The picture you show and numerous others like it imply a rotating air mass, but wait, this effect is caused by lowered pressure in a highly localized area in the center of the tip vortice, which reduces the temperature below the dew point. The visible moisture is carried rearward in the boundary area between the slipstream and the surrounding, stable air, until it disapates because it looses the rotational velocity(of the vortice) to maintain the low-pressure/low temperature zone. The air in the slipstream is not rotating like the tracing of the vortice, but rather resembles a paper tube being extruded from a machine that wraps a flat piece of paper to achieve the tube. The location and direction of travel of each element of visible vapor is more directly behind the location of the prop tip at the instantaeous moment when that vortice portion was formed. In short, I don't see these pictures as proof for or against any spiral slipstream effect.

I didn't follow your statement that the apparent slipstream goes the wrong way to support the theory. Doesn't the apparent spiral, following the rotation of the standard prop, move rearward surrounding the extended crankshaft and perpendicular to the center of the prop disk? (in the situation where the prop disk is perfectly aligned with the airflow to the prop) If it does, then (the argument goes) it would strike the left side of the vertical fin, and absent another, equally effective (area x mean distance from center of mass on the longitudinal axis) surface below the center of mass, one result would be a net force moment to the right at the tail. (nose left) Obviously, everything over-simplified, but assuming other factors to net zero.

Regarding the prop imparting some energy in the direction of rotation, I definitly agree, again going to the wing airfoil analogy, in stable flight, the prop's slipstream moves rearward after passage of the aircraft, and the vortices generated by the wings move generally downward but also slightly forward. This implies generation of a rotational component by a rotating prop in the immediate area of the prop and cowl, and for some distance behind it. My Question relates to whether or not the rotation continues, and if so, how so? It seems that the laws of physics tell us that "a body (element) in motion will continue in the same direction and velocity unless acted upon by an outside force". What outside force changes the direction of our air particles after the prop has passed through them? Curves or spirals require an outside sustaining force, as air particles have both mass and velocity at all times relevant to this discussion.

Can you help me on this?

Wow, lots of interest in this thread! Good stuff.

stek79,

Regarding the Dirty Birdy, it was a classic. Everyone had one back in the day. It inspired many follow on designs. Very true behavior. My guess is the emphasis for appearances to resemble something full scale is what has lead to the outlines that are prevalent today.

mesae,

Thanks for the compliment. This thread is certainly bringing out the rigorous discussion. All kinds of brain cells getting dusted off. Aviation is my hobby and occupation. I very much enjoy the flight dynamics. Flight is a great physics experiment. I mostly scratch design and build. Someday I want to mount a helicopter rotor system on the front of a 3D type plane and have at it! I'm thinking super duper computer radio with everything on separate channels and who knows what kind of mixing? Maybe even tail rotors on the wing tips.

LouW,

I mentioned cyclic pitch to help indicate how powerful P effect can be. But the analogy between airplane and helicopter soon begins to diverge. The difference is, with a helicopter, rotor blade changes control the movement and orientation of the rotor and helicopter. With an airplane, aerodynamic forces from the airframe control direction of flight and the orientation of the propeller. Basically each dances around propeller effects but from opposite frames of reference. A very most fundamental difference.

Once again Gyro forces only come into play when the aircraft is allowed to change heading. Rudder must be used to balance the yaw force of the P effect if heading is not changing. If too little (or much) rudder is applied, the plane will begin to change heading and then and only then do gyroscopic forces introduce themselves. For example imagine a toy gyroscope spinning in its wire cage. When you push on the rim it reacts at 90 degrees because the axis is not restrained. If one person pushes on the cage in one direction and another person pushes on the cage just as hard in the opposite direction, these forces balancing each other. The gyro axis is not disturbed and no reaction at 90 degrees is generated. Similarly rudder balances yaw inducing P effect to maintain heading. Simple statics. Gyroscopic reaction is a dynamic phenomenon.

