The pilot reported that he was taxiing the airplane on the grass beside the runway when the airplane went into a ditch. The airplane's tail and propeller were damaged. The Federal Aviation Administration inspector who examined the airplane reported that the empennage had sustained substantial damage. The inspector reported that the airplane was taxiing on the grass parallel to closed runway 27. The grass was wet and the airplane was too close to the ditch when it slid into the ditch.