Does anyone know of any real aeroplane or wind-tunnel tests that quantifies the effects of asymmetric propeller blade loading (P factor) versus the Spiralling Slipstream effect in creating yaw.
Which effect is greater?
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$\begingroup$ I got the following results today in a Pitts S1S with a 180 HP Lycoming IO 360 and 76 inch diameter Sensenich 2 blade prop with 61 inch fixed pitch. Upright in a 70 kt full power climb starting at about 3000 ft, wings level, ball in the centre. When the right rudder was released and the feet held clear of the pedals, the a/c yawed to the left about 45 degrees quickly. Then with the wings held level with right aileron and holding 70 kts, the left yaw continued in a flat turn at about 9 deg per second and this was allowed to continue for 180 degrees. Continued below. $\endgroup$– ForbesCommented Jan 30, 2023 at 3:28
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$\begingroup$ The Pitts was then returned to 3000 ft, rolled inverted and put into a 70 knot full power climb with wings level. When both feet were held off the rudder pedals, the a/c yawed slightly to the pilot’s left, hardly discernible. $\endgroup$– ForbesCommented Jan 30, 2023 at 3:29
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$\begingroup$ My initial deductions are that the vertical tail was mostly clear of the propeller slipstream when inverted due to the high angle of attack, and that the P-factor (which should have been the same inverted as upright) is a very small effect compared to the Spiral Slipstream effect created by the propeller. The ability to conduct a flat turn without rudder seems to show that the spiralling slipstream has a very large effect at high power and slow speed in aeroplanes like the Pitts S1 $\endgroup$– ForbesCommented Jan 30, 2023 at 3:56
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$\begingroup$ Any aerodynamics experts out there? $\endgroup$– ForbesCommented Jan 30, 2023 at 4:02
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1 Answer
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That depends on Angle of Attack (AOA), as well as other factors, including aircraft design. P-factor is caused by the angle between the free-stream airflow in front of the propellor and the propellor rotation axis. That changes as AOA changes.
Slipstream effect is caused by the impact of the corkscrew airflow created by the propellor impacting the vertical stabilizer.
So, for example, in my Long EZ, where the propellor is at the back, in the downwash from the main wing, P-factor is minimal at most AOAs. And the slipstream effect is nonexistent, since the corkscrew airflow from the prop is all behind the aircraft.
