The home built gyrocopter was being flown by the owner/builder/designer who had an airplane, single-eng, land rating. Shortly after the gyrocopter took off, the engine was operating at a high RPM when a witness heard a pop/noise. Subsequently, the gyrocopter crashed in a corn field about 1-1/2 miles from the departure point. The main wreckage was found inverted. One rotor blade, the oil cooler, and the left vertical stabilizer and rudder were found 60 to 150 feet from the main wreckage. Both of the wooden prop blades had shattered. The oil cooler had been installed below the engine and just forward of the prop. Its upper side had been attached with a single solid mount using an 'L' shaped aluminum strap. Its lower side was fastened with a semiflexible mounting. There was evidence that the upper mount had failed and allowed the oil cooler to move aft into the plane of rotation of the prop blades. Except for the oil cooler bracket, no pre-accident mechanical problems were found.