A young man was scalped and killed by the blades of a model remote-controlled helicopter that chopped off the top of his head in Gravesend, Brooklyn today, sources said. The 19-year-old victim — a well-known model helicopter enthusiast — was on the corner of Shore Parkway and Bay 44th Street around 3:40 p.m., when he lost control of the toy aircraft, it boomeranged and sliced off the top of his head