DURING THE TAKEOFF INITIAL CLIMB, THE RIGHT HALF OF THE ENGINE COWLING CAME LOOSE. THE PILOT STATED TO THE INVESTIGATOR IN CHARGE THAT HE IMPROPERLY LATCHED THE COWLING ON THE PREFLIGHT. A TURN WAS INITIATED TOWARD THE AIRPORT AND ON A 45 DEGREE ANGLE TO THE RUNWAY THE AIRPLANE STALLED AND IMPACTED THE TERRAIN SHORT OF THE RUNWAY.