PRIOR TO THE FLT, THE PLT STATED THAT HE WOULD BE FLYING FORAPRX 1/2 HR. APRX 25 MIN AFTER TAKEOFF, GROUND WITNESSES NORTHEAST OF THE ARPT OBSERVED THE ACFT ON A WESTERLY HEADING, TURNING TOWARD THE ARPT. SHORTLY AFTER THAT, THEY SAW THE ACFT IN AN UNCONTROLLED DESCENT. THEY DESCRIBED IT AS FLIPPING OR CORKSCREWING IN A NOSE DOWN ATTITUDE. A PLT, WHO WAS FLYING AT ABOUT 1350 FT MSL, ALSO SAW THE ACFT SPINNING AT APRX HIS ALT. HE STATED THAT THE ACFT APPEARED TO BE RECOVERING, BUT WAS STILL OUT OF CONTROL, WHEN HE LOST SIGHT OF IT. ANOTHER PERSON IN A WOODED AREA ABOUT 1000 TO 1500 FT FROM THE IMPACT POINT HEARD THE ACFT BUT DID NOT SEE IT. HE SAID THE ENG SOUNDED NORMAL UNTIL SECONDS BEFORE HE HEARD THE PLANE CRASH. AN EXAM OF THE WRECKAGE REVEALED THAT THE LEADING EDGES OF BOTH WINGS WERE COMPRESSED. NO EVIDENCE OF A PREIMPACT/MECHANICAL MALFUNCTION WAS FOUND.