Thanks Dickeybird & Hal. Maybe my hinges are too weak. I have changed to delta hinges of quite thin code 5 plastic, maybe about 25thou thick. I am a bit concerned about heavier plastic hinges as surely they will resist the necessary flexing (?) With the thin hinges & the delta angle the plane flies fine, but the flutter is a nuisance. The rotors are 2 inches chord and 1/4 thick, made of balsa with no other stiffening. Mounting is about 1/3 of chord. The plane can be seen at http://www.geocities.com/tintrax/page6.html (& subsequent page) - the photo shows it before it had the delta polyprop hinges. It is the plane with the red fuselage. I have never seen any autogiros fly, only my own! My own design.
Colin