The gyroscopic forces from our model propellers are relatively small. This is easily demonstrated with an aerobatic type designed to have no roll or pitch coupling when rudder is applied. During flight adjust trims for hands off straight and level at cruise speed. From straight and level hands off, cause a yaw by simply inputting rudder. Does the plane pitch or yaw? If gyro forces were large, the plane would pitch noticeably as well as yaw. Do the same in pitch. From straight and level pitch up and down. Does the plane pitch or yaw. If gyro forces were large, the plane would yaw noticeably as well as pitch. This brings up a misconception regarding the lomchevak (or Lomcovak, whichever spelling you prefer). This maneuver relies on a gyroscopic reaction at near zero airspeed. Our model engines/props don't usually have enough rotational inertia by comparison to perform a true lomchevak, but enough control authority (3D) to do a tumble that seems like one.

All,

P effect for a prop airplane is a multidimensional problem. Previously, I did not consider the rest of the problem. As mesae and MajorTomski correctly point out, besides the AOA change there is also a velocity difference involved. This increases the magnitude of the P effect. And because more lift always comes with more drag, there is a pitching component generated from the additional drag of the blade that is working harder plus the decreased drag of the other blade.

With a normal thrust line, the pitch component goes unnoticed because it gets masked by the control input that is creating the P effect in the first place. Normally our models are stable in pitch and we trim our models for cruise flight. So slow flight or steep climb requires some up elevator. The pitch component of P effect helps the nose up requiring less up to perform the maneuver, but the pilot is still pulling up to perform the maneuver. However, this is part of the reason why there is a difference between power on verses power off stalls behavior. The other part is simply due to accelerated air blowing over the tail.

MajorTomski,

Spiral slipstream is not a significant factor when a plane is in normal flight at speed. But for modern 3D stuff at very slow or no airspeed such as hovering, slipstream effects become significant because there is little else to oppose them. My guess is the early study you mentioned did not look at the static condition of zero airspeed, but maybe it did? Still, 3 degrees AOA is significant, especially when nothing else is going on. By the way, didn't engineers forget about studying props when the jet age began? At least until the fuel shortage hit?? I think the 3D movement has brought subjects like slipstream and P effect into minds of modelers.

gearup,

To address your question of "whether or not the rotation continues, and if so, how so? It seems that the laws of physics tell us that "a body (element) in motion will continue in the same direction and velocity unless acted upon by an outside force". What outside force changes the direction of our air particles after the prop has passed through them? Curves or spirals require an outside sustaining force, as air particles have both mass and velocity at all times relevant to this discussion." The answer is the same thing that keeps the vortex itself going around - vacuum. Air has pressure. Any rotation causes a vacuumed core opposing centrifugal force. And any 2 adjacent bodies of air moving in different directions causes rotation, due to friction/viscosity along the shear.

Multiflyer

Hello guys, very good info, indeed!

I posted a question, since it is a bit hidden at the end of the second page I repost it. If you have any info it would be greatly accepted! The reply should be more straightforward than the first questions ... Again, many thanks to all!

Multiflyer,
do you remember right thrust settings of the Dirty Birdy? Had it less right that today designs? Just a curiosity, the fuselage has an interesting shape!

http://jemarsh.home.mchsi.com/pdm/myplanes13.html

Downwash would be equivalent to the propeller slipstream in general. A "forward" component of motion imparted to the air influenced by the wing would be equivalent to the rotational component of the slipstream.

Thank you all for the interesting discussion, and so onward and upward, with the proper amount of right rudder applied.

Stek79: I also am interested in your question regarding the influence on the spiral effect that might result from significant changes in propeller diameter, so my comments to Multiflier will interest you.

