Stability, Control, and Simulation of a Dual Lift System Using Autonomous R-MAX Helicopters

Cargo missions of the near future will require transportation of increasingly heavy loads to remote locations that are at distances farther, and at altitudes higher, than what can be accomplished by current helicopters. Using two helicopters to carry a single load, termed dual lift, provides heavy lift on demand and avoids the very costly design and development of a special purpose heavy lift helicopter. This paper discusses the stability and control aspects of a dual lift system using autonomous R-MAX helicopters. Unique dual lift modes lie in a frequency range that affect helicopter attitude responses and should be considered while designing an inner-loop (attitude) dual lift controller. One such mode is unstable and is characterized by the helicopters drifting away from each. In order to stabilize this mode, helicopter separation feedback is used. Unlike in a single lift system, there exists only one pendulum mode in a dual lift configuration and a cable angle that is relative to a no swing plane is needed to isolate the swinging motion and improve damping of the payload. With the identified control challenges as a foundation, a multi-objective parametric optimization approach is used to design a controller for a dual lift system that meets a comprehensive set of ADS-33E stability and performance specifications.