Evaluating Painter, can't get gravity to work properly

I am evaluating Substance Painter, and the first object I am working on, I am trying to use the rain particle brush to get a certain look. I have rotated the object in my 3D programme so that the front is not facing the top. (I'm trying to emulate the kind of streaks you would get from fast flight). However, when I pick the rain brush and paint it on the surface, the droplets run down the object in what look like completley arbitrary directions. I have tried to adjust the properties, and although I get the strok to alther, the direction it runds down the object does not. Can you help, need this to work.