Development of a Forced Oscillation System for Measuring Dynamic Derivatives  of Fluidic Vehicles  B. C. Trieu’, T. R. Tyler*, 6. K. Stewa3, J. K. Charnock’, D. W. Fisher*,  E. H. Heim’, J. Brandon, and S. 6. Grafton’*  Abstract  A new Forced Oscillation System (FOS) has been designed and built at NASA Langley Research Center  that provides new capabilities for aerodynamic researchers to investigate the dynamic derivatives of  vehicle configurations. Test vehicles may include high performance and general aviation aircraft, re-entry  spacecraft, submarines and other fluidic vehicles. The measured data from forced oscillation testing is  used in damping characteristic studies and in simulation databases for control algorithm development and  performance analyses.  The newly developed FOS hardware provides new flexibility for conducting dynamic derivative studies.  The design is based on a tracking principle where a desired motion profile is achieved via a fast closed-  loop positional controller. The motion profile for the tracking system is numerically generated and thus not  limited to sinusoidal motion. This approach permits non-traditional profiles such as constant velocity and  Schroeder sweeps. Also, the new system permits changes in profile parameters including nominal offset  angle, waveform, and associated parameters such as amplitude and frequency. Most importantly, the  changes may be made remotely without halting the FOS and the tunnel.  System requirements, system analysis, and the resulting design are addressed for a new FOS in the 12-  Foot Low-Speed Wind Tunnel (LSWT). The overall system including mechanical, electrical, and control  subsystems is described. The design is complete, and the FOS has been built and installed in the 12-  Foot LSWT. System integration and testing have verified design intent and safe operation. Currently it is  being validated for wind-tunnel operations and aerodynamic testing. The system is a potential major  enhancement to forced oscillation studies. The productivity gain from the motion profile automation will  shorten the testing cycles needed for control surface and aircraft control algorithm development. The new  motion capabilities also will serve as a test bed for researchers to study and to improve and/or alter future  forced oscillation testing techniques.  Introduction & Background  Forced oscillation testing is traditionally used to investigate the dynamic derivatives of vehicle  configurations. A model is oscillated one frequency at a time at various amplitudes. Balance data, angular  position and rate data are measured and recorded. The data set corresponding with a model mass and  inertias is reduced to determine the dynamic derivative coefficients. The coefficients are used in damping  characteristic studies, and used in simulation database for control algorithm development and  performance analyses. Test vehicles may include high performance and general aviation aircraft, re-entry  spacecraft, submarines and other fluidic vehicles.  Historically, the forced oscillation testing hardware has been an induction motor driving a crank and  linkage mechanism to provide oscillatory motion. Figure 1 shows an existing system, formerly designed  and built for the 30- by 60-fOOt Langley full-scale tunnel that can be configured to oscillate a model in roll,  yaw and pitch. The system is limited to sinusoidal motion having the frequency controlled by motor speed  and the amplitude determined by the mechanical linkage. Oscillatory frequency may be changed remotely  by changing the motor speed. However, to change amplitude, the oscillation hardware and the tunnel  must be halted and the linkage mechanism manually reconfigured.  ** NASA Langley Research Center, Hampton, VA  Vigyan, Inc., Hampton, VA  Proceedings of the 38 Aerospace Mechanisms Symposium, Langley Research Center, May 17- 19,2006  387