Innovation in Engineering Computational Technology
Chapter
11 W.G. Habashi
Department of Mechanical Engineering, McGill University, Montreal, Canada Keywords: in-flight icing, CFD, droplet impingement, ice accretion, icing certification, anti-icing, de-icing, ice protection.
When super cooled droplets present in the clouds hit an aircraft or a rotorcraft, they form an ice layer whose roughness and form can lead to substantial distortions in the aerodynamic profiles of wings, air intakes, rotors and propellers. Performance degradation can then occur from a combination of increased drag as a result of roughness and flow separation, and a reduced stall angle of attack, with higher and shifted weight being additional issues. Asymmetrical roughness distributions can also cause significant stability and control problems, compounding already reduced aircraft performance. Ice can also block engine inlets and internal ducts and, if ingested or released, damage components, causing power fluctuations, thrust loss, rollback, flameout, and loss of transient capability. Stall is the killer, because current stall protection systems cannot alert the pilot that the margin between a stall warning and an actual stall is significantly reduced and perhaps completely eliminated in icing situations. In-flight ice accretion can be prevented by adding energy in the form of heat (thermal anti-icing, that is, preventing water droplets from freezing), or accreted ice can be cyclically removed using systems operated by pneumatics (mechanical de-icing, that is, breaking the bond at the ice-surface interface) or by thermal means (thermal de-icing, that is, melting ice by internally impacting hot air onto the leading edge of the wings). Unfortunately the complete prevention of ice formation, or its complete removal, is not, and can never be, economically feasible because of the large amount of thermal or mechanical energy required. Moreover, the controlled amount of ant or de-icing hot air bled from the engines is often needed during climb, especially for smaller aeroplanes. In, practice therefore, while some areas of the aircraft are anti-iced, others can only be de-iced and a large part is left unprotected. Such unprotected areas must be precisely determined and the aircraft tested in an icing tunnel, behind an icing tanker, or through flight in natural icing conditions, before being certified for `flying into known icing.' An ironclad solution against icing is prevented by two shortcomings: the difficulty of detecting and/or measuring ice accretion and the necessarily cyclic nature of de-icing an aircraft in flight. Ice detection systems are installed on only a fraction of aeroplanes operating today and are known to be inefficient. Pilots do not trust ice detection systems and simply monitor places where ice accretes: "If I have ice on the windshield wiper bolt," they reason, "I have ice on the wings." Wrong, as the windshield wiper bolt is not a stagnation point! In the case of booted aeroplanes the pilot must wait for some ice to accrete before activating the boots. One would think that the precise amount of ice permitted to accrete would be based on scientific principles, but it turns out to be no more than a rule of thumb (quarter inch to half an inch) depending on the mechanical ability of the boot to `crack' the ice. Half an inch of ice would have vastly different aerodynamic effects on various aircraft, and can a pilot really sense half an inch of ice on portions of the wing hidden from his line of view, especially at night? A second shortcoming is that available power dictates that in-flight de-icing operations be cyclic (for example, wing, tail, empennage, and back) with blackout periods for each component. It only makes sense for the wing to be designed to sustain aerodynamically the inter-cycle ice load that accretes during the wing de-icing blackout period, but this is just now being studied. With major aircraft manufacturers, certification agencies, and research agencies seemingly internationally linked by research in the area of in-flight icing, it is only natural for the public to assume that this aspect of flying has been completely mastered. While these entities are certainly trying their best, the fact is that aircraft and system design and operational procedures still have not totally conquered the in-flight icing problem. Flying in icing conditions continues to result in incidents and accidents, with no aircraft type, size, or configuration being immune. A May 2006 invited article in Aerospace America entitled "Icing Research Heats Up" reconfirms the fact that adverse weather conditions contribute to 30% of all aircraft accidents. More notably, this article which supposedly covers USA and international research does not mention CFD simulation even once, reflecting again the great conservatism that controls the official icing research community, but from which, interestingly enough, industry is breaking free. Thus, the rest of the paper will cover mostly second-generation icing codes, which have been typified by FENSAP-ICE. The article should not be viewed as a review article, as space does not allow. The article will also not address verification and validation, which have been thoroughly analysed in a number of other papers. Finally, results will not be shown independently at the end, but will be interspersed to illustrate each section.
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