I have a little problem with this plane. I entered all data from that model to RF G2 again this time including actual weight, control surfaces, incidence ect - whole nine yards. And model came out a little squirly - not pich stable . For the sake of experiment I inreased the span of horizontal stab and all problems went away.

I checked the numbers again and compared them to original Piper J3 Cub and CoroCub from SPAD website. Here are the numbers:

My CoroCub:
Wing: Span 86 in, Chord 15.5 inch, area 1333 sq inch
Horiz stab: 22 inch, Chord 11 inch, area 242 sq inch
Wing to horizontal stab area ratio : 5.5

SPAD CoroCub:
Wing: Span 76 in, Chord 12 inch, area 912 sq inch
Horiz stab: 22 inch, Chord 9 inch, area 198 sq inch
Wing to horizontal stab area ratio : 4.6

Piper j3 Cub
Wing: Span 422 in, Chord 65 inch, area 27430 sq inch
Horiz stab: 117 inch, Chord 48 inch, area 5616 sq inch
Wing to horizontal stab area ratio : 4.8

So I guess I screwed up on the horizontal stab span - I did not recalculate it [:@]. It should be around 26 inches. When I put that number in RF G2 the Cub few very stable. I wonder if I can just add 2 inches of coroplast at each end of the stab using bamboo sticks and tape (or maybe replace stab tips with a wider ones) or should I just replace the entire stab. Or maybe RF G2 is wrong and the plane will fly just fine.

Edit: I corrected my mistake about Lateral stability. What I ment was pich stability (Def: Longitudinal stability is pitch stability, or stability around the lateral axis of the airplane).