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]

PLEASE you folks are missing the POINT OF MY POST.


IF the spiraling slipstream is real WHY isn't there a set of equations quantifying it?

In my post just above I can find an equation defining the airflow in the plain of the prop spiral, but I can't find anything that defines the actual quantity and angle of the supposed spiraling airflow.

It is all empirical conjecture.

JUST SHOW ME AN EQUATION THAT PROVES IT EXHISTS!

 

[Dan320]

A part of the Force you will never be, if you an equation need to believe...

 

[Boathouse]

Look at the exhaust stacks on many PT6 installations....rotated to take advantage of prop swirl.

They also leave exhaust trails down the fuselage, ask any PC9 mechanic!! Wiped many off myself.

I remember there was a fair bit of aero work done on the Beech JPATS PC 9 derived contender to stop the entrained flow of the prop / exhaust oiling up the canopy.

 

[vorwald]

Sir

There is a section in Helicopter Theory, Johnson, pp 40, that deals with the equations to calculate the swirl in the wake behind a rotor. I believe with minor adjustments, it would apply to a propeller. Most of the swirl is due to lift induced torque, the drag induced torque appears to be a smaller component

A quick look on the nasa tech server, with the search phrase "propeller wake swirl", shows some promising leads, including

Method for calculating effects of a propfan on aircraft aerodynamics at subsonic speeds
by Chandrasekaran, B.; Bartlett, G.
Jun 1 1983

Essentially, draw a control volume around the propeller and through the drive shaft, and then consider
1) conservation of mass
2) axial momentum
3) angular momentum
4) and energy
Assume that the flow upstream is irrotational and has no rotational energy.

From Johnson, the simplest eqn appears to be

Vr = Vh (2 Vh Omega r) / ((Omega r)^2 + vh^2)

Vr is rotational velocity at radius r
Vh is the developed induced velocity from thrust
and is approx vh^2 = T / (2 rho A)
Omega is the rotation speed
r is the radial station, from zero to R

including the drag term gives the additional term

Vr = Vh ( (2 Vh Omega r) / ((Omega r)^2 + vh^2) + 2 Cd/Cl]

Good Luck
John

 

[HerkyJim]

I'm not an engineer; just an old semi-retired multi-engine prop driver. I'm not sure if you can "prove" that the prop slipstream "swirls" but you can sure see it. I think I linked to a picture below. The Hercules Allison/Hamilton-Standard combination rotates the prop clockwise as viewed from aft of the airplane.

Youy can see the little vortices swirling off the blade tips and in turn the little swirls rotating about the axis of the propeller rotation extanded aft.

So we tell the students in refresher training that the idea of critical engine is primarily due to the "swirling" and due to "P" factor also. Gyroscopic effects may come into play for a while, but reaction force due to propeller shaft torque doesn't do much in multi-engine like it might in a single engine airplane.

Some one mentioned above "P" factor during takeoff run...as far as I know, there has to be an angle between the axis of rotation of the propeller and the relative wind for "P" factor to exist. In the airplane I'm familiar with, it looks like the axis of rotation of the propeller is actually inclined down when the airplane is static and more so during takeoff ground run. So there ain't no (or very little) "P" factoer until rotation for takeoff.

My $0.02

 

[GregLocock]

That is a great photo, but I'm not sure that it proves anything. Isn't that vortex trail just tracing the path of the tip of the prop?

Cheers

Greg Locock

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

 

[GregLocock]

Ah, I've just realised, was the aircraft stationary when that photo was taken?

Cheers

Greg Locock

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

 

[HerkyJim]

I guesss it is tracing the vortex trail. I'm no whiz at technical stuff. Math stopped just short of differential equations and physics was "physics lite". I think I remember the explantion of thrust from a propeller--as explained to me those many years ago--as being a result of air being accelerated as it passes through the prop arc.

A bigger picture in the attachment. He's just taking off and going perhaps somewhere around 100 to 130 knots true airspeed at this point.

 

[HerkyJim]

from an earlier posting:

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

This is a C-130A (three-bladed propeller). In fact, the yaw due to slipstream and P-factor is nose-left. The left outboard engine (#1 engine) is the critical engine.

 

[thruthefence]

P factor: This is a problem with powerful "conventional gear" aircraft, ie taildraggers. As the aircraft begins it's takeoff roll, the tail is down on the ground. As it accelerates, under takeoff power, at dome point the pilot pushes the stick forward to raise the tail, reducing drag, & allowing airspeed to increase. At this point the gyroscopic progression forces come into play, causing the aircraft to yaw. Having once worked for a guy who had owned a couple of "warbirds", a P51D & a T28C, both very powerful aircraft with large props, I was told the T28 was a much more benign aircraft on takeoff, altho it's power-to-weight ratio was higher. ( it was a trainer, after all)

 

[zerosum]

Not sure what x-plane is all about, but here is another opinion.

