C152 EXECUTES A GAR AT HWO AFTER ENCOUNTERING DOWNWASH FROM A HELI.

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I HAD JUST COMPLETED 1 HR OF AIR WORK FOR MY BFR WITH A FLT INSTRUCTOR IN THE R SEAT OF THE ACFT AND I HAD FLOWN BACK TO NORTH PERRY ARPT TO DO 3 LNDGS. THE WIND WAS OUT OF 210 DEGS AT 11 KTS. I WAS CLRED BY THE TWR TO DO A TOUCH-AND-GO ON RWY 18R AND ADVISED THAT A HELI WAS JUST OFF THE R SIDE OF THE RWY HOVERING ABOVE THE GRASS AND THAT THE HELI WOULD REMAIN THERE. I ACKNOWLEDGED THAT INFO AND SET UP MY APCH. I SAW THE HELI; BUT DIDN'T THINK IT WOULD AFFECT ME; SO I USED A NORMAL APCH. I HAD 30 DEGS FLAPS SET; CAME OVER THE THRESHOLD AND JUST AS I WAS ABOUT OVER THE NUMBERS; THE ROTOR DOWNWASH FROM THE HELI HIT THE AIRPLANE AND IN A SPLIT SECOND THE ACFT WAS ROLLING TO THE L. I IMMEDIATELY PUT IN R AILERON; RUDDER AND FULL THROTTLE AS THE PLANE FLEW OVER THE GRASS ON THE L SIDE OF THE RWY. SIMULTANEOUSLY; THE FLT INSTRUCTOR DID THE SAME THING; GRABBING THE CTLS; EVEN THOUGH I HAD MADE ALL THE SAME CORRECTIONS. AS THE ACFT STARTED TO CLB; THE INSTRUCTOR SAID; 'I'VE GOT THE CTLS. I'VE GOT THE CTLS.' SO I RELEASED THE CTLS TO HIM. I ADVISED THE TWR I WAS GOING AROUND; INFORMED THE CTLR THAT THE HELI'S ROTOR DOWNWASH WAS BLOWING ACROSS THE THRESHOLD; AND THEN THE INSTRUCTOR GAVE THE CTLS BACK TO ME AFTER ABOUT 10 SECONDS. I CLBED OUT ON RWY HDG OVER THE GRASS. I ASKED THE TWR IF I COULD CHANGE TO RWY 18L; BUT WAS TOLD; 'NOT AT THIS TIME.' I FLEW AROUND THE PATTERN; THIS TIME DETERMINING TO LAND LONG; BUT IT WASN'T NECESSARY BECAUSE WHILE ON BASE LEG THE CTLR TOLD ME TO EXTEND MY BASE FOR RWY 18L. I DID 3 LNDGS AND EXITED THE RWY. THIS SITUATION HAPPENED FOR SEVERAL REASONS. ALTHOUGH I HAD READ ABOUT WINGTIP WAKE VORTICES AND ROTOR BLADE DOWNWASH; I HAD NEVER ACTUALLY EXPERIENCED EITHER PHENOMENA; SO I DIDN'T CONSIDER THAT THE ROTOR DOWNWASH WOULD HIT THE ACFT. I SHOULD HAVE STAYED HIGH ON FINAL AND LANDED LONG; WELL PAST THE HELI. THE CTLR SHOULD NOT HAVE ALLOWED THE HELI TO HOVER SO CLOSE TO THE RWY. ALSO; THE XWIND CONTRIBUTED BY BLOWING THE ALREADY STRONG ROTOR DOWNWASH DIRECTLY OVER THE THRESHOLD OF THE RWY. ULTIMATELY; AS PIC; IT WAS MY ERROR AND I HAVE LEARNED A VALUABLE LESSON. FORTUNATELY; THE OUTCOME WAS POSITIVE AND I USED IT AS A REAL-LIFE CLASSROOM LEARNING EXPERIENCE.