Improving Aircraft Safety: Ice accreted on aircraft surfaces eventually sheds and can cause damage to downstream components, such as impact damage to aft-mounted propellers or ice ingestions at aft-mounted jet engines. The hazard to the aircraft is mostly from large ice fragments formed after long exposure to icing conditions. Aircraft icing certification regulations require that the effects of ice shedding on aircraft safety be evaluated through flight testing. However, ice shedding flight tests in natural icing conditions are very costly and pose many challenges for aircraft manufacturers. There is a strong need for ice shedding simulation tools to support aircraft icing analysis and certification.
The State of Current Simulation Tools: Available ice shedding simulation tools vary from ice trajectory codes that use ice particle weight and empirical aerodynamic data to compute ice fragment trajectories, to unsteady computational fluid dynamics simulations of the flow about an ice fragment to determine the fragment aerodynamic forces for use in trajectory analysis. Typically, simplified analysis tools lack the ability to simulate the physical phenomena associated with actual ice shedding events and therefore their range of application is limited. Consequently, the accuracy of their predictions vary considerably depending on how close the empirical aerodynamic data used in the trajectory computations approximate the actual forces and moments of the fragments being analyzed. On the other hand, the more complex tools require large computational time and effort that render them impractical for aircraft icing analysis and certification where a large number of trajectory computations are needed for the “what-if” analyses. Another drawback of many of the available ice trajectory analysis tools is that they do not model the random phenomena associated with ice shedding events such as the variation of initial fragment conditions at the moment of shedding. These conditions could have a significant impact on the fragment’s flight path. Finally, most of the tools used today for ice shedding analysis have not been validated with experimental trajectory data because experimental ice trajectory databases are not publically available. The development and validation of ice shedding simulation tools that will address current shortfalls in ice shedding analysis is a significant undertaking.
Applying the WSU Methodology: In 2003, Wichita State University (WSU) with support from the aircraft manufacturers in Wichita and the NASA Glenn Research Center in Cleveland embarked on a long-term effort to develop improved ice shedding simulation tools for aircraft icing analysis and certification. During the last six years an ice shedding simulation methodology has been developed at WSU that combines computational fluid dynamics (CFD), probabilistic methods, and extensive experimental aerodynamic data for a range of ice fragments, in a six-degrees-of-freedom (6-DOF) trajectory analysis code. This code has been used to establish boundaries, or limits, of fragments with respect to critical aircraft components. The WSU methodology was used to compute trajectories of simulated leading edge and runback ice shape fragments shed from various locations such as the nose, windshield, wing upper surface, and antennas on a generic business jet aircraft. Typically, 20 to 60 thousand trajectories of a fragment are computed using variations in fragment shed location, orientation, and size. These trajectories are used to determine the likelihood that a fragment will impact a critical aircraft component at a downstream location.
Validating the WSU Methodology: An important step during the development of a simulation tool is its validation relative to an established database (usually experimental) to assess its accuracy, reliability, and range of applicability. To date, detailed experimental data that can be used for a quantitative validation of ice trajectory analysis tools have not been located. Thus, WSU investigators initiated a research effort in 2008 to generate experimental ice fragment trajectory data for code validation.