This thesis summarizes a novel steady-state 3D throughflow method to analyze aircraft nacelle/fan systems. The method adopts the body force concept using source terms to model the effects of fan blades. The method is designed to capture interaction effects between bypass fan and nacelle at off-design conditions, especially for circumferential inlet distortions. The unique feature of this body force model compared to conventional throughflow methods is that it requires no information about the fan blade geometry. Instead, fan performance data is fed into the body force model, and the resulting body force model can predict both nacelle and fan performance under inlet distortion. This method was validated with both axisymmetric calculations and inlet distortion calculations to demonstrate its capability in off-design analysis.