Nonlinear Aerodynamics and Nonlinear Structures Interaction for F-16 Limit Cycle Oscillation Prediction

Abstract

In this paper, a physically-rooted Nonlinear Structural Damping (NSD) model based on a generalized van der Pol model is integrated into ZEUS; a ZONA's Euler Unsteady Solver, to establish a Nonlinear Aerodynamics and Nonlinear Structures Interaction (NANSI) simulation tool for predicting limit cycle oscillation (LCO) amplitude. The parameter involved in the generalized van der Pol model is estimated from the flight test data of an F-16 with external stores configuration at a given flight condition. Once this parameter is identified, NANSI simulation is carried out to predict the LCO amplitudes at all flight conditions. Results show that the predicted LCO amplitudes for the F-16 Typical-LCO and Non-typical-LCO configurations correlate very well with the flight test data, demonstrating that the NSD is one of the key elements involved in the LCO of the F-16 with external stores configurations. 1. Background Several current fighter aircrafts with external store configurations persistently encounter Limit Cycle Oscillation (LCO) problems. LCO is a self-excited, sustained vibration of limited amplitude which can impact a pilot's control authority over the aircraft, ride quality, and weapon aiming. It can also induce structural fatigue and, under certain circumstances, flutter. Denegri [1] provided a detailed description of the aircraft/store LCO phenomenon. Norton [2] gave an excellent overview of LCO for a fighter aircraft carrying external stores and its sensitivity to store carriage configuration and mass properties. Because of this sensitivity, the LCO clearance of a modern fighter aircraft should be addressed for all possible store/weapon configurations. Given the drastic number of such configurations, this effort is a major engineering task in aircraft/store weapon compatibility certification. It requires accurate aeroelastic predictions within a short-time frame as demanded by rapid military responses when facing today’s everchanging international situation. Further, since there can be thousands of store/weapon combinations for a typical fighter aircraft, the LCO predictions must also be computationally efficient to rapidly identify the critical cases. A robust post-processing procedure is also needed to identify a wide variety of aeroelastic response characteristics including flutter, divergence and LCO. It is generally believed that LCO of an aircraft with stores is a post flutter phenomenon that belongs to the so-called supercritical LCO mechanism. When the flight condition of the aircraft is beyond its flutter boundary, the aircraft's aeroelastic system is unstable and a divergent response of the structure occurs if the aeroelastic system is linear. However, if the aeroelastic system is nonlinear and includes a “LCO bounding mechanism” dependent on the amplitude of