Radar Cross Section Modeling and Measurements of Inlets and Cylinders With Skew Blades

Ray based methods give useful scattered field predictions from the skin of the aircraft illuminated by an incident plane wave. They can also be used to predict the returns from simple inlets and cavities but fail when complex terminations such as skewed blades are encountered. As a possible enhancement to the ray-tracing procedure, an auxiliary program based on the modal method was developed to predict the scattering of electrically large and complex jet inlets with engines. For the present development, it is assumed here that these structures can be approximated by a series of rectangular, circular, coaxial and sectoral waveguide sections. The field matching technique is used to give the generalized scattering matrices of the junctions between these waveguide sections. By combining the scattering matrices of the waveguide sections representing the inlet and engine, an overall S-matrix is obtained. Knowing the modes induced at the inlet aperture by the incident plane wave, the scattered fields from the inlet and engine can be readily predicted in all directions. Monostatic RCS measurements of a 0.706 m diameter test cylinder containing 30 skewed blades mounted on a center shaft with a conical hub have been performed at X-band. The dimensions of the structure, the number and orientation of the blades are consistent with those of existing jet engines. Fair to good agreement between predictions and measurements verify the developed software and analytical method used