On April 11, 1998, approximately 1920 central daylight time, a Waxenfelter Thorp T-18 experimental airplane, N18WX, owned and operated by the pilot, was substantially damaged following a loss of control while landing at the Petit Jean Park Airport, near Morrilton, Arkansas. Visual meteorological conditions prevailed, and a flight plan was not filed for the Title 14 CFR Part 91 personal flight. The private pilot and passenger were not injured. The flight originated from Russellville, Arkansas, about 1900. The pilot reported to the NTSB investigator-in-charge that, "very little" cross wind correction was required during the landing to runway 20. During the landing roll, he held the tail off until it came down "normally." A short distance after the tail wheel touched down, the aircraft started to "abruptly" veer to the left. "Normal amount of correction (right rudder) was totally ineffective." As the aircraft continued to veer left, the pilot applied full right rudder and some brake; however, it did not have any effect. The aircraft departed the left side of the runway with "full right rudder, right brake locked, and stick full back." The aircraft was recovering to the right when it slid into a tree, structurally damaging the left wing and left horizontal stabilator. In the enclosed NTSB Form 6120.1/2, the pilot reported that he had accumulated a total of 99 hours in his tailwheel equipped airplane. He further reported that the wind was from 170 degrees at 4 to 5 knots. His examination of the aircraft after the event did not reveal any mechanical problems with the flight controls.