Rain erosion resistance characterizations: Link between on-ground experiments and in-flight specifications

With the increasing velocity of vehicles and the fact that the vehicles must fly even in rainy conditions, environmental problems such as rain erosion become more important. Precise specifications are now being set for the rain erosion of materials in the vehicle design. Different attempts have been done to predict the rain erosion resistance of materials from their mechanical properties but this will give only the order of magnitude of the resistance, which is generally insufficient. Thus this resistance must be known more accurately, that is possible only by experiments. To perform this, different apparatus are existing: rotating arms, linear tracks, water jet generators. Each apparatus present advantages and disadvantages. For example, with rotating arms the advantage of using real water droplets is counterbalanced by the disadvantage of the existing tangential force which can affect the measurements. With tracks, very severe vibrations are induced. With water jet generators, there is no tangential force, but the use of a jet instead of a droplet necessitates the knowledge of equivalence laws. Each apparatus has its own characteristics: water droplet diameter, water concentration, which cover a part of possible rain specifications. In-flight specifications are characterized by generally one rain intensity, a time duration, and a droplet diameter distribution. The link between on-ground conditions and in-flight specifications must be established to determine, from the rain erosion resistance measurements performed on specimens of the material, if the structure built with this material will withstand the in-flight rain erosion specifications. We will present the results of our recent studies concerning the possibilities of linking on-ground characterizations obtained with water-jet apparatus to in-flight specifications.