Helicopter flying could eventually be “jet smooth,” thanks to hub-mounted actuators that counter, at their source, vibratory loads stemming from blade movement asymmetry and blade-vortex interaction. Sikorsky has completed rig testing of a hub-mounted vibration-suppression system (HMVS) that could be in service in 2015, and the company plans to integrate the active system into all its helicopters.
Lord, which has teamed with Sikorsky and the U.S. Army’s Aviation Applied Technology Directorate on the project, describes the dual-frequency HMVS as motorized imbalanced rotors. They rotate at the blade-pass frequency to create centrifugal forces.
By phasing the two rotors, the magnitude and orientation of the centrifugal forces cancel lateral hub vibration, according to Lord. The Cary, N.C.-based company adds that the force generator, controller, sensors and power electronics are contained in a single unit.
Bill Welsh, chief of Sikorsky’s dynamics and internal acoustics group, told AIN that the HMVS is adaptive. “It learns how the helicopter reacts to dynamic load inputs. It applies a sample load from each actuator and analyzes echoes. The algorithm determines what command to apply so that the HMVS load suppresses loads coming from the main rotor,” he explained.
Current passive systems use springs and mass calibrated to tune out the worst vibration frequency in the helicopter. There are usually five of these devices per helicopter, typically in such areas as the rotor head, cabin, nose and cockpit.
The new system suppresses vibration everywhere in the fuselage. Typically, helicopter vibrations are in the 0.1-0.15g range, Welsh said. The HMVS will reduce that number to “0.03g or below–virtually undetectable vibration,” he went on.
Lord anticipates that the adaptive system will also provide weight savings. Current passive systems weigh about 0.2 percent of the aircraft’s empty weight. The HMVS will cut this down to 0.1 percent, according to Welsh.
The system is also expected to improve durability of the helicopter’s parts. While acknowledging that the extent of the improvement “is debatable,” Welsh estimated “one can expect the system to enable a 10-percent reduction in parts removal.”
The HMVS will be slightly more expensive than current systems “because of its electronic content,” Welsh said. However, he expects the weight and durability improvements to offset the increased upfront cost.
The flying testbed for the HMVS is likely to be a Black Hawk or an S-92. The flying testbed will fly in the fourth quarter next year or early in 2012, with 10 to 15 flight-test hours planned. The companies expect the main certification challenge to be demonstrating that a degraded mode, with one actuator failed, is safe.