1 Flight Test of Technology for Virtual Monitoring of Loads Joshua D. Isom isomjd@utrc.utc.com Principal Research Engineer United Technologies Research Center East Hartford, CT, USA Mark W. Davis, James P. Cycon, and James N. Rozak {madavis, jcycon, jrozak}@sikorsky.com Sikorsky Aircraft Corporation Stratford, CT, USA Jay W. Fletcher jay.fletcher@us.army.mil RASCAL Project Manager U.S. Army Aeroflightdynamics Directorate Moffett Field, CA, USA ABSTRACT Technology for real-time virtual monitoring of loads on rotor system dynamic components was validated in a series of flight tests conducted on RASCAL at NASA Ames Research Center in 2010, 2011, and 2012. Real-time software produced estimates of load waveforms for eight key structural loads in the rotor system, including main rotor shaft bending, main rotor shaft torque, push-rod loads, servo loads, and stationary scissor loads. For virtual monitoring of loads using only measured aircraft states, the correlation between the instantaneous estimated and actual loads for dynamic components in the rotating frame was in the range 0.81-0.98, and the correlation between instantaneous estimated and actual loads for dynamic components in the fixed frame was in the range 0.36-0.78. For loads monitoring using the measured aircraft state and the flap motion of one rotor blade, the correlation between instantaneous estimated and actual loads for dynamic components in the rotating frame was in the range 0.92-0.99, and the correlation between instantaneous estimated and actual loads for dynamic components in the fixed frame was in the range 0.86-0.91. The availability of technology for accurate real-time virtual monitoring of loads may enable advances in structural usage monitoring, load-alleviating controls, and structural fault detection and isolation. INTRODUCTION Virtual monitoring of loads (VML) is the real-time estimation of one or more loads on a helicopter dynamic or airframe component. VML is important as an enabler of advances in structural usage monitoring, load-alleviating controls, and structural fault detection and isolation. These three applications of VML have the potential to enhance aircraft safety and reliability, lower aircraft maintenance costs, and reduce aircraft weight. In current practice, the loads that will be encountered by a helicopter structural component during its lifetime are estimated and validated when the helicopter is designed and qualified. Occasionally these estimates are updated in reaction to specific fleet issues. Rarely are actual in-service loads measured. The estimated design structural loads ultimately determine the design life, the appropriate aircraft maintenance schedule, and the weight of fatigue sensitive components. It has been well recognized that real-time monitoring of loads on production aircraft would enhance aircraft safety and lower aircraft maintenance costs. The barrier has been the number, weight, and reliability of physical sensors and data acquisition units traditionally required to do this in a production aircraft on a sustained basis. Presented at the AHS 69th Annual Forum, Phoenix, Arizona, May 21–23, 2013. The nature of the loads encountered by structural components is determined by the way a helicopter is flown. Level flight, turns, pull-ups, and dives create characteristic load profiles. In current practice, structural design loads are estimated for each of various types of assumed load maneuvers. A flight load survey test program is used to create a characteristic load profile for each maneuver and substantiate design load assumptions. The assumed number of maneuvers and the characteristic load profile for each maneuver are then used to estimate retirement times for life- limited structural components. To partially address the aforementioned barrier, many existing on-board helicopter health and usage management systems (HUMS) monitor aircraft state parameters in order to characterize in-service flights in terms of the flight regimes flown. However, the conservative nature of loads allocated to each regime still results in significant uncertainty of in-service load distributions. Acknowledging the importance of monitoring in-service loads and the associated barriers, Sikorsky Aircraft Corporation (SAC) and United Technologies Research Center (UTRC) embarked on a multi-year effort to develop technology for estimating structural loads on many key components using information that is already logged by HUMS. The resulting technology for VML exploits the quasi-periodic nature of helicopter structural loads, and was first shown to be viable by researchers at UTRC. More recently, laboratory tests at SAC showed that the loads estimation algorithms could be computed by the existing HUMS using existing parametric data. The latest milestone in technology maturation is a series of flight tests conducted by SAC and UTRC in collaboration with Army Aero-Flight