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]

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)

 

[vortexman]

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.

 

[Majortomski]

Whooops my bad I meant

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

 

[zerosum]

If you don't think this makes it official, just look at the list of Commissioners.

Seriously, good thread, don't let up 'til somebody finds an old NACA wind tunnel report with a powered prop and smoke for documentation

 

[Majortomski]

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

 

[Majortomski]

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.

 

[zerosum]

Majortomski,

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

 

[thruthefence]

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.

 

[Majortomski]

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.

 

[thruthefence]

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.

 

[Majortomski]

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.

 

[vortexman]

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

 

[zerosum]

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.

 

[Majortomski]

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.

 

[zerosum]

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

 

[vortexman]

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

 

[Dan320]

Vortexman, please explain.

 

[thruthefence]

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?

 

[Majortomski]

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?

 

[Majortomski]

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