## 4 Analysis of Experimental Results
### Load Characteristic Analysis
When the leading edge of the blade contacts ice, the ice debris after milling begins to enter the wake region of the propeller. At this time, the tip vortex becomes more unstable, forming tip vortex cavitation, sheet cavitation, and a mixture of ice debris. When the blade is subjected to full-contact milling with ice, the area of sheet cavitation on the blade surface that exits from the blocked region increases significantly, and the area of sheet cavitation increases as the blade approaches the blocked area. In Figure 8c, with an increase in axial depth of ice-propeller milling, the larger the area of sheet cavitation on the surface of the non-milling blade and the closer the blade to the blocked area, the more evident the increase in the area of sheet cavitation on the blade surface. In addition, it is observed that with an increase in axial milling depth, the filaments of the blade tip vortex become thicker. By comparing Figure 8b,c, it can be observed that the cavitation level of the propeller increases sharply owing to the decompression environment, and the most evident effect is that the tip vortex size of the propeller wake increases significantly. The propeller and mixed paraffin model ice after the test are shown in Figure 10.  
Figure 10: Propeller and mixed paraffin model ice after the test. In order to explore the load variation law of the ice-propeller milling condition in the above three environments, the time-domain change curves of the ice milling load and mixed load of the propeller and single blade were extracted, as shown in Figures 11 and 12. A rotation cycle was selected from the load curve during the stable ice-propeller milling process. Figure 11 shows the time-domain variation curves of the ice milling load and mixed load of the propeller under different ice-propeller milling conditions. From Figure 11a, the periodic distribution of the ice milling load of the propeller is evident when ice-propeller milling occurs in air. There are four peaks of the ice milling load in a cycle that correspond to the milling action between four blades and the ice. However, owing to the shielding effect between the blades in the ice-propeller milling process, the peak values of the axial forces of the four blades are different [8; 30]. When ice-propeller milling occurs in water, the time-domain curve of the propeller thrust has several peaks, and each peak of the wave is consistent with the axial force of the ice milling load on the propeller but is opposite to the direction of the hydrodynamic load; that is, the ice milling load causes loss of propeller thrust. A comparison of the propeller thrust under atmospheric pressure and that under decompression shows that the former is greater than the latter, and the difference between the peak value of thrust under the decompression condition and peak value of the axial force of the ice milling load is smaller, which indicates severe hydrodynamic load loss at the non-contact part of the blade in the ice-blade milling process; that is, there is a serious cavitation effect, which is consistent with Figure 8c. Figure 11b shows that the presence of water significantly increases the torque of the mixed load, but the occurrence of cavitation in the decompression condition decreases the torque of the mixed load. Figure 12 shows the load variation of a single blade over a rotation period for the ice-propeller milling condition in three environments. As observed from Figure 12a, in terms of the blade thrust, when the blade is milling ice (approximately \(\theta=0^{\circ}\)), the presence of water has a significant effect on the thrust of a single blade. Generally, the variation in thrust of a single blade in water is significantly higher than that in air. The main reason is the existence of a non-continuous flow at the contact between the blade and ice, as well as turbulence and separation flow at non-contact. The crushed ice blocks and small ice pieces along with the cavitation bubbles formed a cloud behind the propeller, which further modified loading.