As an 'interested SLF', with no connection with rotary wing flight other than a semi scale Bell 47 R/C helicopter I built - but never really learned to fly - nearly forty years ago, ground resonance is not entirely unknown to me. However, I always understood that gyroscopic precession played a part?
I can understand the rapid escalation of the phenomenon - positive feedback in the system - and the utter destructiveness if allowed to propagate. I can also understand out of spec' springing/damping being a contributing factor, but I don't really understand exactly what triggers the onset? And does gyroscopic precession - a pretty powerful factor - have any influence?
Roger.