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

Abstract

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 remotesensing 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 exceptionally 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 significant 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 constraints. 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, control laws are designed for a single operating condition, and the closed-loop performance is presented across a range of velocities.