A STUDENT PLT IN CESSNA 152, N24516, TOOK OFF ON RWY 18 TO PRACTICE TFC PATTERN WORK & PERFORM TOUCH & GO LNDGS. ALSO, AN ATP PLT IN AEROSPATIALE AS-355F-1, N5796P, DEPARTED THE HELICOPTER OPNS AREA AT THE NORTH END OF THE ARPT ON A PSNG FLT TO A HOSPITAL ABOUT 3 MI TO THE NW. THE TOWER HAD CLOSED EARLIER (AT 1800 CDT). SVRL ARPT PSNL WERE ON DUTY (INCLUDING PSNL AT OPERATOR FACILITIES); THOSE INTERVIEWED, DID NOT RECALL HEARING A RADIO ADZY FM EITHER PLT. AT ABOUT 1846 CDT, THE 2 ACFT COLLIDED ABOUT 1.4 MI NW OF THE ARPT AT AN ESTD ALT OF 500 TO 600 FT. WITNESSES RPRTD THAT THE HELICOPTER WAS CLIMBING ON A NW HDG, WHILE THE CESSNA WAS IN LVL FLT. ONE WITNESS THOUGHT THE CESSNA HAD TURNED ONTO A BASE LEG FOR RWY 12. JUST BEFORE IMPACT, THE CESSNA ENTERED AN ABRUPT LEFT CLIMBING TURN & ROLLED OUT OF THE TURN SLIGHTLY ABV & TO THE RGT OF THE HELICOPTER. SUBSEQUENTLY, THEY CONVERGED & THE HELICOPTER'S MAIN ROTOR BLADES STRUCK THE LEFT SIDE OF THE CESSNA, THEN BOTH CRASHED. THE HELICOPTER WAS NOT OBSERVED TO TAKE EVASIVE ACTN.