How do you cap a rib with a carbon fiber strip? How much strength does it add?

[Accu157]

It's something that has intrigued me for some time, and it's something I might try if it's useful enough, and cost worthy, but, how do you do it? Where do you get the strips for it?

[Barnsey]

I've only seen carbon cap strips used in conjunction with carbon trailing edges, kevlar thread-wrapped carbon/balsa shear web/carbon spars and carbon D boxes, a technique pioneered and used by free-flight flyers for many years.

The carbon cap strips link the leading edge assembly with the carbon trailing edge, resulting in a structure of great lightness but immense strength, especially torsional rigidity. The cap strips themselves are simply cut from sheets of carbon of the right thickness.

I once had the pleasure of spending a weekend with Mike Woodhouse, a free-flight flyer at WC level, to see first-hand just how these structures are fabricated. Mike showed me how I could make a replacement T tail stabiliser/elevator assembly for a model I had which needed substantial nose weight. The foam/glass/veneer original weighed 80g and was pretty torsionally rigid. Mike's sample weighted 5g before covering and was bulletproof. One of the most worthwhile 1800km journeys I've ever undertaken :D.

Mike sells all the products needed for such structures from this Aladin's Cave (http://www.freeflightsupplies.co.uk/).

John

[Ollie]

If the balsa rib has medium density, it has compression strength of 1700 pounds per square inch (PSI) along the grain and 300 PSI cross grain. However, the tension strength is about two times more.

The strength of unidirectional carbon with epoxy depends on the layup process, wet with tow, wet with Uni-web, prepreg, compressed (or not), Pultruded Carbon, and Modulus (standard, medium of high). So, the carbon compression strength can vary from ~75,000 PSI to over 300,000 PSI. With very, very small cross section area (skinny) carbon compared to its length, the problem is buckling far before it uses its compression strength. So the skinny carbon needs its support to prevent bucking.

The team work with balsa and carbon makes a strong, stiff and light structure such as rib.

I suggest use manufactured carbon prepreg only 0.007" sheet and strip it with the rib thickness (1/16 or 3/32 inch) depending the size of the rib and spacing between ribs. Glue the balsa to carbon strip with epoxy or thin CA. See:
http://www.deltronix.net/cgi/acp_display.exe
Or
http://www.cstsales.com/carbon_fiber_uni_laminates.html#925

Prepreg carbon sheet 0.007" x 1/16" has a compression strength of about 95 pounds!!!

[Mark Miller]

One of my Soprano wings came with a slight twist in it. I tried to take the wist out by the usual heating the covering method but it would not stay. The only way was to carefully strip off the carbon rib cap from one side of the rib and weight the wing down to a flat surface. I then CA'd the cap back on and it is still straight and in use today. It was amazing how much torsional strength was on that wing panel using so little carbon.

Mark

[Accu157]

Thanks guys! Sounds like a done deal to me. :D

Ah, a related question.

None of the planes I've seen using this design had a hollowed-out rib. Is there some engineering reason why I shouldn't hollow it out when using a carbon rib cap?

[oakman7004]

Thanks guys! Sounds like a done deal to me. :D

Ah, a related question.

None of the planes I've seen using this design had a hollowed-out rib. Is there some engineering reason why I shouldn't hollow it out when using a carbon rib cap?

The reason is probably the time, needed to hollow out the rib. If you would have laser cut ribs them mabye you would program that process.

And the effect(weight saving) is probably not that much, in a 100-150" design...

It's just not worth it!

cheers Jonas Ekman

[Ollie]

"Is there some engineering reason why I shouldn't hollow it out when using a carbon rib cap?"

You don't get it? Read and understand post #3 again.

With very, very small cross section area (skinny) carbon compared to its length, the problem is BUCKING far before it uses its compression strength. So the skinny carbon needs its SUPPORT to prevent BUCKING. If you take away the SUPPORT (like balsa or rigid foam) the skinny carbon doesn't work to resist the compression load.

How much SUPPORT? It depends on how much you hollow out the ribs. That's why you test the sample or you learn to apply engneering math. If you want specific answers, then you must give specific numbers like specific dimensions (thickness, shape, height and length) and materials (balsa density, which epoxy and which carbon).

