Two little tests will fix the problem.

First stick a prop blade into a pan of water & stir it using a cylindrical movement approximating blade motion in the air -- what happens to the water.

Next, go out to your hanger, start up your Extra 300 & rev it up. Turn on the smoke & hold it up over your head -- then tell me what you see.

Probably moot points -- Red seems to have interjected something that will wreck the argument -- NASA evidence!!


What about the pro-verse roll?

Majortomski wrote: The twist of the propeller slipstream has been experimentally shown to exist, no doubt about it! (See my previous post for links)
As I previously stated, the twist angle is of the order of a few degrees and thus you will not see the slipstream spiral around the fuselage simply by observing an aircraft in flight by eye. In controlled experimental conditions however, the twist is easily measured.

When confused by theories trust nature!

/Red B.

Yep Red B finally found what I was looking for however...

The old report from the 30's finally gives an equation to the amount of twist imparted in the slipstream, and at cruise it is only 3 degrees. Doesn't say which way, or what happens at slower airspeed and higher angle of attack.

The second report, and now I've got to go hunt down a full copy of it, basically assumes that the swirl exists because the left wing stalled before the right wing by surprise...the three degrees from the first report! But they didn't go into the detail of how was the test aircraft rigged, or was it a damaged airframe in the first case.

Now under this new light I have a new set of criteria to examine. But I'm beginning to believe that over the course of the years the two minor forces involved in slow flight yawing (spiraling slipstream and angle of attack on the propeller) have been over emphasized the the significant forces actually at work (propeller rotational velocities and P factor thrust) are being almost completely over looked!

My take, with the engine axis in line with the main axis of the airplane, and thus passing through the CG torque creates pure roll to the left. Off set the engine centerline to the right and its axis of action will now pass to the left of the CG, somewhere inboard of the wing tip. The result is that the airplane will both roll left and pitch down. Take the extreme right offset solution, the engine is now pointing 90 degrees to the centerline of the fuselage (basically pointing toward the right wingtip, like some of my landings) and the torque will cause the plane to nose down. But we will have finally eliminated it from the left rolling problem. 'course the plane won't fly this way either

In all cases torque is a constant. It can only be countered with a torque in the opposite direction. With respect to the question no amount of right offset can make engine torque effects go away, just move them someplace else.

P-factor is a differential thrust across the face of the blade due to localized changes in relative velocity and angle of attack. Sadly in most literature being published, only the angle of attack portion is being taught. First pardon my old school English measurements; I never made the transition to metric ;-)

A propeller spins at a certain RPM, for our discussions let’s only look at the tip of the propeller. For a given RPM the propeller tip is moving a specific velocity in feet per minute. For example a 4-foot propeller the tip is 2 feet from the hub. The distance the prop tip is traveling is the circumference of the circle or PI x D; (3.1416 x 4 = 12.58 feet) Now if the engine is turning a typical 2400 rpm on a Cessna type aircraft, the tip speed works out to 12.58 feet (one revolution) x 2400 revolutions per minute or 30, 200 feet per minute or for smaller units (dividing the minutes by 60) 503 feet per second. (88 feet per second equals 60 miles per hour) so the prop tip is doing 340 miles per hour.

Lift is a function of the following equation: Lift (or in the case of a propeller Thrust)
T= ½ CL � V2 A
CL is the Coefficient of lift. The CL is a function of the airfoil shape and its angle of attack; this is determined by wind tunnel work.
� RHO the density of the air
V2 the velocity through the air squared
A area of the wing. In the case of a propeller it gets more complex because there is an integration of the different angles of attack and speeds along the length of the blade. If we confine our discussion to just the tip, we only have to work with one value. Note that the Velocity is a squared value in the equation so a little a small change in velocity causes the most change in lift.

An airplane in a steady state climb, at a constant angle of attack (say 5 degrees and a constant airspeed (say 120 mph or 176 fps) causes the propeller to strike the on coming are at the same angle of attack the as the wing. From vector analysis that on coming airflow has a component that acts perpendicular to the front of the propeller and a vector that acts in the plane of the propeller’s rotation. If our airplane has a 5 degree angle of attack then from trig, the component of the in coming 120 mph air is 10.4 mph acting in the plane of the rotation, and 119.4 MPH perpendicular to the plane of rotation. We only need to focus on the 10.4 mph component.

