Development of an Aerodynamic Model and Control Law Design for a High-Altitude Airship

Lighter-than air vehicles are an attractive solution for many applications requiring a sustained airborne presence. The buoyancy force provides an energy-free form of lift, offering a non-traditional approach to long-duration missions for which traditional aircraft are not well-suited. Potential applications include roving or hovering surveillance and communication utilities for both military and commercial use, and a variety of remote-sensing instruments for the scientific community. In particular, the Missile Defense Agency plans to utilize unmanned airships at high-altitudes to provide a long-duration missile defense presence around the coast-line of the United States. Operated at 70 kft, each of these “high altitude airships ” will fly above all regulated air-traffic for several months to years, will reside in a steady atmospheric regime, and will utilize solar energy to provide all required power. Two key objectives for this type of mission are that the unmanned airship have excep-tionally long endurance, and that it operate with a sufficiently high-level of autonomy. In order to achieve these objectives, a robust guidance and control system is required, capable of auto-piloting and controlling the airship under an extremely wide range of atmospheric and wind conditions. The successful design of such a system first requires an accurate model of airship dynamics across its expansive flight envelope, and a representative model of the expected disturbances. The dynamics of an airship are markedly different from traditional aircraft, with signif-icant effects from added mass and inertia, and a much higher sensitivity to wind. In this paper, a typical airship configuration is first sized to meet energy balance and mass con-straints. The geometry of this configuration is then used to develop a general aerodynamic model for the airship. The equations of motion with added mass and inertia are developed, and the open-loop dynamics are analyzed across a range of flight conditions. Finally, con-trol laws are designed for a single operating condition, and the closed-loop performance is presented across a range of velocities. I