A double amputee was arrested at his Brooklyn nursing home after he was allegedly caught raping and beating another patient - a 78-year-old bedridden grandmother, police said. The suspect, Samuel Irving, 43, a new patient at the Greenpark Care Center in Fort Greene, was discovered by a staffer sexually assaulting the elderly woman as she lay helplessly in her bed at around 8:30 p.m. Friday, a hospital official said.