X2™ Load Alleviating Controls

The Sikorsky XH-59A, X2 Technology™ Demonstrator, and S-97 Raider™ helicopters employ closely spaced coaxial rigid rotors which constitute what's known as an Advancing Blade Concept (ABC) helicopter. The rotors of these are subjected much larger vibratory bending stresses in flight than would occur for articulated rotors of similar size. The design rotor separation is driven by the tip clearance that results from maneuver requirements and blade stiffness. X2™ Load Alleviating Controls (X2™-LAC) constitute a patent pending technology employing the use of sensors, control laws, and independent rotor controls, to automatically reduce rotor loads and therefore reduce blade vibratory stresses, increasing blade tip clearance (minimum distance between the blade tips at any cross-over). The increased tip clearance can be used to reduce the blade stiffness required, or reduce rotor spacing. This has the potential to improve a mix of attributes including range, speed, weight, and/or maneuverability. The technology has been shown to work using flight simulation analysis. This paper describes the history of rigid rotors, leading up to the X2 Technology™ Demonstrator. The control system is described, and how it differs from a standard coaxial control system. It then explains the reasons for the higher loads associated with rigid rotors, showing flight test and simulation data showing the effects and limitations these higher loads impose. Finally, it describes how X2™ Load Alleviating Controls work, simulation results, and benefits expected for this type of aircraft.