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Effect of Heavy Rainfall on Performance
The witnesses testified that the rainfall, which was occurring at the time and place of the accident, was the heaviest experienced in a great many years. These statements were substantiated by the fact that small streams in that area were filled to overflowing, bridges were washed out, and highways were flooded. In view of this evidence the Board sought and received a report from the National Advisory Committee for Aeronautics as to the effect of heavy rainfall on airplane performance.
Calculations were based on an estimated rain density of 50 grams per cubic meter. This rain density is equivalent to a rainfall of 1.4 inches per minute if the falling velocity is taken as 12 meters per second. According to a recent report prepared by the United States Weather Bureau this is maximum rain density likely to be experienced anywhere in the eastern part of the United States and represents extreme conditions of actual rainfall in a cloud burst.
According to the National Advisory Committee for Aeronautics report, the increase in weight due to the accumulation of rain, as well as the impact of rain, assuming a mean rain drop velocity of 20 feet per second, is negligible.
Using a rainfall of the same intensity, calculations were also made to determine the effect of the drag which might result from rain impinging on the frontal area of the airplane. The results show that this effect, while not negligible, is not likely to force an airplane down. The drag effect varies with the size of the rain drops, but, assuming that the rain consisted entirely of large drops, the power absorbed by the rain while substantial, would be less than the power reserve available. Even if the reserve power were not drawn upon, the path angle and rate of descent
