We address main issues concerning the limitation of load cycle speed in a typical airframe full-scale fatigue testing (FSFT) of rotary wing aircraft (RWA) in a systematic way by means of developing a predictive and adaptive model-in-the-loop (MiL) testing method. Current typical control systems employed for the RWA FSFT are based on a reactive-style feedback loop via hydraulic servo-cylinders and sensors, which are insufficient as the frequency, speed, and number of actuators of the test increases. This inefficiency stems from complexities caused by large airframe displacements, airframe inertial effects, actuator cross coupling, and phase lag caused by system response times. This MiL testing method is somewhat new in the airframe FSFT for RWA, and may provide advantages in improving the performance of actuation and sensing systems if the numerical and real parts of the test system are to interact correctly. TDA will utilize its recently developed high performance 6-axis hexapod actuation system to build an airframe MiL testing system composed of a main fuselage test article as a physical system and a virtual model of a helicopter tail section. This virtual tail section is to be integrated to the physical system to form a complete airframe test article. We particularly picked the tail section as a candidate of the virtual model because its long and flexible structure produces very large deflections, posing substantial difficulties in a typical airframe test. Our proposed MiL test system will have more than required 15 actuators and 20 sensors, and its advanced dynamic control system with the enhanced hexapod actuator will meet and exceed the 10 Hz loading frequencies and 100 in/sec actuator speeds. Based on our investigation and research effort in Phase I, we will develop a concrete plan to develop a scaled MiL test demonstrator in Phase II with the dynamic control system that can achieve higher cycling rates and faster test speeds compared to those achievable by current reactive control systems. This reduced scale high-speed test demonstrator will enable us to evaluate the system scalability and verify the performance envelope of a full-scale system.
