P-factor may be too excessive

[legoboyvdlp]

Hi there, I noticed that XDraconian made a complaint about the new c172p FDM with the RC version,

C-172 has excessive P-Factor and/or Left-Turn Tendency. My gear is packed up, so I'm flying with auto-coordinated mouse control. Mitigation: None. I was able to takeoff, but for novice pilots this would be unusable. Not sure if anyone else has experienced this problem - my configuration is non-standard so I don't want to rule out that this may be isolated to my build.

Well, personally, I think the propwash is a bit too strong, but that's just the feeling I get. I imagine this is because he is using auto-coordinated mouse control.

it0uchpods as well, told me that the p-factor was far too much, and the engine seems "weak". Well, I have a hard time sometimes to maintain speed / rate of climb at a suitable level if I am heavy and took off from a short runway, but I can't say I have noticed a weak engine. He does say the problems are "new", so it might have been introduced by my issue-124 - I trust not!

[algefaen]

At takeoff you have to have a considerable amount of right rudder in the real thing, especially if you're doing a short field take-off and climb at Vx. Not saying that we have the exact right parameters today, but ... flying a real c172 with an auto-coordinated mouse would be really hard.

[gilbertohasnofb]

@dany93 what are your thoughts on this?

[tonghuix]

it0uchpods as well, told me that the p-factor was far too much, and the engine seems "weak".

Yes, I agree with the p-factor is too strong. To experience and reproduce this 'effect':

1. Set no wind
2. RPM 2000 or less
3. Level off at a low altitude, and well trimmed.
4. Release the joystick, at least release rudder pedal

Now you will feel the plane has a little bit RIGHT turning tendency, yes RIGHT direction not left! Very little left rudder could compensate it easily.

I also feel the propwash is a bit too strong, no need to fix this IMO.

[dany93]

Thanks @algefaen for your indications (I miss that). At implementing this feature, I asked for opinions and feedback on this effect IRL and the simulation. I was particularly worried for beginners. All the more that the evaluation is mainly based on pilot's feeling, personal memory (not quantitative). See https://github.com/c172p-team/c172p-detailed/issues/947#issuecomment-361001283, https://github.com/c172p-team/c172p-detailed/pull/948#issuecomment-363423977. I only had responses from @tigert. And the opinion (not based on RL experience) from @gilbertohasnofb. My opinion (feeling) is that it is not so far from reality. I have the impression that XDraconian's setting for the rudder is inadequate (not reaching 100% efficiency). With the 180 hp, default gear, default load, at start, I need at maximum about 40% rudder only at very low speed, quickly decreasing above 10 - 20 kt. I'm strongly against the auto-coordination in FG.

However, I'm still a bit worried for beginners, and (less, although still) for many users who control the rudder by twisting the JS. With the mouse, people need to know the left button control. Not straightforward. No problem with rudder pedals.

However, the main question remains: realistic or excessive? And the general opinion (mine included) is: if reality is difficult, the simulation must render it.

[tonghuix]

I'm strongly against the auto-coordination in FG.

@dany93 agree with you!

Well I prefer realistic, for beginner who only using mouse, it is suggest that coordinate with keyboard [NUM 0] and [ENTER] as rudder.

[gilbertohasnofb]

However, the main question remains: realistic or excessive?

Yes, that's the question.

I need at maximum about 40% rudder only at very low speed, quickly decreasing above 10 - 20 kt.

I am a bit surprised, 40% does sound like a lot. But then again I have an impressive one single hour under my belt on a 152 so I can't really comment on that. We really need experienced pilots for this. Also, we could consider taking a look on how strong the P-factor is on other sims. Does anyone have access to X-Plane or FSX?

However, I'm still a bit worried for beginners, and (less, although still) for many users who control the rudder by twisting the JS. With the mouse, people need to know the left button control. Not straightforward.

