Load Distribution Control and Swing Angle Feedback for an Autonomous Dual Lift System with Flight Test Results

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. The benefits of dual lift include increased lifting capability, increased range, and and the ability to fly at higher density altitudes, but are contingent on proper load distribution between the helicopters. This paper discusses the development and flight validation of a load distribution control concept to equalize the cable tension between the two helicopters in flight and a swing angle feedback controller that increases the damping of the load's swinging motion while in a hover. The load distribution controller allowed the dual lift system to maintain the RMS of the cable tension error to be less than 1.5% of the external payload weight when flying a racetrack pattern at 10 knots as compared to a RMS of 4.3% when no distribution control was active. In addition, the swing angle feedback controller increased the damping ratio of the external payload's swinging motion by more than 50% when compared to a dual lift system without augmentation.