A Queens teenager’s obsession with high-powered model helicopters ended in gruesome death yesterday, when one of his beloved toy whirlybirds spun out of control in a Brooklyn park and scalped him, sources said.

Roman Pirozek, 19, — a chopper aficionado who bragged about his exploits on YouTube — was piloting the small machine at Calvert Vaux Park in Gravesend at around 3:40 p.m., when something went horribly wrong, cops said.

The toy aircraft turned toward the teen as he tried to pull off an unusual stunt. It hit him at high speed with the rotors spinning into his head and taking out “good chunk,” law enforcement sources said.

A handful of his friends watched in horror.

“I was playing at the park and we came into the clearing and we just saw a body on the floor,” said Maria Delgado, 13. “He was just decapitated — the whole top of his head was gone,”

Pirozek was pronounced dead at the scene, the FDNY said.

The park, on Shore Parkway near Bay 44th Street, is a sanctioned flying field by the Academy of Model Aeronautics (AMA) and popular spot for model flyer aficionados.

“I saw Roman this morning putting his helicopter [into] the station wagon, he seemed happy,” said neighbor Victor Tommaso, 68. “He would travel with it and he’s a real professional. He grew up flying [model] planes with his dad. Flying a helicopter was his passion. He grew up loving those planes.”

Pirozek was a member of the Seaview Rotary Wings club, whose members regularly fly planes at the park.

At their Woodhaven, Queens home, his family broke down in tears while talking of the loss.

“He was the best brother, he had the biggest heart,” sobbed his sister, Amy Pirozek. “I know today he went out flying by himself, it was his day off. He always protected me and was a good brother.”

The avid flyer had his own YouTube channel featuring many videos of his flights.

In April, he wrote on his channel page that his new blades “feel more stable and can handle the sudden and harsh maneuvers very well, and they are also very agile and smooth.”

Just last month he posted that he was “loving” his Trex 700N model — “one of the best nitros that I have ever flown with so much power.”

One helicopter he features on his YouTube page retails for over $1,000.

City Council member Dominic Recchia, who represents the 47th District, came out to the park to urge the Parks Department to put a moratorium on flying model aircraft until an investigation into the cause of death is completed.

Additional reporting by Erin Calabrese