N511 is taken. It's currently assigned to a 2004 Kimball James KIMBALL MCCULLOCOUPE registered in Mckinney, TX.
See the full aircraft record →A tail number only frees up if the owner deregisters the aircraft or moves it to a different N-number — and even then the FAA holds it before releasing it to the public.
Tails one nudge away from N511 that aren't assigned or reserved: