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Is Propeller Spiraling Slipstream a myth or provable fact?

Is Propeller Spiraling Slipstream a myth or provable fact?

Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
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?
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

ok ...
a propeller producing thrust is rotating.  the swirl velocity of the wake of a propeller is easily calculate (if you know how).  it is no myth.

in WW2 several twin engined planes had propellers rotating in opposite directions, to balance out the wake.

in modern times, engine power increased so that some applications have counter-rotating propellers (Tu-44, Gannet), the point being that counter-rotating propleers reduce, significantly, the swirl of the slipstream.

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
"the swirl velocity of the wake of a propeller is easily calculate (if you know how).  it is no myth."

Well I've been looking for roughly 20 years now and I haven't found that methodology.

"in WW2 several twin engined planes had propellers rotating in opposite directions, to balance out the wake."

Please, name ONE.  The P-38 had counter rotating props to resolve other aerodynamic problems, but they rotated in the "wrong" direction, they made two critical engines instead of one.

All of the WW-II vintage aicraft with "H" tails have the tails in the upper half of the supposed swirling slipstream which would make the condition worse, not better.

I believe most of the modern counter rotating props are due to length of landing gear vs available power to be absorbed issues.

Sorry rb, but your answer is the canned answer that I always find in the performance and pilotage books, but no one actually knows the math to prove the swirl exists.

Thanks

T

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

I don't know if is true or not - thoughs I supect it may.

Cessna always put a little more dihedral in the right wing (less lift) to offset the torque of the engine - plane trying to turn left.  This may also have been done becasue of what you say.  They always told me it was for torgue.

One day as a student pilot - I added power way too quick - apparently - and almost shot off the left side of the runway.  Plane too slow and dihedral efeect had not yet come into play.  I am still here - so no big deal.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

ok, the propeller blade is an airfoil at incidence.

The propeller blade is an airfoil, creating lift and drag.  These forces resolve into thrust and an inplane component acting in the tangential direction.  These in-plane forces (from different blades) are in phase, all acting CW or CCW depending on the rotation of the blade.  These cause the swirl.  It is possible that there is no readily available math for this swirl velocity 'cause it depends on the after-body.  Also I'd anticipate that the swirl velocity is significantly less than the axial velocity of the slipstream, so you may not feel it.

"Well I've been looking for roughly 20 years now and I haven't found that methodology."  tonight i'll look up my 4th year propulsion engineering text; i told my wife they'd be usefull one day.

I think i had the P38 in mind ... if the props are rotating in opposite directions, why are they "two critical" ones ??

as for counter-rotating props, yes in part, the engine power needed more than a single propeller, as you say, the radius of the prop is limited by many things, and the number of blades is also limited; however, it is a huge complication to arrange for counter-rotation ... why would they do this if there wasn't a performance pay-back ?

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
A critical engine in a multi, esp twin engine airplane is the one that if it fails the remaining engine causes the most advers yaw due to the P factor thrustline running down the outside of the nacelle.   In the case of the P-38 both engine turned outboard over the top, so that if one fails the other always causes the most adverse yaw condition.   

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

i was thinking about your question on my drive home ...

the drag component on the propeller blade (acting in a tangential direction) is minimised in modern propellers due to pitch control; this optimises the airfoil force vector to be in the thrust direction.  older fixed pitch propellers would be set to maximise thrust at a particular airspeed, and would be inefficient at different speeds (wasting energy in creating swirl).

i was also thinking about the classical picture of a prop in flight, with the nice cork-screw motion of the tip.  i think this is the case for a prop with pitch control.  without pitch control, the drag of the prop is sort of causing the prop to "skid" through the air.

another reason there isn't a "canned" equation to calculate the swirl is that it is dependent on the airfoil of the blade.

i'll give you the P-38, what about my observation on the counter-rotating props ?  what about my drag forces ?

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

As a net effect on the control volume of air it must be a fact by conservation of momentum. The motor exerts a torque on the aircraft, this must be counterbalanced by a torque applied to the air by the prop.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

IE critical engine: On an aircraft that has both props rotating clockwise when viewed from the cockpit: The distribution of thrust across the 'disk' is not equal, at higher angles of attack, such as in a climb, the descending blade side develops more thrust. Thus, the r/h engine is further away from the center line of the aircraft, and has a better " mechanical advantage " for yawing the aircraft, thus making the L/H the "critical engine" if it fails. Critical because you have to add more rudder to control the yaw, then you would had the r/h engine failed.Any unnecessary induced drag when you have just lost half your thrust, is indeed "critical".  Two light twins aircraft that I am aware of,( Piper Seneca & Beech Duchess ) the r/h engine rotates counter clockwise, viewed from the cockpit. eliminating the 'critical engine'. The P-38 also had counter rotating props, tho they rotated L/H CCW, and R/H CW, giving you TWO critical engines??

Anecdotal unscientific evidence of 'spiraling' propeller slip stream:
Look at any turboprop powered aircraft. The soot deposits from the exhaust travel aft down the fuselage in a distinct spiral pattern.
  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

"as for counter-rotating props, yes in part, the engine power needed more than a single propeller, as you say, the radius of the prop is limited by many things, and the number of blades is also limited; however, it is a huge complication to arrange for counter-rotation ... why would they do this if there wasn't a performance pay-back ?"


The performance payback is the additional power available when it isn't limited by propeller size.  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

you missed the point surestick ... having decided to use two props to absorb the engine power (mind you there are many other ways to do this other than 2 props), you "could" have both props turning in the same direction (out of phase by 1/2 a blade-spacing), though i don't think it's been done; typical practice i think is to have the props counter rotating which is a significant mechanical complexity and cost, so i reason there must be a reasonable pay-back (which i think is recovering the swirl).  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Is it too far off base to associate this question to the differences between pusher and tractor type engine systems.
Simplified:
One of the ideas of a pusher (rear mounted) engine is to eliminate the vortices from the propellers from striking the fuselage (which causes noise, among other things).
This would be considered a spiral of air around the fuselage that would go all the way to the tail.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

As a matter of semantics;

The large, powerful engines, Turboprop, or recip, that use two props rotating co-axially, but in opposite directions, are referred to as "contra-rotating".

