Dr. James Valentine, a fluid dynamicist whose work was subsequently recognized by the National Research Council's Transportation Research Board as one of the five outstanding aviation research projects in 1994, found that a thin water film forms on the airfoil surface by the fraction of the raindrop that is not splashed back. The raindrops form small impact craters and surface waves in the water film, which roughens the airfoil surface. As with ice or frost, this creates additional surface friction on the boundary layer, and thus increases the negative effects on the separated boundary layer. The net effect is a loss of lift, an increase in drag and a premature separation of the boundary layer, all of which have negative effects on an airfoil's performance. These are most pronounced in high-lift configurations with flaps and slats deployed, and are most severe at high angles of attack, which is the aircraft's normal configuration while landing. Thus, when the downpour occurs when an aircraft is in a high-lift configuration with leading edge slats and double-slotted trailing-edge flaps extended, the airfoil experiences a decrease in maximum lift of up to 18 percent, an increase in drag of up to 40 percent, and perhaps most importantly, a decrease in stall angle of attack of up to eight degrees. An "average" airfoil typically stalls around 17 degrees, thus one with a contaminated surface could stall at just nine degrees AOA.
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