[Accu157]

I guess there is such a thing as a stupid question? I understand it now.

[bwalt822]

That failure mode is called buckling BTW. When I see it in caps a few times I feel the need to correct.

[nuevo]

Couldn't resist that urge, huh? :rolleyes: From 2 years ago, no less.

[bwalt822]

Crap, I didn't see the date and there was no way I was that deep into this forum. I hate it when people do that. Oh wells. I swear this thread popped up next to the forum title.

[nuevo]

sorry, no ill will. I just thought it was funny.

[T.D.]

This reminds me of how much Ollie is missed around here.


T.D.

[schrederman]

Yep, we miss Ollie. This is still an interesting read. From what I heard about the process during manufacture of these models... They make a balsa block, wide enough to make say, 100 ribs, with a CNC router that is the shape of the rib they need... say R-14... They use a sheet of cf the same size and glue it to the block, top and bottom. Then they slice the ribs off, ready to use. Now that have 50 sets of R-14 for their tapered wing. No tedious gluing of tiny strips to 3/32" ribs.

Jack

[ozmo01]

Well I learned quite a bit reading this ...soooo THANKS :D

[ChuckA]

It's always good to bring back good threads like this one.

Chuck

[Mark Miller]

I have seen it done both ways. If you look at a Topaz for example the ribs are certainly cut from a block with carbon bonded to the top and bottom. The Soprano was not. They were individually put on the rib after the rib was put into the wing. You could see where if the carbon and rib had any difference in the thickness from the saw it was the same for both. On the Soprano usually the carbon was slightly wider than the capping.

BTW...I see the last activity for Ollie was Oct. 26th 2008 so at least he is still reading. Hi Ollie, we miss you very much.

Mark Miller

[dharban]

I have had good success in making carbon caps by taking .03 inch unidirectional carbon composite (the stuff that is already epoxy impregnated and cured), covering it with masking tape and then cutting it into rib width strips. You can then use medium to thick CA to glue to the rib and then peel the masking tape off for a clean, smooth finish.

Happy Flying,

Don

[JimHSoars]

Don.

So unidirectional carbon composite, even though it is pre-impregnated with epoxy, is flexible enough to follow the curve of a rib?

Jim

[rdeis]

If it's thin enough, yes.

I use 1mm round stick and 1mmx3mm stick on curves all the time in my foamies, and that's much thicker than the stuff that's been discussed in the thread so far.

[dharban]

Don.

So unidirectional carbon composite, even though it is pre-impregnated with epoxy, is flexible enough to follow the curve of a rib?

Jim

No problem at all. The product I have used is from Aerospace Composites at this site:

http://www.acp-composites.com/acp-ucfl.htm

I have used .007" (CL1-36 .007"x6"x36" typ.) and .003" (CLX-36 .003"x6"x36" typ.)

Happy Landings,

Don

[airporter]

To make things even easier go to the new owners of Peck Polymers (A2Z) and look up carbon pultrusions under 'Model Airplane Items'. Near the bottom of the page you'll find two widths of carbon that are ideal for capping ribs and for laminating other structures: 1.5 x 0.12mm, 2.0 x 0.13mm and 3.0 x 0.13mm.

http://www.peck-polymers.com/store

I've also included a picture of a much modified Red Herring I recently completed. The cores are 1.9# EPP. In this case the ribs are capped with the 3.0 x 0.13mm material. The rudder is also laminated using the 1.5 x 0.12mm carbon. Other sizes of carbon are used for the ribbon main spar and the trailing edge stiffener.

I've also built a Bug and a Gambler AG using this technique. If there's interest I can take and post pictures of these planes.

airporter

[JimHSoars]

airporter,

Please post the pics of the Bug and Gambler. Especially during the building phase with no covering.

Jim

[SUNSQUINT]

subscribing to this thread...

I have also been playing around with carbon caps-- both on new built-up wings and repair of topaz wings. Since I have a couple thousand feet of 3k and 6k tow, I'm thinking of pulling the tow through an epoxy bath and roller-set to lay down on the ribs.I would like to build a Bubble dancer and cap it.;)

Has anybody had any luck with this method?