As the propeller rotates in this in coming air it sees that 10.4 mph component reverse direction. At 12 and 6 o’clock it is zero, at 3 o’clock it’s PLUS+10.4 mph, at 9 o’clock it’s MINUS-10.4 mph.

Now add these two values to the propeller tip speed from the above discussion. At 3 o’clock the prop tip now has a speed of 350.4 MPH (340+ 10.4) and at 9 o’clock the tip now has a speed of 330.6 MPH (350-10.4) if we plug just these two values into our equation above

T= ½ CL � V2 A

And use an arbitrary 0.1 for CL, 0.01 for RHO and 10 for area we get
T @3:00=(.5)(0.1)(0.01)(350.4)(350.4)(10)=1,227 pounds of thrust (not a real number because of using the “1s” just something to work with)

But at the 9:00 position
T @9:00=(.5)(0.1)(0.01)(330.6)(330.6)(10)=1093 pounds of thrust

A difference of 134 pounds of thrust pulling harder at the right propeller tip causing a turn to the left.

And that is only due to the difference in rotational velocity. Buried in the CL term is the angle of attack. It to changes as the prop spins by +/- the 5 degree incoming air. Its effect are additive to the rotational values.

That is where P- factor comes from. Now for the first curve ball the strongest thrust is at the 3:00 position, that causes the highest airflow down the right side of the fuselage.

But due to gyroscopic precession it also pulls the nose of the airplane up!

Drat, I did this in word, and I just noticed the symbols dissappeard

[]

Lost me on that one. 'splain the pro-verse adverse rudder roll concept

thanks

MajTom's P-factor explanation was dead on -- simple really



Proverse roll is pretty simple to understand -- the aircraft rolls in the direction of rudder input (note rudder, not aileron). The roll will subsequently cause the AC to turn. This is what you would normally expect to see -- it is a sort of naturally occurring self-coordinating turn mechanism. Model trainers almost always exhibit this behaviour -- right rudder, for example, results in a right yaw & a near-simultaneous right roll, giving a nicely coordinated turn without even touching the ailerons. Causitive factors include, but are not limited to; dihedral, swept back wings, & substantial upward vertical displacement of the aerodynamic center from the vertical C-of-G (high-wing pendulum effect).

Adverse roll is just the opposite -- the aircraft rolls away from the direction of rudder input. Right rudder produces a right yaw, but a LEFT roll. This is counterintuitive & it gives inexperienced pilots a very bad time. The turn must be coordinated by using both rudder and aileron together (in the same direction). Causitive factors include; forward swept wings, low wings, and anhedral (or simply insufficient dihedral).


I'm going to use proverse roll to down-play the Major's claimed effects of P-factor in explaining roll-neutral aircraft at power-on near-stall conditions

You are right about the origin of the P-effect MajorTomski. To many people think it is associated with torque or gyroscopic effects.

I have only one comment:
Gyroscopic precession only occurs during the yaw. Thus the nose up pitching moment you were discussing will only occur if the yawing tendency caused by the P-effect is left unchecked and is allowed to yaw the aircraft.

It seems that we have come to an agreement :-)

/Red B.

Well, guys, as I said in my original post, I'm pretty much a newbie to this airplane stuff (started in summer of 2004), but with the help of my physics from 40 years ago I more or less follow. It's been interesting, really. This has helped my understand the need for right rudder on trail dragger takeoff. Much of the explanation I had gotten elsewhere was muddled in the fog and misunderstanding of the those who tried to explain. Some things are just inherently complicated.

We may have shaken some other newbies who ventured into this, even though it's a beginner's forum. I'm sure they either moved on or will recover.

Thanks for the discussion.

Hello again Red, this one formented in my head for a day or two. Share your thoughts on this one. If the aircraft is in a steady state climb. i.e. the typical light plane in a 10 minute climb to 2000 meters, then the P-factor thrust will be a constant force down the right side of the fuselage, AND re-acting on the prop at the 3 oclock position. Gyroscopic procession then will imply that the force will react on the plane through the propeller/engine crank shaft at the 6 oclock positon, right?

