C172 PLT ON DEP FROM NON TWRED ARPT EXPERIENCED A CONFLICT WITH A NORDO MEDICAL HELI.

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AS WE PREPARED TO TAKE OFF FROM RWY 34 AT ZZZ WE OBSERVED THE MEDICAL HELI TRANSITION FROM SW TO NE AT WHAT APPEARED TO BE TFC PATTERN ALT; WITHIN 1 MI OF THE ARPT. THIS IS NOT UNUSUAL; BECAUSE THEY FREQUENTLY TRAVEL BTWN ZZZ2 AND ZZZ3. HOWEVER; THERE WAS NO COM FROM THE HELI AS IT MADE ITS TRANSITION TO THE NE. HAVING SEEN THIS; WE PROCEEDED TO ANNOUNCE OUR INTENTIONS ON CTAF. WE TAXIED TO RWY 34 AND DEPARTED. ON UPWIND APPROX 800-900 FT MSL; WE OBSERVED THE HELI AT OUR ALT XING OUR FLT PATH FROM R TO L APPROX 1000 FT IN FRONT OF US. THE HELI HAD BEEN PREVIOUSLY OUT OF SIGHT DUE TO SURROUNDING TERRAIN DURING THE CLBOUT. WE WERE FORCED TO EXECUTE A 90 DEG TURN TO THE L TO AVOID A COLLISION COURSE WITH THE HELI. THE HELI THEN PROCEEDED IN A WESTERLY DIRECTION AND DSNDED INTO A FIELD TO MEET LCL EMER PERSONNEL. AT NO TIME DURING THE ENTIRE EVENT DID THE HELI MAKE COMS ON ZZZ CTAF. IT SEEMED AS IF NO REGARD WERE GIVEN FOR THE PROX OF AN ARPT. SHORTLY AFTER THE INCIDENT; WE CONTACTED ANOTHER ACFT ON THE CTAF TO INFORM THEM OF THE HELI'S PRESENCE. THAT ACFT ACKNOWLEDGED OUR XMISSION; LEADING ME TO BELIEVE OUR XMITTER/RECEIVER WAS FUNCTIONING PROPERLY.