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Emerald Group Publishing Limited
Copyright © 2008, Emerald Group Publishing Limited
Testing a search-and-rescue helicopter under icing conditions is some times a risky business. Recently, when Canada's Department of National Defence (DND) tested their new Cormorant helicopter by flying it through a spray cloud generated by a specially equipped US military helicopter, as advised by NRC Aerospace, they were 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.
“It's very important that we create a realistic type of ice” stated NRC Aerospace Icing Specialist, Dr Myron Oleskiw. “In the tunnel, we can do just that by controlling the ice growth conditions very precisely. When we put these samples on the spin rig, we'll be able to determine reliably the adhesion strength and then transfer that knowledge to the numerical side of the project, which will develop 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. “My model emulates the behaviour of individual droplets, their trajectories, their stochastic motion on the surface after impingement and, finally, their freezing” he explained. “The model predicts a somewhat random ice structure that corresponds well to observed ice shapes. It's also very good at predicting different types of ice, such as rime and glaze, a feature not found in existing models. Furthermore, 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 NEC's propulsion and icing wind tunnel to determine how well the numerical model j 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, which will be produced using modified spray systems in the same wind tunnel. The knowledge generated will enable authorities to incorporate SLD conditions, into their certification requirements.
It cannot 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.
“This project will provide the knowledge helicopter manufacturers need to better design their ice protection systems to enable their aircraft to fly safely over a wider range of flight conditions and to expand that protection to a larger fleet of helicopters” Oleskiw stated. “NEC's combination of unique facilities, existing icing expertise, and cutting- edge numerical capability offers them a set of tools to solve whatever icing problems they might come up against.”