The problem with mouse is that it's very easy to use the left click to centre the aircraft while taking off but you need to release it in order to rotate, and as soon as you lift off you will invariable move the mouse a tiny amount sideways and thus resetting the rudder. This effectively has the effect of someone releasing the pedal as soon as the aircraft rotates. My take offs with mouse are always a bit wobbly... But IMO that's not a problem, realism comes first.

As for the beginners bit, back in the days of Microsoft Flight Simulator they had an option to enable/disable the P-factor entirely. Would this be also possible in FG?

[dany93]

@tonghuix wrote:

Now you will feel the plane has a little bit RIGHT turning tendency, yes RIGHT direction not left! Very little left rudder could compensate it easily.

I realized it just yesterday. A pity that no one else noticed that before the stable FG release. The slight rightwards roll tendency is due to (my fault, sorry) I forgot to restore the aileron trim after restoring the previous value for aileron efficiency. I have to commit a fix.

The need for right rudder at climbing with full throttle is well known and common. I've even seen that turning left needs only less right rudder pressure, not left rudder.

[tonghuix]

@dany93 I opened a new issue #1113 to track fixing right turning tendency.

[legoboyvdlp]

Hi, Josh reports:

In the real plane, all I had to do, was rest my foot on the pedal , and not on the other, and it will stop the pfactor

even at full power, it was such a slight depression, in FG I have to do 20% of the pedal... its very wrong

All I needed to do to stop the p factor, is rest my foot on the pedal one side ever so slightly more than the other. Barely any pedal movement needed

I asked him about rudder trim, perhaps his flying club has extensive rudder trim applied

[dany93]

@tonghuix wrote

Yes, I agree with the p-factor is too strong Level off at a low altitude, and well trimmed. Release the joystick, at least release rudder pedal Now you will feel the plane has a little bit RIGHT turning tendency,

Can you explain why you think this is due to the p-factor? Or what makes you think that the p-factor is too strong? If I understand well, you are observing a (slight) roll or / and yaw tendency, rightwards, at level flight, at a stabilized cruise speed around 90 kt. (do you confirm?)

Firstly, I do not confirm your observation. At 1900 - 2000 RPM, level flight, released controls, stabilized airspeed about 88 - 90 kt: the aircraft is almost accurately neutral in roll and yaw. I found no preferred leftwards or rightwards tendency. Letting the preset aileron trim at 0.044 and rudder trim at 0.02.

But I don't feel the issue (if there is) is in this behavior.

My opinion:

• The p-factor makes the aircraft yaw leftwards (not 'turn' rightwards),
• The p-factor is a weak contribution (much weaker than the spiraling propwash), and most of all it is even weaker at this attitude (AoA ~ 2° here).

If you look at the prop files (under Engines), you will see that there is no p-factor set. I'm not sure about a non-zero default value, but I would guess that it comes to zero effect. I let you search if you want, for me that's not an issue. JSBSim Thrusters, FG propeller.

I can be wrong or not having understood something well, but, please, can you elaborate?

[Octal450]

Even with neutral rudder trim, all I had to do to cancel P factor at ~2200RPM was to just rest my root a little harder on the pedal. Mostly right.

In the FG one, I have to twist my joystick 20-30% (0.2-0.3 in /controls/flight/rudder) to achieve the same effect.

This is on the latest master, and a few times back.

Kind Regards, Josh

[dany93]

@tigert wrote in this message

Yeah, right rudder is needed. Also it is present on climbing and turning to left crosswind leg (low speed, climbing turn to left) where you need no left rudder at all — or actually you just slightly ease the right rudder a to keep the ball centered.

[Octal450]

Yep, exactly. Only a slight input is needed, nothing like 20-30% in my experience. I can't exactly confirm since of course, at home I am using a joystick, not a real plane, but this is what it seems like for me. I remember as only some slight pressure.

Josh

[Octal450]

By the way, I am talking about cruising, despite the fact that this c172p-detailed has right rudder trim set.

[dany93]

@it0uchpods wrote,

In the FG one, I have to twist my joystick 20-30% (0.2-0.3 in /controls/flight/rudder) to achieve the same effect.

