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How does p-factor affect jet engines, if at all? If it doesn't, why?

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  • $\begingroup$ @Bianfable I saw that answer before posting up the question, but it’s a little bit vague and does not really address p-factor entirely in my opinion. $\endgroup$
    – Kolom
    Jan 29, 2021 at 12:06
  • $\begingroup$ Think about what P-factor is and you'll know the answer to this. $\endgroup$
    – Zeiss Ikon
    Jan 29, 2021 at 12:16
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    $\begingroup$ @Bianfable there's going to be some P factor in a turbofan, since it's just a fixed pitch prop with oodles of blades, to the extent that flow at the blade AOA varies with body AOA. How much this occurs will be a function of the cowl inlet's efficiency at redirecting the inflow. Downstream guide vanes shouldn't make any difference; it's the asymmetrical lift from differing AOAs of the blades which affects the strength of the downwash from the blades so the fan is making more thrust on one side, just like any propeller. I think there is SOME P factor, but not enough to have a noticeable effect. $\endgroup$
    – John K
    Jan 29, 2021 at 20:42
  • $\begingroup$ @JohnK OK, I retracted my close vote. Feel free to add that as an answer... $\endgroup$
    – Bianfable
    Jan 29, 2021 at 20:55
  • $\begingroup$ This question is relevant and could lead to discussion of gyroscopic effects when pitching (a rapidly rotating) jet engine. $\endgroup$ Jan 30, 2021 at 0:28

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Since P Factor is primarily variation of AOA of a thrust producing blade when its rotation axis is not aligned with the air stream, you should see some P factor in a turbofan engine, to the extent that the inlet flow strikes the front of the fan from an angle.

I say "some" because the cowl inlet is going to align the inflow to be more perpendicular to the fan face than if the fan was in the freestream (where it becomes just a propeller with lots of extra blades), but it's hard to say how much. In any case, the flow is probably not quite 90 degrees to the fan face at higher body AOA, so it's safe to say there is probably a little bit, but since P factor is not accounted for in turbofan installations, what P Factor exists is not enough to matter (maybe if you stuck a turbofan on the nose of a Cessna 172, you might notice some P Factor effect).

On a turbojet there will be no P Factor at all since the thrust is coming form flow accelerating out the burner can, hot section and tailpipe, not from lift producing propeller/fan blades at the front.

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    $\begingroup$ With a turbofan on the nose of a -172, P Factor certainly won't be the only interesting new "effect" that you're suddenly going to notice! $\endgroup$
    – Ralph J
    Jan 30, 2021 at 12:22
  • $\begingroup$ "Maybe" is not an answer. $\endgroup$
    – Pilothead
    Jan 30, 2021 at 20:37
  • $\begingroup$ "Maybe" just jokingly refers to a "turbofan 172". The answer is, logically there has to be some, but not enough to matter, and "some" might be a 5% effect, or a .5% effect, but in any case it's not enough to matter on aircraft equipped with TFs. $\endgroup$
    – John K
    Jan 30, 2021 at 20:48

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