A twin engine aircraft having propellers that turn in opposite directions, are said to have " counter-rotating " props.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Majortomski,
If the wake isn't spiralling, what then is the cause of the need for counter rudder application at take-off? In a climb, the downgoing blade will perhaps have greater lift and try to turn the a/c. However, in a take-off roll in a nose-wheel a/c with little or no AOA to begin with, the nose still wants to swing immediately after application of power. Spiralling theory explains well what's going on even if it may be incorrect. Let's tuft the nose and windscreen and see for ourselves!  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

dan, there's some contribution from propeller gyroscopic forces

but the OP has gone quiet ... maybe he's thinking, maybe resting, maybe pining

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Ahh, now you've all started down the path!

The aircraft yaws on take off due to p-factor.  The THRUST of the propeller is higher down one side of the fuselage than it is on the other side.  

If the spiral is true, then the momemt the aircaft lifts off the ground, then it would ROLL right with 5 times  the force of the yaw to the left.  We always see this spiral drawn in side profile.  Draw it in top profile and the completely ignored effects on the wings and the horizontal stabilizer show the plane rolling the direction the opposite of the yaw.  But this is never shown or discussed.

The prop tip spiral mentioned above also goes the wrong way to support the theory.

Furthermore, the prop is nothing but an airfoil.  Are we saying that the wing induces air particles to move forward in the same direction as flight aft of the trailing edge?

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

If we consider what is actually happening (as opposed to the usual wind-tunnel mind picture) in flight we see that the wing is moving through still air.
The wing profile and aoa causes higher pressure on the bottom side and lesser pressure on the upper side to achive lift, and in doing so displaces air downward. It is not entirely unthinkable that the forward motion of the wing will cause a (small) forward movement composant of the displaced air. That would be the equivalent of a spiralling propeller wake.
   

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

To add to my previous post, in the wind-tunnel mind picture the airspeed is decreased on the bottom side (and increased on the upper), if we say the free-stream airspeed is some discrete number and the lower side airspeed is less, mathematically that is the same as saying "a wing travelling through still air will impart a forward motion to the air deflected by the wing." Of course the main air movement is in the downward direction.
This reasoning supports the spiralling wake theory.
The magnitude is another matter. Perhaps by relating chordwise pressure charts to resultant airspeeds could give us an indication. CFD?   


       

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

What about where you have axially coincident contra rotating props -eg Fairey Gannet? There is no net torque at the engine mounts, (ideally), so no need to accelerate the propstream rotationally.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

what forces are being applied to the airflow by the propeller ? ... lift and drag.

the ideal condition is when the resultant is in the thrust direction.  old (fixed pitch) propellers achieved this for one airspeed, modern (variable pitch) propellers can achieve this over a much wider range.

in the off-design condition there is a thrust (fwd) component and a drag (tangential) component.  the tangential component is causing swirl.

i had to think a little about the propeller on the plane, flying forward; alot of the apparent rotation of the flow through the propeller is taken up by the forward motion of the plane, remember the pictures of the propeller tip advancing (cork-screw like) through the air.  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
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?

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

" 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.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
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?
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.   

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
"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.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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

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

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
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

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

"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.  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
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.
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
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!

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


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

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.

[img http://i240.photobucket.com/albums/ff143/Ithaca37/vorticesII.jpg]

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

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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?

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Greg Locock

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RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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

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Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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)   

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.

  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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 ...

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
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.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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.
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
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.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

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

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RE: Is Propeller Spiraling Slipstream a myth or provable fact?

IRstuff,
if the slipstream isn't moving relative to the surrounding airflow, where is the engine power going ? and how is thrust being generated ?  maybe "moving" is meant to be read as "rotating, spiralling", 'cause it is obviously accelerated.

Majortomski,
i'm having trouble with the double negative "wrongly atributed to a non spiraling slipstream" ... the 1st time a read it i thought you meant that the slipstream IS spiraling (which i think everyone replying has said) but then i thought that your OP implied that you didn't think the slipstream spiralled.  you originally asked for a theoretical equation demonstrating the effect, but the replies indicate (to me at least) that the effect, though real, doesn't lend itself to a theoretical equation due to the influences of specific designs (fuselage shape, prop diameter, prop section, etc)

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


Surestick,

Torque has a completely different effect; it only acts in the roll axis, which is not what we are discussign here.  P-factor does exert a moment in the yaw axis.  Whether the slipstream impinging on the fin or the P-factor is a larger effect depends on the situation.  Having said that, the P-factor  has a moment arm of less than the prop's radius, while the fin has a much longer moment arm.  I would guess that the slipstream is the larger effect most of the time.  In any case, the question at hand is whether the effect of the slipstream is a myth, not whether it is the most important effect.

vortexman

Majortomski,

I doubt it.  The slipstream is definitely helical; do you think the higher velocity that might be caused by the P-factor would still result in that velocity differential by the time this spiraling slipstream gets to the tail?  Doubtful.  If so, that might also be a factor, but not to the exclusion of the spiraling slipstream effect.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Whooops my bad  I meant

 Which I believe has, since the publishing of stick and rudder, been wrongly atributed to a  spiraling slipstream.  

 
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Interesting link, first time I've ever seen the angle reffered to as the "Sidewash angle".   

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
And a google search for "sidewash angle" comes up with several papers, all concerned with reducing the yaw angle of an aircraft in steady flight to zero. Yet once again in lengthy mathematical disertations on how we size vertical tail volume so that yaw angle remains low, they add the mythical "and oh some consideration must begiven to the increase in sidewash angle due to propeller slipstream.  Again an undefined afterthought in a discussion concerned with angles less than 10 degrees.

One would think that if I'm optimizing the configuration of the aircraft for the most cost effective cruise, I would not wag such a value and trust a now out-of-alignement rudder to just take care of it.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Majortomski,

Have you done a good search on "propwash"?  That's what we used to call it when Wilbur taught me to fly.  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Have you ever taken a close look at, say a Piper Turbo Lance? Engine canted off to the right ( aircraft's right ) maybe 3 degrees? Why would they do that? Do you think they just got their fixtures crooked, back in aviation's murky past, and had  an aviation writer make up a "myth, so they wouldn't have to re-tool? Talk to anyone who's flown a V-35 Beech with the IO-550 Continental conversion. Big paddle blade prop absorbing nearly 300HP, pull the prop rpm back, tries to fly half a ball out. Not roll, but yaw. This aircraft was originally designed with a 185 HP engine. No deflection of the engine, R or L.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
All down thrust and right thrust can be the result of trying to offset P-factor not the spiraling slip stream.