Thank you, Josh.

I checked. 180 hp, default load. Climbing, full throttle, rudder (pedals) for keeping the slip ball centered. Values from/flight/controls in the Internal Properties 60 kt: rudder about 0.2, 80 kt: rudder about 0.1 to 0.14.

[wlbragg]

Hi Josh, nice to see you!

nothing like 20-30% in my experience

Even though it's all we have, comparing a control device input to a live rudder pedal system is going to be really subjective because of it's calibration and any loading it might have VS any trim or loading the rudder system may have. And one person's/system may be different than someone else. Maybe a better way to say it is, what is slight pressure on a joystick or control device, 1 5 or 30 deg?

Can this pull to the left be changed on the runway and not affect the inflight characteristics?

I don't know about the feel of the real thing but I use a keyboard and mouse exclusively and I can easily control the c172pm, not so much with some of the other aircraft.

Although I will say that the rudder itself appears to be loaded if I remember correctly. Visually my rudder doesn't have the same range of movement right as it does left. I found this to be true while working with the hydo system. So that has to have an effect on all of this.

I'm not trying to influence the discussion other than to make sure we don't over react without considering everything.

[wlbragg]

Speaking of "Rudder Trim Set" isn't that a feature (dialog) we want to add? Would that allow users to gain some control over this?

[dany93]

@it0uchpods wrote

By the way, I am talking about cruising, despite the fact that this c172p-detailed has right rudder trim set.

Ah... I understood at climbing...

As I wrote, the current version needs its aileron preset trim to be corrected (from 0.044 to 0.02). But that does not change the rudder trim. If the aileron trim is correct (0.02), I can fly at cruise velocity (about 2300 - 2400 RPM, 110 - 120 kt) with released commands, slip ball centered. I've preset the aileron and rudder trim for these conditions.

[Octal450]

Hi @wlbragg, thanks nice to hear from you! I agree, it could be subjective, my joystick axis is rather sensitive, and not very precise, hence why I can only give a range.

By slight pressure, I was talking about the real pedal, (I was flying mostly PA28, but flew the C172P a few times before my medical problem started). For the sim I was refering to the input amount on the rudder property, which according to my quick glance at the fcs, would deflect the rudder 3.2-4.8 degrees with neutral rudder trim.

@dany93 I'll do another test later today, I don't quite remember what my alt and speed were, but I know I was around 2200RPM.

Best Regards, Josh

[dany93]

@wlbragg wrote:

I'm not trying to influence the discussion other than to make sure we don't over react with out considering everything.

Thank you, @wlbragg, that's my feeling too.

Speaking of "Rudder Trim Set" isn't that a feature (dialog) we want to add? Would that allow users to gain some control over this?

That would be perfect, but the real C172 does not not have them... What to do? Aileron trim is the more important. I have trim settings for the three axes on my JS, very comfortable. All the more with joysticks, of which the springs have more returning effect than RL commands.

[tonghuix]

No one test 160hp ?!

Right roll / yaw tendency is more obvious at 160 hp. Is it caused by 'weak' engine power?

[Octal450]

@dany93 add it, but make the dialog option greyed out when the aircraft is moving. (<enable> tag)

Kind Regards, Josh

[Octal450]

@tonghuix @dany93 I was using the 160hp, didn't not mention that, sorry.

[dany93]

make the dialog option greyed out when the aircraft is moving

Good idea. Like that, it comes to a ground setting. (however, I know about nothing on these dialog options)

[tonghuix]

I remember the real c172 has a fixed rudder trim at the tail of the rudder, engineer could adjust it on ground.

According the doc: https://www.redskyventures.org/wp-content/uploads/2018/01/C172-Training-Manual-31-Aug-2014.pdf (Page 28)

Models before 1977 and after 1996 had a fixed rudder trim. The models in between have an adjustable rudder trim tab. The C172RG has an adjustable trim wheel.

