Is Propeller Spiraling Slipstream a myth or provable fact? 1

[Majortomski]

Hello ladies and gentlemen! I’m a total newbie to this forum, and I have used the search engines here on the forum to no avail in finding an answer to my question.

Just so we’re all on the same page I am referring to the phenomenon of spiraling slip stream the theory that the propeller induces a spiral of air around the fuselage that strikes the fin/rudder as some angle of attack that causes a yawing force. Said to be cancelled if there is a sub rudder or if the rudder is placed outside the slipstream as on an Erocoupe. Supposedly present all the time. This is not to be confused with the turbulent spiral that is visible off a propeller tip in humid air, which flows the wrong way to support the theory.

The reason that I question whether or not it is a myth is because I have never seen this phenomenon quantified. The aerodynamics of an airplane are cookbook plug and crank mathematical operations. Take a set of interactive equations, plug in a bunch of numbers, and it cranks out the answers of area and angle of attack for all of the flight controls. The one thing missing in all those equations is the mathematical definition of the slipstream. Such that for a given horsepower, a given number of propeller blades we should get an answer as to how much the fin should be offset to correct for this supposedly ever present spiral. By the way before the publishing of “Stick and Rudder” this theory didn’t exist.

Now to be honest I have seen one brief equation mentioned in a very old NACA which was summarized as the angle of attack of the vertical fin due to this effect, was at MOST 3 degrees off centerline, again an insignificant value when considered against the extreme yaw encountered by most S.E. aircraft in a climb.

So, have any of you ever seen this effect quantified?

 

[Majortomski]

Let me focus the thoughts again.

The spiraling slipstream theory says that the rotation of the propeller on a single engine aircraft causes the airflow to spiral around the fuselage. This spiraling air in turn causes an angle of attack on the vertical fin which causes the aircraft to yaw.

My problems with this theory.
1. What got me thinking about how wrong this might be is simpley if the spiraling airflow changes the angle of attack on the vert. fin then it MUST likewise change the AoA on both halfs of the horizontal stab, and the wing. If you draw an image of this then the aircraft will also ROLL with even greater magnitude that the illustrated yaw, and the roll will be in the direction OPPOSITE of the yaw.
Yet when I fly a single engine airplane I do not experiance roll, only excessive yaw in slow flight.
2. The effect is not quantified. If it does exist, then for a given power and a given set of propeller blades I should find a set of equations to minimize any trim drag effects caused by this spiraling slipstream... Yet in 20 years of looking I've yet to come close to finding any such methodology.
3. The spiraling slipstream effect is first described in "Stick and Rudder", prior to this book there is no mention of the phenominon.

So to get back on topic, why do you think it has not been quantified, only accepted as fact?

 

[btrueblood]

" then it MUST likewise change the AoA on both halfs of the horizontal stab, and the wing."

It does, which is the reason why the P-38 had its props spinning the way it did. Rotating the other way caused much higher drag, and reduced the top speed.

Roll for a single prop is counteracting the engine torque, and in any case is likely a reduced effect relative to engine torque. Do you not have to adjust aeleron (roll) trim for different power settings in single-engine craft?

That the effect is likely negligible on tail surfaces I cannot argue with, I have not personally done the calculations. That spiralling slipstreams ahead/behind rotating machinery are real I can argue. You can actually even see the entry slip stream spiral in front of todays turbofan engines, if ground conditions are right -- a rain-wet taxiway, high humidity...the condensation forms at the core of the vortex, and you can see the core bend over and touch the ground ahead of the engine. As the engine spools up, and the plane accelerates down the runway, the vortex moves farther and farther ahead of the plane....neat things, vortices.

 

[Dan320]

Majortomski,

The reason for not quantifying the effect is probably that it. in most cases, is not crucial for design. A rule-of-the-thumb offset of the fin or a trim surface on the rudder is usually sufficient. Why, the rudder pedals are constantly being pushed left or right by the pilot and he only cares that the ball is in the middle, right?
Next time you go flying, attatch a tuft to the windscreen and see the magnitude during different phases of flight. I will anyway.
A note; On the Saab Safir, a single engine trainer/tourer the rudder has quite pronounced wash-out, i.e. the top rib is not parallell to the bottom rib. This is built in rudder trim.
The designer, A.J. Anderson must have known something as it is unlikely he did it on a hunch.