Down thrust principly minimizes trim changes with power changes, BUT it also has the rarely thought about effect of decreasing P factor in a climb.  Thus needing less right rudder.  Right thrust eliminates the rest of the need to fix P factor induced yaw.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

I say again: P-Factor is the gyroscopic precession component that causes yaw when the aircraft is pitched up or down. Unless you continue loading the propeller disk, that is keep a constant pitch command on the aircraft, the P-factor is transient. It simply is not a factor in trimmed, cruise flight, with a constant power setting. Pilots of the old WWI rotary engine aircraft, could almost turn in their own length by pitching the aircraft in combination with rudder & aileron, due to the gyroscopic forces on that large rotating mass on the nose.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
I agree, mostly.

However P-factor has two components; the forces affecting the airframe through the gyroscopic precession on the propeller, AND the resultant additional thrust or airflow down the right side of the fuselage or nacelle.  If there were no force created by the propeller at the 3 o'clock position, (in a climb) then there wouldn't be any gyroscopic forces to begin with.

And as you stated, the gyroscopic effects are mostly transitory with pitch and yaw changes, but the additional thrust is a constant in a high power, high alpha, low speed climb, so therefore there is a constant pitch up component to the P-factor on the face of the propeller in a climb.
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


The P-factor has nothing to do with precession.  It is the differential thrust caused by differential angle attack of the propeller blades as a result of a non-zero pitch angle.

vortexman
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

I agree with vortexman.  There can be no precession unless there is an inertial change (in our case, rotary motion around either the pitch axis or the yaw axis) and that's not what Majortomski's claim is about, at least as I understand it.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
In a climb, due to diferent localized velocity and angle of attack acting on the propeller, P-factor, more localized thrust will be created on the right side of the plane.  It is a constant.  It is a force that is constant acting on the propeller.  That force created at the 3 oclock positon on the prop disc will, by gyroscopic precession react with the plane at the 6 oclock position, contributing a pitch up component.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Not if there is no rotation in either pitch or yaw.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


Majortomski,

You are mistaken about how precession works.  Rotation in one axis results in torque about a separate axis.  Torque in one axis does not result in rotation in a separate axis.  In any case, this phenomenon would not be relevant to your original question.

vortexman

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Vortexman, please explain.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Vortexman, thanks; I stand corrected. A "senior" moment I suppose. However, does anyone know what transpires in a "pusher" propeller configuration? Is the yawing component (attributed to "spiraling slipstream") absent due to no structure aft of the prop to act upon?  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
A friend owns a vari-eze, says it still needs a touch of right rudder in a climb, but not as much as a pusher.

My guess on this one is the wing acting as a huge flow straighener  on the upper half of the prop arc. So any yaw inducing pfactor will only be the result of thelower half of the prop in "free" air.

As to the gyroscopic issue above.

If a constant force is applied to a spinning gyroscope at one point what is the reaction of the gyroscope?

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Wow, I'm goofey, anyone know how to edit one of your own posts on this board?

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


Dan320,

I'm not sure what you want me to explain, so I'll try to clarify both points.  First, the P-factor is a phenomenon completely unrelated to gyroscopic precession.  P-factor is simply a thrust differential resulting from the propeller operating at a non-zero angle (usually in pitch) to the oncoming flow.  This non-zero angle causes the propeller blades on one side to operate at a different angle of attack than those on the other side, so that more thrust is generates on one side than the other.

Gyroscopic precession causes a torque to be generated about one axis (say Y) when the spinning body (say it's spinning around the X axis) is rotated about one of the other axes (say Z).  If the spinning body is rigidly fixed in the Y axis, then no rotation would result in that axis.  Sorry, it's hard to be clear without a diagram.  The point is that a rotation of the spinning body results in a torque, not vice versa.  I'm sure there are good websites that illustrate this better than I can.

vortexman

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


thruthefence,

Yes, without an asymmetric fin, or any fin, there would be no such effect.

vortexman

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Gentlemen,

I commented on the gyroscopic issue because it is a result of the P factor thrust.

You all agree that P-Factor produces a thrust on one side of the fuselage.  That thrust, that force is a reaction on the propeller, a spinning gyro.  That force just doesn't disappear.  It is a constant in a climb and thus the propeller reacts through precession by adding a pitch up component equal to the thrust of the P-Factor.  They are interrelated but as stated above have no relevance to my original question.

I propose that there is no spiraling slip stream.  The increased velocity of the air down one side of the fuselage causes the same effect as if the slipstream were spiraling.

Your thoughts please?

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


Majortomski,

Why do you think there is no spiraling slipstream?  Does it just seem incredible to you, have you done some measurements, did someone knowledgeable make this assertion to you?  In your original post, you mentioned a NACA paper that talked about a 3 degree angle of attack on the fin due to this phenomenon.  A wing, which the fin is, operating at a 3 degree AoA will generate quite a bit of lift.  Why wouldn't you think that a fin exerting that much force laterally would induce significant yaw?

From another perspective, what do you think the flow generated by a spinning propeller looks like?  Do you think it's a jet with no spiraling motion?  I can assure you that this is not the case.

I would be happy to give you as useful a response as I can, but I think you need to help me out by explaining what you can't accept about the "spiraling propwash theory".  All I really get out of your last post is that you propose that the spiraling slipstream doesn't exist, and another phenomenon explains the yaw.  Why do you believe that?

vortexman

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

And respectfully, Majortomski, you need to learn the fundamentals of inertial precession and how it relates to aircraft handling.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Let's look at these in slightly reverse order:

"Why wouldn't you think that a fin exerting that much force laterally would induce significant yaw?"  

Oh I do fully believe there is a HUGE amount of yaw at high power and high angle of attack situations.  I just don't believe it is due to the spiraling slipstream.  It is due to P-factor and P factor alone.

"Why do you think there is no spiraling slipstream?  Does it just seem incredible to you, have you done some measurements, did someone knowledgeable make this assertion to you?"

I ask the question to see if someone has a simple answer.  I ask the question because if it does exist, it is one of the extremely rare phenomena in aviation that does not have a simple-to-find mathematical solution. Why?  I have found mathematical formula to predict the velocity in the downstream flow vortex off of that propeller but not how much twist (if any) it imparts to that flow.

