C150 AND PA28 HAD NMAC DEPARTING EUL.

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INCIDENT OCCURRED AT CALDWELL INDUSTRIAL ARPT. DURING PREFLT; I SAW 3 OTHER ACFT USING RWY 12. I TAXIED TO THE DEP END OF RWY 12; BEHIND A YELLOW AND WHITE PIPER TRI-PACER. THE TRI-PACER ANNOUNCED ON UNICOM THAT HE WAS DEPARTING ON RWY 12; AND WOULD STAY IN THE PATTERN. THE TRI-PACER TOOK OFF; I WAITED WHILE A C180 OR C185 MADE A STRAIGHT-IN APCH AND LANDED ON RWY 30. WHILE I WAS TAXIING ONTO RWY 12; A PIPER ARROW RPTED HE WAS '5 MI E.' I ANNOUNCED ON UNICOM THAT I WAS 'DEPARTING RWY 12; LEAVING THE AREA NBOUND.' THE ARROW THEN ANNOUNCED HE WOULD ENTER R DOWNWIND FOR RWY 30. I TOOK OFF ON RWY 12 AND TURNED N APPROX 1/2 MI PAST THE END OF THE RWY. AFTER TURNING N; I SAW THE ARROW ON DOWNWIND FOR RWY 30; ON A DIRECT COLLISION COURSE. I DSNDED SLIGHTLY; FLEW DIRECTLY UNDER THE ARROW; THEN RESUMED CLBING. CONTRIBUTING FACTORS WERE: 5 ACFT HAD USED RWY 12 IN PREVIOUS 10 MINS. THE CESSNA AND ARROW IGNORED THIS AND LANDED ON RWY 30. ALSO; THE ARROW RPTED HIMSELF TO BE '5 MI E;' AND WAS IN THE PATTERN; EBOUND; APPROX 90 SECONDS LATER.