 

[Dan320]

In "Aerodynamics for Naval Aviators" the authors state that (my words) lateral control during take-off can be the critical design condition for the rudder. This suggests that there may have been proprietary methods at the manufacturers of the day.
The Pipers and Cessnas are just not powerful enough to run in to these problems.

 

[rb1957]

i wonder if it would be easier to see the slipstream flow from a ship screw propeller ? ... this would be doing fundamentally the same thing as a plane propeller ...

 

[Dan320]

I remember reading about some wartime a/c being so powerful that you had to restrict T/O power to what could be handled. Seems that otherwise the a/c even with full opposite rudder would go sideways to the point where it ran off the runway or the tires were forced off the rims.
I believe this is due not to slipstream spiralling but to the propeller blades closest to the ground being less effective, making the upper blades "go heavier" through the air and pushing the nose to one side. Maybe all a/c to some degree crab into the air? That is until the propeller is high enough from the ground.

 

[Majortomski]

zerosum, the Xplane link you posted is nice and clear but please notice he(they) just reitterate the accepted norm.

Again we only see the interaction of the vertical fin and the swirling slipstream. They are ignoring the rest of the aircraft.

If the spiraling slipstream is true. Then in slow flight in a single engine aircraft, where you have to hold full right rudder to counteract its effect and the effect of P factor, then one must also hold in massive amounts of LEFT aileron to counter the RIGHT roll that MUST be formed by the same slipstream that is caused LEFT YAW. BUT you don't do this in a small plane.

Again draw the diagram from the top and look at the AOA of the wing and the Horz stab if the spiraling slipstream is true there's a force and an effect that magically disappears.

PT-6 SOOT PATH

I work with a fleet of Be-300's I have a picture of the outboard sides of both nacelles. The gas exhaust paths are identical and symetrical.

 

[Dan320]

The effect you are talking about may well be there, but it is swallowed up by the much more powerful roll authority of the ailerons. You just hold the stick where it needs to be to fly level. Also, engine torque works opposite to your tail rolling moment, maybe they cancel.
Some twin engine a/c have dihedral on the stabilizer to get more equal flow conditions on the left and right side. If there was no spiralling, this would be unnecessary.
Could'nt see the soot picture, but if the spiralling flow gets caught in the upsweep in front of the wing I think there will be so little left of it that you can't see any difference btw left and right side of the nacelle.

 

[vortexman]

Majortomski,

There is definitely significant yaw resulting from the helical flow generated by the prop impinging on a non-symmetric (top-to-bottom). This is most noticeable when the airspeed of the plane is low, and the engine is operating at high power, because under these conditions the net angle of the flow around the fin deviates the most from the thrustline. Perhaps this effect is not typically quantified using specific equations because it is so simple to predict and deal with. If there were an engineering need to do so, and I would bet that it has been done on numerous occasions, the equations would simply involve the velocity of the flow (magnitude & direction) around the fin, the CD & CL of the fin, and the moment arm of the fin. This wouldn't involve any data that isn't already known, and no new equations need to be derived. Because it has been determined that this yaw is manageable by the pilot, perhaps no detailed analysis is needed, and pilots are simply trained to use the rudder as needed. Alternatively, this analysis might be routinely done as part of the stability & control analysis of a new aircraft, just to assure that there is adequate rudder authority. The mechanism behind this phenomenon is sufficiently straightforward that there is not really any room for debate about whether it's a 'myth'.

vortexman

 

[Surestick]

Vortexman,
In a low speed, high power scenario P-factor and torque are going to be big factors and can be used to explain the same effects that you would see from a spiraling slipstream.

 

[Majortomski]

MY point exactly!

P-FACTOR generates higer localized thrust down the right side of the fuselage. Which inturn creates lower pressure on the right side of the fin and as emperically seen while flying a yaw to the left. Which I believe has, since the publishing of stick and rudder, been wrongly atributed to a non spiraling slipstream.

 

[IRstuff]

Don't the photos prove that the slipstream isn't moving against the surrounding air? If it were, wouldn't the trails have complete dissipated into a swirling cylinder of smoke? The fact that the trails are individual and distinct says that there is little or no movement of the air behind the props.

TTFN

FAQ731-376