This is fairly timely for me becase I found myself in this exact situation for real about two weeks ago. As a cautious pilot with a couple thousand hours I was flying a fixed wing aircraft under marginal daytime VFR, totally smooth air in light rain under a cloud base of only 1500 feet, terrain was up to about 300 feet in spots, the rain gradually got heavier until forward visibility was zero, could still see the ground OK, one minute later the ground was gone too and it was solid dark grey IMC murk and then it got bumpy. It seems I had flown into a patch of cloud cumulus off the edge of a biggish storm that went right onto the trees. I pushed on for about another minute hoping to pop out and it started to feel a bit weird and then decided it was all just too scary and turned around to get back out, had partial leans, almost instantly felt sick, nothing on the panel seemed to be matching up with what I felt in my guts. Felt like I was turning but wasn't, tried to get "level" in my guts, that made the AI turn me the other way. It is a disgusting feeling. Through luck I only spent a few minutes feeling totally out of control but finally came back out of the gloom and saw some ground and all the sicknesss vanished. Without any pride I can tell you I nearly messed myself during that few minutes. My encounter was honestly unintentional but guys and girls, just don't ever do it on purpose because it is an experience that you will really not enjoy. I was very lucky and learned my lesson. One thing that freaked me out is how engine noise seems to double due (to reduced optical sensation ?) and that just adds to the confusion. Looking at a clear AI image of a plane turning and honestly believing that you are straight and level is just horrible.
stay safe.
Vic