I have an older BVM Balsa Bandit. I recently flew this jet at one of the local jet events. After landing the jet I noticed some significant stress cracks in both wings.. Just wondering if anyone else has experienced this?? I'v removed sections of the bottom wing skins and added two carbon fibre spars to strengthen the wings.. I priced a new set of wings but could not justify spending the money on a jet of this age as it was around $1250 plus shipping for a set of composite wings. I figured I was better off buying a new Bandit..

If you can glue end grain balsa between ribs and to new spar that will give you more strength. Looking at the picture of your repair it seems like the breakage is above the landing gear area. And maybe caused from some rough field landing. I have had experience with that on my buddies. Dan Avilla BVM Rep

I have a Balsa bandit and had the same issue until I found out the cause. In my case it was because of rough landings and rough runway. What caused it in my case was the retract flex plates flexing (doing their designed job) but they were coming in contact with a small section of the upper skin rib that was not removed or lowered enough to allow the flex plate to flex without making contact. When the flex plate flex's aft enough the gear is swinging upward and contacts this small section of rib ( the section of rib that is partially removed under the flex plates) After grinding down this rib a bit more that solved the problem. I also doubled up my flex plates and have not had an issue since.

Kelly has the correct answer, and on the BVM website there is a tutorial for the cracked wing skin fix. No need to remove the skin either. Your addition of the carbon blade in the leading edge "D Tube" area isn't the solution to the problem, either.
Extra weight forward of the problem area won't help the flex plate from cracking the skin.
You did not mention whether you have a Balsa Bandit or a Super Balsa Bandit...Both are older.....I will assume it is the original version, which I always refer to as the "Classic Bandit", as you can see the crack in the photos.

When the "Super Balsa Bandit" was introduced, a optional construction step was added to strengthen the upper wing skin to prevent cracking during hard landings. Page 18 of the manual shows the optional text concerning the addition of a piece of 6 ounce fiberglass cloth immediately above the retract unit. The "Classic Bandit" manual does not contain this addition.

When I built my last Super Balsa Bandit, (Which I still have, updated with composite wings now) I vacuum bagged a square of carbon fiber cloth to the inside of the skin before construction was started. This really helps.
Your second photo shows the crack above the gear unit without the glass inside patch. I recommend adding some 6 ounce cloth or a patch of 5.7 ounce carbon fiber cloth to stiffen the area above the gear unit. Go from rib to rib, forward to the main spar, and aft to the aft spar if possible. Apply the cloth to the length of the crack. Adhere it to the skin with epoxy resin.

As for the top surface, please follow the link to see the method we recommend for patching a crack on the outside of the wing skin. The link explains all of the steps necessary to accomplish the task. The method includes all wings, balsa and composite.

http://www.bvmjets.com/Pages/skin_patching.htm

I hope this helps.

Not to pile on here either, but unless you tied the carbon spars you added to more than just the wing ribs, you really haven't added any strength to the wing at all.

One step you might consider is calling BVM and asking if they have a Balsa Bandit wing kit laying around that wasn't sold. It looks like they've dropped the BB as a kit option but they may still have parts in inventory.

I concur with Harley and Doug and have experienced the exact same damage with my Classic Balsa Bandit after a less than perfect landing. A long crack in the upper surface of the wing skin and one of the recessed ribs in the UC bay had also split. The flex plates themselves were still in one piece but had started to de laminate. There was enough movement to do this damage.

I repaired it as per Harleys suggestion. The top skin was simply CAed together and repainted. I did add some carbon tows to the recessed rib, in an attempt to get some fore/aft strength.

One tip I can give is to regularly inspect your flex plates, especially after a good bounce. As I said, they can partially de laminate without breaking. I have had this happen and not bothered to replace them. On the next flight a nice gentle landing has caused a complete failure of the plates, with resulting damage to the upper wing skin.

BTW, landing a Bandit is pretty easy, but there is a technique that you need to stick too. It took me little while to master it, but now have no more trouble with damaged gear.

Thanks for the responses gents. Much appreciated.. I haven't finish the front spars as yet.. I have done some carbon reinforcing to the top skin of the wing in the undercarriage areas.. Stared re-sheeting the areas that were removed..

That carbon cloth on the skin will keep the skin from cracking.
Now to get the balance of the skin glued down, faired, glassed and painted. Get a nice piece of .010 Poly Ply on the outer skin as shown in the "Skin Patching" tutorial....

I've decided to give the trusty Balsa Bandit a full makeover .. Here is a couple of pics to show the progress..

Done some strengthening around the inlets and also glassed on the elevators as I wasn't to happy with the way they were mounted..

Bit more progress on the Banit refurb.. Both stabs fully glassed on and pretty much finished. Inlets removed, cleaned up and re-glued back in place and now working on the wing fitment to the fuselage. Does anyone have any pics of how the Banidt doors mounts etc??

I also picked up a Bals Bobcat which I will refurbish at some stage to.. Too many projects!!