T

As I understand it, the gyroscopic precession effect is dependent on the rate of change in pitch or yaw. As long as the differential thrust caused by the P-factor is counteracted by the rudder so that the aircraft is flying along a straight line and not yawing, there will no be gyroscopic forces tending to pitch the nose of the aircraft up or down. Gyroscopic effects only occur when the there is a change in the orientation of the gyro’s plane of rotation. You can take a gyro and transport it north/south, east/west, or up/down, without causing any precession, as long as the gyro’s plane of rotation remains parallel to the original plane of rotation. Thus gyroscopic forces will only be felt when the aircraft is flying along a curved path. In other words: In order for gyroscopic precession to occur a net torque must be acting perpendicular to the axis of rotation.

/Red B.

“And as shown in the photo in post #20 above, when you can see these spirals off the prop, they are going the wrong way to support the myth.”

“Ahh but... The swirl theory is based on the assumption that the propeller imparts energy to the air particles that causes them to move in the same direction as the propeller. This is the source of the clockwise rotation. The problem with this is that it violates airfoil theory. Air particles moving over an airfoil are forced down and AFT. To make the slipstream work they'd have to go down and FORWARD!”

On the first statement, you cannot make any conclusions about velocity from the still photo. All we can determine is that the tips of the props have scribed a spiral path with respect to the ground and airframe in recent history. This says nothing about the actual airflow. There is no way to conclude that the spiral is twisting at all, much less one way or the other from that photo. It is however a cool pic.

On the second, that is dead wrong unless I missed something. Actually, what violates airfoil theory (not to mention the laws of thermodynamics) is what you present as correct.

First, if what you say is true, then air has a net velocity increase rearward. I.E. it gains velocity as it leaves the airfoil. If true, this net gain must react on something, i.e. the airfoil. Then the accelerated air would increase the velocity of the airfoil, pushing it faster thru the air. So once you start the prop spinning, or the wing moving, it would accelerate all on it’s own until and you could turn off the power plant. That is perpetual motion… not possible. Violates the laws of thermodynamics.

I believe the hang up is with the downward wash off an airfoil when it generates lift. Yes, the velocity of the downwash is rearward and down (with respect to lift), however, the net velocity of this air mass is moving slower than the clean air. It must… that is drag. Look at the velocity distribution of any airfoil, you will always find that the air mass at the TE of the airfoil is moving slower than the pre-airfoil air.

Then when we have a rotating wing generating lift (a propeller), the downwash will still be slightly slower than the velocity of the airfoil, it must. This will cause the slower air leaving the airfoil to follow the path of the airfoil in the same direction. Once we leave the rotating coordinate system (i.e. sit in the plane or watch from ground) the air velocity from our vantage would then be rearward and in the spiral pattern, with the rotation in the same direction as the prop movement. I think it is the jump from a rotating coordinate system to a fixed (ground or plane based) system that messes with people.

I still don’t know if there is any significant effect to yaw from this, but unless someone can point out were my logic failed, I believe the wash is in the direction of prop rotation, which could possibly lend weight to the spiral slipstream causing left yaw on standard rotating engines. Even after my proof that the spiral wash is indeed the direction I originally indicated, I am beginning to believe that while the some effect is there, p-factor dominates. Thoughts? Cheers.

I’m really curious in finding an answer to this issue, so I did some stone knife and bear skin testing. I’ll be the first to admit what I did isn’t the best test, and I don’t have wiz-bang measuring instruments like a research lab would have, but it is the best I can muster.

I have an indoor electric I just completed that I know has thrust set to 0/0. I verified that as best I can. I also have a lazy-susan contraption I made to help set revo mix on micro helis. I strapped the plane to the lazy susan such that movement is restricted to yaw only.

I fired up the motor and the plane instantly yawed to the left. It took about 10-15 degrees of right rudder to cancel the yaw. There didn’t seem to be a big change in rudder required based on throttle.

I then fixed thread to the top and bottom of the rudder and fixed the lazy susan so it could not move and I set the rudder back to neutral. When throttle was applied, the string at the top was blown rearward and to the right (looking from the top) and the bottom string was blown the opposite way. For this to happen there must be a pressure differential across the rudder fin that is opposite top to bottom. To me this clearly indicates a spiral slip stream with a rotation in the same direction of the prop, i.e. counter clockwise as viewed from the front. From watching the threads, I can see where with proper rudder fin design it may be possible to effectively cancel these forces such that the yaw component is null. In the case of the test plane, I’d have to increase rudder/fin area below the fuse centerline, or decrease area above.

