I had an interesting discussion with a fellow CFI who is very experienced about left turning tendencies. Is it true that the phenomenon of the Spiraling Slipstream is a myth? I try not to teach this to my students (as much) as I am not a 100% sure if it's a fact or fiction.

I used to teach it because it was testable in an oral exam for private, commercial, and CFI tickets. In reality, it may play a minor role in yaw moments, with gyroscopic procession, P-factor, and reactive force causing most of the left turn tendencies. In fact, the FAA texts refer to Spiraling Slipstream as a theory, so I framed it such and moved on.

I would wager it's there, and go on to say it's extremely hard to describe, given it's 3-d flow (spiral). Nearly every explaination I see tries to describe it in 2-d terms (hitting vertical tail, causing yaw to left, or that it changes relative wind, increasing angle of attack and lift to the right, also increasing lift on left horz. stab, increasing roll to right).

I'd say gyroscopic precession is just as minor and irrelevent 99% of the time. I do some aerobatics on the side, and that's where I really start to notice it, during quick/radical pitch changes. Rotating or climbing? Not so much. One of the big points by the FAA is that a tail-dragger is kind of "worst case scenario" for rotation, due to the slipstream, p-factor, gyroscopic precession, and torque, all of those "make it go left", where as for a tricycle, only p-factor, spiraling slipstream and torque "make it go left". This due to taildraggers initially pitching down, vs tricycle initially pitching up. If it weren't for "all these adding together" for the taildragger, I question whether they'd even be in there.

Then there's spiraling slipstream for multi-engines and "critical engine factors"...all I got to say to that is: "seriously?"...talk about ridiculously unimportant...

Most of these "factors" are designed out of modern aircraft or not really imortant due to how they manifest, and the P-factor becomes the real important one.

Theory: spiraling slipstream circles around the fuselage rearward, striking the side of the vertical stab, resulting in a turn/yaw in that direction.

Where it can be argued is (and I may be completely wrong on this, so feel free to correct me)...the vert stab gives the a/c stability about the vertical axis as we all know. If the tail were pushed/displaced in one direction, the opposing side of the vertical stab is now exposed to more airflow and pushed back to its normal position (positive static stability). This is where "for every action, there is an equal and opposite reaction" comes into play. My point is, I think spiraling slipstream COULD be considered one of these tendencies, but I understand why some would argue that in practicality, it is not.

That photo is a great visualization of prop geometric efficiency (slip?), but how many of those blade tip vortices are actually spiraling around the fuselage?

Flying Magazine - Google Books

Peter Garrison talks about it in "Technicalities" Flying Magazine, July 2001.

Correct. Those are nothing more than vortices created from the tips of the prop that are staying in place as the aircraft creeps forward. Because its a picture and you can't see it moving it creates the illusion of spiraling slipsream moving toward the ttail

Just observing from the plane I fly, I think it might be a lot. I fly a turbine powered spray plane and after a few hours of operation, soot from the exhaust is visible primarily on the right hand side of the fuselage and on the left horizontal stab and elevator. Not saying the rest of the plane is free from it, but definitely more on those areas.

Even though it might be theory, still teach to the PTS. The FAA doesn't want to hear it.