Multiflier: Thank you very much for your very informative comments, and without taking issue, I would like to examine the concepts a little further. In general, props are designed with a given pitch speed for fixed-pitch props. If we assume that to be the case for this discussion, then it follows that the slipstream is primarily a cylindrical column of air generally moving aft at the initial velocity of pitch x rpm less innefficiencies. The friction of the column where it interfaces with the surrounding air and the fuse would create boundary turbulence at those interfaces, the control surfaces, stabs and wings would provide a vaning effect acting to straighten the flow at the expense of imparting a corresponding rolling moment to the airframe, and then we have the asymetric impact of the spiral slipstream on the aerodynamically unbalanced vertical stab/rudder assembly. I would venture a guess that the same effect would be assumed to act on any net variation in the distribution of the fuse side profile above or below the prop centerline.

Now we have this revolving column of air moving aft and we want to take a look at it at a column cross section that bisects the verticle stab. Minus the vaning corrections of the preceeding wings and fuse, and frictional losses from passing over the fuse, combined with the frictional losses of the column interfacing with the surrounding air, it should look pretty much the same as it did right behind the prop. IF it is still a rotating column, what portion of that column is striking what portion of the vertical stab at the assumed 3 degree angle? You see, if the whole column is rotating at uniform radians/minute, as assumed, only one small area at one particular distance from the center of rotation would be traveling at any given angle if the whole column is moving at the same relative velocity. This is similar to the fact that the constant pitch prop generating the slipstream has continually changing angle of the airfoil relative to the plane of the disk as it moves out from the hub so that all portions travel forward through the relative airstream the same distance each revolution regardless of their distance from the hub. Are we moving forward here?

The slipstream is generated by a propeller with uniform rotating angular velocity (rpm) whether at the center near the hub, or near the tip of the blade. Do you think the rotation speed of the column is the same as the rotation speed of the prop? Unless it is, there appears to be a transition taking place, whereby the rotation of the column slows rapidly the further it gets from the prop. I believe that we are dealing with the properties of a viscous fluid (air) here that most closely moves with the prop at the prop's boundary layer interface with the airstream, and that the spiralling component rapidly degrades due to the combination of friction losses to surrounding air, tip vortices off of the prop, vaning effects of the flying surfaces in the slipstream, and the absence of any additional rotational energy being introduced after the prop interface. That being said, I also assume that the process of degrading rotation of the slipstream is a function of several factors, among which are: 1: elapsed time after prop interface during which friction losses accumulate, 2: initial diameter and mass of the slipstream, and 3: velocity of the slipstream relative to the surrounding air. Do you think I'm on the right track here?


If these suppositions are accurate, we can imply the following:
1: Spiral slipstream will generally have greater effect upon short aircraft, than those with longer fuselages.
2: Spiral slipstream will have diminished effect at higher airspeeds.
3: Spiral slipstream will be diminished by smaller prop diameters.
4: Spiral slipstream is not one of the major factors affecting most prop driven aircraft.
5: Spiral slipstream likely exists near the prop, but it's effect and even possibly it's existance diminishes as we move aft.
6: Canard Pushers are not affected at all.

HAPPY NEW YEAR ALL

Hello,

Hope everyone had a great new years day. We got in some good flying this morning until it started to rain again.

Stek79,

I don’t remember the details of the Dirty Birdy or others of that type from that time. I want to say about 1 or 2 degrees was the usual? Joe Bridi planes are still available at www.bridiairplanes.com

Gearup,

All you write in post #56 makes sense. No doubt lots of gradients and integrals and fancy math to actually figure spiral slipstream local velocities. My guess is that the spiral doesn’t really vary so much with prop diameter, pitch, or rpm, but more with power? A wing in flight not only makes downwash but some forward wash too. In reaction to drag, the air gets dragged along a bit. Basically a prop is a wing flying sideways to make lift forward. So basically the drag from lift production is what drags the air into a spiral as it is accelerated rearward through the prop. Faster airspeed straightens the flow but more power must be transmitted to fly faster. So the spiral might even increase with speed? My guess is since the rotation is comparatively slow, the wing probably doesn’t straighten it out much??

