Just went through this see this thread over on Stuka..

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=323579&mesg_id=323579&page=

My solution doesn't have the rock hard torsional rigidity of Al Rabe's but it was easy to do, the materials were easy to get, they are as straight as an arrow and allot stiffer than they would have been with only wood. I am covering them with .2 ounce carbon matt applied with finishing resin thinned a little with acetone. Have one side curing as I type this, in a couple days both sides will be covered and cured enough to handle.. I fully expect them to be the straightest, stiffest flaps I've ever used. When they are finished I'll weight them and report back..

Rather than mess with CF cloth, epoxy, and the like, I've simply double-covered my built-up flaps with silkspan.  What we want is monocoque-type stiffness, which you get easily with silkspan and Brodak paints.

Floyd in OR

The problem I've always had with just silkspan and dope is keeping them straight. If you don't apply dope exactly the same on both sides you have a good chance of ending up with a banana. This is the reason I used epoxy to apply the CF instead of dope.

If the flap is straight to begin with, and if you use a product that does not shrink then it should be straight when its dry. hence epoxy, Ok so lets see what the problem really is, the problem is that we want stiffer flaps with no weight. Ok so solutions, glass fiber and epoxy. can be very heavy. and likely overkill. How about carbon mat, or veil, well since the fibers are non directional, you gain only a small amount of strenght from them. especially if you use dope to adhere it you wont gain much more than silkspan though you may think you do because its "carbon fiber" .  SO you are adding a fair amount of weight with marginal strenght increase. To maximize the strength, you need to align the strength of the material with the lines of stress. So, then we have to decide what are we trying to accomplish. well longitudinal rigidity isnt really all that important since we use enough hinges to keep it from flexing tha tway. In fact, i challange that making the flap to rigid from tip to root is a bad thing, especially if any flex is present in your wing, which I would imagine is likely in most wings under high load. Ok so we need to eliiminate the torsional twist.
this means that the fibers in the matrix to most maximize their strenght to weight ration shouls be on a diagonal bias. fibers aligned any way but diagonally are not helping to add to our solution they are only addign weight, and soaking up resin . so, align your cloth on a bias, use .75 oz cloth and only enough resin to bond it effectifly. the optimal ratio of cloth to resin is one to one. This means for every square yard of .75 oz cloth, there should be .74 oz of resin. Now in a hand layed matrix this is virtually impossible to obtain so we are going to have a bit of excess weight,but not much. there is another option. One thing that is lacking with Nitrate or dope, is a good matrix bond and the contiual shrinkage that occurs with dope. Howa bout if we used another more easily controlled catalyzed product. One thing I have been experimenting with is using urethane clear as a bonding matrix. It has some very good properties. It is catalyzed and does not shrink further once cured. it si easy to flow onto a surface. Might be worth considering. It will eliminate the problem of uneven shrinkage as well since it doesnt shrink. You may actually consider using it with silkspan or CF veil. Just remember that you wont get as much strenght with veil as you would with FG cloth. but it will also be potentially lighter.

Greg,
It sounds like your flaps are flat with no taper? If you glass tapered flaps and then weight them on a table they will have a twist in them when they are cured. Just wanted to make sure that was clear lest someone try this with there tapered flaps and end up with a trim problem.

Am I the only one that drills holes in the flap and glues toothpicks in to stiffen them up? I do it on profile fuses too but use either dowels, K&S hollow alum tube or bbq skewers.

I'll offer this because it was such a surprise to me recently. I am building a nobler. I planned on carbon veil covering the flaps. I apply it to nitrate doped surface (3 coats or until grain is no longer raised) with Nitrate dope thinner dabbed thru the veil. Thanks to Bob McDonald for the technique. Anyway, I get really cheesed off at banana shapped flaps and had resigned myself to making another to replace the banana I had. But I thought I'd try something different that absolutely never worked with water or any other method I have ever used to remove a warp. It did work here and extremely well.

I applied the veil to both sides as close time wise as I could. Once I had it down with the thinner I applied a coat of dope... no waiting. Then I sat it on my glass building table, cupped side up, and sat a quart paint can on top of the cup and let it set overnight. Now trying to remove a warp like that with water or even just weight as you know normally does not work at all. I was amazed the next evening to find the thing perfectly straight! Flap is in its second coat of silver awaiting another wet sanding and still straight.

bob branch

Wayne

Nope. Quick and dirty. I knew it wasn't going to work anyway. After 3 coats of clear and light sanding there was no vistage of the can remaining. It just worked. I don't like to mix wax into any layer that is going to be finished. Too much risk of contamination down the road. After dealing with Amine blush in boat building, its an issue I am always aware of. Everything seems to seperate off glass very easily.

bob

OK, I build extra rigid flaps with carbon torque tubes for the leading edges,.  This reply, however is to offer a bit of new experience that I had with my new Mustang flaps.  After installing the torque tubes I usually add a couple of layers of light glass and Zap epoxy resin, thinned one "A", one "b", and one part acetone.  Pairs of flaps usually weigh about 5 ounces.  I don't worry much about weight as, in my opinion, weight added to flaps results in an airplane's ability to carry much more weight through competitive corners than flap weight added.

This time, however, I made up some test panels using thinned Zap epoxy and some with thinned clear Klasskoat epoxy paint.  Some panels had talc added for effect.  I was surprised at the stiffness obtained with the Zap epoxy thinned 100% with acetone.  I half expected it not to cure fully.  It not only cured and sands well but the extra thin mixture penetrated deep into the wood and gave remarkable rigidity.  My new flaps, with CF tubes are amazingly rigid with only one layer of glass and 100% thinned Zap, enough so that I'm stopping with only one layer of glass and saving nearly 1 1/2 ounce over my typical flap pair weight.  My mixture here is one "A", one "b" and two parts Zap finishing resin.  I didn't like the thinned Zap with talc added.  It seems the epoxy mostly penetrated the wood leaving the talc on the surface with something like a sandpaper texture, hard and difficult to sand.

I also liked the Klassskoat epoxy paint mixture of one "clear", one "hardener", one talc, and two Parts Klasskoat thinner.  This mixture went on well by brush and sands very well.  I used it on bare wood aft of the wing to save finishing weight.  With an epoxy hardened substrate on my Mustang, now I'll will be using Acrylic primer surfacer.

Al
 

I w

1/2 ounce.

Al