On June 4, 1999, at 0930 central daylight time, an Aero Commander 500, N711NE, owned and piloted by a private pilot, received substantial damage during a wheels-up landing on runway 29 (5,100 feet by 75 feet, dry concrete) at Effingham County Memorial Airport, Effingham, Illinois Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 flight was operating on an instrument flight rule (IFR) flight plan. The pilot reported no injuries. The business flight originated Sumner County Regional Airport, Gallatin, Tennessee, at 0800, en route to Effingham County Memorial Airport. The pilot reported that while in the traffic pattern he had lowered the landing gear and completed a checklist consisting of seven items, one of which was the landing gear. He stated that on the base leg he began to add flaps. He touched down on runway's centerline; at which point, he began to hear a low vibration. He added that when the throttles were retarded to the idle position, the gear warning horn sounded, which he initially thought was the stall warning horn. The pilot stated that during the aircraft's recovery, he had selected the master switch to the "ON" position, and the landing gear warning horn sounded. He then stated that when he lowered the landing gear handle the landing gear extended. The pilot described the placement of the flap control and landing gear control as being separated by a distance of approximately five inches.