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We had a nice sunny day today, so I went flying. Last time I flew, I did some stall speed testing, using an experimental four leg GPS method, and I was puzzled that the results were about 3.5 kt different from the earlier testing I had done using the same method. After thinking about it a bit, I realized that I had possibly done the stalls with the mixture set differently than from the earlier flight, and that this might possibly have resulted in a different idle speed, and a different effect from prop discing drag. I wanted to investigate this today.

We were under the influence of a strong high pressure area, with the Ottawa altimeter setting at 30.81, and fairly cool temperatures (–10°C or 14°F), so the density altitude at Smiths Falls was very low (3200 ft below sea level, or about 10% higher density than sea level with standard temperature). The plane was a veritable rocket ship under these conditions, reaching over 140 kt before the end of the 4000 ft runway.

The air was smooth, so I did my testing at 4500 ft barometric altitude (about 3700 ft pressure altitude). First, I did numerous stalls with mixture at full rich, and then repeated with the mixture set to roughly where I remembered it from the previous flight. I noted that the stall IAS was about 3 kt lower with the mixture rich than it was with the mixture leaned. The engine rpm at the stall was about 80 rpm lower with the mixture rich than with it leaned, which supports my theory that variations in prop discing drag signficantly affect the airflow over the inboard wing, leading to variations in stall speed.

Then I did four four-sided box pattern stalls (i.e. 16 more stalls) with the mixture rich - the results, when corrected to 1800 lb weight, are in between the results from the previous two flights. I suspect the engine idle rpm at the stall may vary with altitude, which may affect the stall speed on my aircraft. I’ll continue the science project on future flights, doing testing at a range of altitudes.

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