I have also been looking for a thread on repairing the all pultruded trailing edge on the Topaz I inherited. Is there one on this forum? It is the same construction as the AVA's and others.It looks like I will have to de-bond the entire trailing edge and replace it instead of repairing the 3 fractures....:eek:

[pmccleave]

Guys,

I got a chance to look at Jim's Bug and Gambler up close and personal last weekend and I can tell you right now, he has the process figured out. Both planes were as nice or actually nicer than any of the AVA's, Topaz's or Sopranos I have seen. Of course Jim's planes are the standard in which I continually strive for in all of my builds. I have not gotten there yet but with Jim's encouragement and occasional constructive criticism I am getting better with every build.

See Ya,

Pat McCleave
Wichita KS

[tonyestep]

"...repairing the all pultruded trailing edge on the Topaz..."
-------------
Well, I don't know how serious your damage is, but here's how I fixed the t.e. on an Ava.

Get some stiff flat carbon strip material. CST and ACP have it, but for this purpose you can also use the stuff sold at your LHS. There's some that's about 2 mm x 1/2 mm, which is great for this purpose -- stiff as hell and easy to work with. It's much more stout than the Topaz or Ava t.e., but that's fine.

Take off enough covering so that you have a clear shot at everything. If the t.e. is just cracked and the ribs are intact, there's very little to do; just splint the breaks with a 1" length of your repair carbon, held on with CA.

If the ribs are broken, carefully take everything apart, saving all the pieces. Take the caps off the rib, leaving them stuck to the D-box. Then rebuild the ribs, filling in missing pieces of wood as necessary. Don't worry if it's flimsy. To make it strong again, make a rib doubler just slightly smaller and glue alongside the repaired rib.

Next fix up the t.e., splinting everywhere it needs it. Try to get it nice and straight, but don't worry if it's a little wavy.

Finally, glue the caps and ribs back to the t.e. Clamp the t.e. straight as you do.

When it's all done, you'll have added only a few grams of material, and the ribs and t.e. will be stronger than before.

Re-cover it and you're done. The repairs will be hardly visible through the covering, and if you do a neat job of replacing the cut-away covering, the whole thing will be unnoticeable.

[SUNSQUINT]

Thanks tonyestep.
I did uncover well beyond the break sand CA'd the TE at the three snapped places. I haven't splinted yet. A couple ribs were busted up and I tried to puzzle them together in place. The best I could do didn't get me back to the original foil shape.I think your idea of pulling the ribs out and leaving the cap strips in place may work well...that would allow me to cut new ribs altogether.

I'll try the splints on the TE and see how they come out. I tend to try to do repairs to the extent of being undetectable ( yes, a little anal...).

I bought/inherited the Topaz after the owner managed to do a snap-roll followed by a lawn dart while still on the winch...:eek::censored:. Nice trick.:cool: The pod is gone and the TE was damaged, but the rest is in nearly new shape.

This thread on cap strips is something I have been toying with and I think I will try a few plain ribs this weekend. The original cap strip method on the Topaz is exactly as Mark Miller said in post 17. Carbon uni laminated to a block of rib stock and then sawed into sliced ribs.

The AVA's, Graphites, Pulsars, and I think Organics I have checked out appear to have the caps added individually. It is probably an acceptible method for the single model, but I think these models in production must have some fixturing involved to make it feasible.

I think I can lay down some wet tow and get a good, flat cap strip...we'll see.

Mark

[SUNSQUINT]

airporter, I too would very much like to see the pics of your Bug and Gambler!!

[airporter]

Sorry for the slow response, but a bout of the flu and someone deciding that one bounce was enough to stop me from receiving emails from this thread slowed things down.

I'll describe the Bug in this response and the Gambler in the next.

The plan view shows the addition of the carbon capstrips and the half ribs. If you have the full ribs cut and glued to the spar the half ribs are easily made by fitting rectangular blanks between the full ribs. Using the carbon caps as a sanding guide you can quickly reduce the half ribs to the right shape. This view also shows the carbon glued to the front side of the trailing edge.

