Center of buoyancy moves along with pitch attitude variation, which causes Helium gas inclination. In this paper, movement of center of buoyancy and corresponding variation of longitudinal static stability were observed. The effect of separating wall, which is placed in the envelop to minimize movement of center of buoyancy was also investigated. Installation of separating wall was proved to be essential for current design, because movement of center of buoyancy aggravates longitudinal static stability. Investigation of longitudinal static stability for various speeds reveals a 50m airship is statically stable only in a low speed regime.