Peter,
that the Piaggio Avanti changes its pitch significantly upon entering IMC
I have flown the Avanti one and two from 2005 to 2008 all over Europe - there is no such pitch up entering "humidity". The only pitch-up the Piaggio knows is shortly before touch down when you retard the throttles due to the braking effect of the props, so you don't need to break the descent rate with elevator input. True is what Whiskey Bravo says, strong rain increases airfoil roughness ... not too critical when you climb out in a jet with V2 plus 20kt - but when you experience an engine failure after V1 and you climb out at max gross with V2 in torrential rain, you gonna have a problem ...
Sorry Peter, you did not mention the pitch change direction - I made a pitch-up out of it ...
My dad used to tell us about maneuvering between little rain showers in his Dragonfly homebuilt by turning when he felt the nose down trim change. It was a comfortable aircraft and would carry two at 150 mph cruise on 3.5 gal/hr - this was one of the compromises that allowed it.
I understood the front airfoil was more sensitive to rain basically as a result of it needing a shallower lift coefficient versus AoA slope, for static pitch stability.