 No, I haven't had the opportunity to evaluate the rotor downwash formula proposed earlier in the post, and no I haven't found that actual NASA (NACA) paper to see where they base the 3 degrees on.  (Higher airflow down the side of the fin could also be interpreted as an angle of attack)

And the biggest flaw in the theory of spiraling slipstream is, as previously stated (see my post of 29 Sept 08 11:15), it is almost universally demonstrated in one plane only.  It is always illustrated from the side.  Again draw it from the top, include the wings and horizontal tail and you should see that the same HUGE force that causes full rudder deflection in slow flight should also be causing an equivalent HUGE rolling moment to the right. And that just doesn't happen.  Like the gyroscopic forces discussed above we have a situation where a force just disappears.

I'd like to know if someone has done this research and has an answer before I go down that prolonged path.

Thank you for your time, keep it coming.



Likewise
"And respectfully, Majortomski, you need to learn the fundamentals of inertial precession and how it relates to aircraft handling."

I reiterate sir, what is the effect of a constant force applied perpendicularly to the edge of a spinning disc?
Where is a helicopter's rotor blade at maximum lift if the pilot inputs stick forces to pitch the aircraft nose down and thus move forward?

Keep it coming guys this is fun (well for me at least)
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

ie helicopter control inputs: for our purposes here, lets use an American aircraft, say a MD500, formally a Hughes, or OH-6. sitting in the crew seat, looking up, the rotor appears to rotate in a clockwise fashion. To tilt the rotor disc forward, and transition out of ground affect, the swashplate, causes the blades on your left (approximately 90 degrees to the desired heading )to increase their angle of attack, and on the right, to decrease slightly. common sense tells you the rotor disc will tilt to the right, but because of the gyroscopic precession, the rotor disc tilts up behind you, and down, in the direction of flight. Lots of other stuff happening about now, but the tilt comes 90 degrees in the direction of rotation. The controls are configured in such a way that you push the cyclic in the direction you want the disc to go, but the forces on the swashplate are 90 degrees off the desired effect.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

<what is the effect of a constant force applied perpendicularly to the edge of a spinning disc?>

If the spinning disc is an inertially free body with no other forces at work except a torque, it will precess.  But that is not a good representation of an aircraft in stabilized fllight; there are many other forces and momemts at work to prevent any rotation about any axis (here pitch and yaw is of interest), therefore there is neither precession (a motion) nor a torque that is inertially caused  by the rotating prop.  As pointed out earlier, there can be torque caused by an uneven flow field through the propellor disk.

I am not comfortable commenting on helicopter rotors since most are articulating and I never had to study rotors

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

"I ask the question to see if someone has a simple answer. "

"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. "

Try reading previous replies.

Cheers

Greg Locock

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RE: Is Propeller Spiraling Slipstream a myth or provable fact?

i don't think it's as simple as that ... there is torque reacted between the engine and the fuselage, but isn't that torque created within the engine (within the cyclinders) and doesn't the fuselage react the torque with the airflow ?

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Yes, but the airflow MUST rotate in response to the torque applied by the prop.

Draw a control volume around the air including the prop (ie through the crankshaft) but not the fuselage. If tehre is no net acceleration in the airmass in the control volume then the surface of the control volume must be exerting an equal and opposite torque.

Equally, if the aircraft is not accelerating in roll, then a control volume that includes the rest of the engine, and the air around the fuselage, must exert a torque equal and opposite to the crankshaft torque at the surface of the control volume.

Everything else is, as they say, details.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

zerosum,
Talk to any old salt who has flown a say FG-1 Corsair, big 14 ft dia. prop, 3000 HP & a extreme nose high attitude at rest, & they will tell you that there are gyroscopic forces at work when you pick the tail wheel up under power. The same forces are at play during aggressive maneuvering, such as combat.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


Is there no one in this community with access to Computational Fluid Dynamics software? That might give some answers.
I am set to tuft the nose of my a/c but hasn't gotten round to it yet. Perhaps someone with more reliable weather could do that too?

Both methods should "prove" the spiralling to exist, and CFD even the magnitude.


 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Ok, let's try to close the torque issue and go back to the existence of the spiraling slipstream.

I only mentioned torque related issues in my posts in an attempt to close the loops on the discussion.

ZEROSUM; on the contrary, the spinning propeller on an aircraft in a steady-state high-power high-alpha climb IS a good representation of gyro and it's results.  In this one condition there is constant force acting at the 3 o'clock position on a spinning disk. That force MUST react with the aircraft in some manner.  The only point I was trying to make is that it doesn't disappear.  Through precession that force imparts a pitching up moment through the propeller blade, into the engine shaft and the rest of the aircraft. That force is easily countered by the elevator. That is my only point on the precession issue.   I brought in the helicopter, as thruthefence pointed out only flys because of the precession of all of the inputs on the rotor disk.

GREGLOCOCK:  Pardon me, but I have read all of the posts, and the best one so far was from VORWALD back on September 12th, and as I've said above, I haven't had time to play with the equations or the other references.  But let's look at your last two statements by this same logic, at some point moderately below a moving aircraft, say at ½ a wing span, we should sense the localized free-air-airflow now moving in the direction of the aircraft because it was displaced by the wing, is this a correct assumption on my part?
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Would the effect be visible by 'tufts' or smoke streams etc in wind tunnel etc?

For instance the circulation of air round a wing isn't easily demonstrated by such means as I recall.

KENAT,

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RE: Is Propeller Spiraling Slipstream a myth or provable fact?

thruthefence,,,

No denying that there are gyroscopic forces at work, and resulting precessional components in the stability & control arena, during maneuvering.  Doesn't have to be a big prop. Compressors and turbines, too. There are obviously inertial forces at work in the flight regimes involving any kind of rotation, including what Majortomski has eluded to, precession.

Rotate about more than one axis simultaneously and the situation gets complicated and occasionally dangerous.

But we digress, I believe he is referring to stabilized flight.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Regarding the "swirling slipstream" theory being originated by Wolfgang Langewiesche in "Stick and Rudder", which I believe was published in 1944. I recently found a reference to the phenomena ( or myth, if that pleases you ) in an old aviation textbook published in 1941; "Aviation" vol 1 of 6, published by the "American technical Society" on page 162, in the chapter discussing the design of rudders, & vertical stabilizers, the authors state: "As the air is driven backwards from the propeller, it is given a swirling motion, and at the same time is deflected downward by the downwash from the wings.The swirling motion of the air causes it to impinge upon the left hand side of the rudder and fin, in the case where the rotation of the propeller is clockwise, as viewed from the pilot's seat. See figure 10 " ( fig 10 being a quaint pen-and-ink drawing of a fixed gear cabane strut monoplane, with the airflow shown striking the tail.) I must emphasize, this is apparently a "trade school" series of books, not a NACA paper, but the information had to come from somewhere. My point in submitting this, is not to prove or disprove the theory/myth of 'swirling propwash', but to show that perhaps it pre-dates "Stick and Rudder". While Langewiesche published magazine articles (Air Facts) before "Stick and Rudder" was published, it's hard to believe the publishers of this large work (24 inches on my bookshelf ) gleaned their information from the popular press.  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
In the FAA library here in Oklahoma City, S&R was the first published book that I came across it referred to.  However, again the point that is over looked is the theory is always used to only explain one phenomina: the need for right rudder in a climb or in slow flight.

