High-speed raindrop impact erosion poses a significant and persistent threat to the structural integrity, aerodynamic performance, and operational safety of modern aircraft. During flight through precipitation, high-kinetic-energy water droplets relentlessly strike critical surfaces such as leading edges, radomes, and engine fan blades. This can lead to material degradation, including pitting, cracking, delamination, and optical/functional coating failure, ultimately compromising flight performance, increasing maintenance costs, and shortening service life. As aircraft continue to operate at higher speeds and in more diverse meteorological conditions, a profound understanding of rain erosion mechanisms and the development of effective protection strategies have become paramount in aerospace engineering. This review aims to consolidate current knowledge, critically evaluate existing approaches, and identify future research frontiers to enhance aircraft durability and safety in rain environments.

This article provides a systematic overview of the state-of-the-art in rain erosion research, encompassing three core aspects: mechanical mechanisms, experimental techniques, and numerical modeling. Mechanistically, the transient fluid-structure interaction process is dissected into key stages: initial water-hammer pressure generation, lateral jetting, and stress wave propagation (including longitudinal, shear, and Rayleigh waves). The review synthesizes classical and improved models for impact pressure, highlighting their assumptions and limitations. The cumulative damage process, characterized by an incubation period, acceleration, maximum rate, and decay stages as per ASTM standards, is detailed. Critical influencing parameters are analyzed, including impact velocity (following a power-law relationship with erosion rate), impact angle (affecting normal/shear stress components), droplet size, surface roughness, and material properties (especially coating-substrate adhesion strength). Experimentally, the review categorizes and compares major testing methodologies: simulated real rainfields (wind tunnels, rocket sleds, flight tests), accelerated specimen methods (waterjet apparatus, whirling arms, ballistic tests), and accelerated droplet methods (single/multi-jet apparatus, pulsed jet systems). A comparative table outlines the advantages, limitations, and applicable speed ranges of each facility, noting that single-jet and whirling arm devices are most prevalent for fundamental studies and material ranking, respectively. However, a lack of standardization hinders direct data comparison across different apparatuses. In numerical modeling, the evolution from traditional mesh-based methods to advanced techniques is traced. The limitations of the direct FEM (finite element method) with eroding elements for handling extreme fluid deformation are discussed. The review emphasizes the growing adoption of meshless methods, particularly the SPH (smoothed particle hydrodynamics) method and its coupling with FEM (FEM-SPH), for simulating droplet splash and fragmentation. The coupled Eulerian-Lagrangian (CEL/ALE) method is recognized for its balance in efficiently modeling large fluid flow and structural response. Emerging trends in multi-physics simulations, integrating CFD (computational fluid dynamics) with structural and thermal analyses, are also explored for a more holistic understanding.

Significant progress has been made in qualitatively and quantitatively understanding rain erosion. Experimental methods enable the simulation of rain environments and observation of damage, while advanced numerical techniques offer deep insights into transient mechanics and material response. Key conclusions indicate that erosion severity is governed by a complex interplay of fluid dynamics parameters (velocity, angle, size) and target material properties (hardness, toughness, interfacial adhesion). Future research should transition from phenomenological description to predictive science. Critical gaps and prospects include: (1) Mechanism Deepening: Developing multi-mechanism coupled models that account for strain-rate effects, thermal-mechanical coupling, and microstructural evolution under cyclic impact. Cross-scale studies linking molecular dynamics with continuum models are needed. (2) Advanced Materials & Design: Moving beyond passive coatings towards the design of "smart" protective systems with functionalities like energy dissipation, self-healing, or adaptive surfaces. A systematic theory for coating-substrate system design under impact is required. (3) Unification of Testing & Simulation: Establishing standardized test protocols and benchmark cases to improve data comparability. Developing "virtual testing fields" or digital twins that integrate sensor data, multi-physics simulations, and machine learning for accurate life prediction. (4) Integrated Multi-Physics Modeling: Advancing beyond one-way coupling to fully coupled simulations that concurrently solve fluid-structure-thermal-acoustic interactions during the entire erosion process. Addressing these challenges will pave the way for the functional-load-bearing integrated design of next-generation aircraft surfaces, ensuring reliability in rain environments.