Development of an integral twist-actuated rotor blade for individual blade control

Abstract

An integral twist-actuated rotor blade has been developed for helicopter individual blade control (IBC) applications. A I/6th Mach scale CH-47D blade was designed, fabricated, and tested in hover at the MIT Hover Test Stand Facility. The design incorporates active fiber composite actuators (AFC) within the composite spar to induce a distributed twisting moment along the span. The anisotropic actuators are oriented at 45° to the blade axis to maximize the shear stresses generated. A baseline model blade was modified to obtain design targets of 4° of peak-to-peak tip twist, less than 20% added mass, matching stiff­ness properties with the exception of torsional stiffness, and ensuring adequate strength for forward flight. As a part of the development, active fiber composite (AFC) were subjected to structural integrity tests in order to optimize the material system and qualify the actuator for the rotor application. A new manufacturing process was developed for producing the actua­tors required for the integral blade. The capabilities of the actuators were defined and shown to be suitable for the integral blade application. Based on a passive model blade manufacturing procedure from Boeing, a manufacturing process was developed for the integral blade. The design and manufacture were validated and improved in three half-span blade section tests. With the final blade section, the twist actuation performance was demonstrated near predicted levels and damage tolerance was demonstrated in combined loading tests which simulated limit loads. The twist actuation performance of the active blade was evaluated over a range of rotor speeds, actuation frequencies, and blade loading conditions in hover. Transfer function data were collected from input voltage to blade twist and induced vertical hub shear. Changing test conditions had little affect on the measured performance, though blade flap­ping mode dynamics had a significant effect. Actuator electrical system failures limited the quasi-steady twist actuation during tests to 0.8° peak-to-peak. Sectioning of the rotor blade revealed manufacturing defects as the probable cause. This project successfully demonstrated the effectiveness of integral twist actuation in Mach-scale hover testing, sup­porting the need for further investigation of the concept for IBC.