PVT PLT OF A PIPER J3 CUB WAS INTERCEPTED BY A GOV HELI AND HERDED BACK TO A NEARBY UNCTLED ARPT AFTER PENETRATING CLASS C AIRSPACE.

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I WAS FLYING E ALONG THE AMERICAN RIVER ON THE S SIDE OVER GOETHE PARK. I MADE A 180 DEG TURN TO THE L. UPON COMPLETION OF THE 180 DEGS; I NOTICED A ROTARY WING ACFT COMING DIRECTLY AT ME APPROX 1/4 MI AWAY AT THE SAME ALT. I DSNDED AND DEVIATED MY COURSE TO THE S TO AVOID THE OTHER ACFT. AS I PASSED THE OTHER ACFT I BEGAN TO CLB BACK TO MY ORIGINAL ALT. I ALSO NOTICED THE OTHER ACFT WAS A (LAW ENFORCEMENT) HELI. THE (LAW ENFORCEMENT) HELI CIRCLED TO FOLLOW AND BEGAN FLYING IN FORMATION AS CLOSE AS 400 FT. I MADE NUMEROUS CLRING TURNS IN ORDER TO SEE WHERE THE OTHER ACFT WAS AND EACH TIME BOTH I AND THE HELI HAD TO TAKE EVASIVE ACTION TO AVOID A COLLISION. THIS CONTINUED FOR 30 MINS UNTIL I MADE IT BACK TO MY HOME BASE Q96. DURING THIS TIME; AS THE HELI WAS HERDING ME BACK TO Q96; I WAS FLYING IN THE VICINITY OF CLASS C AND D AIRSPACE. SINCE THE J3 CUB I WAS FLYING IN HAS NO RADIO EQUIP; I WAS UNABLE TO CONTACT APCH. HOWEVER; THERE WOULD HAVE BEEN NO NEED IF I WASN'T CONCENTRATING ON AVOIDING A COLLISION WITH THE ACFT VIOLATING FAR 91.111 (A) AND (B).