Below you will see an FAA approved navigation chart which clearly shows an FAA-approved helicopter route over Prospect Park and Park Slope.  They even have a name for it.  They call it the "Park" route.  Although the helicopter pilots are supposed to follow the blue "highways in the sky" routes, they often times deviate.  As anyone who visits Park Slope and Prospect Park knows, that means they fly directly over the Park and the Historic neighborhood of Park Slope on regular intervals.