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I had day off on Friday, and took advantage by doing a firewall forward inspection and then a short flight. I finally had weather that allowed me to climb high enough to gather some more stall speed data using a four leg GPS method. The results from this flight is a real puzzle when compared to the previous test a few flights ago.

The two test points (four legs each) looked to be excellent quality, with extremely low standard deviation of the four calculations from each test point (0.2 kt and 0.0 kt for the two test points). The two test points gave similar speeds, with only 0.1 kt difference between the two points.

The two test points on the earlier flight also appeared to be reasonably high quality, with low standard deviation (0.8 kt and 0.2 kt). The two test points on that flight giving results within 0.3 kt of each other. But, if I correct the stall speeds from the two flights to the same weight, the stall speeds from Friday’s flight are about 3.5 kt faster than the stall speeds from the earlier flight.

My aircraft has a three blade MT aerobatic prop, and it has a huge amount of prop discing drag with the throttle is at idle, with the prop control fully forward. I suspect that the airflow over the inboard wing is disturbed in this condition, and that results in the stall speed being several kt faster than other RVs with more typical props. I don’t recall where the mixture control was during the stalls on the earlier flight, where the stalls were about 2000 ft higher (7000 ft vs 5000 ft). I wonder if the rpm at idle, and thus the discing drag, might vary depending on where the mixture control was. I’ll experiment with this on the next opportunity to try to get to the bottom of this puzzle.

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