Is Propeller Spiraling Slipstream a myth or provable fact? 1

[Majortomski]

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

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

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

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

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

 

[rb1957]

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.

 

[Dan320]

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

 

[zerosum]

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).

 

[KENAT]

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 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[zerosum]

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.

 

[Majortomski]

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

T

 

[KENAT]

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 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[thruthefence]

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.

 

[Dan320]

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.

 

[thruthefence]

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

 

[Dan320]

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

 

[Majortomski]

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.

 

[Dan320]

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

 

[Majortomski]

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?

 

[Majortomski]

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

http://www.airliners.net/search/pho...umbnails=&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.

???

 

[Dan320]

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.

 

[Dan320]

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.

 

[zerosum]

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?

 

[KENAT]

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 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[Dan320]

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.