Silvaire - interesting theory. Could be the case. Can't remember having it in the Seneca or Duchess, but then again I was busy with my training and didn't have much time to get vertigo! I used to get the same feeling whe I trained in helicopters as well - down low was fine, but anything above 1000ft would start to feel very bad. It was much worse in helicopters for some reason. I've subsequently found a thread in Rotorheads about this and apparently it's a pretty common thing.