THE INSTRUCTOR (CFI), WHO SURVIVED, WAS FLYING THE ACFT FM THE RGT FNT SEAT WHILE THE OWNER/PLT WAS IN THE LEFT SEAT. THE FLT HAD PROGRESSED NORMALLY UNTIL THE PLTS RTRND TO THE ARPT TO LND. CLNC WAS RECEIVED TO LND ON RWY 9L. ACCORDING TO THE CFI, HE LOWERED THE LNDG GEAR, SELECTED 10 DEG OF FLAPS & NOTED HE NEEDED EXCESSIVE BACK PRESSURE ON THE CTL YOKETO KEEP THE ACFT FM DSCNDG. ALSO, HE STATED HE WAS UNABLE TO RELIEVE THE PRESSURE WITH ELEC OR MANUAL TRIM. HE ASKED THE PLT/OWNER TO USE HIS ELEC TRIM (ON THE LEFT YOKE), BUT THIS DID NOT HELP. THE CFI STATED HE RETRACTED THE FLAPS & INCREASED PWR, BUT THE ACFT CONTD TO SETTLE. SUBSEQUENTLY, IT HIT A TREE & A POLE, THEN IMPACTED THE GND & STRUCK A VEHICLE BEFORE STOPPING. A FIRE ERUPTED & ALL 3 OCCUPANTS WERE BURNED WHILE EVACUATING THE ACFT. THE PLT/OWNER DIED FM HIS INJURIES. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FND DRG THE INVESTIGATION. FLT TEST DATA CONCERNING THRUST- DRAG RATIO SHOWED THAT INDUCED DRAG INCREASES RAPIDLY BELOW 90 KTS. WITNESSES SAID THE ACFT WAS LOW/SLOW ON FINAL APCH.