Freezing out danger

Friday, January 08, 2010

Didier Guffond, of French aerospace laboratory ONERA's Physics, Instruments and Sensing department, looks at some of the ways in which military aircraft are protected against ice deposits

Atmospheric ice is a meteorological hazard that may affect an aircraft's operations. It is formed when supercooled droplets impact on the front part of different structures. Ice deposits on aircraft surfaces affect the aerodynamic performance of lifting and control components, directly influence the safety of flight, and have been the cause of numerous fatal accidents.

Icy conditions are characterised by three criteria: the Liquid Water Content (LWC) corresponding to the mass of water (in grams) in an air cubic centimetre, the Outside Air Temperature (OAT), and the droplet size (Mean Volume Diameter – MVD). Icing conditions are found in strato-shape clouds (stratus, altostratus, cirrostratus and nimbostratus) characterised by a great horizontal band, low thickness and a low LWC and cumulo-shaped clouds (cumulus, altocumulus, cirrocumulus) characterised by a low horizontal band, a great thickness and a high LWC.

The freezing of liquid water gives out energy (latent heat) that should be transferred into the air by evaporation, sublimation, and convective heat exchanges (all of these things depend on the heat transfer coefficient). In cold cases, this process occurs very quickly and water freezes on impact, and the ice (rime ice) is white and relatively aerodynamic. In warmer cases, only a part of the latent heat can be transferred locally, some water runs back aft of the impact location, and the deposited ice (glaze ice) is translucent and not aerodynamic.

The protection of aircraft is achieved by de-icing or anti-icing systems. Generally, anti-icing prevents the formation of ice permanently and is used for large aircraft by using hot air from the engine, and turboprops are protected with de-icing systems, which remove the deposited ice from the surface using a pneumatic 'boot'. Electrical de-icing systems are generally used to protect helicopter rotors.

On military aircraft:
• Fighter jet wings are not protected. Due to their high speed, the total temperature is too high to permit an ice deposit. Even if it occurs, the engine power permits it to negate the aerodynamic degradation. Only the probes are electrically heated to assure efficient anti-icing protection;
• Transport aircraft are generally military versions of civil aircraft with the same protection for the wings (pneumatic boots or hot air);
• The rotor blades of military helicopters are protected with electro-thermal systems – a de-icing system for the main rotor and an anti-icing system for the tail rotor. The anti-icing system requires between 2-5W/cm2.

The engine intakes are protected either by anti-icing electro-thermal systems or by bypass, which prevents the ice deposits;
• No tactical Unmanned Aerial Vehicle (UAV) in the army inventory possesses a de-icing capability. As the amount of ice deposited increases as the size of the body decreases, UAVs are very sensitive to icing, in spite of their low velocity. Their power also limits the available energy;
• Protection of UAVs is a very difficult challenge. One characteristic of the aircraft is to fly at very low speed, ie. the heat transfer coefficient is relatively low too: it tends to be prone to 'runback ice' (ice aft the impact area) on all the upper surface seldom seen on aircraft or helicopter blades. Thermal anti-icing systems for such areas can be greater than the UAV capability. Other systems, like electro-expulsion, are possible but studies need to be conducted to determine precisely the size of the protection system, ie. all the upper surface or not. Some UAVs are protected with an anti-freeze solution, which suppresses the water freezing point, and is pumped through the porous leading edges of the wings. This system gives good, but limited, protection.