On April 12, 1999, about 0945 eastern daylight time, a Whitman Tailwind W-10, N173RS, registered to a private owner, operating as a 14 CFR Part 91 personal flight, crashed during a forced landing in the vicinity of Wimauma, Florida. Visual meteorological conditions prevailed and no flight plan was filed. The airplane sustained substantial damage. The private pilot and private pilot passenger reported minor injuries. The flight originated from Lakeland, Florida, about 15 minutes before the accident. The pilot stated she was in cruise flight at 1,000 feet when the airplane experienced an airframe vibration. She reduced power with negative results, and a forced landing was selected. During final approach she observed a ditch which crossed the forced landing area. She added power to miss the ditch however, the main landing gear collided with the ditch. The airplane continued forward, nosed over, and rotated to the left 90 degrees, and came to rest inverted. The source of the reported airframe vibration was not determined.