Happy New Year

Multiflyer

spiral slipstream--that's when you can spit a chaw from the pool table to the spittoon-without hitting the guy at the end of the bar.
Those who fly /have flown a LOT of pattern --already know that as much as all this is fun to fantasize - in the real world -a pattern
plane is constantly changing heading and attitude
Count the seconds where absolutely unchanged attitudes are held --don't forget the hold corrections needed for a breeze/wind.
For years - attempts have been made to come up with magic design where the effects of prop/ trim etc., can be cancelled
No one has done it - and it will never be done.
Simple reason is that unlike a computer model where the breezes and airpockets and temperatures are -non existant -- the model at a contest must be capable of ADAPTING--constantly .
The best design -is simply one which can be moved-in any axis easily and smoothly.
Trying to cancel a applied force such as the "twisted column of air" is futile --it is constantly changing in speed and direction
Maybe not in some airliner chugging along at a fixed speed /altitude load etc., but the contest model has a completely different set of priorities
Setting engine thrust to x angle -may work for one flier yet be a terrible hinderance for another .
The "easy" setup if the plane is light - agile - is one which is reasonably neutral - typically on the stable side of neutral --with enough built in drag AFT such that the model changes little in speed thru up/down lines.
The more constant the speed - the less difference will be seen in any maneuver. also the LESS the model will tend to fall off line in power maneuvers -such as upward lines.
setting the plane up zero zero zero ain't all bad especially with some current radios where the throttle can be mixed to add corrective trim (very delicately).
Sorry - no theories -no conjectures -just past experiences.

Good points guys! You know aero pretty well, much more than me! Thanks to all.

So you suggest that smaller diameter will produce a smaller spiral slipstream... any ideas about pitch?


Thanks again, and happy new year!

I agree completely with the above statements. I have heard and read many comments/articles about how to trim a pattern/IMAC airplane so it will not yaw/roll under all conditions and it cannot be done with a single-engine propeller driven airplane. The very large tail moments of pattern planes are attempts to mask the yawing/rolling effects generated by a spinning propeller, but they can never be completely eliminated under all flight conditions, not even with careful program mixing. If you set the plane up for zero yaw in hands-off level upright flight, the airplane will yaw left in a non-vertical positive climb unless the pilot adds right rudder, and will yaw right slightly in a non-vertical, positive power off downline, unless the pilot applies left rudder. The amount of rudder to apply depends on the power being produced, the airspeed and the AOA (AOA used loosely here).

The following discussion assumes the airplane is trimmed to fly wings level and zero yaw during aerobatic "cruise". At the top of a loop, the airspeed is less than at cruise, unless there is so much excess power available that the pilot "cruises" at less than maximum airspeed and adds power as the airplane climbs, in order to maintain a constant speed. Since without instrumentation and real-time telemetry this is probably impossible, I think we can assume the airspeed at the top of a loop is at least slightly less than at the bottom, if the pilot uses full power for the entry and at least the first half of the loop. Anyway, since the airspeed is less at the top, yet the power setting is normally the same, the aileron trim opposing torque is less, and the airplane will attempt to roll left near the top of a loop. The spiral slipstream is also slightly more compact, so there might be a bit of left yaw as well. Gyroscopic Precession might be more pronounced for the same reason (though the angular velocity is less since the airspeed is less and the radius of loop is constant), and opposes the left yaw force due to slipstream. P Factor is minimal since AOA is near zero at the top of a loop unless it is very tight and high positive G is maintained. There is no way to mix this correction in without messing up most other flight conditions, unless we have attitude telemetry between the plane and transmitter, or some form of autopilot. Every loop I have performed in full-scale propeller-driven airplanes (including full-scale aerobatic competition) has required right aileron near the top to keep the plane from rolling. It is interesting that the left-rolling torque effect is greater than the right-rolling stator effect on the wings/tail at the top of a loop, at least for the Decathlons, Citabria, and C-150 Aerobat I have flown, as well as every model airplane I can think of, including mid-wing airplanes. The F-16 I flew (once, for 20 minutes during a military incentive ride) did not, since there is no appreciable torque, P-effect, or spiral slipstream produced by the turbine engine.

