Results of a perturbation analysis correlating the flows entering and exiting an aircraft inlet system

An investigation of aircraft inlet performance sensitivity to approaching flow conditions was conducted for ground test applications. The approach was to correlate flow conditions at an inlet reference plane (IRP) in front of an inlet and conditions at an aerodynamic interface plane (AIP) at the engine face. A wind tunnel data set obtained for a subscale F-15 fighter inlet provided IRP and AIP measurements suitable for the correlation. The IRP data included flow angularity and Mach number distributions, and the AIP data included average total pressure recovery, total pressure distribution, and turbulence levels. The data were obtained at numerous combinations of free-stream Mach number, angle of attack, angle of sideslip, and inlet-corrected mass flow.

The AIP parameter chosen for the investigation was inlet average total pressure recovery, which is an indication of inlet performance. The IRP parameters chosen were average angle of attack, average angle of sideslip, average local Mach number, and inlet mass flow. One condition in the test matrix was selected to be a baseline condition, and the other points in the matrix were considered to be perturbations from the baseline. Multivariable-least-squares-regression methods were used to develop a mathematical relation between the IRP and AIP parameters. The resulting equation was evaluated for suitability by a four-step approach. First, the coefficient of multiple determination, "R²," and the residual mean square, MSE, were computed. Second, the results were compared to a recovery difference criterion of 0.01. Third, the equation was used to predict values for several data points not used to develop the equation. Fourth, the results were compared with trends in existing wind tunnel data. The equation was determined to be suitable for studying inlet sensitivity to IRP parameters.

Inlet sensitivity was studied relative to using the IRP as a control point in inlet-engine ground tests. Each IRP parameter was systematically varied while holding the others constant in the equation. The results showed that the inlet is sensitive to changes in the IRP parameters although not for all ranges. For each of the IRP parameters, there was a range of values in which the inlet total pressure recovery was not sensitive. This result implies that studying other inlet AIP parameters, such as distortion and turbulence, should also be done.