Spiraling Slipstream
 

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?

Technicalities, Flying Magazine July 2001
 

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.

Who can trust this article?
He misidentifies one of the most famous and easily recognizable airplanes of ALL time :eek::D
He might as well call a P-51 a 'Jug'!

USMCFLYR

In all seriousness - good article. I always find Peter Garrison's articles to be informative.

Just my thoughts, but I do not think spiraling slipstream is a very big factor in left turning tendency. P-factor, torque and precession are bigger forces. The latter only shows up when there is a change in pitch rate. In terms of percents, something like 50/30/variable/10, with the last number being slipstream.

He probably had seen:

http://lockthewelderdotcom.files.wor.../hellcat-2.jpg

PW - please repost. The picture didn't come through.

USMCFLYR

Isn't torque only an issue due to bearing friction (and that's compensated for with engine mounting usually) and when accelerating/deccelerating the prop? I know the size of the prop and the power behind it can have a big effect obviously. Maybe it's just me, but when I took off in conventional twins by going full power, then brake release, it never seemed to take off to the left nearly as much as if I went full power while rolling.

try this USMC http://lockthewelderdotcom.files.wor.../hellcat-2.jpg

I like the other one better :D, but yeah - your probably right.

USMCFLYR

I would think bearing friction is a trivial quantity. You can turn an engine with your bare hands, compression and all- there isn't a lot of friction there.

Torque is not dependent on rate of RPM change like gyroscopic precession is. P-51s and other high displacement engines may produce a lot of torque, but it is constant for a given RPM, disc load etc.

PHAK (Pilots Handbook of Aeronautical knowledge) mentions that since airplane wheels are small and often a bit low on air, high torque engines compress the left wheel on takeoff roll (conventional twins, single engines) making ground friction on the left a significant left turning factor.

In cruise torque shows up as a constant roll force, which is as you mention is countered by engine angle and aerodynamic design tweaks. High torque engines on slow airplanes produce a lot of residual roll. Roll of course is a turning function in airplanes. Stand in front of Caravan for example, and notice the angle of the engine is noticeably to the right.

http://i284.photobucket.com/albums/l...os/caravan.png

In piper meridians the engine is mounted 1-2 degrees to the right. I have also seen the soot stains on those, as well as on TBM's and King Airs, Cheyennes, etc

Slipstream, with regards to high powered military single engine propeller aircraft, in particular the navy fleet, was a critical design consideration. Being most critical at low speed and high power, such as landing on a carrier.

It's not a theory; we know it to exist. Here's a quote from a design book written in the late 40s:

On the flight instructor level, aerodynamics for naval aviators chapter on direction stability and control covers the subject well. Noting that slipstream rotation and/or spin recovery are the typical critical criteria placed on rudder control in a single. Though I suspect they too are referring to high powered singles used for carrier landings.

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FWIW, this is how I teach it:

Torque: A left rolling tendency, not yawing, that is the result of the airplane reacting to the spinning propeller. It's effects are small enough to be ignored unless viewed at full power and at, or very near, stall speed.

Gyroscopic Precession: A left turning tendency during the days where tail draggers ruled the fleet. In todays tricycle aircraft it too can be ignored. Though I depict it visually using a stick, held vertically, to represent the propeller/yoke, having the student grab the top with one hand and bottom with the other, and then push on the top/pull the bottom. We then rotate the stick/propeller 90 degrees and see the right side is pushed forward, left back -- a left yaw. (This is the classic raising of the tail in a tail dragger aircraft)

Slipstream and P-factor: This is what we contend with during takeoff roll and departure climb out. At cruise design corrects for them or our legs would get quite tired.

P-factor is of no factor until we leave the ground and the fuselage has an angle of attack to the relative wind. Slipstream is what requires use of the rudder during the takeoff roll. In flight a combination of these two require rudder corrections during full power climbs, go arounds, or any other high powered low speed flight.

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You may notice the particular interest I've put into mentioning when a pilot must contend with these conditions. That is because it is my position that we are not trying to teach the pilot a complete theory behind these 4 conditions. Instead, we try to show them enough to 1) not be confused/scared and 2) understand and accept their existence.