As I alluded to earlier, it is possible to "cruise" at a low power setting and increase power so that the speed at the top of a loop is the same as or greater than at the bottom, which will cause different corrections to be required. However, even if the airspeed is exactly the same at the top as at the bottom of a loop due to power variation, there will still be a slight yaw correction required (which you could see if you were sitting in the airplane), because P-Factor will be less, due to the lower G loading and therefore lower AOA at the top. The higher power setting will also affect the slipstream and I don't know for sure what effect that will have.

These effects are not huge, so the casual sport flyer might not bother to correct them, or even understand or notice them, which is fine. Full-scale IAC aerobatic judges are generally well-trained, and often are full-scale competitors themselves, and they know what to look for, so if the pilot doesn't make these corrections, it will be reflected in her score.

P.S. A good computer model includes wind and gust effects, as well as the four primary yawing/rolling effects generated by propellers: Torque, P-Factor, Spiral Slipstream, and Gyroscopic Precession.

Regarding the spiral slipstream, I don’t think smaller diameter necessarily gives smaller spiral slip stream? It would if rpm remains constant, the bigger prop stirs more air. But for equivalent power, smaller diameter means smaller amount of air must be affected more vigorously??

Regarding what is the best amount of right thrust to use, Mr. Hanson above is exactly right, there is no magic best answer. By definition a single spinning prop is an asymmetrical confluence of forces. All prop effects change as flight conditions change. There is an optimum configuration for each individual flight condition, but not one for all conditions an aerobatic plane sees. A design parameter such as right thrust can be varied to optimize or "bias" a design for easier handling during a particular condition, but at the expense of more corrective input needed during other conditions. For example, if you personally are good at steering straight during inside and outside loops but are not so good at holding verticals, then more R thrust might be better for you. If you don’t hold heading during inside and outside loops very well but can fly the heck out of verticals, then probably little or no R thrust could improve your own overall performance.

A good pilot will understand the effects involved and therefore understand the control inputs necessary to correct. “Good� of course meaning able to carve out the maneuvers consistently and exactly as requested. The good pilot will understand the design “biases� that have been built into a particular aircraft and then behave accordingly at the controls. A good understanding of what effects dominate during individual maneuvers, and which design variables compensate for which effects, enables a pilot to choose a more optimum set of design "compromises" for flying a particular type of aerobatic sequence.

Multiflyer

Reply to Stek79:

If you intend to minimize the effect of spiral slipstream, and if you desire to present a steady velocity flight, then it follows that you need to use a prop pitch that will minimally pull your plane at that velocity during the most power-intensive part of your routine while providing the capability for extra margin needed for control and adjustments during the flight.

Prop drag, like wing airfoils, is composed of parasitic and induced drag. Parasitic drag increases approximately with he square of prop velocity and induced drag increases primarily with pitch relative to the airstream the airfoil sees. In regular flight, the comparison is when you increase pitch angle and get "begind the power curve". The more total drag you generate, the more spiral effect and torque you will feel and the more power you will need from your engine. Virtually all airfoils have a most efficient angle of attack, at which they produce the highest ratio of lift (thrust for Props) to drag. If you desire to minimize "wasted power" going to produce torque and spiral slipstream that is not generating lift(thrust), you need to select a prop pitch that is optimized for the airspeeds you intend to use. In most cases relating to aerobatic aircraft, that will be a pitch that provides the greatest static thrust near the engine's peak torque rpm.
If the peak thrust prop does not provide enough forward speed for certain portions of the intended flight, you will need to increase pitch as needed, while maintaining a "flying airfoil" at the minimum flight speed you intend to use.

In most cases, you will be balancing lots of factors. Longer props mean more tip velocity, more noise, but they generally make more lift from available horsepower because accelerating a large volume of air by a smaller change in velocity is more efficient than accelerating a smaller volume by a larger change in velocity. We previously discussed that all things being equal, a smaller prop might generate a smaller amount of spiral slipstream, but that might have to be traded to get the other factors you need.