Now my question on all this is what really caused the yaw? Is it really slip stream or something else? P-factor cannot be an issue in my test as there is no forward movement at some AOA, so the thrust symmetry is equal left/right. I’d accept a slight thrust misalignment of the prop due to my measuring instruments (I’m only good to about ¼-1/8 degree), but even if we assume I’m out a full ¼ degree, would this require 10-15 degrees of rudder to correct? Somehow I don’t buy that. My gut keeps coming back spiral slip stream. I believe my test showed there is a yaw effect and a spiral slip stream within the confines of my test (zero airspeed, etc.) But are they related? This is really messing with me. Can anyone find something wrong with my conclusion?

Cheers.

I like it.

I had considered putting ribbons on the rudder/fin myself as a test, but have been deterred by the weather. For me, your simple demonstration passes the first order "smell test."

For my money, the important factors creating a force for left yaw seem to be (in no particular order):
* Spiral slipstream on asymmetric tail feathers (more rudder/fin above than below the thrust line)
* P-factor in high-alpha attitude (axis of prop rotation is above velocity vector)
* Uneven loading on landing gear caused by prop torque (takeoff, esp on grass)

There may be second, third order effects, but that's probably splitting hairs. These might include ground effects on the slip stream as it affects the tail feathers and the P-factor.

Once airborne and in level flight only the first factor would apply. Forces from gyroscopic precession would come into play when there is movement about the yaw or pitch axis. These should be easy to calculate (for some) based on prop mass, prop rpm, and speed of yaw or pitch. Their effects, however, are almost certainly masked (to the pilot) by the control inputs otherwise needed to manage the airplane through the maneuver.

Go back to Red B's post #50. The two papers he sites finally mathematically show a sprial to exist. BUT

BIG BUT

it is only 3 degrees off the centerline of the plane, hardly noticeable at all

Then there's John's bearskin test, great idea by the way. I'm always designing my test apparatus to the point nothing gets built or tested.

I haven't the faintest explantion for that one.

Isn't it amazing--how the seemingly simplest question becomes such a deep academic debate? Fun.....

Wow -- I've been away for a week & this very interesting debate is still raging. Congrats JohnW on doing that lazy susan test. It is something that I was pondering, but you actually bothered to do it -- not to mention the more obvious tuft test.

It would appear that the effects of spiral slipstream are empirically confirmed. [sm=thumbup.gif] I guess that the fixation on full scale engineering has caused a few blindspots. Big planes need really big lazy susans so why bother with all of that costly, messy & unprofessional full-scale empirical testing when we can confirm numerically that P-factor is the culprit.

Hmmmmm. I bet that there is more comming.

I catch the drift of your post BritBrat, et. al. My gut reaction is there is a scale factor here in play, which may explain why the issue seems to be basically moot on large people carrying planes. The small electric I tested has a large prop compared to wing span and a lot of power for the airframe size too. It is also operating at a much lower reynolds number than any people carrying plane. This may be a significant factor. I also believe that if the spiral slip stram factor does exist, is velocity dependent, i.e. it is most significant at zero forward airspeed, as in my lazy susan test. I however have no proof other than my gut reaction based loosely on a semi-educated guess. While I keep pushing the spiral slip stream idea, I'm still not convinced spiral slip is a major player, and indeed it might all be p-factor, even with the models, but the lazy susan test has me second guessing myself as the results were more dramatic than I expected. I'm wondering if maybe the test apparatus interfered with airflow. I plan on retesting again in such a way to eliminate that possiblity this evening. I will post my results. Cheers.

JohnW, I don't think that P-factor is particularly significant. Given its druthers, even a trike model will slew left -- & that absolutely is not P-factor. With a trike, during take-off, P-factor only appears at rotation & then only briefly. With a tail dragger, it is present while the AC accelerates with its tail down -- right up to the point when it lifts its tail -- no more P-factor until rotation -- and again only briefly.