 

[crysta1c1ear]

Someone mentioned accelerating the propeller and its effect on the plane - the need to apply the rudder etc.

I believe that is due to conservation of angular momentum between the propeller and the body of the plane. If the propeller accelerates clockwise in front of you, then the plane will try to accelerate counter-clockwise.

So accelerating propeller mass is also a reason, as I understand it, for sometimes having counter-rotating propellers on twin engined planes, as well as propeller torque from steady state flying.

Now what about spiralling air from the propeller? Pushing air backward pulls the plane forwards. Similarly, the propeller spiralling the air one way will tend to spiral the plane the other way. If some of that air then hits the wings and the tailplane, it can only reduce the effect the propeller originally had, rather than attempt to roll the plane the other way.

(To counteract the rotational effects from the propeller, some planes will have a wing profile on the rudder, with the propeller trying to roll the plane one way and the rudder trying to roll it the other way.)

If spiralling air hitting wings and rudder just reduces the roll induced by the propeller, I'd guess that's why it isn't quantified. It could easily be massaged away by just lowering a figure for the effects of torque on the plane from a steady speed propeller. Also, if the effect is noticable, I bet it depends on speed too, as at some speed a spiral to the rear wings or rudder might be blocked by the front wings, and at another speed might not.
That would make it complicated to quantify.

 

[crysta1c1ear]

Oh, so let me give a direct answer to the question.

Is Propeller Spiraling Slipstream a myth or provable fact?

I'd say it is provable fact.

Non-symetrical rudders are there to counteract torque from the propellers running at a steady speed and that torque is the equal and opposite reaction to throwing air backwards in a long helix rather than completely straight.

 

[Majortomski]

Ahh but if it is provable where's the math or the photos proving it exists????

 

[GregLocock]

A rotating non-accelerating single propeller exerts a torque on the fuselage via the engine mounts therefore it must exert a torque on the air.

The only way a fluid can resist a torque is by rotating.

There, all the physics you need in two sentences.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930092572_1993092572.pdf

for measurements

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Majortomski]

Thanks for the report; I'll soak it up soon.

From a previous post:

"(To counteract the rotational effects from the propeller, some planes will have a wing profile on the rudder, with the propeller trying to roll the plane one way and the rudder trying to roll it the other way.)

If spiraling air hitting wings and rudder just reduces the roll induced by the propeller, I'd guess that's why it isn't quantified. It could easily be massaged away by just lowering a figure for the effects of torque on the plane from a steady speed propeller. Also, if the effect is noticeable, I bet it depends on speed too, as at some speed a spiral to the rear wings or rudder might be blocked by the front wings, and at another speed might not.
That would make it complicated to quantify."

Now we see the other point of confusion on this issue. Many lay folks and some of us too; begin to confuse the three distinctly different effects from TORQUE, the effects of P-FACTOR and the supposed effects from the spiraling slipstream.

Torque only can cause ROLL; it is mostly corrected by slight differential lift on the wings.

P-Factor causes YAW at other than cruise angle of attack. It can only be countered by the fin and rudder.

Spiraling Slipstream also only supposedly causes YAW. AND THAT IS MY PROBLEM with the problem! If you draw a COMPLETE free body diagram of the supposed spiraling slipstream it should cause a ROLL of the aircraft IN THE SAME DIRECTION as the rotation of the propeller, thus countering torque! Many aviation writers have gotten hung up on the intense YAW on the takeoff roll, and the hard over rudder needed in slow flight. IF the hard over rudder IS a result of the spiraling slipstream then WHY is the MASSIVE ROLL due to the slipstream MISSING.

