I have built and flown an 18.5% scale Farman Biplane (1909 version) and have crashed it on 2 of its 3 flights due to a severe pitching instability that develops shortly after a beautifully controlled take-off. It seems that as a certain speed is developed the front (controlled) canard starts to "fight" the rear stabilizers (fixed). The plane goes into ever-increasing poipoising pitches until it noses in.
I have tried the CG between 33 and 40% of the chord. The final dive is steep, making me think it is nose heavy (the rear stabilzers are fully lifting sections), but I really don't know.
The main planes and the rear stabilzers are set at +3 deg. The motor is zero down-thrust, zero side-thrust.
The forward canard is generally very responsive, except in the final nose-over dive, where it seems unable to overcome the final nose-over dive.
The airfoils of wings and rear-stabilizers are thin curved plates, the canard a very thin Clark-Y type, just as per original.
Can anyone help please?