Ultrasonic shear wave anti-icing system for helicopter rotor blades

A new conceptual non-thermal anti-icing system for helicopter rotor blades is introduced. Theoretical calculations predict that certain ultrasonic modes generated by horizontal shear waves produce sufficient interface stresses between a host structure and ice to remove the accreted ice layer. Experimentally obtained ultrasonic shear dispersion curves are compared to theoretical predictions for a multilayer system formed by steel and ice layers. Shear electromagnetic transducers (EMAT) are used to generate shear horizontal waves in the structure. The results experimentally validate the theoretical predictions for shear modes on a 9mm steel plate and the same plate covered with a 3.5mm ice layer. In order to obtain frequency domain dispersion curves, two dimensional Fast Fourier Transforms are applied to 64 waveforms formed by 16,384 sample points each. The experimental results agree with theoretical predictions for the cases of having a 9mm steel plate and 9mm steel plate covered with a 3.5mm ice layer within 5% and 8%, respectively. Experimental tests are performed in a fabricated de/anti-icing prototype formed by piezoelectric actuators and an aluminum plate. Studies on the effects of ultrasonic waves on ice bonding strength at different frequencies and amplitudes are conducted. As the driving amplitude of the shear actuator increases the ice shear adhesion strength decreased. As the driving frequency becomes closer to the ultrasonic resonance frequency of the system, the adhesion strength of the attached ice layer decreased. 100% reduction in well accreted ice shear adhesion strength was reached when the actuator was driven at its resonance frequency and at an amplitude of 450V for a period of 90 seconds. The shear actuators melt a 1.5mm thick ice layer in a time period under 5 minutes when driven at the first ultrasonic resonance frequency of the system (130KHz) and at an amplitude of 450V. At the same driving conditions, the actuators prevent water from freezing on an aluminum host structure at temperatures under - 150 C. Ultrasonic vibration affects regions up to 8cm away from the actuator location.