If an airship does not have enough lift, it will sink under gravity. By angling the nose down, this can lead to a gliding forward flight just like a conventional glider. If an airship has excess lift, it will rise. By angling the nose up, this can also lead to forward movement. In this way, an airship which periodically alternates its buoyancy between positive and negative, while adjusting its attitude accordingly, can gain almost continuous aerodynamic forward thrust. Thus, flight proceeds in a leisurely vertical zig-zag pattern. Because no energy is consumed directly in creating thrust, the principle allows for flights of long duration, although at slow speeds. The proposed Hunt GravityPlane is a hybrid airship designed to take full advantage of gravity gliding.