Reply to Dick Hanson: BTW, the glue that holds the chaw in that tight spiral is good ol' surface tension. Let's hear it for the fearless leader just going out and slaying those dragons. You can hear the dying gasps as we speak. "Zero, Zero, Zero ain't all bad". You'd think someone plans to fly with other than right side up orientation. That's just terrible, why how can you suggest - -- --- --? You have an aerobatic design, symetrical as you can make it, zero wing incidence, zero stab incidence, engine crank on the trust/drag lines, shoulder wing at center line, airfoil stabs at center line, big wing tapers with a deep cord at the fuse, straight leading edge, constant cord ailerons so the ratio of aileron to wing cord varies as we move outward from the center of rotation, neutral stability about the cg in all axis of rotation, no aerodynamic stability, stays where it's put, goes where it's told, lightest possible prop. Sounds a lot like an Edge 540! Flies the same upside down or upright, Right KE or left. Same magnitude corrections, adjusted for attitude. Gotta love it!

Reply to Multiflyer: Is this a great hobby or what? We can choose how we want to tumble at the top of those maneuvers, whether by aerodynamic inputs to the surfaces in the propblast, or by building and using a 2 lb prop so the gyroscopic precession does it for us!

All the best in '06

Regarding the spiral slipstream, I don’t think smaller diameter necessarily gives smaller spiral slip stream? It would if rpm remains constant, the bigger prop stirs more air. But for equivalent power, smaller diameter means smaller amount of air must be affected more vigorously?? -Multiflier

I absolutely agree that accelerating a smaller diameter column by a greater change in velocity can use a comparable amount of power. There remains, however, another factor. If the difference in air velocity between the two situations is considered, we see that there is more rapid loss of energy due to friction from the smaller, faster column. This factor is, I presume, the main reason we do not see direct jets being used in heavy transports, but rather the much larger diameter enclosed fans which transfer the high velocity jet to a much larger volume of air that is then discharged at a more optimum velocity nearer to the aircraft's design cruise speed.

How this factor works out when applied to the spiral slipstream remains to be analyzed, but we know that if the same thrust is generated, the change in velocity of the smaller slipstream must be significantly greater. We do not know if the smaller prop is turning faster resulting in a faster rotating slipstream, or if it is being generated by an increased pitch with increased induced drag and likely, therefore, a more concentrated rotational component resulting. If we assume that the smaller prop is sized and pitched to be equally efficient to the larger one, then the likely result is a more concentrated spiral slipstream. I do think, then, that we would see the more highly concentrated force, moving at increased velocity relative to both the airframe and the surrounding air, degrade at a rate faster than the less concentrated force. This would argue for lesser affect at the removed distance of the vertical stab. No?

This may be a rhetorical question but have you ever noticed that the oil streaks on your RC aircraft always flow exactly in a straight line from the nose to the tail?

Good point. The effect of spiral slipstream alone is small but still significant. What power setting are you using prior to landing? One that allows the slipstream to straighten out, thereby straightening the oil streaks. Also, the oil is in the boundary layer, which has a very low velocity relative to the fuselage surface, and does not necessarily reflect the behavior of the air 1/2" or 1" away from the fuselage. Still, a very good observation, and worth considering.

gearup,

Along time ago a fellow modeler built a scale 60 size spitfire. It was way way tail heavy. To compensate he had a spinner machined out of solid brass. I'm talking about 1 and a quarter pound flywheel on a glow 60! Engine took like 5 seconds to spool up and same to slow back to idle. The plane never flew. Since then I've always wanted to try on with a huge rotating mass up front.

Multiflyer

why?
there actually were craft full scale that had a huge rotating mass up front
and like yer buddies Spit - a very poor compromise -but back in 1914-17 powerplants were very heavy and the lightest of the bunch were the monosupauppe (one valve ) two strokes which had the prop anchored to the crankcase -the entire case and cylinders spun with the prop
These were fairly lethal fighters- (they killed a lot of their pilots.)

