AEROBATIC FLT OVER ARPT RESULTING IN NMAC WITH ACFT MAKING ENTRY INTO TRAFFIC.

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TEN MI S; I CALLED UNICOM; ANNOUNCED MY POS/INTENTIONS; AND ASKED FOR RWY/WIND INFO. THE UNICOM OPERATOR DID NOT REPLY UNTIL MY SECOND CALL. SHE SAID WIND FAVORED RWY 6. AS I WAS PASSING OVER THE CENTER OF THE ARPT PREPARING TO ENTER LEFT DOWNWIND; A SMALL LOW-WING SPORT PLANE APPEARED DIRECTLY IN FRONT OF ME (ABOUT 500 YARDS AWAY) MAKING A VERTICAL CLB FROM BELOW ME. HE (SHE?) MADE TWO VERTICAL ROLLS AS HE PASSED THROUGH MY ALT; DID A HAMMER HEAD STALL MANEUVER ABOVE ME; AND THEN DSNDED STRAIGHT DOWN THROUGH MY ALT IMMEDIATELY TO MY L FRONT AND QUITE CLOSE. I DIDN'T TAKE EVASIVE ACTION BECAUSE IT ALL HAPPENED SO FAST; AND I HAD NO WAY OF KNOWING WHICH WAY HE WAS GOING NEXT. HE DISAPPEARED FROM VIEW AS I TURNED ONTO L DOWNWIND AND SUBSEQUENTLY LANDED. CHKING LATER WITH THE FSS AND THE FSDO; I LEARNED THAT THE ARPT WAS HOME TO AN AEROBATIC GROUP WHICH HAD AN APPROVED AEROBATIC BOX THAT WAS ACTIVE AT THE TIME JUST E OF THE ARPT. I DO NOT BELIEVE THIS GUY WAS IN HIS BOX. WHETHER OR NOT HE WAS; THAT WAS A DUMB PLACE TO PUT AN AEROBATIC BOX. IN ANY EVENT; HE SHOULD HAVE BEEN MORE CONSIDERATE OF US ORDINARY PLTS. THIS CLOSE CALL COULD PROBABLY HAVE BEEN PREVENTED IF: 1. THE UNICOM OPERATOR HAD WARNED ME OF THE AEROBATIC OPS SHE KNEW TO BE IN PROGRESS. 2. I HAD CHKED NOTAMS BEFORE FLT. 3. THE AEROBATIC BOX HAD BEEN PLACED WELL TO THE W OF THE ARPT (AND V-51 AIRWAY WHICH OVERLIES THE ARPT) WHERE THERE ARE HUNDREDS OF MI OF SPARSELY POPULATED FLATLAND. 4. THE AEROBATIC PLT HAD HAD THE COURTESY TO USE HIS RADIO TO WARN THE REST OF US OF HIS INTENTIONS. CALLBACK CONVERSATION WITH RPTR REVEALED FOLLOWING. RPTR DID NOT TALK WITH PLT. LARGE HANGAR ON THE ARPT INDICATES THAT AEROBATIC ACFT ARE BASED AT THIS FAC. NON TWR ARPT AND HE WAS MONITORING CTAF BUT HEARD NO CALLS FROM THE AEROBATIC ACFT. MISS DISTANCE WAS LESS THAN 500' AND HE HAD NO TIME FOR EVASIVE ACTION. DID DISCUSS WITH FSDO BUT DID NOT FILE A NMAC RPT. THE AEROBATIC AREA IS JUST TO THE NE OF THE ARPT BUT STILL OVER LAND.