Aerodynamic and Stealth Characteristics Analysis for a New Type of S-Shaped Inlet With Inner Bulge
Authors: Zhang Xinyu , Yang Qingzhen , He Yubo , Wang Xufei , Zhang Saile 
Published: January 2021

Abstract
Searching for the optimal solution of the comprehensive performance of the aerodynamic and stealth characteristics of the aircraft inlet has become the trend of the development of the aircraft inlet due to the rapid development of radar detection technology.
The aircraft inlet is a vital source of scattering due to its own cavity structure characteristic. Accordingly, reducing the electromagnetic stealth characteristic value of the stealth aircraft inlet can be served as the main way to reduce the RCS value of the whole airplane.
Accordingly, a new type of stealth aircraft inlet that incorporates the design of the inner bulge is designed in this paper, which significantly reduces the value of radar-cross section (RCS) of the inlet by transforming the three-dimensional shape of the inlet.
The design of this kind of inlet is based on the parametric rapid reconstruction geometry method that produces a geometry profile of the inlet through controlling a limited number of characteristic parameters. At the same time, the profile coordinates of the inner bulge is decided by means of three parameters, including the area of inner bulge, the circumferential distance and the radial offset.
This paper calculates the aerodynamic characteristics of the seven S-shaped inlets, including six S-shaped inlets incorporated the inner bulges with different parameters, and an ordinary S-shaped inlet without the inner bulge as a control model.
Total pressure recovery and distortion index are used as the indicators for judging the flow characteristics of these inlets to evaluate the effects on the aerodynamic characteristics of the stealth inlet after incorporating the inner bulge, which plays an important role in further evaluating the influence of the working environment of the compressor after the inlet.
These S-shaped inlets with inner bulges on them resulting a decrease on total pressure recovery by 0.69% to 1.14%, at the same time, the total pressure distortion index at the exit of inlet is increased by 14.77% to 26.85%. Overall, the aerodynamic performance of the inlet with the design of the inner bulge can’t be seen a large decline.
What’s more, this paper also compares the electromagnetic properties of the S-shaped inlet models with the inner bulge with the inlet without the inner bulge, using the algorithm of iterative physical optics (IPO) to evaluate the stealth characteristics of the models. This method is a kind of high-frequency approximation method for analyzing the scattering characteristics of the cavity scattering, with the high calculation accuracy as well as the high calculation efficiency. At the same time, the calculation takes up less memory, this is simply because that the mesh is coarser in calculation when using the IPO algorithm.
The resulting inlet design achieving a dramatic improvement in stealth performance that the value of RCS is reduced by up to 97% after incorporating the design of inner bulge. In general, the stealth characteristics of optimized stealth aircraft inlet has greatly improved.