Two robust controller designs are presented for a twin lift helicopter system (TLHS). The TLHS configuration consists of two UH-60A Sikorsky Blackhawk helicopters jointly lifting a heavy payload. The first design presented considers the case in which the tethers connecting each helicopter to the load are equal in length, and the second considers the case in which the tether lengths are unequal. Both designs are based on a seven degree of freedom model linearized about hover. The primary objective of each controller is to minimize the control action and pitching motion required to stabilize the helicopters as they perform elementary manuevers. Initial controllers developed by solving a mixed sensitivity H~∞ problem are analyzed for robustness to model uncertainty using the structured singular value μ, and new controllers are developed which are more robust with respect to the uncertainty.
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