The pilot reported that the airplane’s airspeed was about 20 mph slower than normal when he turned on final approach. After clearing obstructions at the end of the runway the pilot started to flare for a landing when the airplane suddenly dropped and impacted the runway surface. The hard landing partially collapsed the left main landing gear and resulted in substantial damage to the fuselage. The pilot reported that there were no mechanical malfunctions or failures associated with the airplane prior to the accident and noted that the accident could have been prevented if he had maintained a safe airspeed.