Avoiding Lockout Instability for Towed Parafoil Systems

Abstract

Many towed systems consist of a parent platform moving along the ground, air, or water surface connected to a towed vehicle via a tether line. Motion of a towed parafoil system can be complex and is driven by motion of the parent platform, canopy control inputs, and wind disturbances. A particularly problematic flight dynamic instability for this system is canopy lockout, in which the canopy attains a large lateral offset and bank angle resulting in high line tension. It is possible to use left and right brakes to return the system to its nominal position; however, if the lateral offset and bank angle are too large, it is not possible to restore the system to a nominal position, and the lateral offset and bank angle grows until impact with a ground surface. This paper explores the lockout phenomena using a multibody simulation and identifies passive means to avoid the instability by locating the tether connection point forward on the cradle in combination with sufficiently high cradle–canopy yaw stiffness. Moreover, it is shown that an active control system that regulates canopy roll angle with parafoil brake inputs can eliminate the instability for the tested configuration.