Navion pilot reported landing on the runway gear up and was assisted by airport personnel in tugging the aircraft to the hangar. It was a hot day and the pilot reported not following the checklist procedures properly; and did not confirm the landing gear was down and locked.

Narrative

I cranked up the Navion to do a check on the newly installed airspeed indicator. The run-up was uneventful and I departed from Runway XX due to unusually westerly winds. The day was HOT; above 90 degrees; high humidity; and a density altitude above 1500 ft.; I left the canopy open to get air circulation. I normally do a once around the pattern with a touch-and-go to test all systems and if all checks out; I may go for a leisurely flight up at a cooler level. On doing a left downwind approach procedure I noticed the gear horn was not audible due to the noise from the open canopy. I struggled with it until I felt I could barely hear it. I completed the rest of my prelanding check and set up for a 65-kt.; full flap approach to landing. Everything was just right; airspeed indicator was dead on 65 kt. However; I did not check on the 3 green lights; trusting that the horn was not making noise. The plane landed gently just above stall but made a scraping sound - my heart sank! I turned fuel boost pump; fuel valve selector; electricals and breakers all off. Complete silence following. Airport personnel assisted in raising the plane high enough to drop the gear and subsequently tug the aircraft to its hangar. Airport Authority Representative along with an FAA Safety Inspector showed up to check on me; airplane and records.Needless to say; I ran this video through my brain over and over and I feel I know why this happened. The almighty checklist needs to be followed. Had I been returning from a more involved flight; I would have pressed the button on the Avidyne to show the prelanding checklist. Do not focus on just one instrument - in my case the airspeed indicator; but rather scan the panel and always look for the infernal THREE GREEN LIGHTS. Follow recommended routines without variance. The old manual says; 'On downwind pull the throttle until you hear the horn; then push forward to quiet the horn and lower the gear.' That is the complete procedure just for the gear. I had modified that years ago and what I would do is slow the airplane under 87 kt.; max flap and max gear speed; and then lower the gear. This eliminated the blaring horn. Hearing the horn is important! Why? Because if you don't hear the horn then you don't have safety redundancy; the blaring horn tells you have a problem; no 'three green lights' tells you have a problem! As I slowed down; the horn never went off; even with the throttle at idle. The horn failed! The checklist failed! The instrument scan failed! See what's there and not what you want to be there! Thinking back at the whole thing; I don't think that horn ever blew. I wanted it to blow; the way it should; but I don't think it did; even on the ground with a dead engine and wheels retracted; dead silence!