On August 25, 2019, about 1725 eastern daylight time, a Piper J3C airplane, N49614, was substantially damaged during a forced landing to a field near Russiaville, Indiana. The pilot and passenger were not injured. The airplane was owned and operated by a private individual under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no Federal Aviation Administration (FAA) flight plan had been filed for the flight. The personal cross-country flight departed Frankfort Municipal Airport (FKR), Frankfort, Indiana, about 1715, and was en route to Huntington Municipal Airport (HHG), Huntington, Indiana. According to the pilot, he departed FKR, after a fuel stop, on a multi-leg cross-country flight. About 15 miles east of FKR, he noticed oil on the side window and the oil pressure gauge indicated a loss of oil pressure. The pilot was not sure that the engine would continue to run if he diverted to the nearest grass strip, so he executed a precautionary landing to a field. The pilot landed the airplane with the plowed furrow lines; however, the pilot was not able to stop the airplane before the end of the field. The pilot attempted to ground loop the airplane; however, the right main landing gear collapsed and the airplane nosed into a fence and corn crop. The right wing ribs and spar were substantially damaged. A post-accident examination of the airplane revealed that the oil cap was missing from the engine. Subsequently, an airport employee at FKR found an oil cap on the ramp and the pilot confirmed that the oil cap belonged to the airplane. The pilot stated that he added oil to the engine while on the ground at FKR. He added he was in a hurry to depart so not conflict with skydiving traffic, he did not take the time to verify that the oil cap was properly reinstalled. No other mechanical anomalies were noted that would preclude normal operations.