Aircraft ice analysis, as defined here, involves a set of analyzes comprising: the analysis of sub-cooled water droplet uptake from atmospheric clouds, the analysis of growth and ice formation on adiabatic and unprotected surfaces, the analysis of ice and rain protection systems on thermally or mechanically protected surfaces, but also the analysis of aerodynamic degradation due to ice formation on aircraft surfaces.
When an aircraft flies through a cloud of sub-cooled droplets, they strike the exposed surfaces and may freeze immediately due to the metastability condition, forming “rim” ice, or they strike and freeze later due to the effect of convection, forming the “glaze” ice.
Ice formation can cause: lift degradation, increase in weight, increased drag, ingestion of ice by engines, ice in the propellers, anomaly in sensor indications, loss of aircraft control aand blockage of the pilots’ field of vision.
Due to its adverse effects, the aircraft may have ice and rain protection systems installed. They can be thermal, more common, or mechanical. Some aircraft do not have anti-icing or de-icing systems, so they need to prove that the ice formed does not significantly affect the operation or that they can safely escape the atmospheric ice condition.
Medium and large commercial aircraft have frost protection systems on their wings, tails, engine intakes, windshields and their probes or sensors. Surveillance and defense aircraft may not have antenna and radome protection systems. Small aircraft, also called general aviation, or urban mobility VTOL aircraft, may not have conventional ice protection systems.
Some other types of atmospheric effects, also considered in ice analysis, are the capture of ice crystals by pitot or freezing rain probes.
To assess the collection of water from aircraft surfaces, it is necessary to know in detail the operation and aerodynamic performance of the aircraft. The icing and icing protection systems design are highly integrated with the aircraft's systems and their aerodynamics affecting drag, lift, weight and flight quality. Therefore, they are directly related to flight safety.
You need to know the airspeed from the altitude, but also the range of local angles of attack for each speed. Thus, the liquid water content (Liquid Water Content – LWC) and the median volumetric diameter (Media Volumetric Diameter – MVD) are estimated using the envelopes in Appendices C and O of the FAR 25, which define the atmosphere of ice conditions for aircraft designs. Subcooled droplets exist between 0˚C and -40˚C, so the ice atmosphere is within this range ranging from sea level to approximately 30,000 feet. There are shorter, more water-laden clouds and longer clouds with less liquid water content. Droplets larger than 40 mm in diameter are called Super Large Droplets or SLD (Super Large Droplets).
The ice protection systems, chapter ATA 100-30, can be of the anti-icing type, which prevents the formation of ice, but has residual water formation, and of the de-icing type, which allows ice to form and then melts or expels the ice formed. The first is a system that operates in a steady state and the second operates in a transient state in cycles of accretion and expulsion or ice melt.
On commercial aircraft, the thermal ice protection system is more common. It acts to control the temperature of exposed surfaces by melting or preventing ice formation.
For ice analysis, DATCOM+ Pro JSBSim, CFD++, LEWICE 2D and 3D software, and tools developed by ATS for 2D and 3D ice formation and protection systems are used. Depending on customer availability, Ansys FENSAP software can also be used.
The certification of ice formation and ice protection systems is a high risk for the development of new aircraft in accordance with FAR 25 or 23 and its circulars and supplementary documents. It is supported by SAE, MIL or ASTM standards specific to this type of certification.
Ice protection system analyzes can be performed on wings, empennages, engines, propellers, windshields, probes and sensors.
Besides that, the analyzes need to be supported by tests in wind tunnels, ice formation tunnels or by in-flight tests. Tests are used to validate numerical tools and to make a more realistic check on some operational points. Numerical tools generally shorten the testing campaign by decreasing experimental operating points, which lowers the total cost of the design and certification campaign.
ATS has experience in the design, certification, testing and simulation of ice formation and ice protection systems. The tools developed by ATS are unique in the market and validated with experiments recognized by the certification authorities.
Applications in the transport industry cars, planes, trains, helicopters, VTOL and ships.
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