ARIOUS means that have been tried to eliminate the annoyance of ice on aircraft windshields are briefly discussed by Mr. McBrien. The three chief losses that have to be compensated when heat methods are employed result from the formation of ice due to:1. The impact of subcooled water droplets. 2. Evaporation freezing. 3. Heat lost to the air flowing over the windshield.
The development of the double windshield currently used by United Air Lines is discussed, with mention of the shortcomings of the several transparent materials used in the installation.
Experience during the past two winters has proved that adequate heat can be supplied the double windshield from the cabin heating system of the DC-3 airplane to prevent ice from obscuring vision in all except the most severe conditions encountered in scheduled operations.
The optical characteristics of the two-pane installation are somewhat inferior to those of a single pane but are felt to be more than offset by improved ice protection and by increased strength to resist bird-strikes.