AFTER LNDG, THE PLT SEEMED CONCERNED ABOUT THE ACFT'S WT & ASKED THE ATTENDANT TO SVC THE FUEL TANKS THE BOTTOM OF THE TABS. AFTER LOADING WITH 3 PSGRS & BAGGAGE, HE TOOK OFF ON RWY 27. WINDS WERE RPRTD TO BE FM 260 DEG AT 20 GSTG 30 KTS; LOW LEVEL WIND SHEAR & TURBULENCE WERE RPRTD IN THE AREA. ACCORDING TO WITNESSES, THE ACFT SEEMED TO ACCELERATE SLOWLY &IT LIFTED OFF AFTER ROLLING ABOUT 1500 TO 2000 FT. THEY NOTED THAT IT WAS CLIMBING AT A SLOW RATE & TURNED LEFT (TO THE SOUTH, THEN EAST) AT 150 TO 200 FT AGL. ONE WITNESS NOTED THE ACFT'S NOSE WAS HIGHER THAN USUAL. ANOTHER RPRTD THE ACFTWAS 'STAGGERING: A SLOW PORPOISING MOTION NOT GAINING AIRSPEED.' THE ACFT THEN BANKED LEFT, DSCNDD & IMPACTED, RGT MAINGEAR 1ST ON A HDG OF 080 DEG. AFTER INPACT, FIRE ERUPTED, & THE ACFT SLID 105 FT & BURNED. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FND. THE ACFT WAS ESTD TO HAVE BEEN LOADED TO 3680 LBS WITH A CG OF 89.4 INCHES. THE CERT MAX GROSS WT WAS 3600 LBS; THE CG LIMITS WERE FROM 81.0 TO 87.7 INCHES.