On May 27, 1998, at 1135 central daylight time (cdt), a Waters Vortex gyroplane, N1025Y, impacted the terrain during a forced landing after losing rotor rpm while en route to a private landing strip in Davis, Illinois. The pilot received minor injuries and the aircraft sustained substantial damage. The aircraft was being operated as a personal flight under 14 CFR Part 91 when the accident occurred. The flight originated in Monroe, Wisconsin, at 1125 cdt. Visual meteorological conditions prevailed at the time and no flight plan was filed. The pilot stated that shortly after departure from Monroe Municipal Airport while at about 1,200 feet above ground level, he felt the gyroplane slowing down and he noticed a loss of rotor RPM. He initiated an emergency descent and elected to land straight ahead in a cultivated field. The pilot said he realized he was not going to clear the trees. He initiated a left 180 degree turn and continued his descent to where the gyroplane impacted the terrain. The pilot reported that the left landing gear dug into the soft plowed field and the aircraft rolled over onto its left side. Post accident inspection of the gyroplane by Inspectors from the Federal Aviation Administration's Milwaukee Flight Standards District Office failed to reveal any mechanical failure and/or malfunction of the gyroplane or the engine which would have prevented normal operation. The pilot installed a pre-rotor system on the gyroplane the weekend before the accident. Post accident inspection of the pre-rotor system failed to reveal any anomalies.