A lot of the foam 3D models are flying around with the CG slightly behind the Np to enhance their stunting. BUt then they seldom try to fly in a non stalled "normal" manner for more than a few feet. Technically if the CG is right on the Np it will fly inverted with no input required but other things such as center of overall drag come into the picture as well. The center of drag in normal flight can be a de- or pro- stabilzing effect every bit as much as elevator trim. When you get down to this level of finickyness the little things will bite you.