The purpose of flying a plane with a large rotating mass would be to experience the large gyroscopic influence. For fun. And to learn what a real lomchevak is all about. During WWI the difference between rotary powered fighters and others was fundamental tactical knowledge. The rotarys could turn on a dime in one direction but not so good in the other.

Multiflyer

I think that is where I started my comments on this thread, way back at number 42. (oil on the fuse from a blown oil filter) We can now be known as the oil slick contingent of this thread.

Mesae: good observation, and, BTW, does the apparent inability of the spiral slipstream to overcome gravity and cause the oil slick to climb up the side of the fuse mean it doesn't exist? Probably not, but rather just that the model was flown in the normal attitude and the oil started on the bottom of the fuse initially. Are we having fun here?

Multiflier: I'm with you on the experiment, and guess a good test bed would be a DR-l so it could really be scale in handling to, not just looks. I saw a thread somewhere about a scale engine built like the old ones, you know, ignition interrupt instead of throttle, rotating cylinders, etc. Would be some project, but I'm not sure my thumbs are up the the rapid adjustment and it might be difficult to get a sim to properly handle the unusual forces for practice. I'd sure like to see it done though. It would no doubt make a neat film segment.

Dick Hansen: I think we would have a different effect from the weighted spinner due to the distribution of the weight very near the center of rotation. (than a heavy tipped prop or rotating round engine cylinders) This would cause the gyroscopic effect to be diminished by comparison. I think any of the WW1 fighters (and a few of the later ones also) have a short enough nose moment to provide the opportunity to demonstrate gyroscopic precession in a 30% or so model, without really having to add much more weight than is already needed to make them fly decent. We just need to put it in position to rotate through the largest practical radius at the highest safe speed, then pitch or yaw and hang on! Looks more like a project for giant scale warbirds than SA.

Guys,

I really thank you all for the EXCELLENT infos! Dick, I understand what you mean, thanks for the experienced reply! I agree with you, really thanks for that.

Gearup, really good point about prop drag! So, since parasite drag increases with the square of prop angular velocity, and induced drag is linear with pitch (I think...), I ask: can we say that if we want to reduce slipstream we should primarily concentrate on reducing rpms, rather than pitch?

IMO, it isn't worth trying to minimize the effect of spiral slipstream alone in designing an aerobatic airplane. Just use the rudder like it should be used!

Also IMO, there is altogether too much emphasis on mixing things in or out with models. Full-scale aerobatic pilots (like myself) do all the mixing manually. That way, we can use just the right amount for all conditions (in theory). And we don't use snap-roll buttons either. I know we are already too far down this road to go back, but I wish there was no electronic mixing of any kind allowed in model aerobatic competition (hard to enforce at this point I suppose). That would re-emphasize excellent design and piloting, and make differences in pilot technique more apparent to judges, since they have no way of knowing how much of a maneuver's score is due to electronic mixing versus conscious stick wiggling.

An example of practically institutionalized reliance on electronic mixing is the top-skin aileron hinging present on some manufacturer's composite-wing airplanes. This introduces a differential effect by presenting more aileron above the wing than below, for identical degree deflections. This is not ideal design for an aerobatic airplane and requires opposite (electronic or mechanical) differential to compensate, if ideal rolling performance is desired.

I know I'm in the minority but I can wish, can't I?

When those composite models first hit the market - they suggested a convoluted tape hinge line seal -I tried a number of em -all with negative results - so now ---- the guys that like these planes point with pride to the "screetch "they produce in a snap roll.

But with people not being stupid, opposing fighter pilots soon learned to counter it. When the nose of a rotary-powered airplane pitched up, they stomped the rudder and filled the sky to the left of the airplane with lead.

But the oil streaks I’ve seen show no indication that they had any path other than a straight line from the front of the aircraft to the back. If this spiral airflow exists one would think that at least the oil on the vertical and horizontal stab areas would show the spiral airflow. This oil would not be in the boundary layer around the fuselage.