Accurate Radar Cross Section Modelling of Jet Inlets & Engines

Most of the industrial software codes used for the prediction of the radar cross-section (RCS) of aircrafts are based on ray tracing. While ray based methods give useful scattered fields from the skin of the aircraft illuminated by an incident plane wave, they largely fail to predict the returns from cavities onboard. As a possible enhancement to these ray-tracing programs, an auxiliary program based on the modal method was developed to predict the scattering of electrically large and complex jet inlets and engines. It is assumed here that these structures can be approximated by a series of rectangular, circular, coaxial and sectoral waveguide sections. 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 wave, the scattered fields from the inlet and engine can be readily predicted in all directions. Such software has been developed. Monostatic RCS measurements of a 0.706m diameter test cylinder containing 30 skewed blades mounted on a centre 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 existing jet engines. Good agreement between predictions and measurements verify the developed software and analytical method used. This software could generate the database of RCS returns for a given engine over a wide range of aspect angles.