ICE PROJECTILE LENGTH PREDICTION OF SHED ICE FROM ROTOR BLADES

Open Access
Author:
Schneeberger, Grant M
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 17, 2016
Committee Members:
  • Jose Palacios, Thesis Advisor
  • Richard Randolph Auhl, Committee Member
  • Philip John Morris, Committee Member
Keywords:
  • ice
  • rotorcraft
  • shedding
Abstract:
Ice shed from rotor blades can be a ballistic concern for conventional helicopters, tiltrotors, wind turbines, and propellers. In conventional helicopters these ice projectiles can cause damage to the empennage and tail rotor, and in a tiltrotor configuration fuselage damage can be a concern. In both types of rotorcraft the ice could be ingested by the engine. All of these can lead to a catastrophic failure of the rotorcraft. A significant amount of research has already been conducted on adhesion strength of ice, ice shedding, projectile trajectories, and impact damage. This thesis will discuss research conducted to predict the ice projectile length of shed ice leaving the tip of a rotating blade. Knowing the size of the ice projectile is critical during the design of ice protection systems and critical structures that could be impacted by shed ice. Two models (based on Euler-Bernoulli and Timoshenko beam theories respectively) were developed to predict the length of shed ice as it slides past the tip of the rotor blade due to the effect of centrifugal force. The failure mechanism of the ice was determined to be due to direct stresses created by the bending of the ice shape as it hangs over the rotor blade tip. These direct stresses exceeded the ultimate strength of the ice once the overhanging ice structure reached a critical length. An Euler-Bernoulli model for ultimate tensile strength failure was compared to a Timoshenko shear failure model. Comparison of the models confirmed failure due to direct tensile stress. Given the large discrepancy of the reported values of ultimate tensile strength of ice found in literature, this value was experimentally measured in this research for impact ice shapes. The Pennsylvania State University's Adverse Environment Rotor Test Stand (AERTS) was used to create ice at representative in-flight icing conditions. Such ice shapes were removed from the rotor and used to determine the tensile strength of impact ice. The ultimate tensile strength of impact ice was found to be 0.685 MPa with a relative standard deviation of 38%. The same facility was used to conduct experiments to visualize ice sliding down the rotor blade upon shedding and the breaking up of the ice projectiles was quantified during rotation. The cross-section of the ice shapes was photographed prior to inducing shedding. These ice shape were then digitized and the second moments of area and centroid were found for the accreted ice shapes. Electro-thermal heaters were used to induce the accreted ice to shed during rotation and high-speed cameras were used to capture the breakup of ice as it traveled over the tip of the rotor. Quantification of the break lengths of the ice projectiles was obtained. Using the measured cross-sectional properties of the shed ice cross-sections and the ultimate tensile strength of ice experimentally quantified, the model was able to predict the break length within 11% of the experimentally measured lengths for the entire data set. Knowledge of the cross-sectional properties closer to the fracture face of the ice projectile results in predictions within 8.6% of the experimentally measured lengths as compared to 13% for cross-sectional properties estimated using the ice shape at the tip of the rotor blade.