The fuselage is 1/16" balsa with 0.005" carbon between the 1/16" balsa doublers in both the nose and wing openings. I also added 1/4" triangular stock to the bottom corners of the fuselage for strength and shape. The fuselage was glassed with 2 oz/ft² glass cloth applied and filled with thin CA. Paint is automotive lacquer from a spray can. The canopy is retained with a pin in the front and a magnet at the rear.

The wingtip views show in a bit more detail the rib capstrips and the reinforcing of the wingtip for the launch blade. Note that there is a strip of carbon that is laminated around inside the cutout of the wingtip. Also, the launch blade area is reinforced with two layers of 0.005' carbon, top and bottom. And although this may be overkill, the launch blade is tied to the carbon tube spar with a piece of 1/32" piano wire that wraps around the launch blade and extends about 1/2" inside the spar.

The receiver is an AR6300 with two D-44 servos and a 4 x 160 NiMH battery. Any questions about receiver range were answered on the third or fourth flight when I specked the Bug in a thermal. I've also flown the Bug a lot on the slope on lighter days and have managed to go from one end of the Rock Pile to the other, about 1,000' each way, with no control problems. The pushrods are 0.025" carbon in stretched, bondable teflon tubing. I use a large pin bent 90º for the servo end of the pushrod and a homemade piano wire clevis for the control surface end. The connection between the carbon and the end pieces is accomplished with heat shrink and CA.


It's not real obvious, but the cutout in the horizontal stabilizer is lined with a piece of 1.5 x 0.12mm carbon ribbon.

This is the second Bug I've built. The first one used the standard ribs and flew quite nicely, but I felt it could use a thinner airfoil, so with this one I thinned the airfoil to 7%. When I've flown it against the first one I found that I gained a little launch height and gave up very little if anything in hang time.

This version of the Bug weighs 103 grams or 3.6 ounces giving a 3.2 oz/ft² wing loading.

Perhaps a note of interest; the UltraCote Light covering added 8 grams to the wing.

I do use a launch switch that adds a tiny bit of down elevator and a noticible amount of right rudder.

[airporter]

The Gambler was selected by the Minneapolis club as a Winter project last year. I don't know how many finally flew, but I enjoyed the build. As usual I could not leave the kit in it's original from. I did NOT modify any of the aerodymanics, but I DID modify all three major areas of the structure.

The fuselage is 1/16" balsa with 0.005" carbon between the 1/16" balsa doublers in both the nose and wing openings. I also added 1/4" triangular stock to the bottom corners of the fuselage for strength and shape. The fuselage was glassed with 2 oz/ft² glass cloth applied and filled with thin CA. Paint is automotive lacquer from a spray can. The canopy is retained with a pin in the front and a magnet at the rear.

I could not bring myself to laminate the carbon tow to the outside of the structure so I substituted precured carbon strips in all the locations that called for the carbon tow.

In the wing I placed 0.005" x 0.187" carbon between the D-tube sheeting and the spar web, top and bottom. Although the building sequence gets a bit tricky, I allowed the carbon strips from the tips and center section to overlap for stronger polyhedral joints. These spar reinforcements also extend out to the end of the wingtip so the launch blade is also tied into the spar. I also replaced the hardwood leading edge dowel with a carbon tube. Please note the 0.5 x 3mm carbon reinforcement glued to the front side of the trailing edge. When the carbon capstrips overlap the trailing edge it's surprising how much torsional rigidity is added.

The wingtip views show in a bit more detail the rib capstrips and the reinforcing of the wingtip for the launch blade. Note that there is a strip of carbon that is laminated around the outside of the wingtip. Also, the launch blade area is reinforced with two layers of 0.005' carbon, top and bottom.

You'll note that there is a lot of empty area in the nose. Originally I used a Berg 6 receiver with a full length, but light wire antenna. After my experience with the Bug I changed to the AR6300. This requred moving the receiver under the wing to acheive the balance point I wanted. I again used two D-44 servos but used a 4 x 120TA NiCad battery. The pushrods are 0.025" carbon in stretched, bondable teflon tubing. I use a large pin bent 90º for the servo end of the pushrod and a homemade piano wire clevis for the control surface end. The connection between the carbon and the end pieces is accomplished with heat shrink and CA.

