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?

 

[zerosum]

A different propwash discussion, same conclusion

 

[Majortomski]

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!

 

[vortexman]

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

 

[thruthefence]

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.

 

[btrueblood]

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.

 

[zerosum]

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.

 

[btrueblood]

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

 

[zerosum]

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.

 

[Majortomski]

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

 

[thruthefence]

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.

 

[Majortomski]

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

 

[rb1957]

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.

 

[zerosum]

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.

 

[zerosum]

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

 

[thruthefence]

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

 

[shebly]

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.

 

[Majortomski]

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

 

[gruidae]

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).

 

[thruthefence]

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.

 

[Dan320]

Trimmable rudders are commonplace.