In high power, low speed, high Alpha situations (particularly with a large dia prop), P-factor appears again. However, I don't use right thrust in my models, & high-Alpha left turns just aren't a real problem until essentially at stall (if at all), then torque & spiral slipstream become the big players.

I just dont buy P-factor as a serious problem in model flight. With a full scale Corsair or Thunderbolt -- yes I'll accept it, but with a 40-size model trainer -- nope.

I ran the lazy susan test again, but this time I suspended the plane ½ wing span above the lazy susan with a 1” diameter tube. This gave the plane airflow which was undisturbed less the 1” tube for ½ wing span below and basically infinity for my purposes in all other directions.

There was still some left yaw, but it wasn’t nearly as large as the first test. It took about 3-5 degrees of right rudder to correct, with the most rudder needed at mid throttle settings. Aileron had no effect. However, elevator did. Up elevator seemed to not make any diff, but down elevator seemed to nearly cancel the left yaw. The threads on the rudder again showed there was a spiral flow, at about 2-3 in degrees.

Again, this is stone knives and bear skin testing, but from this I think it is safe to conclude that if the spiral effect is a factor in yaw for typically flight, it is a small one, at least on the particular model I tested. It may also be possible to conclude that the spiral effect seemed be enhanced in ground effect and/or from my test stand.

To me at a personal level, this seems to debunk spiral slip stream as a significant factor, which really only leaves p-factor. Sorry it took me so long to get to that point, but I don’t consider lack of equations proof and the reverse spiral explanation which couldn’t be true threw me off track some.

The problem I now have is similar to britbrats observations in his last post. I've flows a lot of planes, but in recent years they are mainly limited to 2M pattern ships. Reasonably fast, clean, high power. I typically need to carry 1.5 to 2 degrees of right thrust for good yaw tracking. Why? In level flight the positive AOA on the thrust like is probably about zero. Same for verts. So p-factor can't be a major player can it? If p-factor is significant then when comparing level flight (low AOA) and a loop (high AOA), I should see a significant yaw, yes?. But I don't. Are we back to spiral slip stream? This is making my head hurt.

Cheers.

John, your two lazy susan tests cover the areas of interest. The "directly-mounted" test reflects on-ground behaviour -- what could be expected during take-off. The results also reflect what is observed in the real world -- substantial rudder input requirements for tracking during take-off.

The suspended test is representative of an in-flight situation, & again, the results fit with the real world -- very little effect in flight at low AOA, as would be expected.

I think that your empirical tests strongly support the spiral slipstream theory.

Is it possible to conduct a test at a positive AOA? You may have a powerfull fan available to add low-velocity free-stream airflow (near stall conditions). You would probably need some flow-straighteners to approximate the real thing.

This test might adequately demonstrate the add-in the effects of P-factor.

I thought about doing a dynamic test as you indicated. I have a fan and I almost set it up, but I was concerned I would not be able to get clean airflow from a fan not to mention I have two pattern ships I want to finish by new years that are sucking up my time. Like you suggested, I'd have to construct some vanes to clean the airflow from the fan. This is probably beyond the effort I'm willing to go to at this time, not to mention I'd need to use the garage for a project that large, and last night our temp was -9 F.... way too cold for me to be putzing around in the garage. At some point, I would like to investigate these effects more. Cheers.

Took a long time to get through it all... had to skip some!

I don`t Believe the P-factor is dominant. It will have no effect when the plane has no speed. That`s when the yaw is greatest isn`t it? Surely there is lots of yaw in my plane on startup. ( Tried it )

And surely when flying level and on cruise speed there would be no p-effect and still the propeller is 1-2degrees to the right to achieve straight flight... Actually when the propeller is pointed downwards, in this situation p-factor would yaw the plane to the right... ..and still the propeller is 1-2 degrees to the right for straight flight...

What about fitting a motor to an arrangement with no tail and see if there is yaw and then attach a tail and see if theres yaw and then flipping so the fin comes underside and observe if the yaw is opposite... Who could argue then?

The size of slip stream effect will be affected by forward speed, throttle setting and on prop size and rpm so concluding just to 3degrees with no info on these parameters will give little assurance to me.

I find the discussion interesting! I think thats what discussion forum`s all about

Stian