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?

 

[vortexman]

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

 

[vortexman]

thruthefence,

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

vortexman

 

[Majortomski]

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?

 

[vortexman]

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

 

[zerosum]

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

 

[Majortomski]

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)

 

[thruthefence]

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.

 

[zerosum]

<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

 

[GregLocock]

"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

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

 

[rb1957]

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 ?

 

[GregLocock]

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

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

 

[thruthefence]

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.

 

[Dan320]

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.

 

[Majortomski]

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?

 

[KENAT]

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,

Have you reminded yourself of faq731-376 recently, or taken a look at

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

 

[zerosum]

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.

 

[thruthefence]

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.

 

[Majortomski]

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?

 

[btrueblood]

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.

 

[zerosum]

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