Surveys of the fluctuating wall pressure were conducted on a sub-scale, thrust-optimized parabolic nozzle in order to develop a physical intuition for its Fourier-azimuthal mode behavior during fixed and transient start-up conditions. These unsteady signatures are driven by shock wave turbulent boundary layer interactions which depend on the nozzle pressure ratio and nozzle geometry. The focus however, is on the degree of similarity between the spectral footprints of these modes obtained from transient start-ups as opposed to a sequence of fixed nozzle pressure ratio conditions. For the latter, statistically converged spectra are computed using conventional Fourier analyses techniques, whereas the former are investigated by way of time-frequency analysis. The findings suggest that at low nozzle pressure ratios –where the flow resides in a Free Shock Separation state– strong spectral similarities occur between fixed and transient conditions. Conversely, at higher nozzle pressure ratios –where the flow resides in Restricted Shock Separation– stark differences are observed between the fixed and transient conditions and depends greatly on the ramping rate of the transient period. And so, it appears that an understanding of the dynamics during transient start-up conditions cannot be furnished by a way of fixed flow analysis.
Several years of earlier research was conducted for the U.S. Air Force, related to the impact that warhead-induced damage had on the aeroelastic integrity of lifting surfaces and in turn the resulting upset of the complete aircraft. This prompted us to look at how similar aeroelastic events and aircraft upsets might be triggered by ice accumulation on specific parts of the aircraft. Although seldom studied, icing can also significantly impact the aircraft’s aeroelastic stability, and hence the overall aircraft stability and control, and can finally result in irreversible upset events. In this latter context, classical flutter events of the lifting surfaces and controls can occur due to ice-induced mass unbalance or control hinge moments and force reversals. Also, a loss of control effectiveness caused by limit cycle oscillations of the controls and lifting surfaces may appear, due to significant time-dependent drag forces introduced by separated flow conditions caused by the ice accumulation. A review is presented in this article on the mechanisms that initiate these ice-induced upset events when considering the class of small general aviation aircraft. The review is based on literature and earlier experimental work performed at The University of Texas at Austin. Two commonly observed ice-induced aircraft stability and control upset scenarios were selected to investigate. The first upset scenario that is presented involves an elevator limit cycle oscillation and a resulting loss of elevator control effectiveness. The second upset is related to a violent wing rock or an unstable Dutch Roll event.