They always draw the diagram illustrating the forces affecting the tail, but also always totally ignore the same forces and their effects on the REST OF THE AIRPLANE.

The theory has a hole in it because it sumarily dismisses these huge forces: the roling forces imparted to the airframe that are at lease equal to the magnitude of the yawing force from the fin.

Where do these forces disappear to, or what is used to counter act them?

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

One last time: the rolling forces that act on the wings due to a spiralling slipstream, act in opposition to engine torque, and so come out in the "wash" of the aileron/trim settings required to counteract the engine torque.  You can argue that the magnitude of the spiralling slipstream forces, relative to engine torque, are small, and I'd agree.  But the slipstream rotates, you can see the rotation in smoke or vapor trails behind both prop and jet engines.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Almost certainly an old Langley NACA report from the thirties will have the propwash empiricals documented

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Zerosum from the last link

"You don't notice the effect of the helical propwash in cruise, because the aircraft designers have anticipated the situation. The vertical fin and rudder have been installed at a slight angle, so they are aligned with the actual airflow, not with the axis of the aircraft."

Show me one drawing with this design feature on it!

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


Majortomski,

The helical slipstream doesn't have much effect in the roll axis.  The main reason for this is the one pointed out by btrueblood: The engine exerts torque on the air passing through the prop, and then the swirling air partially cancels that torque out by exerting some of it back on the wings.  There is certainly some net torque, and it is trimmed out using aileron.  This torque would be relatively small, and ailerons have a lot of roll authority, so they can easily deal with it.

The link posted by zerosum has a great writeup, and comments on P-factor as well as gyroscopic precession.  I highly recommend that you read it.

I think your characterization of the helical slipstream as a source of yaw as a "theory" is incorrect.  A theory is not needed here.  This is phenomenon, and it has been widely observed and conclusively explained.  The explanation is simple, understandable, and consistent with widely understood aerodynamic knowledge.  For some reason, you seem to have an axe to grind here, and I suspect that you are way past the point of saying "Yeah, that makes sense", so I won't hassle you anymore.

vortexman

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

I suggest we, as a group, present this problem to "mythbusters" on the Discovery Channel.

I remember how they "proved" the obvious fact of a runaway prop driven aircraft cutting strips out of an aircraft fuselage.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Here are some NACA reports dealing with the subject at hand.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930081810_1993081810.pdf

The authors discuss the computation of roll, pitch, and yaw coefficients for a single-engine airplane including the effects of twist (rotation) of the slipstream due to propellor operation, and compare calculated results to test data, showing good agreement.  References are given for sources that they used to calculate twist angles for the prop.  The report specifically mentions that spreading of the slipstream due to shear/turbulent dissipation was ignored, but that they also assumed 1/2 of the propwash twist was cancelled by the flow-straightening effect of the wings based upon data from ref. #11 in the report.

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

Powered model tests for the XB-28, a twin engine medium bomber, where they discovered that props rotating up in the center gave better lift and effective thrust; this data was supposedly used by designers of the P-38 to determine which way the props should rotate on that bird.  More references for determining stability characteristics of powered models.
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

This is from the Princeton Aero Dept, one of the foremost centers of technical excellence in the field of aircraft stability and control in the country.  I doubt that any of their Profs would propagate any myths regarding their chosen field.   

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Sorry, "calculate twist angles for the prop"  should be "calculate twist angles for the propwash or slipstream".

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Good (re)seaching btrueblood.  The first NACA report (for instance see Fig 14C, easy to see the asymetry due to the propwash) should should finally dispell any myths.   

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Vortexman, your statement

"The explanation is simple, understandable, and consistent with widely understood aerodynamic knowledge.  For some reason, you seem to have an axe to grind here, and I suspect that you are way past the point of saying "Yeah, that makes sense", so I won't hassle you anymore."

But it doesn't make sense.  I have no axe to grind.  I am searching for someone to clearly explain the problems this theory creates that are either totally ignored or magically dismissed with the wave of a hand.

Spiraling slipstream continues to appear as a simply stated fact, but there is no easy to find mathematical proof that it exists.  There are no computations in aircraft performance that acknowledge its existence and thus its effective elimination.  This one point flies in the face of the other fact that we have mathematical solutions to virtually EVERY other aspect of aviation.  

From the link you posted earlier:
From 8.4 "You don't notice the effect of the helical propwash in cruise, because the aircraft designers have anticipated the situation. The vertical fin and rudder have been installed at a slight angle, so they are aligned with the actual airflow, not with the axis of the aircraft."
THIS kind of teaching makes my point exactly!  Show me the equations to design in this mythically perfect angle. He's teaching pilots to believe the engineers have taken care of him, and the topic doesn't even exist in textbooks.
He continues to expand on spiral and minimize P-Factor:
"8.5.3 Initial Takeoff Roll
There are quite a lot of myths surrounding P-factor. For some reason, P-factor gets blamed for the fact that typical aircraft require right rudder on initial takeoff roll. This is impossible for several reasons.
·    Nearly everybody these days learns to fly in nose-wheel type aircraft, which means the propeller disk is vertical during the initial the takeoff roll. Since there is no angle between the relative wind and the propeller axis, P-factor obviously cannot occur.
·    Now let's suppose, just for sake of argument, that you are flying a taildragger, in which the propeller disk is actually non-vertical during the initial takeoff roll. Common experience is that the most right rudder is required at the very beginning of the takeoff, before much forward speed has been achieved. The FAA Airplane Flying Handbook (reference 16) says this is because P-factor is worst at low airspeeds. But wait a minute — real P-factor is proportional to airspeed. In the initial moments of the takeoff roll, there is no relative wind, so there can't possibly be any P-factor. Of course, if you are taking off into a headwind, there could be a little bit of P-factor — but does that mean if you take off with a slight tailwind there will be a negative amount of P-factor, requiring left rudder? Don't bet on it.
The real reason that you need right rudder on initial takeoff roll is because of the helical propwash, as discussed in section 8.4. P-factor exists in some circumstances, but it cannot possibly explain the behavior we observe during initial takeoff roll.