The tail surface treatment is a bit much, but I think it looks cool and fits with the style of the rest of the structure. Both tail surfaces start with the stock parts. First I laminated 0.13 x 1.5 mm carbon, followed by two laminations of 1/32" balsa, another layer of carbon and finally two more layers of balsa. This is all done with thin CA. Round off the leading edges. Next add the 0.005" x 0.187" hinge line reinforcement to both surfaces, top and bottom. Now glue the 0.13 x 1.5 mm bracing to one side of each surface. The second side is aligned by pushing a pin through the balsa alongside each end of the upper strip. Now glue the lower strip next to the pins. When you have all the braces added take a new #11 blade and rough cut all the openings. With various shapes of flat and round sanding strips smooth the openings to a neat appearance.

This version of the Gambler weighs 173 grams or 6.1 ounces giving a 3.3 oz/ft² wing loading.

Perhaps a note of interest; the UltraCote Light covering added 15 grams to the wing while the glass finish added 8 grams to the fuselage.

I do use a launch switch that adds a tiny bit of down elevator and a noticible amount of right rudder.

Were I to build another Gambler I'd probably thin the airfoil like I did on the Bug.

I've undoubtedly forgotten something, but ask away.

airporter

[SUNSQUINT]

Sorry it took so long to get this posted. Work got in the way again.

I posted some pics of a tail I designed last week on Schrederman's thread (here (http://www.rcgroups.com/forums/showthread.php?t=951820&page=4)) on the *&^$^@# design process and used it to try out the carbon cap strips.

I was all set to do a wet bath roller system for pulling tow, but ran across a Dave Brown Carbon fiber strip product at my LHS....so i gave it a try since it was something that would be much more available to the average guy. It is Part # CFSP-5230.

The strip is a pre-cured ribbon of unidirectional fibers (just called uni) and measures .007" x .500" x 66". It was $7.50 for the package and is enough to conceivably do an entire model. It proved to be good quality and easier to work with than I expected:).


Here are some pics of the Process...

[SUNSQUINT]

Here's what I found...

the fit of the wood-work needs to be gap-free. Specifically the tops of the ribs must be flush with the leading and trailing edges. You don't want the carbon bridging over gaps.
when choosing where to apply the carbon, consider that it offers most strength in tension.
the main strength gain is by tying the leading and trailing edges to the rib with an overlap of at least 1/16".
there is less waste of the ribbon if it is split in long lengths and glue and trim as you go.
Medium thick CA is better than super thin. This gives just a little more working time, and also a better bond. (the pics show me using a blue label which is thin, but somebody:mad: had left it out, and it was a little thick. It turned out to be a good thing....)
Would I do it again?... Yes!
For the whole tail assembly, adding the carbon was maybe an extra 1/2 hour. It will be less the next time. The finished product is every bit as good as what's on my Topaz, Pulsar, or AVA.

One thing I did forget to do however, is get a before and after weight.
The tail is still quite light and very strong, so I guess I don't care. I've been already asked to make 3 more sets of the same, so I can get more details if needed.

hope this helps...

Mark

[SUNSQUINT]

airporter,
those are some very good posts on your Bug and Gambler. Is the tail on the Gambler carbon over the foam and windowed out?:cool: How stiff is the whole arrangement? It looks super light. I may have to try it myself...

looking at the link to Peck Polymers, their strip is probably the way to go for a larger project...I think I'll get some.

Item Code: Strip1_5/20
DPP Pultrusion Strip 1.5 x 0.12 mm x 20m Roll
Carbon fiber/epoxy pultrusion. Strip material for rib cap stripping and genral reinforcement. Sold in 20 meter rolls

Get more details on this product from the manufacturer www.DPP-Pultrusion.com (http://www.dpp-pultrusion.com/)
Our Price: 31.95


Thanks for the posts:D
Mark

[airporter]

[QUOTE=SUNSQUINT]airporter,
those are some very good posts on your Bug and Gambler.
Thank you.

Is the tail on the Gambler carbon over the foam and windowed out?:cool: How stiff is the whole arrangement? It looks super light. I may have to try it myself...

