1. However, the bulk of this work is concerned with rain erosion resulting from high speed droplet impacts with the aircraft surface.' The first reported investigation of the effect of heavy rain on the perfo ance of aircraft was published in 1941 by Rhode? In this report, the author concludes that added drag of rain impacting the aircraft exacted the greatest performance penalty, but that this penalty did not seem to be a safety concern. This conclusion, however, was reached without considering the decreased performance margins which exist during landing. This author has been unable to find any additional published work until 1983 when Luers and colleague Haines published two articles discussing heavy rain penalties on airraft- followed by a subsequent article by Luers.' The conclusion of this work was, that under extremely heavy rain, the rain roughened wing could suffer a decrease in stall angle with maximum lift coefficient reductions of 30%. In the same time period, Calarese and Hankey 6 published an analytic study which showed that under heavy rain conditions the lift of an airfoil actually increased. Their analysis, however, neglected droplet interactions with the wing surface and the resulting splash back and surface roughing effects.
2. The remaining published investigations on performance penalties associated with operation under heavy rain conditions are being presented at this session. They include th esults of a recent test program conducted by NASAP-' on a flapped NACA 64-210 section. These tests confirm at subscale that performance penalties can occur when the airfoil is operating in very severe rainfall. Another paper demonstrates that under heavy rain conditions, laminar airfoils such as those used on gliders, but tested at subscalf, are also susceptible to a performance penalty.
3. been found dealing with this phenomenon. The work by Rockenbach and Alexander 14 have examined oblique impact with a moving film and have considered the ejecta patterns. Drop sizes were approximately 4 mm and impact velocities were unfortunately low and equal to the termiM1 velocity of about 9 mlsec, while film thicknesses were from onefourth to four-fourths of the droplet diameter. while the results are not applicable to splash back from an airfoil since the impact velocities are so low, the interesting observation was made that the volume of ejecta droplets exceeded the incoming drop volume. A more relevant work was undertaken in 1975 by Povarov et al.15 in the USSR. Here millimeter sized drops were allowed to impact a rapidly rotating disk and the interaction of the droplet with the boundary layer was investigated by