A PVT PLT & INSTRUCTOR (CFI) WERE ON A LCL FLT WITH A 3RD OCCUPANT ABOARD. THE PURPOSE OF THE FLT WAS TO PROVIDE DUAL INSTRUCTION TO THE PVT PLT SO HE COULD OBTAIN A MULTI-ENG RATING. AT 1818, 1 OF THE PLTS CONTACTED HOUSTON TRACON & REQDRADAR ADZY & SAID THEY INTENDED TO DO SOME AIR WORK IN THE AREA. THE LAST TRANSMISSION WITH THE ACFT WAS AT 1847 WHEN THE CTLR ADZD THEY WERE CLR OF OTR TRAFFIC. APRX 4 MIN LATER, RADAR CONTACT WAS LOST. THE ACFT WAS FND WHERE IT HAD IMPACTED THE GND IN A RELATIVELY FLAT, WINGS LVL ATTITUDE. AN INV REVEALED EVIDENCE THE ACFT WAS SPINNING LEFT WHEN IT CRASHED. RADAR DATA SHOWED EVIDENCE THAT THE PLTS HAD MADE A SERIES OF STALLS JUST BFR THE ACFT ENTERED A RAPID DSCNT FMAPRX 7800' (DENSITY ALT APRX 10,250'). FOR THE ACFT'S WT & CONFIGURATION, STALL SPEED (VS) WAS DETERMINED TO BE 81 KTS. THE PUBLISHED MIN CTL SPEED (SEA LVL CONDS) WAS 81 KTS & THE INTENTIONAL 1 ENG INOP SPEED (VSSE) WAS 86 KTS.