Propeller-ice Impacts Measurements with a Six-component Blade Load Sensor

The impact of ice on a propeller is a complex process, which most likely results into a milling or crushing process, or a combination of both. The highly dynamic forces during an impact can change rapidly in amplitude as well as direction. Little information can be deducted from conventional test setups with rigid propellers, especially forces on the individual blades are seldom successfully measured. The presented paper will outline the challenges which need to be overcome to measure these impacts and present results of actual propeller ice impact measurements.Classic designs for propeller testing involve rigid propellers with force measurements located at the base of the propeller. From this type of setup, two problems arise. First of all, the blade area and the size of the impacted ice can be similar. Even in the case of a high quality measurement, it will be hard to derive which part of the measured forces contributes to which blade. Secondly, in practical test situations the natural frequency of the entire propeller mounted on a force transducer can be relatively low. Impacts contain high frequency energy, causing the entire propeller assembly to vibrate and thereby obscuring a correct measurement of the impact loads.
A measurement setup has been designed to overcome both problems. Rather than all four blades, only a single blade is mounted on the force transducer. This particular force transducer is capable of measuring forces in six egrees of freedom. The other propeller blades are mounted directly on the shaft. This measurement setup enabled MARIN to overcome the first problem, any impact visible in the measured signals are solely due to forces on the instrumented ‘key’ blade. In AARC’s ice basin a series of tests were performed where propeller ice impacts were measured. Synchronised high speed video recordings were used to gather more insight in the complex phenomena that occur during these short events.