Residual pitch oscillation (RPO) flight test and analysis on the B-2 bomber

摘要

Current limitations in aerospace industry state of the art analysis capability to accurately predict unsteady transonic aerodynamic loads has led to unexpected flight test anomalies. Historical examples include the RB-57F body-freedom-flutter, the HIMAT canard oscillation, the B-1B vortex shedding, the F-111 TACT aircraft buffet, and the F-16 Limit Cycle Oscillation (LCO). More recently, the B-2 experienced an unpredicted residual pitch oscillation (RPO) during low altitude high speed (LAHS) flight testing outside the operational envelope. An 11 sortie flight test program was flown to better understand the phenomenon and define the RPO on-set boundary. The oscillation was characterized by a rapid decrease in damping over a very narrow Mach number range and exhibited "hump" mode characteristics for many configurations. Chase plane video of the B-2 during control surface excitations appeared to show a moving shock visible in the condensation cloud over the engine nacelles. Analysis of all available data indicates that the RPO is most likely induced by an oscillating shock. Augmented linear model results correlated well with frequency responses from selected flight conditions. Nonlinear modeling results showed promising comparisons with the flight data and potential for improved transonic analysis capability.