 The powerplant and propeller examinations indicated that the engines were operating normally but that a propeller system malfunction occurred" which allowed the left propeller's angles to be oriented nearly perpendicular to the direction of flight, resulting in insufficient thrust and higher drag on the left side.The NTSB conducted a testflight in an EMB-120 with the left engine having the propeller control mechanism set to a similar mechanical condition but blocking the propeller blades from moving below 22 degrees to not endanger the flight crew.It was found that the flight crew would have been unable to perceive any problem with the airplane until the propeller blade angle was between 24 and 26 degrees.