Aircraft Ice Accretion Due to Large-Droplet Icing Clouds

Atmospheric clouds are defined by factors such as water droplet size, liquid water content, and air temperature. In natural clouds, droplets are not all the same size, but rather are of various diameters. A statistical factor called median volume diameter is used to describe the relative droplet size distribution for a particular cloud. Typically, individual droplets range from 2 to 50 micrometers in diameter, and clouds usually have median volume diameters of less than 35 micrometers. Generally, it is thought that droplets tend to precipitate out as they reach 100 micrometers in diameter. However, under certain conditions, icing clouds with median volume diameters as high as 170 micrometers, with individual droplet diameters as large as 400 micrometers, may exist.

Large-droplet icing tests are being conducted in the Icing Research Tunnel with aircraft wing models of various lift profiles and element configurations--both with and without ice protection devices. Very peculiar ice shapes and ice types have been observed in Lewis' Icing Research Tunnel studies. On models with no ice-protection equipment, thin ice forms and breaks into pieces. These pieces then slide around slowly over a film of water on the airfoil. On the ice-protected models, small water rivulets flow aft from the leading edge to an ice ridge that forms aft of the ice protection. Here, some of the water freezes while some of it gets blown off the edge of the ridge, subsequently impinging on the model further aft, creating strange ice nodules that grow normal to the model surface. All these tests are being supported by an analytical effort including computational studies using LEWICE, a Lewis-developed code that simulates aircraft icing.

In addition, Icing Technology Branch members have been involved in Air Force and Federal Aviation Administration (FAA) icing tanker flight testing at Edwards Air Force Base and in hearings and investigations being conducted by the National Transportation Safety Board concerning the commuter aircraft accident of 1994. We are working to increase understanding of the large-droplet ice-accretion phenomenon as well as helping the aerospace industry to address the situation.

Comparison of LEWICE ice accretion prediction to actual ice accretion on an airfoil model in the Icing Research Tunnel under large droplet conditions.