For 7 years I flew a STOL equipped C-182, for my check ride and every other year after that, during that BFR, there we'd sit full power about 20 degrees nose up, right knee locked and shaking holding full right rudder against the stop to keep the ball centered. Airspeed has disappeared under 40 KTS. I'm adjusting pitch to maintain a heading. Too much pitch and we start to go left because there is no more rudder to stop the airplane from going that way.

If I believe the statement the above author makes in full context of his complete article, then while I was replicating the conditions of a take off, I was literally fighting ONLY the effects of the spiraling slipstream (or helical propwash as he calls it) and not P- Factor.

IF his argument is valid, then as I've repeatedly stated, the helical propwash should be causing a roll to the right, of the same massive magnitude as the yaw that I'm fighting that is trying to drive the aircraft left.   You folks have proposed that this massive roll is quite conveniently and continually negated by the torque from the engine at ALL power settings and airspeeds leaving only one mysterious un-countered remnant of the helical propwash to cause the only effect present, a massive yaw to the left.

That my friend is what doesn't make sense and why I continue to challenge you all to challenge blind faith in a very old but unchallenged aviation statement.

BREAK

Yes, btrueblood's posts were extremely helpful, but they also contradict other previous posts.  I stated that I thought Stick and Rudder was the source of spiraling slipstream, yet one of these documents predates S&R, and the other is a wartime document that I doubt could find its way into the hands of the non-military.

BREAK
One other argument in favor of spiraling slipstream is the H shaped tail assembly on multi-engined aircraft.  Frequently this tail configuration is touted as designed to improve control over single finned aircraft by keeping the fin and rudder in the high-speed airflow of the remaining engine.  Please note that if the designers believed in spiraling slipstream, virtually ALL of the examples I've posted below should have vertical fins that rest completely submerged deep in the propwash to ELIMINATE any yawing effects from the slipstream.  Yet all but ONE of these examples have fins located in such a way that would MAGNIFY the negative yawing effects of the slipstream.

Beech 18
http://richard.ferriere.free.fr/3vues/c45f_3v.jpg
OV-1
http://richard.ferriere.free.fr/3vues/ov1_mohawk_3v.jpg
Bf-110
http://richard.ferriere.free.fr/3vues/me110c_3v.jpg
B-25
http://richard.ferriere.free.fr/3vues/b25c_1_3v.jpg
P-61
http://richard.ferriere.free.fr/3vues/p61_3v.jpg
Potez 63-11
http://richard.ferriere.free.fr/3vues/potez6311_1_3v.jpg

The ONLY twin that comes close to having the fin and rudder buried deep enough in the slipstream is the SKYVAN
http://richard.ferriere.free.fr/3vues/short_skyvan_3v.jpg

So, there is just one more commonly used argument that invigorates my quest for a clearly defined solution to this "twist" in aviation literature.

Keep it coming.
Thanks

Tom
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

ya know, there are other considerations to the design of the eppanage, some of them structural, some of them aerodynamic. I don't think you can realistically look at some renderings from "Janes" or whatever,& pronounce then inappropriate for the "spiral propwash" myth, oops, I meant "theory". Any conventionally designed aircraft (this rules out flying wings & pushers) are going to have at least some sort of fuselage back there that reacts to the air being accelerated past it. For nearly a hundred years now, backyard tinkerers, mechanics, and finally engineers have designed for propwash impinging on the aft L/H fuselage & vertical stabilizer. Do you really think at least one of these talented people wouldn't have debunked this "conspiracy" by now? Geeze, I would think he'd win the Collier Trophy! I can see the headlines in Aerospace News & Space Technology now:  "So & So discovers swirling propwash a myth!!!" Don't think they haven't tried, fer gosh sake, this is another source of induced drag, & everyone tries to get away from that, in the search for efficiency. Was there an argument when Whitcomb began his work? Drag from tip vortices?? Thats a myth! Show me the proof!

I was once told, that arguing with "true believer" is alot like wrestling with a hog, in a mudhole, you don't make a lot of progress, and soon you come to realize the hog is enjoying the process.  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Kind of like the never ending plane on a conveyor belt problem????

Two schools as to the answer: yes it will, no it won't and neither school will accept the solution from the other side.


In the case of this thread, again the NACA papers and the Helicopter down rotor equations have given me the newest clues in my search, I have to study them for a while.

Again I ask you all to keep an open mind that all the above arguments agree that  P-Factor does cause a higher velocity airflow down one side of the fuselage.  That higher velocity airflow could cause the same yawing effect on the vertical stabilizer that is claimed to be a higher angle of attack caused by the spiraling slipstream.  Just maybe the NACA engineers were working to a forgone but inacurate conclusion.  

Just like styrofoam can't possibly punch a whole in a carbon fiber leading edge.

Our job as engineers is to validate facts.  Just sometimes it is worth the effort to go outside the box and look at very old problems in completely new and different ways.

Thank you all for your input time and courtesy.

Have a safe, sucessful and blessed life

Tom Solinski

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

i'd like to take a different tack on answering this ...

consider a propeller in isolation (ie no fuselage or wing effects).  the drive shaft from the engine has torque in it, which the propeller has to react with the airflow.  this implies a couple of drag (circumferentially directed) forces.  i think this is an "interesting" point in a couple of ways ... i had earlier supposed that pitch control could remove the (inefficient) drag component of the propeller aero-force so it could provide pure thrust; i guess pitch control allows the propeller blade to adopt efficient configurations producing more thrust for a given torque (that the drag component is set, determined by the torque).

another thought is, consider the airflow through the prop disc ... do you really think that the airflow can go straight into the disc and come out in the same straight direction (which i think is required if the slipstream doesn't swirl) ?

adding in the real world fuselage afterbody and the wing obviously affect the slipstream in such a way (i think) as to make the problem intractable to mathematical solution; i doubt there is a closed form equation solution (like was originally requested at the beginning of the thread).

one last point ... designers have been offsetting the fin either in anticipation of a problem, or in response to a flight test demonstrated problem.  i doubt anyone thought they were carefully determining an "optimum" angle, i think they were reacting to experience.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

I  took the opportunity this weekend to visit the Museum of Naval Aviation and  look up the empennage of a bunch of high-powered conventional tail-draggers.  Forgot my camera, but here are my observations:

Around 25-30 aircraft in the above category inside the building.  Quite a few were suspended from the roof so were not in the sample.  Maybe ten – twelve where I could really check alignment.

