On May 10, 2014, about 1215 central daylight time, a Pazmany PL-4 light sport airplane, N44PL, was substantially damaged during a forced landing near Yates City, Illinois. The pilot sustained minor injuries. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The local flight originated from the Tri-County Airport (2C6), Yates City, Illinois, at 1145.The pilot reported that he had completed a 20 minute flight in the local area, after which he returned to the airport and completed a landing and takeoff (touch and go) without incident. He remained in the airport traffic pattern after the touch and go. On the downwind leg, the pilot stated that he felt a complete loss of thrust as the engine speed increased, which he interpreted as a loss of engine power to the propeller. He established the airplane in a glide and set-up for a forced landing. During the glide, he increased engine speed without any accompanying increase in propeller thrust. He subsequently executed a forced landing to an open field; however, a hard landing resulted. The accident airplane was powered by a 65-horsepower Volkswagen engine, with a Culver Props 2-bladed, fixed pitch propeller. The installation included a Valley Engineering propeller speed reduction unit. The reduction unit utilized a cog belt/pulley arrangement to transmit power from the engine to the propeller. The most recent conditional inspection was reportedly completed on December 12, 2013; the airframe had accumulated 423 hours total time at the time of the inspection. The engine had accumulated 62 hours since overhaul, with 53 hours since the inspection. The propeller reduction unit had also accumulated 62 hours time in service. A postaccident examination revealed that the pulley supporting the propeller remained securely attached to the mounting bracket. However, the pulley had moved downward within the bracket slot creating slack in the cog-belts. The pilot/builder reported that the reduction drive unit was purchased from a private individual in combination with the engine, carburetor, and intake system. There was no documentation related to the reduction drive unit provided by the seller. The pilot/builder contacted the manufacturer of the unit to confirm compatibility to his installation. He also noted that the reduction drive components were not modified in any way prior to installation. The manufacturer stated that the reduction drive unit installed on the accident airplane was an earlier design. They noted that the installation included a fitting to support the pulley to prevent it from moving within the bracket slot during operation. However, no formal documentation was reportedly available for the initial design. He also noted that the mounting bracket used in the current design configuration does not include a slot. The fitting to support the pulley, preventing any downward movement within the bracket slot during operation, was not installed on the accident airplane.