My trainer, a Sig Scamp, would tend to go back to level if placed into a dive and I let go of the sticks. It was because of the flat bottom. As it picked up speed, the wing would generate more lift, which would tend to make it level out. This is much less likely to happen on a semi-symmetrical wing. It would also tend to level itself out side to side. Needless to say, the transition to my second plane was interesting since it was more likely to go where it was pointed.