## 1 Introduction
In terms of water entry under non-ideal penetration conditions, which includes oblique water entry, water entry with an attack angle and water entry of asymmetric projectiles, currently, a large number of studies are focused on the water entry of asymmetric projectiles and oblique water entry. [PERSON] [36] performed oblique water entry tests using projectiles of various shapes, and had the following findings: when the projectile nose is moving in the water, the fluid exerts a lift force on it because of the cavitation caused by projectile motion; due to the pressure difference between the upper and lower surfaces of the projectile, cavitation occurs on the two surfaces is not symmetrical, and then the amount of water displaced by the projectile on the free surface side is less than the other side; thus, the pressure on this side is lower, resulting in asymmetric pressure on the upper and lower surfaces, thus exerting a lift force on the projectile and causing projectile ricochet; the nose shape of the projectile greatly affects the ballistic stability in water: the flat-nosed projectile has better ballistic stability underwater, the sharp-nosed projectile is prone to ballistic instability, and the ogive-nosed projectile tend to overturn in the early stage of water entry [37; 38; 39; 40]; compared with symmetric nose, after the asymmetric nose enters the water, a non-axial component exists in the hydrodynamic force on the nose [41], and an overturning moment acts on the projectile, changing its attitude angle. This further affects the hydrodynamic force on the projectile, leading to the nonlinear increase in the overturn and yaw of the projectile [42; 43]. The more asymmetric the projectile nose is, the worse the attitude and trajectory stability of the projectile is, and the projectile is prone to instability and inclination, which leads to yaw. There are much less studies on the water entry by a projectile at an attack angle. [PERSON] [44] used numerical calculation to simulate the vertical water entry by a projectile at a small attack angle, and found that under certain attack angles, the projectile tail contacts with the cavity, thus reducing the ballistic stability, and that the spinning of the projectile in water has little impact on ballistic stability. However, [PERSON] [45; 46] found that the spinning motion of the projectile induces a lateral force on the projectile, resulting in a curved trajectory. [PERSON] [47] studied the water entry by the underwater vehicle through numerical simulation and revealed that positive attack angles suppress projectile whipping, whereas negative attack angles aggravate this phenomenon; therefore, ballistic change easily occurs under negative attack angles. [PERSON] [48] conducted a study using the LS-DYNA program and found that greater attack angle leads to faster axial velocity attenuation of the projectile, and thus greater overturning angle and projectile velocity. [PERSON] [49] studied the oblique water entry of projectile at small attack angles with LS-DYNA, and discovered that the attack angle determines the direction and magnitude of the moment on the projectile nose. To sum up, detailed investigations on the residual characteristics, pressure load characteristics and structural deformation of projectiles have been conducted through the ideal penetration test of the water-entry projectile. During the oblique water entry, entry with an angle of attack and water entry of an asymmetric projectile, ballistic yaw will occur. Among these cases, the penetration at an attack angle has been less investigated, and numerical calculation is the main method adopted. Few experimental studies have been carried out on the water entry by high-velocity projectiles with attack angles because the initial attack angle is difficult to realize in ballistic tests and is not conducive to ballistic control.