If you draw the typical yaw due to slipstream diagram we see the top of the airplane with the spiral flowing from right to left, causing a yaw to the left, usually illustrated as a large arrow pointing to the right off of the rudder. Now draw what isn’t drawn. From the front of the airplane looking aft, you will see that big arrow pointing to the left, and if the slipstream is affecting all of the flight control surfaces you should see the same size arrow pointing down off the right horizontal stab, up on the left horizontal stab, all with the same massive force as the full right rudder deflection needed in slow flight. All causing a massive roll to the RIGHT. Add to this the arrow on the Right wing pointing down and the arrow on the left wing pointing UP. MORE roll to the right. Yet in slow flight, where the spiraling slipstream is taught as the cause of the need for massive right rudder the rolling factor is completely missing. IF the spiral is true, and it IS the source of yaw, then why don’t we teach the roll component?

 

[rb1957]

"Spiraling Slipstream also only supposedly causes YAW." ... who says so ?

 

[zerosum]

As thruthefence has alluded to: an engine leaking oil in flight always produces spirals down the nacelle/fuselage.
Seems like pretty convincing evidence to me. Ask any WWII B-17 or B-24 crewchief.

 

[zerosum]

Note the name Ragallo on the NACA report GregLocock posted. Not that it affects the validity of the theory, just a famous name from the past.

 

[Majortomski]

"As thruthefence has alluded to: an engine leaking oil in flight always produces spirals down the nacelle/fuselage.
Seems like pretty convincing evidence to me. Ask any WWII B-17 or B-24 crewchief."

Well, not a B-17, or B-24 but I do wipe a lot of oil off of a DC-3/C-47/R-4D-7 and it doesn't wrap around the nacelle as predicted. It just blows straight back and all over every thing in its way.

 

[Majortomski]

"Spiraling Slipstream also only supposedly causes YAW." ... who says so ?

See "Stick and Rudder" the empirical source of the myth.

 

[Majortomski]

RE: Stick and Rudder above, Sorry I don't know how to edit my previous posts

Ironically the offensive text is printed here:

http://www.ercoupe.org/SticknRudder.asp

 

[thruthefence]

In my post, "anecdotal and unscientific" evidence of spiraling slip stream , I alluded to turboprop aircraft- not "oil leakage" but soot deposited on the nacelles, and to a lesser extent on the fuselage. Best example is the Beech, or Raytheon, if you will, "Kingair" series.

 

[Surestick]

"Majortomski (Aeronautics)
2 Sep 08 13:27
"As thruthefence has alluded to: an engine leaking oil in flight always produces spirals down the nacelle/fuselage.
Seems like pretty convincing evidence to me. Ask any WWII B-17 or B-24 crewchief."

Well, not a B-17, or B-24 but I do wipe a lot of oil off of a DC-3/C-47/R-4D-7 and it doesn't wrap around the nacelle as predicted. It just blows straight back and all over every thing in its way. "

Same thing with the oil/exhaust stain down the belly of any Cessna you look at. It doesn't spiral around the fuselage.

 

[dgapilot]

Something to consider, I've seen sheetmetal damage from vibration (air impulses) on the lower side of the left wing (metalized tripacer, and nowhere else. I have also seen several Pawnees with similar damage to the fabric surface of the right upper wing. Kind of makes you think that there is a spiral air flow around the airplane and that it isn't smooth, but quite turbulant.

 

[thruthefence]

More anecdotal unscientific observations from the field: On a single or twin engined aircraft, conventional eppanage, ( as opposed to a "T" tail); without exception, the outboard, L/H elevator hanger bearing will exhibit wear long before any other hinge bearing. Since I was a wee lad, this explained to me as having to do with the propeller slip stream. I seriously doubt these old farts ever heard of Langewiesche. This my date me a little, but in the old black & white TV days, there used to be a watch commercial, where a watch was strapped to a outboard prop, and taken for a lap around the lake. It was filmed in slow motion ( I suppose so tou can see the watch on the prop blade ) and as this thing passes thru the water, the trail of bubbles coming off the prop is a perfect helix.

 

[Majortomski]

I have absoutely no problem with the issue that there is a helix coming off the prop, infact you can see it clearly here:

http://farm3.static.flickr.com/2077/1555927397_121e7387b7.jpg?v=0

BUT that spiral, caused by the vortex off the prop tip turns THE WRONG WAY to support the spiraling slipstream myth! If you follow the angle of those spirals it would strike the LE of the Vert Fin on the Right side, causing higher AOA on the fin to occure on the left side, thus yaw to the right not the left as inidcated by the myth.