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?

 

[thruthefence]

When the spiraling prop vortex disappears.............

http://www.youtube.com/watch?v=VDGIwy-XIWU&NR=1

 

[thruthefence]

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.

 

[Dan320]

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.

 

[zerosum]

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.

 

[Dan320]

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.

 

[Majortomski]

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

T

 

[Hansmeister]

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

 

[Dan320]

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

 

[Majortomski]

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.

 

[rb1957]

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.

 

[shebly]

Milne-Thomson is available on Google Books, but the preview stops several pages short of the desired item! Here, however, is not a proof, but a test.

 

[btrueblood]

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

 

[shebly]

Which one of us has the shortest "short term memory"?

http://www.icmrt07.unina.it/Proceedings/Papers/c/13.pdf

Proof of propeller vorticity.

 

[Co]

Greg has it right on

 

[FeX32]

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,

Fe