Once their existence is accepted it will be of far greater value to have a complete understanding of when these conditions require particular attention and input from them. And that is during takeoff roll, climbs, go arounds, and any other slow speed and high power condition.

Most texts explain that torque causes a left yaw during takeoff roll due to more pressure/drag on the left tire as a result of the left rolling tendancy. There aren't a whole lot of other forces that can explain why a conventional twin likes to "go left" so much on the takeoff roll, definitely not "spiraling slipstream". The spiraling slipstream, if significant to any extent, will go "backwards", how's that going to be some kind of major influence with a twin?

I also highly doubt spiraling slipstream (being significant) due to how much negative p-factor can affect an aircraft (it's not directly related to the aircrafts AOA, it's directly related to the aircrafts flight path vs props flight path, which are very different obviously).

You have most participants of these boards on a limb as far as the proportion of the classic four left-turning forces on a typical SEL airplane. P-factor is a big one, no doubt. Torque has to be large for high-torque airplanes. For a tricycle airplane, I would give slipstream a third place ranking. Several users said they have seen soot or spray indications that support spiral slipstream theory.

Regarding torques effect on rollout, I'm not sure, I can't claim one way or another. What I know is I've not read a statement like the one made in PHAK from any source I'd consider reputable on this subject. Though the concept makes intuitive sense, I wonder if its magnitude is negligible.

Where I run into a mental block with the scenario PHAK presents is that full aileron for a crosswind departure doesn't exhibit an appreciable effect on the need, or lack of need, for rudder during the rollout. I would guess at the later portion of the takeoff roll, with speed on the order of 40-50 knots, full aileron would yield a far greater rolling force than engine torque forces.

If the props are counter rotating, as many are, then spiraling slipstream can be ignored. However, so could the effects of torque on wheel weight and p-factor.

If the twin has a critical engine, then slipstream and torque effect would come back into play. As well as p-factor when in flight.

That is why I said fuselage AOA. :) Which isn't exactly accurate as most engines are canted downward a few degrees. In other words, p-factor at the initial start of the rollout, prior to raising the nose, would present with a right yawing tendency.

That's just my point, the engines are mounted away from the fuselage in my "conventional twin" scenario, how does the spiraling slipstream from the left engine "jump" backwards and right and "hit the tail"? Why doesn't it go "straight back"?, and if not, then is it really widening that much as it's moving backwards (and maintaining the same magnitude?). I just don't see that as realistic. I don't see them behaving the "same" with such radically different engine mounting configs (single eng vs conventional twin), unless those charateristics are due to something else (torque during takeoff roll and P-factor during flight).

But I agree with the torque on takeoff, and I think I found myself turning the yoke a little in that direction during takeoff roll in a high powered single.

I cannot say with any degree of certainty. I will make note that most of the books I've been reading do mention slipstream in a twin. However, it is not expanded upon since asymmetrical thrust is nearly always the critical design consideration.

Do recall, though, that it's not about air from the slipstream striking the tail and pushing it. It's about the slipstream creating a sideways component relative to the free stream flow. Thereby causing a slight change to the effective relative wind interacting with the vertical tail.

With that said, some thoughts/questions come to mind: Is the slipstream in a twin still close enough to change the rudders local AOA? Does the slipstream hold a uniform diameter, or does it widen as it travels backwards? Consider that the both engine in a critical engine configuration will move air to the right at the top of the slip stream.

P P-factor
A Accelerated Slipstream
S Spiraling Slipstream
T Torque

It ain't rocket science folks...

Single engine:

The propwash is absolutely rotating when it leaves the blades. One way I came to that conclusion because the exhaust leaving a jet engine is not rotating due to stators, among other reasons.

If the plane was parked, breaks on and engine speed high and constant, would that mean the left-turning tendencies of "gyroscopic effect", engine torque and p-factor would not be realized? Any stresses on the airframe and gear would be from spiraling slipstream only.

(Got an argument for engine torque? Line two planes up, one behind the other. Have the propwash from the front plane wash other back one, which isn't running and take the stress/strain measurements)