No, the carbon is over balsa. It is very stiff and quite light, even when using contest grade balsa to start with.

I must disagree with your comment that the carbon is only good in tension. As long as the carbon is held straight it adds a lot of compressive strength. The 'hold straight' part is the key. When you glue thin carbon to anything you must be sure and glue the whole length of carbon. If not held along the full length this will show up when you cover the part. As the carbon is placed under compression it will bow up away from the part - VERY irritating when you're covering the second side of the surface when it happens.

I'm impressed with what you've done with your parts.

Porter

[SUNSQUINT]

Thanks airporter,

I think you are right and I should have tempered my statement you commented on... I meant for our purposes of capping ribs and strengthening light weight framework, the design process should be focused on the carbon strips being used in tension and less in compression. While carbon does have good compressive strength, it does not compare to the tensile strength. The delamination at the glue joint and off-axis buckling you mentioned are key...

If there is an occasion where the carbon can be placed in a straight line and with a good continuous glue joint, there is good value. These kinds of places occur, for example, on the tops and bottoms of spars, the opposing sides of vertical members of fins and others. These see bending loads. While being bent, one side sees tension and the other see compression.

In the case of spars, it should also be noted that carbon on the bottom usually sees tensile loads while the top sees compression loads. A well designed spar will have more carbon on top to make up for the lower yield in compression.Look to the Bubble Dancer and others...
It does seem a little counter-intuitive. Maybe the analogy of pushing a rope versus pulling a rope will help.

Another thing to consider is that carbon is still heavier than balsa. Balsa's compressive strength to weight ratio in the grain direction is hard to beat. It is still is a desirable material for shear and compression webs in both models and full size aircraft.

The overlapping of the strips onto the neighboring structural member is useful in reinforcing and following the load paths as well.

That all being said, these little strips of .005"-.007" are a very effective way of improving torsional, bending, and tensile strength to otherwise fragile frame work. This can be done with very little weight penalty and even result in weight savings if designed properly.

Now go out and try it...have some fun!

[WEB01]

I am resurrecting this thread with a question. I am completing the construction of a modified Majestic where I have followed Dr. Drela's recomendation of stretching the V-tail by 4 inches on each side simply by spreading the ribs. I showed the V-tail to my club members and, while some thought it would be fine once covered... several thought that it was dangerously lacking in torsional rigidity. The methods described here were one suggestion on how to improve this situation. My question is that Sunsquint has suggested that the most strength is added by tying the rib caps into the leading and trailing edges. I noticed that he has a balsa/carbon laminated leading edge in his example and since my leading edge is just balsa... is capping the ribs going to be enough to improve my torsional rigidity?

Ward

[SUNSQUINT]

WEB01,

The torsion strength is much improved with carbon on the leading and trailing edges, especially when the carbon ribbon or flat strip is orientated like in the leading edge in my pics (perpendicular to the plane of the stab). It helps with both bending and torsion. If your stabs are not yet built, this is definately the way to go. you only need about 1-2 layers of .007" strip to make a big difference.

There is still some torsional improvement by just capping the outsides of the LE's and TE's parallel to the stab, but not quite as much. The improvement here is more in bending. If your stabs arer already built, this would be the way to go.

Capping the ribs adds significant strength to the overall structure, especially with the tie-in overlap.

Covering does offer torsional strength, but it does matter which film you use. Ultracoat is good, but Monokote is better from a strength perspective. More rigid.

I hope this helps....while you are at it, why not take some pics and post your findings to the thread? Though I didn't start the thread, I would like see it continue.

Mark

[StevenatorLTFO]

Mark,

What did you use for the trailing edge?

Steve

[SUNSQUINT]

Steve,
I used some carbon rectangular stock measuring .100" x .045". I don't quite remember where I got it, but I think it came from the Midwest display case in most LHS's. This tail group was for a 2 meter plane of my design. I usually buy lots of sizes in bulk for whatever military UAV projects come around....

[Ich Bin]

This thread has been very helpful on carbon reinforcements to plane construction. I really appreciate all the help and info shared.

Thanks again.

E