And also more picture and explanation: http://www.tennesseeaircraft.net/position-and-hold-a-rudder-option/

[wlbragg]

I know we go for realism but you could add a checkbox in the dialog, something like "developer mode" or "allow access while in the air" so it could be used to tune the aircraft live while in flight? Either that or maybe a few common preset choices that cover the ranges were discussing here.

[Octal450]

Honestly I'd rather leave it not, to minimize cheating. If you want that, I would add an optional "add-on" kit of some kind that allows control from the cockpit, rather than adding a cheat.

Best Regards, Josh

[wlbragg]

Honestly I'd rather leave it not, to minimize cheating.

At the very least then I would have a inactive slider unless on the ground and three radio buttons labeled something like... Authentic Sim Optimized Custom

"Custom" contains the slider setting.

My whole point being I feel we need to hint to the user that this is a subjective setting and there are some difference of opinion on how this should be set in a simulator VS real life.

I could live exclusively with an inoperable slider while inflight, but don't see the necessity to limit options.

Although there is something to be said for having to land and have your mechanic make adjustments a few times to get it the way you like. That has a certain appeal as well. 😈

[gilbertohasnofb]

I know we go for realism but you could add a checkbox in the dialog, something like "developer mode" or "allow access while in the air" so it could be used to tune the aircraft live while in flight? Either that or maybe a few common preset choices that cover the ranges were discussing hear.

@wlbragg I am very much against the idea to be honest. There is nothing 'developer-like' in tuning the aircraft in the air, developers (i.e. us) can very well set properties manually when developing.

As for the beginners bit, back in the days of Microsoft Flight Simulator they had an option to enable/disable the P-factor entirely. Would this be also possible in FG?

@dany93 sorry to bump my question but would something like this be possible in FG? Then we could have a toggle enable/disable the P-factor, similarly with what we do with the complex engine management.

[Octal450]

That is possible, yes, but I don't think it's a good idea...

[gilbertohasnofb]

@it0uchpods care to say why? @dany93 showed some fear about beginners and so do I, and other sims have handled it in the way I describe.

[wlbragg]

toggle enable/disable the P-factor

That is basically the same thing I am talking about, It could just as well be a checkbox choice in the Rudder Trim GUI along with the trim slider that is only available on the ground. So you would have the mechanics "Rudder Trim" GUI and in that GUI you have a slider (only available while on the ground) and also include the option to turn off the P-factor entirely. @dany93 , is it as easy as a property that cancels out a value that causes or adjusts the P-factor?

By the way, I don't see a real difference in "cheating" in a GUI with a slider that can be accessed while in the air or a menu choice that effectively "at will" does the same thing.

[dany93]

@gilbertohasnofb wrote

sorry to bump my question but would something like this be possible in FG? Then we could have a toggle enable/disable the P-factor

Sorry for not still having responded, but I wished to to do some tests before. However, responding to the other questions ate my time. I did not forget your question.

Cancelling the spiraling propwash effect as an option (I say it again: this is not p-factor) would be easy in the FDM. Although I'd like someone to do the option box. Before, I would like to check that the aircraft does not become unbalanced with the canceled propwash. I don't want to add extra and optional trim values for this situation. The FDM and its tuning (and redoing it at each change) is already complicated enough like that.

@wlbragg wrote is it as easy as a property that cancels out a value that causes or adjusts the P-factor?

Yes

            <function name="aero/coefficient/Cnspw">
                <description>Yaw_moment_due_to_spiraling_propwash</description>
                <product>
                    <property>aero/function/qbar-propwash-psf</property>
                    <property>metrics/Sw-sqft</property>
                    <property>metrics/bw-ft</property>
                    <value>-0.05</value>
                </product>
            </function> 

[gilbertohasnofb]

@wlbragg I don't think that's basically the same thing at all. I think that giving a complex option to fine tune the rudder trim position in order to find a value that might cancel these effects (something a beginner won't find out by the way) while being in the air is unrealistic and complex. Having an ON/OFF toggle is friendly and we already do it for the complex engine procedures and oil management.

