During cruise flight in smooth air, with the autopilot engaged, the experimental category amateur-built airplane suddenly encountered a single occurrence of rudder flutter which lasted two to three seconds and then ceased. One passenger described the “frequency of vibration as about 10 Hz” (cycles per second), and described the sound as “BRAAP.” The pilot disengaged the autopilot and continued to his original destination for an uneventful landing. A subsequent examination of the airplane found substantial damage to the stringers, frames, and empennage skin in the area just forward of the horizontal stabilizers. The pilot, who was also the builder of the airplane, had added an electrically actuated rudder trim tab several years earlier. Most of the weight of the rudder assembly was aft of the rudder hinges and the builder had not installed rudder counterbalance weights forward of the hinges. In addition, he had recently installed struts on the bottom of the horizontal stabilizer. Airworthiness standards for certification of normal category airplanes require that they must be shown to be free from flutter. Airworthiness standards for certification of experimental category airplanes do not require that they meet those standards.