Guys,
My Dr1 developed a hairline crack at the juncture where the lower wing butts up to the fuselage cradle. It was apparent because I noticed the monokote was wrinkled, as if I had overtightened the wing into the mounting bolts. Upon inspection I found the trailing edge closest to the fuse was loose under the covering. When I stripped it back to do the necessary repair I found that there was only soft balsa spars where you would normally find hardwood. The smaller spars were cracked and as stated the thin balsa sheeting had a hairline fracture running about 1/2 the width of the wing. I CA'd everything and cap stripped the inboard sections. As a prevenative I opened up the other side and did the same thing with the CA and cap strips. Let me lastly state that I did not crash or land the plane hard! It might be to your benefit to keep an eye on your lower wings to catch this problem if it occurs. Plane is an outstanding performer otherwise!

Happy Flying!

LOOPMAN