How was this scene with a zombie getting its head cut off by helicopter rotors filmed?

FX Artist Tom Savini did the FX for Dawn of the Dead. He casted the head and molded it into foam latex and fishing line were added. He pumped blood through Jim's clothes to fake part of his forehead. The helicopter blades weren't running at the that time. An optical effect was added later in post-production.

From WIKIA page for the movie

One of the creative ways of killing people Savini came up with was the decapitation in which a zombie stands on boxes in the Monroeville Airport and gets part of his head chopped off by the helicopter rotor. A friend, Jim Krut, had a naturally low forehead and Savini asked if he'd want to take part in the movie. Krut said yes and Savini started off by casting his friend's forehead. Then he built it up higher which would give Jim a more normal-sized forehead. After molding the piece in foam latex, fishing line was applied to the sliced sections. While assistants pulled the line, and the chunks of skull seemed to tear away, Savini pumped stage blood through Jim's clothes up to the fake portion of his head, while hiding behind the on-screen boxes. The blades were never on — an optical effect added in post-production.