On December 25, 1999, about 1400 Eastern Standard Time, a homebuilt RV-4, N8ZZ, was substantially damaged during a forced landing in Florence, New Jersey. The certificated airline transport pilot received minor injuries, and visual meteorological conditions prevailed at the time of the accident. No flight plan had been filed for the local flight, from Northeast Philadelphia Airport (PNE), Philadelphia, Pennsylvania. The personal flight was conducted under 14 CFR Part 91. According to the pilot, the flight originated about 1330. Engine start, taxi, and engine run-up were all normal. About 30 minutes after takeoff, the pilot felt a "buzz," which lasted about 2 seconds. The airplane then shook violently, so the pilot retarded the throttle to reduce the shaking. He then turned the magnetos off, one at a time, and reapplied power, but the shaking returned with each power application. Carburetor heat was also applied, with no effect. The pilot then tried to add "enough power to hold altitude," but again found that its application resulted in violent shaking. "Rather than risk shaking [the airplane] apart," power was again reduced to keep the airplane controllable. The pilot attempted to guide the airplane to a nearby airport, but could not hold altitude, and headed towards a field. During the approach to the field, the vibration was so severe that the pilot had trouble seeing the airspeed indicator. Nearing the landing, the pilot reduced power, "to see what I was going to hit." The pilot saw that the field had been plowed, and attempted to align the airplane with the furrows, but couldn't due to the airplane's approach angle. Upon touchdown, the landing gear collapsed, and the wing spars were damaged. After the accident, the pilot found only one of the two wooden propeller blades. According to a Federal Aviation Administration inspector, after the airplane was removed from the accident site to a storage facility, one of the wooden propeller blades was missing. There was no evidence of a bird strike. The propeller had been bolted onto the hub with the correct torque's, and the cowling had been found securely fastened to the airframe. The engine was also examined, with no significant anomalies noted, and there was no indication of airframe flutter. The owner of the airplane stated that there were only 5 to 6 hours of flight time on the Aymar-Demuth propeller. "This prop was beefy - built to handle the torque." The propeller was the third one on the airplane, which was built in 1994. The owner had flown the airplane to Oshkosh in the summer of 1999, and while there, a crack had been found in one of the blades. The manufacturer shipped him a 160-horsepower-rated loaner propeller to get the airplane home, and once there, the 180-horsepower accident propeller was installed. The owner also noted that after every time the airplane was flown, he cleaned and polished the propeller, and that before the airplane took off on the day of the accident, it was "perfect."