Four aircraft with the vertical stabilizer definitely offset. 1)   P-40 Tomahawk. 2)  Hellcat 3) Bearcat (The Wildcat was hanging, couldn't check) 4) AD-1 (A-1H Model) very pronounced angular offset.  Maybe permanently trimming the vertical for cruise was a Chance Vought and Douglas thing. I couldn't discern offset on the Grumann fighters.

The verticals on the remaining aircraft appeared to be centered on the fuselage centerline.  None were offset the opposite way.  All props on the offset-aligned aircraft  rotated clockwise looking forward as did all the other aircraft I checked.

The technical order showing how to jig and structurally align the A-1 should still be around in somebody's archives and would be the final clincher about propwash effects, at least for me.
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Maybe Grumman used a non-symetric airfoil for the vertical.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

"very pronounced offset"  Think about the mission of these aircraft- carrier based, huge prop, powerful engine.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Milne-Thomson gives a mathematical treatment of the slipstream in "Theoretical Aerodynamics, 4th edition, Dover, pp 235-242. Yes, the propellor induces radial, axial, and TANGENTIAL motions to the slipstream, and is thoroughly described.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Thank you Shebly I'll have a hunt for that one.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

GregLocock was right on to raise the issue of the torque applied to the AC by the engine/prop. It is apparant that the question, as originally posed is a complex issue. I was always impressed by the fact that experienced pilots, at take-off positioned their aircraft at an angle to the runway (by eye-ball as much as 10 degrees) to compensate for this effect. My take on this was that at low speeds and high prop thrust the AC would be rotated (to the left)and  as power was applied the AC would rotate to the desired heading. This is one of the advantages of counter rotating props - it ellimnated this effect - especially important when takeoff was to be accomplished in close proximity to other aircraft. This is, of course, more noticable on aircraft of high power to weight ratios (or more specifically high prop thrust to weight{prop diameter/blade pitch). With no engineering credentials I will add that correcting for this effect - the rotating component of the prop wash - must also vary, from acceleration (as in climb conditions) to cruise and any fixed rudder offset must be a compromise(it cannot be a steady-state solution).  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Concerning the vertical stabilizer; is anyone aware of an aircraft that had a movable vertical stabilizer for trim purposes? There are tons of trimming horizontal stabilizers flying, but I don't think I've ever heard of one for the vertical.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


Trimmable rudders are commonplace.

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

i think thruthefence is talking about a horizontal stabilizer that pivots on the fuselage (as opposed to trim panels as Dan320 mentions).  H.Stab's are pivoting, or "flying", to correct for very large CP changes for high mach numbers (like fighters) and for large CG movements (like civilian transports).  V. Stab's aren't designed to pivot (except maybe in some fighters) cause the lateral loads don't show the same large changes; a rudder is sufficient.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


Not sure of what thruthefence is getting at, but to introduce the significant complication of an all-moveable stabilizer, be it horizontal or vertical isn't done unless it's absolutely unavoidable. For slipstream effects trimmable rudders have proven sufficient where needed.
Why, in low-power a/c the pilot uses the rudder trim actuators for walking up to the cockpit!
Have a nice week-end.

Dan
  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

thruthefence, rb1957,

I have never heard of a trimable vertical stabilizer, most likely for the reason stated by rb1957.  

Lots of recip single-engine fighter pilots learned to not reach full throttle until there was enough airspeed to ensure rudder effectiveness. Even go-arounds could take some technique to execute with good control (I am told).

  
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

F35 has an 'all moving' vertical stabilizer if I recall correctly.  Actually it has 2.

I'm not sure about F22 though.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Yeah, Kenat, you're right.  I had mentally discounted a solid slab configuration for yaw control. I need to jump ahead fifty years to the present.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
The YF-12/A-12/SR-71 family have slab rudders/all moving vertical fins

T

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Good point Majortomski, I'd forgotten about them.  I wonder if any other high speed A/C had them, I believe X15 did.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Yeah, I was referring to a movable vertical stab, as opposed to a trim tab, and no, I'm not planning on building an aircraft so equipped; just curious.   

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


May I point out that none of the mentioned a/c is propeller driven and none of their tails will ever encounter a spiraling propeller wake.
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Not to worry, Dan320, as they don't exist anyway............

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


Well, I believe they exist. For me it's a matter of faith. You have to believe in something these days...

  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
AHHH but...that IS the whole point of this thread!

Faith is belief in things unseen.   Every other aspect of aerodynamics is mathematically documented.  And except for the refrence up on 9 November no one else has put forth mathematical proof that the spiral exists, it's a faith thing that I'm questioning.

I'm still working on getting a copy of that book to see what it says.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


The reason I believe is that the spiraling slipstream spirit manifested itself in a 3 degree offset of the vertical stabilizer of the Seabee I used to own.

Reverend D
The Congregation of Believers in Un-Straight Airflows and other Such Stuff
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
But Dan, looking at the Seabee side view:

http://www.republicseabee.com/Images/Seabeesideview.jpg

By the "faith" theory there's absolutely no need to have an offset vertical fin.  The fin and rudder are burried symetrically in the spiral, therefore the upper half and the lower half of the fin, by faithfull assumption cancel out the effects of the spiral off of the propeller.

Comments?

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
AND the problem is further confounded by the issue that the Seabee prop turns the wrong way too!

http://www.airliners.net/search/photo.search?a=1&aircraft_genericsearch=Other%7CRepublic%7CF-84%20Thunderjet%2FThunderstreak%2FThunderflash&amp;keywords=RC&keywrange=&amp;nr_of_rows=111&amp;first_this_page=15&page_limit=15&amp;sort_order=photo_id+DESC&amp;thumbnails=&engine_version=6.0&;nr_pages=8&page=

All the photo's I'm looking at indicate that the prop on your Seabee should cause the spriral to flow counter clockwise, demanding LEFT rudder if the faithfull theory is to be followed.

???

