Investigation of ice shedding properties of superhydrophobic coatings on helicopter blades Stefania Tarquini a,1 , Carlo Antonini a,b , Alidad Amirfazli c , Marco Marengo a, ⁎, Jose Palacios d, ⁎ a Department of Engineering, University of Bergamo, Dalmine, BG, Italy b Laboratory of Thermodynamics in Emerging Technologies, Mechanical and Process Engineering Department, ETH Zurich, 8092 Zurich, Switzerland c Department of Mechanical Engineering, York University, Toronto, ON M3J13P, Canada d Department of Aerospace Engineering, The Pennsylvania State University, University Park, PA 16802, United States abstract article info Article history: Received 12 July 2013 Accepted 19 December 2013 Keywords: Helicopter icing Blade de-icing Superhydrophobic surfaces Icephobic surfaces Ice adhesion strength Ice regimes The state-of-the-art of icing protection systems for helicopter rotor blades is based on active thermal de-icing systems that require large amounts of power. This work focused on assessing the potential icephobicity of superhydrophobic coatings as an alternative passive strategy. Ice shedding tests were conducted in a helicopter blade icing chamber, to simulate atmospheric icing conditions. Ice accretion and shedding were tested on four different materials, including two common metals and two superhydrophobic materials, with the objective of evaluating icephobic potential for anti-icing purposes. Coating test results showed a strong influence of temper- ature and surface roughness on the ice adhesion: the strength increased when temperature decreased and rough- ness increased. Ice regime was independent of the type of surface used, but superhydrophobic surfaces resulted in a thinner ice shape in comparison with common metals, which resulted in a shorter shedding time, especially in rime ice conditions. The relationship between ice regime and adhesion load showed that ice adhesion load substantially increases in rime ice conditions, demonstrating that ice regime is an important parameter in the ice adhesion process. Additional results showed that superhydrophobic surfaces were associated with a decrease in the adhesion load with respect to the baseline materials ranging from the 16% to the 70% in the best case; but this reduction may not be revealing for practical applications as ice reduction mechanisms need to be first understood. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Flight into adverse weather conditions is a critical operational issue for helicopters. Icing environment leads to potentially dangerous ice ac- cretion on helicopter blades, which causes a change in the airfoil shape and performance degradation due to decreased lift, increased drag, and increased torque and blade vibration. Furthermore, ice shedding from blades due to centrifugal force poses a ballistic danger to the helicopter and creates large vibrations due to imbalanced rotors (Palacios et al., 2011). Helicopter rotors are more susceptible to icing than fixed wing vehicles of similar gross weight, because their operations occur almost exclusively at low altitudes, between 1000 and 4000 m, where the at- mosphere contains supercooled water droplets, leading to an increase in icing potential (Palacios et al., 2011). The state-of-the-art for ice protection is based on electro-thermal systems that operate as de-icing systems: they activate intermittently to melt the ice layer in contact with the solid surface and to allow ice removal by centrifugal and aerodynamic forces. However, they present several drawbacks: first, electro-thermal systems operate cyclically (to limit power consumption) allowing ice to accrete up to 6–7 mm in thickness prior to removal; and second, these systems only cover the leading edge of the blade, where most ice accretes, leading to the poten- tial formation of the so-called “runback ice”, caused by liquid water flowing in the aft direction. Moreover, they require high power inputs (~4 W/cm 2 )(Brouwers et al., 2011) obtained with high weight devices, unsuitable for smaller helicopters (Coffman, 1987; Yaslik et al., 1992). Further, these systems usually rely on high thermal conductivity of lead- ing edge materials, which is not adaptable to new generation erosion resistant polymer based leading edge materials (Palacios et al., 2011). The need for alternative solutions drove researchers and industry to explore the use of different active and passive strategies to prevent or mitigate ice formation. On one hand, different active technologies such as piezoelectric actuators (Ramanathan et al., 2000), Electro- Impulsive De-Icing (EIDI), and ultrasonic anti-icing devices (Palacios et al., 2006, 2008) have been investigated. On the other hand, the sur- face coating approach has the advantage to be passive and therefore with the final goal of being a low power, low weight and reliable de- icing system. In literature, two different coating strategies for icing mitigation can be found: the first one is based on the use of icephobicity, the property Cold Regions Science and Technology 100 (2014) 50–58 ⁎ Corresponding authors. E-mail addresses: marco.marengo@unibg.it (M. Marengo), jlp324@psu.edu (J. Palacios). 1 Present address: Telespazio VEGA Deutschland, Europaplatz 5, 64293 Darmstadt, Germany. 0165-232X/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.coldregions.2013.12.009 Contents lists available at ScienceDirect Cold Regions Science and Technology journal homepage: www.elsevier.com/locate/coldregions