For aircraft designed for Flight Into Known Icing (FIKI), protection of certain components will be required. The extent of protection will depend on the sensitivity of each component to the effects of icing. There are various means of providing protection, including pneumatic boots (commonly used on turboprop aircraft), hot bleed-air systems (used on the majority of current large transport aircraft wings and nacelles), electrothermal systems (which can be operated in either anti-ice or de-ice mode, and are found on some propellers, intakes and helicopter rotors, as well as the Boeing 787 Wing ice Protection System), electromechanical systems (found on some business jets and UAVs/drones) and fluid systems (generally found on General Aviation type aircraft).
Due to the move towards more electric aircraft, the use of electrothermal systems is becoming more prevalent. This is an area where AeroTex have a particularly high level of experience. Helicopters have traditionally been the platforms which have utilised electrothermal IPS the most, both on the blades and for engine inlets. Anti-ice systems prevent the formation of ice, and can be further broken down into running wet systems and fully-evaporative systems. A running wet system aims to keep the protected extent clear of ice, maintaining the surface temperature slightly above zero. These systems generate runback ice aft of the protected region, which can still be detrimental to aircraft performance and handling qualities. Fully-evaporative systems aimto evaporate all of the impinging water, leading to a completely clean surface. Whilst giving the highest aerodynamic performance, fully-evaporative systems have high energy demand. Where power availability is low, or sensitivity to ice accretion is not as high, de-ice systems can be employed. These allow ice to form on the surface and then periodically heat the surface to remove the accretion. Great care is needed in the design of such systems to ensure that adequate shedding is achieved without generating high levels of runback further aft, and also taking into account temperature limitations of the structure.
The chordwise and spanwise extent of the ice protection is generally based on an impingement analysis, but also may need to take into account the under-lying structure. De-ice schemes require very careful design to minimise the size of each zone (to minimise the applied power), whilst adhering to limitations on maximum inter-cycle ice thickness, runback thickness and maximum temperature. Temperature instrumentation can often be required to be embedded into the structure to provide closed-loop feedback control.
All of the necessary aspects can be investigated using a transient solver. Many commercial tools allow the transient response of a amulti-layed structure to heat input to be assessed. However, there are very few which include ice growth, melting and shedding on the outer surface. AeroTex utilise a specific code to perform analysis of electrothermal IPS to allow the full top-level architecture to be developed. This can be peformed for wings, rotors, propellers, intakes/inlets and windshields. The below cases show very simplified designs where the leading edge region is permanently heated, whilst zones further aft on the aerofoil are cycled.
One of the key outputs from the simulation tool is the ice accretion which forms on the surface. Generally, limits are defined for the maximum thickness of ice which can accrete on the surface, both on the protected extent (inter-cycle ice, which regularly forms and sheds) and behind the protected extent (runback ice, which remains after the icing encounter). The example output shown here has a permanently heated parting strip at the leading edge highlight, which remains free of ice, and cyclic zones further aft on the upper and lower surfaces. The size of each zone, its power density and the power sequence are all design variables which canbe modified to produce an optimised system. Smaller allowable ice thickness generally results in higher overall power demand.
The structural temperature response is an important part of the design. Heating a zone for too long or at too high a power density can lead to runback ice generation. This needs to be minimised, since it will have an aerodynamic performance degradation even after exit from the icing condition. In addition, safe material temperature limits must be adhered to so that structural integrity is maintained.
The surface temperature response helps to review how much margin is available. If the surface only just reaches 0°C during a sequence, sensitivity to manufacturing tolerances may be too high, leading to poor performance in-service. Conversely, if the surface temperature remains above 0°C for too long after power is removed, runback ice can be generated.