How do you cap a rib with a carbon fiber strip? How much strength does it add? - Page 3 - RC Groups
Thread Tools
Nov 21, 2008, 07:04 PM
the lift is always in the sun!

Here goes...

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) 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 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...
Last edited by SUNSQUINT; Nov 21, 2008 at 08:35 PM.
Sign up now
to remove ads between posts
Nov 21, 2008, 07:39 PM
the lift is always in the sun!

Lessons learned...

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 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...

Last edited by SUNSQUINT; Apr 23, 2009 at 09:37 PM.
Nov 21, 2008, 07:48 PM
the lift is always in the sun!
those are some very good posts on your Bug and Gambler. Is the tail on the Gambler carbon over the foam and windowed out? 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
Our Price: 31.95

Thanks for the posts
Last edited by SUNSQUINT; Nov 21, 2008 at 07:57 PM.
Nov 22, 2008, 06:03 PM
Registered User
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? 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.

Nov 22, 2008, 09:15 PM
the lift is always in the sun!
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!
Last edited by SUNSQUINT; Nov 22, 2008 at 09:32 PM.
Apr 22, 2009, 12:59 PM
Registered User
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?

Apr 23, 2009, 09:34 PM
the lift is always in the sun!

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.

Apr 23, 2009, 10:55 PM
AKA - The "Flywheel"
StevenatorLTFO's Avatar

What did you use for the trailing edge?

Apr 23, 2009, 11:18 PM
the lift is always in the sun!
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....
May 13, 2009, 11:13 PM
Registered User
Ich Bin's Avatar
This thread has been very helpful on carbon reinforcements to plane construction. I really appreciate all the help and info shared.

Thanks again.

Aug 30, 2010, 03:38 PM
Registered User
krles's Avatar
I know, this thread is old.
But brought me some good informations.
I cyaned a roving to the ribs on my homebrew like-Ava stab and rudder. IMO not a good idea (wretched thumbs). The SUNSQUINT method seems to be better! I will try it on the wing.

Aug 30, 2010, 09:28 PM
Proud member of LISF and ESL
Interesting thread. Never saw it before. Glad it came up again.
Aug 31, 2010, 11:35 AM
the lift is always in the sun!
That's a beautiful job on the AVA style stab! It makes me consider doing one for my AVA to replace the older style....

I'm guessing that you didn't like doing the rovings or tow because it is both messy and very hard to keep flat and that right? The other thing to consider is how straight you were able to keep the stab while gluing the carbon down. If you start with a twist or bow, there is no getting it out....

So are your parts acceptable as they are, or are you going to redo them? They sure look good in the pictures. Good luck and let us know how they go.

Sep 01, 2010, 11:18 AM
volare est vivere
ray foley's Avatar
hi there from Toledo

Hey Mark

That is a great post on the c/f reinforced tail components. Could we get that put up as a "sticky"? Great stuff like that can help us all and it would not get lost, as it were.

I am now confident that I can follow your lead quite successfully, thanks so much!

ciao -rjf

ps: KRLES, beautiful stab work, really. -rjf
Sep 13, 2010, 09:23 PM
.: Looking for Thermals :.
Ricardo RW's Avatar
Hey krles,

Nice fuse, can you give us more details and pics?

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion were do you run the antenna on carbon-fiber boom bosco49 Radios 6 Jan 27, 2006 04:56 AM
How Much Do You Increase Thrust With Two Motors WillowPtarmigan Power Systems 3 Sep 30, 2003 09:03 PM
carbon elevon do you do them? gouch Foamies (Kits) 7 Aug 25, 2003 01:50 PM
How do you make decals with a computer? Andy W Scale Kit/Scratch Built 50 May 24, 2003 10:12 PM
How do you balance a PicoJet with NIMHs?? Dunc Foamies (Kits) 7 May 17, 2003 10:34 AM