So you would have the mechanics "Rudder Trim" GUI and in that GUI you have a slider (only available while on the ground) and also include the option to turn off the P-factor entirely.

Something like that, but I would strongly suggest to add that toggle to the aircraft menu as it has nothing to do with the rudder (it's just us removing a realistic complication from the FDM). Also, it will make company to the complex engine procedures over there.

Cancelling the spiraling propwash effect as an option (I say it again: this is not p-factor) is easy in the FDM. Although I'd like someone to do the option box.

@dany93 Great to hear that. If you open a PR and tell us the property that will toggle that either me or @wlbragg can help you with the GUI, should be fairly simple.

[dany93]

I've edited my previous message when you wrote. I think you will find the response for the property.

[gilbertohasnofb]

@dany93 Sorry but I don't understand, you pasted the code of a function but I don't understand what property should I use. I tried searching for aero/function/qbar-propwash-psf but I can't find it!

[wlbragg]

I guess I am a bit confused then. All I really need to know is if the spiraling prop wash can be canceled, or at the least toned down, with a rudder trim tab adjustment and if that is a realistic option? Also, if that adequately addresses the users issue. If so then that is all we really need to do, yes?

If not, then I do see your point that removing the spw is effectively the same type of option as to having a complex engine procedure choice and in that example it is a different feature or option.

[wlbragg]

@gilbertohasnofb I think we can create a function that just cancel out the aero/coefficient/Cnspw property by overriding it to 0 wherever the GUI option is set to "No SPW effect" There is actually a couple different methods we could employ to achieve that. I guess @dany93 can let us know if he just needs the GUI bool property to use an he can decide how he want's to implement it in the FDM?

[gilbertohasnofb]

@wlbragg as far as I know most aircraft do need a bit of rudder input during take off. Also, as some pointed out, the amount of rudder input does change throughout the take off as the speed increases. My proposal is to have a toggle which would dumb down the aircraft and remove some of these complex FDM effects. On top of that we could also add a menu option to change the rudder trim tab, though as some pointed out this is normally not carried out by the pilot but by a mechanic. I would suggest that if we do implement this new dialog that we also add a reset button or something like this since people will invariably mess up their setup more times than not.

[wlbragg]

add a reset button or something like this since people will invariably mess up their setup more times than not.

Another reason for a couple "standard" radio button choices which will override the slider.

My point also being, if we could get away with a rudder trim tab setting that effectively " dumb down the aircraft and remove some of these complex FDM effect" you would have no need for it in the main GUI, but I do see how you think it fits better in that location than in the Rudder Trim Tab GUI. So I am OK with either.

[gilbertohasnofb]

@wlbragg first, I don't think we can find an option to rule them all. But even if that's possible, a beginner would have to figure out what this 'rudder trim dialog' stuff is then find what option gives them the result they want. An ON/OFF toggle on the other hand in the aircraft menu is possibly much more trivial.

Anyway, it's my two cents. Since you and @dany93 are the ones actually implementing this then I think it ultimately should be your choice in case we have different opinions.

[Octal450]

@gilbertohasnofb Sorry for not responding, because, unlike removing a feature, or a cheat, you are talking about removing a component of the aerodynamics. Won't that contradict the point of a detailed project if you can just yank the realism out? It doesn't take much effort to stop it (in the real plane, at least), so I don't think it's a good solution. Things like MFSX default C172 have crappy FDMs, so it doesn't matter, and stuff like the A2A simulations C172 doesn't have such an option, as it's built into their realistic flight model (IIRC)

Best Regards, Josh

[tonghuix]

I agree with @it0uchpods , removing a component of the aerodynamics is a stupid idea, even a optional toggle at GUI dialog.

What the issue concern about is IMPROVE the realistic, not DECREASE it. So what we should do is just reduce p-factor or/and propwash. For none-joystick player, or joystick not detected, we could give them an option, or give them a suggestion that they could play it in Game mode, but not realistic as it real get, or tell them choose another aircraft.

