PLT OF BE90B DEPARTING NON TWR ARPT ON IFR CLRNC HAS CLOSE ENCOUNTER WITH A VFR ACFT ON APCH TO LAND.

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LIKE ALL INCIDENTS; A SERIES OF CIRCUMSTANCES LED TO SIT WHICH WAS POTENTIALLY DANGEROUS. EVEN THOUGH ON WEEKENDS IT IS ONE OF THE BUSIEST IN SOUTH CAROLINA; HILTON HEAD ISLAND'S ARPT DOES NOT HAVE A CTL TWR. SINCE THE WIND WAS CALM; PLTS WERE USING BOTH RWY DIRECTIONS. SINCE THE WX WAS MARGINAL VFR; BOTH IFR AND VFR OPS WERE TAKING PLACE. ALTHOUGH I PLAN TO INSTALL ONE; MY ACFT DOES NOT HAVE A TCASII. I WAS GIVEN AN IFR RELEASE BY SAVANNAH APCH CTL (121.1 MHZ) TO DEPART RWY 3 FOR AN IFR FLT FROM HXD TO CHO. THE CTLR DID ADVISE THAT THERE WAS A VFR ACFT IN THE AREA. AS I RECALL; I MADE A BRIEF ANNOUNCEMENT ON UNICOM (123.0 MHZ) BEFORE TAXIING ONTO THE RWY BUT; QUICKLY; TRANSFERRED TO BEAUFORT APCH CTL (125.6 MHZ). SHORTLY AFTER ROTATION; I SAW A LIGHT TWIN ON APCH TO RWY 21. I BROKE TO THE R AS DID THE PLT OF THE OTHER ACFT. OUR 2 PLANES PASSED NO CLOSER THAN 1000 FT. I CONTRIBUTED TO THE SIT IN 3 WAYS. FIRST; I SHOULD HAVE QUIZZED THE SAVANNAH APCH CTLR ABOUT THE EXACT LOCATION OF THE VFR ACFT. SECOND; I SHOULD HAVE HAD A BETTER PRACTICAL UNDERSTANDING THAT AN IFR RELEASE AT AN UNCTLED FIELD DIFFERS FROM THAT OF ONE AT A CTLED FIELD. AT A THEORETICAL LEVEL; I UNDERSTAND THE DIFFERENCES. HOWEVER; I REACTED AS IF THE AIRSPACE WAS BOTH CLR AND FULLY PROTECTED. FINALLY; I SHOULD HAVE USED THE UNICOM MORE PROACTIVELY. RATHER THAN MAKING A PASSIVE ANNOUNCEMENT; I SHOULD HAVE ASKED WHERE THE VFR TFC WAS AND WHAT WERE HIS INTENTIONS. I THINK THE CTLR SHOULD HAVE ALERTED ME AS TO EXACT LOCATION OF THE VFR ACFT. HAD HE DONE THAT; I OBVIOUSLY WOULD NOT HAVE DEPARTED. ALSO; PLTS SHOULD HAVE A BETTER PRACTICAL (AS OPPOSED TO THEORETICAL) UNDERSTANDING OF ATC IFR DEP AND ARR LIMITATIONS AT UNCTLED ARPTS. I WONDER IF THE TRAGIC ACCIDENT AT QUINCY; IL; LAST YR WAS NOT PARTIALLY CAUSED BY A PLT'S MISUNDERSTANDING OVER AN IFR RELEASE? I HAVE BEEN ATTENDING FLT SAFETY ON A REGULAR BASIS FOR 4 YRS. NOTHING HAS BEEN TAUGHT TO ME IN EITHER THE BARON OR C90 KING AIR PROGRAMS ON POTENTIALLY DANGEROUS SITS LIKE THE ONE I RECENTLY EXPERIENCED. I WILL BRING THIS UP WITH THEM AND SUGGEST THAT THEIR PROGRAMS BE MODIFIED.