Hollow CF Wing repair advice.

[DaveH]

I've got two inner panel wing LE repairs to make to a Claymore and a Kappa 3.5.  The easier (less extensively damaged) to repair is the Claymore, so I thought I'd start with that for my first hollow wing repair.       I decided to use pre-finished thin CF sheets.  A separate patch is needed for the top and bottom as it can't be persuaded to bend around the LE radius.   This also requires that the LE is shaped in filler etc.      The Claymore is nearly done, just some sanding to carry out on the LE. I'm fairly pleased with it so far.    

My plan was to strengthen and stabilize the surrounding wing surfaces, build a sub leading edge with CF tube and balsa, then to sheet the top and bottom in 0.2mm CF sheet.  Once the LE is shaped to my satisfaction, I had planned to make small hole in the bottom surface of the patch and inject expanding foam.   I had mentioned this in another thread and read that the expanding foam can distort the shape as it expands...   Does anyone have experience of this method?    

[MikeE]

The last LE / D box patch repair I did, was several years ago and it still is close to invisible. Time is a good test of the techniques used as shrinkage or movement etc is likely to start to show the repair if it's movement is not similar to the originals.

As a starting point I always try to use similar weights and qty,s of cloth, within reason of what I have available.

To hold the shape I have used several different methods over the years, from moulding the until piece over a similar shaped piece of wing panel and the inserting the new material into the wing with butt joints. A small ledge created internally first out of balsa sticks. The proved very successful and still looked good after many years.

The last one I put wettted put cloth onto the repair site. Inside the wing I put a piece of foam from a washing up pad. The outer surface was then pushed down with a plate and tape on the top surface until it was flush at all points. Again very successful. The foam was later removed by fishing it out with a hooked wire, through I hole made in the wing root. The hole was filled with end grain balsa.

But what ever you do, if the model flies against then repairing can be a  experience worthwhile experience.

[DaveH]

That's interesting MikeE,  lots of different techniques.   

Your last repair of, I guess, using external parts to make a mould, then applying wet cloth over a sponge, later removing the sponge will leave you with a thin skin.  That's pretty much what i have ended up with, although it looks like a stuck-on patch (which is what it is).  It's not a pristine model, so i don't really mind what it looks like.    

Now that the LE is shaped (I just bought one of those palm sanders for this job, my wife was quite hopeful that some decorating was about to take place ), it looks good, I do not have to inject foam, but had planned to so in order to "practice" for the bigger repair.....    maybe I'll leave it as is.       

[MikeE]

I actually used a several pieces of thin cloth cut so the weave was layed at different angles, wetted out and layed as one. so that the finished cloth was as close as possible to the original in its feel and strength. 

The external part was a piece of very thin epoxy board held in place and shaped by the existing wing structure on each side of the break. The foam simply pushed back against this. Very simple.

[DaveH]

I had a go at filling the void between the repair sheeting on my more damaged wing.  It didn't go as expected, although I am pleased with the results (eventually).   I assume that the foam needs something to allow it to cure, moisture or some air, neither of which were available inside the airtight void of the wing.   It didn't set or expand as I had hoped, but it did leave a thick coating of foamy skin on the inside top surface, and (I assume) the bottom surface.     

I appear to have got away with it.. The repair is very acceptable and probably better looking than if i had repaired it the usual way.    

Since I finished, i found that there is some expanding foam available that has an expansion of only 5% over its extruded volume.  Maybe next time. 

 

 

[EssexBOF]

I have done some of this type of repair see https://www.barcs.co.uk/forums/topic/11172-tarannis-x9d-radio-failure/

I used pink foam to fill the void then a layer of CF Multi directional over the top.

If I had to do it again, I would tack some light 1/16 balsa sheet onto the inside of the wing, with it projecting into the area to fill, approx 4/5mm. Then cut another piece of balsa to fit the shape of area to fill. The thickness of the sheet needs to be just under the surface of the original wing. Add a false L E from balsa, to follow the plan form.  Then lay a carbon cloth light weight covering epoxied to the sheet, top & bottom. Fill any area to achieve a match to surrounding areas. 

 

[DaveH]

On my more damaged wing, I believe that the damage was made much worse as the two interlocked planes hit the ground, compacting the X formation of the wings and crushing my wing badly on the top of one side of the impact and on the bottom of the other side.   It was all very creased and flexible, but too much to cut out, I thought.  These areas had a 1.5mm balsa doubler fitted to cover the crushed area.  I guess that it might not have been too much work to extend the balsa across the open area and fill from there,  maybe next time.