A CESSNA 150 AND A HOME BUILT SKYBOLT ACFT COLLIDED WHILE ON A DOWNWIND LEG OF A TRAFFIC PATTERN TO LAND. THE SKYBOLT WAS A COMPETITOR IN A LOCAL AEROBATIC COMPETITION EAST OF THE ARPT. THE C-150 WAS RETURNING FROM A STUDENT PLT TRAINING X-COUNTRY FLT. BOTH ACFT HAD RADIOS AND HAD ANNOUNCED THEIR POSITIONS IN TRAFFIC. BOTH WERE FLYING STRAIGHT & LEVEL AT APRX 1800 FT AGL WITH THE SKYBOLT SLIGHTLY HIGHER. THE ACFT CONVERGED SLOWLY AT ABOUT THE SAME AIRSPEED. AN OFFICIAL OF THE COMPETITION WARNED THE ACFT OF THEIR PROXIMITY. THE CESSNA PLT BOTH SAW & FELT HIS PROP CONTACT THE TAIL SECTION OF THE OTHER ACFT AS IT CAME FROM BEHIND AND ABOVE HIM. THE SKYBOLT PLT FELT THE IMPACT AND THEN SAW THE CESSNA BELOW HIM. BOTH ACFT WERE ABLE TO LAND BUT THE SKYBOLT GROUNDLOOPED AFTER LANDING DUE TO DAMAGE TO THE LANDING GEAR. THE COMPETITION CONTESTANTS AND NORMAL TRAFFIC WERE INTERMIXED IN TRAFFIC WITH NO ATTEMPT AT SEGREGATION BY DESIGNATION OF DEDICATED AREAS FOR EACH.