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


Well, it does in fact have an offset fin. The lower half of the slipstream hits the fuselage which is straight, and the upper half hits the fin, just like any airplane. The fuselage isn't much of a wing due to its shape, the fin is probably more effective in producing lift with an AOA.
Come to think of it, we once sucked oil into the manifold one fall to avoid corrosion in the cylinders. I have a video of that and it shows the oily smoke from the exhaust go up on the right side and down on left side. It doesnt spiral, it just "bends" when passing the propeller and then continues straight but at an angle to the airplane long. axis. The exhausts are fairly close to the prop hub.

  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


Majortomski,
You probably wrote your latest posting while I was writing mine, so I didn't read it then. You do have a point about the prop going the wrong way. Maybe the rear fuselage isn't so ineffective after all. Or maybe the Seabee is a special case. I have seen the smoke puffs from the exhaust go in different directions so in my mind there is no doubt about spiral wakes.

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Dan 320:

Did you really have to use left rudder to counteract torque?
I believe that there are (or used to be) aircraft handling quality standards that such a situation would have violated for an aircraft such as Seabee.  But you said it never got type-certified in the US?

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

I could be wrong but aren't half the posters still mixing up the large scale 'turbulent spiral' with the bound vortex this (possibly) generates?

Is'nt this latter what the OP asks about?

I'm wondering if the effect of the bound vortex is dwarfed by that of the overal 'turbulent spiral'?

 

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Zerosum,
Right rudder.
The Seabee was type certified in the US, it was built by Republic Aircraft Company in Farmingdale, New York.
Republic as in P47...
Who said it wasn't?

Kenat, I thought the bound vortex around the individual blades caused the large-scale spiral.

To get back to the question in the 1st O.P. I think we by now should be able to agree that there probably is such a thing as spiraling slipstream, but that it is not the only force or phenomenon occurring. Spiraling may in some flight conditions have the larger influence on the aircraft, in other conditions it may be some other force as for instance the P-Factor.

I am also convinced that the 3 or so degrees of offset on a fin is sized for cruise and that in climb we have to use the right leg to counteract the stronger spiral (and/or P-Factor) due to the higher engine power output.

I fear this will go on for ever.

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

To segue onto the Seabee certification comment- I believe the confusion comes from the "airliners.net" link, where one of the photos describe an aircraft where a "Corvette LS6" engine was installed in place of the venerable Franklin, thus earning an "experimental" certification. I bet that cured that old "underpowered" reputation. I once worked for an outfit that operated a "Twin bee", powered by two Lycoming tractor engines ( no, not THAT kind of tractor) no idea if that pesky prop wash business affected it.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


Re. the Twin Bee, I have always wondered about the single engine performance. The original 'bee had 215 hp and isn't exactly overpowered, the twin 2X160 hp and if one of them quits there will only be 160 hp available. And increased drag from the dead engine and prop. Sounds marginal at best.



 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Dan320:

re "Who said it wasn't?"  I misread the "airlinerslink.net" site referenced above, sorry.  I'm well familiar with Republic Aviation having flown the Thud for awhile.  Just wasn't familiar with the Seabee.  Interesting airplane.  

I know we're up to around 140+ posts on the thread, but now your example of a pusher prop turning the 'wrong' way with right rudder required for torque offset does indeed tend to upset the cut-and-dried propwash theory.  (And the vertical being offset leading edge to the left(?) makes it even more interesting). This subject certainly had to be well-explored in the 1930's-1940's. We just haven't yet tapped into the motherlode of information on the subject yet, unless Shebly's referenced book (above) is it.  I am also trying to find a copy of Perkins and Hague's book titled something like "Aircraft Stability and Control" probably Princeton Press circa early 1950's.  Bet there is a write-up on there about propwash.

Going to have to do a search for one of these aircraft (Seabee) in the Museum of Naval Aviation. They might just have one in the back forty somewhere.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?


As far as I can see P+H only state (Directional Control 8-3):

"Slipstream Rotation - The slipstream behind the propeller has a rotational component which changes the angle of attack on the vertical tail and will create sideslip if uncorrected by the rudder. The critical condition for slipstream rotation is for high power at low speed. see fig 8-11. counter-rotating propellers, of course, obviate this factor."

In the Longitudinal Stability chapters they say something about difficulties trying to treat them analytically.

One can say they treat most aspects of stability and control except slipstream effects. For the above reason.

When I visited Pensacola they did not have any Seabee's on exhibit. However, in the New England Air Museum at Bradley Airport,CT, there at least used to be one.

 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Seabee's were post ww-ii and probably never used by the military so it wouldn't be at Pensacola.

T

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

The original premise of this thread was the quantification of propeller swirl in the slipstream.

There are similarities in flow patterns between aerodynamics and hydrodynamics.  Boat props do indeed create a swirl effect.

Aircraft swirl effect may be minimal, probably due to viscosity and density differences to that of water.  As a result, the effect is considered minimal, and of little consequence, and not studied further.  My best guess.

When I first got my plane, I tufted the right nacelle (and wing), and observe on the nacelle the flow pattern was similar as if smooth water was flowing over it.  That is, the tufts on the inboard side "flowed" inboard, and the outboard tufts "flowed" outboard.

I've tried to post a picture.  It was taken at 97 mph, near Vmc

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Mmmmm...NACA Tech Reports...

While searching for other information, I found this:
http://naca.central.cranfield.ac.uk/reports/1947/naca-tn-1483.pdf

In this report you will find tests done on a P40 to measure tail-loads. There is a large difference in lift btw l/h and r/h stabilzer halves, in some cases the direction of lift is opposite on left and right. Hard to imagine anything at work here but spiraling slipstream.

Dan  
 

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

(OP)
Hmm the images on pages 42 and 43 make my point from the early post.  If the slipstream is spiraling then in addition to the yaw to the left caused by the AOA on  the vertical fin, then there should also be a ROLL to the right caused by the differential loading of the horizontal surfaces.  The diagram says this is the case.

What the report doesn't say is did they check for any distortion of the horizontal stab on the aircraft before or after it was modified with the pressure ports.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

ok, so assuming the plane wasn't bent, then we all agree that that there is a spiraling slipstream producing effects that are not ameanable to precise calculation but are highly dependent on the installation under consideration.

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Shelby,

was gonna give you a star, but that's the same report that I posted a link for on 17 Oct 08 11:38  

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Greg has it right on

RE: Is Propeller Spiraling Slipstream a myth or provable fact?

Interesting conversations. My first thought is the effect of an inertial torque as some of you mentioned.
However, the thought that the rear stabilizers were offset on some previous aircraft's designs has one searching for a documented reason.

ttfn,

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