Since the beginning of aviation icing is a serious safety issue which every aircraft manufacturer has to deal with. Water can exist in liquid form even at sub-zero tem-peratures. In unfavorable weather conditions supercooled water occurs in clouds and turns into ice instantly when it encounters a cold aircraft surface. If ice then accumulates the sensitive aircraft aerodynamics are disrupted which can lead to a significant drag increase, a change in the lift characteristics and decreased control surface efficacies (see Figure 1).
Today’s aircraft are already prepared for ‘normal’ icing. Anti-icing systems on the leading edges of the wings, engine inlets etc. prevent large-scale deposits of ice. Such systems have been part of the certification process for decades – no larger aircraft is approved without it. Things are different under so called SLD (Supercooled Large Droplets) icing conditions. These relatively rare conditions are characterized by large water droplet sizes greater than 150 microns and can occur when snowflakes from a high altitude layer of cold air fall through a layer of warm air, where they melt, and are then supercooled in a lower cold air zone. If an aircraft encounters them, the droplets dissolve or break up, then freeze and accumulate. They can be thawed off in places where the anti-icing systems are installed, but ice can still form behind the systems.
In November 2014 a new certification regulation for aircraft that also targets icing caused by SLD has been introduced. It initially applies only to smaller aircraft, but larger aircraft may also be affected by this in the future. As this is of great interest to aircraft manufacturers, a solution is needed. The current lack of calculation methods, test facilities and, ultimately, flight data on the phenomenon of SLD is the reason for the industry, various scientific institutions and universities to invest in research and development.
The development of high quality aircraft simulation models from flight test data using system identification techniques (Sys-ID) is an important research topic with a long tradition at the DLR Institute of Flight Systems (DLR-FT). With respect to the exper-tise available in this topic a connection between the Brazilian aircraft manufacturer Embraer and DLR-FT has therefore been established for several years. Begin of 2013 an initial 3 years research cooperation was launched concerning recent DLR research initiatives into aircraft icing modeling and the development of Online Sys-ID methods (project SuLaDI).
Because of the major effects of icing on the aerodynamic properties and flight per-formance, this is termed a ‘degenerated aircraft configuration’. Understanding the behavior of degenerated aircraft under different types of icing is important for increasing pilots’ situational awareness and thus preventing accidents. The flight data acquired by the project will be used to develop new flight dynamics models. These will then be applied to the DLR AVES flight simulator. Figure 2 shows an example of different drag polars for the clean aircraft and for 2 icing conditions measured in flight test. The red curves show the models developed using Sys-ID. As long term perspective an incorporation of appropriate icing scenarios into the standard pilot training is considered.
→ DLR magazine 145: An icy affair