On April 26, 2008, about 1645 mountain daylight time, an experimental Canup Titan Tornado S, N706BC, experienced the separation of a portion of its wood propeller while climbing to cruise altitude about five miles north of Council, Idaho. The private pilot, who was the sole occupant, was not injured, but the airplane sustained substantial damage to the propeller. The 14 CFR Part 91 personal pleasure flight, which departed Council, Idaho, about five minutes prior to the propeller failure, was en route to McCall, Idaho. The airplane was being operated in visual meteorological conditions. No flight plan had been filed. According to the pilot, while climbing through about 5,500 feet mean sea level (MSL), she heard a loud bang, followed by a severe vibration of the airplane. She immediately turned back toward Council Airport, and after she was sure she had the runway made, reduced power and continued the descent to a successful full-stop landing. After shutting down and inspecting the airplane, the pilot discovered that about four inches of the tip of one propeller blade was missing, and that the other blade was cracked along almost its entire span. Inspection of the Sensenich experimental-use-only W58DJL-50 wood pusher propeller by a Federal Aviation Administration Airworthiness Inspector revealed that the blade had failed at a point where it had been damaged by a foreign object. Further inspection revealed that at the point of failure there was a sharp-edged indentation about one-half inch long and about one-sixteenth to one-eighth inch deep.