons. Light and extremely easy sanding to a fine base for the finish coats

FWIW2U

MattK

MTK

Suggestion for you....Try a different prop design. The tips shown herein are way too wide and will waste a lot of engine power into tip vortex and drag. Narrow then up to half of what you have, tapered from around the 60% station out. Your thrust will improve if the largest area is around the 60% station and tapered from there. Also lower the pitch at the tip but increase it at the 60% station. The engine will run happier with a more efficient prop design.

Nice mold though. Did you machine it or cast it from ally or did you have someone else do it? My molds are made of composite materials but do want to make some molds from ally

MattK

Ed Smith

Hi Matt,

Maybe some prop information is in order.

The prop shown is one of a range meant for FAI Pylon Racing. At the time they were used the engines were tuned pipe .40 size racing engines using FAI fuel and turning up to 29,000 rpm. The prop molds were designed and cnc machined by APC.

APC has experience in these matters.

Ed S

MTK

[quote]ORIGINAL: Ed Smith

[quote]Suggestion for you....Try a different prop design. The tips shown herein are way too wide and will waste a lot of engine power into tip vortex and drag. Narrow then up to half of what you have, tapered from around the 60% station out. Your thrust will improve if the largest area is around the 60% station and tapered from there. Also lower the pitch at the tip but increase it at the 60% station.



Ed, I recognize the type of prop. You may want to look for Jack Norris on the web. His book on prop design is eye opening. His prop designs are winning large scale races by wopping margins. I learned a few things from him and am applying them in larger models than 40 sized racers.

Jack Norris is a real guy just like us. You can call him and chat and he loves to chat in the subject of interest. He was quite complimentary of APC's efforts in the larger, Pattern sized prop designs.

I didn't realize that APC (Bergdorff I think?) would cut ally molds. How much may I ask? And how did you go about asking?

MattK

Ed Smith

Hi Matt,

At the time I made those props I was asked by a member of the US F3D team to produce carbon props for him. I agreed to do so provided I was supplied with molds. That individual made an arrangement with APC to design and produce the molds. I was supplied with six molds from which I made a number of props. When the project was over I was told I could keep the molds and and use them as I wish. In actual fact no money was involved.

Ed s

daven

My Feedback: (1)

Back to the carbon veil for a moment.

Say for instance, someone laminated the complete INSIDE of a fuse with Carbon Veil, would their be issues with interference to a 2.4 ghz receiver with the antenna inside the fuse?

iron eagel

Thanks Matt for your reply.


Dave,
I have yet to find out for myself what the issues with radio's are. I have gotten this reply from someone who built a B1b totally from carbon fiber in a layup using carbon fiber mat.
http://www.rcuniverse.com/forum/m_7133321/tm.htm

I also got these responses in another forum regarding this issue:

http://www.rcuniverse.com/forum/m_76...tm.htm#7644679

I am currently doing a build of a modified Somethin Extra that I am covering the entire exterior of the plane in carbon fiber veil, I should have some hard info myself in the near future.
Right now I have gotten to the point where I am about to do the layup of .2 oz veil on the fuselage...

it is located here:
http://www.rcuniverse.com/forum/m_6641498/tm.htm

MTK

Dave,

The epoxy paint I use is KlassKote. It's great stuff and I highly recommend it.

It is possible to get interference in terms of lost frames with the 2.4 gig systems inside a carbon fuse. Several things must happen just right for that to occur but it is possible. When building the fuse you may want to take extra care and think ahead as to where and how the antennas will be mounted and pointing. If there's a clear path for signals to get to the antennas, it will work fine.

That's the reason it is recommended that 2.4 gig antennas be mounted at right angles to one another.

MattK

iron eagel

If you read this it has some interesting data about how different 2.4 Ghz rx's work that may be helpful as well.