I'm certainly not surprised that anything with a Fox 35 for power would have a resonance problem. Two even more so!

I've only ever built one twin, a c/l model for twin Norvel 061's, and they ran as smooth as a baby's bottom.

I have recently seen a full size Catalina run up on our local lakeside hardstand. At full RPM with the pilots holding it on the brakes with full down elevator (as in the pic)

the rear tail section, from just behind the side bubbles, resonated at least a foot either side at the tailplane tips.

Presumably this doesn't happen when it's flying!






Interestingly an RTF electric r/c Catalina in our club shows serious stress marks across the same rear fuselage sections.

Resonance has also proven to be a problem with some Oliver Tiger type diesel engines on 1/2" thick Aluminium alloy plate based engine test stands.

Vibrations with amplitudes of 1/2" have been reported. It was cured by making the plate mounting less rigid.

Resonance is certainly a complex business.

Ray