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Getting a handle on rotorcraft icing

Testing a search-and-rescue helicopter under icing conditions is sometimes a risky business. Recently, when Canada’s Department of National Defence (DND) tested its new Cormorant helicopter by flying it through a spray cloud generated by a specially equipped US military heli-copter, as advised by NRC Aerospace, it was in for a shock. In one particular trial, ice that had built up on the main rotor broke off and struck the tail rotor where it dislodged more ice, triggering severe tail vibrations in the process – and alarm among the crew. Finding a way to prevent this from happening in real situations became a priority. It grew more acute when a search of existing ice shedding studies turned up little useful data.

NRC Aerospace and DND therefore decided to set up a jointly funded research project to study helicopter icing and then use their new knowledge to develop predictive tools. The five-year study will explore various aspects of the problem, including ice formation, ice shedding and ice trajectories. Trends in ice adhesion and shedding will be identified through small-scale experiments in which ice samples will be grown on an object in the NRC Altitude Icing Wind Tunnel. The object will then be attached to the rotating arm of a spin rig, located in a cold room, and spun around until the ice breaks off.

The knowledge gained from the tests will aid in developing a predictive tool for ice shedding. That tool will be created using an award-winning, patented morphogenetic model devised by Dr. Krzysztof Szilder, a senior research officer at NRC Aerospace. His technique is unique among those that already exist because it predicts the structure of ice, rather than assuming its bulk properties. For example, the model's discrete and random characteristics enable it to successfully simulate, for the first time, a complex peculiar structure called 'lobster tail' ice that can form on swept wings.

In a later project phase, full-scale tests on a helicopter tail rotor will be carried out under realistic conditions in NRC's Propulsion and Icing Wind Tunnel to determine how well the numerical model predicts ice shedding. Experimental and numerical results will be used to improve de-icing systems on actual helicopters.

NRC researchers also plan to extend the full-scale rotor ice accretion and shedding studies to supercooled large drop (SLD) icing. The knowledge generated will enable authorities to incorporate SLD conditions into their certification requirements.

The consensus is that these studies can’t come too soon. Canada is internationally known for its helicopter operations, which in winter are frequently carried out in icing conditions – particularly off the east coast when sea states get too rough to safely ferry people by boat to and from oil platforms. De-icing systems exist but they're expensive to install, their weight reduces payload, and their complexity results in more frequent breakdowns. Better, more cost-effective ice protection systems are therefore increasingly being sought by helicopter operators and manufacturers.