[algefaen]

If you want an unrealistic arcade mode setting it might be better to set the propwash, p-factor and maybe also torque components constant values that are adjusted to counteract the default right rudder trim setting.

Personally I don't like the idea of rudder trim sliders, adjusting the rudder trim tab is not something you do regularly. Power users can tweak it through the property dialogs already. @tonghuix, I don't think there's enough "evidence" here to say that the p-factor / propwash is wrong, other than the aileron trim setting that @dany93 forgot.

It's hard to gather good data on how much rudder pressure you're giving in a real aircraft. In real life you may have more pressure even if you don't move the pedals that much, because the slipstream is pushing on the rudder. An easier experiment would be to climb/cruise/descend at various RPM settings with feet off rudder, and get a video of the slip/turn indicator.

[gilbertohasnofb]

Ok guys, this is starting to rub me the wrong way. First, I am taking a deep dislike to some of the language used here. Second, the amount of messages posted lately is just extremely large. This is not a forum, this is a repository where we are attempting to develop an aircraft. If we have to go through piles of messages then we are losing development time. We welcome feedback but please do not comment on every single message... Also, please consider commenting only if you are adding something to the discussion. This is not a public forum which we get features voted or shot down so please do respect that.

Won't that contradict the point of a detailed project

The 'detailed' term came from our initial idea of improving the 3D model and textures, so it refers only to that. But it's just a word, though we do subscribe to maximum realism and you do know that. But with that said and as I pointed it out a million times, we already have noob friendly options in the menu. I am not suggesting dumbing down the FDM to everyone, just adding an option to make life easier beginners if that's possible and if Dany (our FDM specialist) think it's suitable and if the rest of the team agrees.

Things like MFSX default C172 have crappy FDMs, so it doesn't matter, and stuff like the A2A simulations C172 doesn't have such an option, as it's built into their realistic flight model (IIRC)

It does matter and you are missing an important point: this is the default aircraft of FG and the first point of contact with anyone installing the sim. You can't just compare it to A2A, the aims are completely different. We already offer a much more realistic default aircraft than any of those sims, but being a default aircraft it can easily have additional options to make life easier for beginners. And I do think this should be our call.

removing a component of the aerodynamics is a stupid idea, even a optional toggle at GUI dialog.

Please do not call my ideas and comments stupid.

What the issue concern about is IMPROVE the realistic, not DECREASE it.

Also, no need to shout, in particular to the developers who are the ones that have made this aircraft as realistic as we can.

If you want an unrealistic arcade mode setting it might be better to set the propwash, p-factor and maybe also torque components constant values that are adjusted to counteract the default right rudder trim setting.

I don't want an arcade mode. Simply some of us (namely me and @dany93) have expressed concerns about new comers not being able to handle the aircraft due to some specific characteristics of the FDM.

Personally I don't like the idea of rudder trim sliders, adjusting the rudder trim tab is not something you do regularly. Power users can tweak it through the property dialogs already.

Now this is something that I do agree with and which I had previously expressed to @wlbragg. But again, it's ultimately his call.

[Octal450]

I apologies if I said anything which offended you, that was not my intention.

If you do add such a thing, as long as this option is not enabled by default, I don't really have a problem with it.

I agree with @algefaen, you should have this toggle disable few other advanced FDM features then also, not just one thing. I'd do a toggle called "Advanced FDM" or something, because then when checked (1), you can just add it as a product in the functions, and when the switch is off ,it will be multiplied be 0, aka no moment created.

Kind Regards, Josh

[tonghuix]

Okay, I want to change my opinion here. I do not intend to attack anyone, sorry for my aggressive words before.

What about add a new state and make it default, which is disabled "advanced" FDM, and make it mouse friendly. And also increase realistic for other states.

I also believe if remove the aileron trim code which is @dany93 forgot removed before, no rudder trim slider is needed.

[dany93]

@wlbragg @gilbertohasnofb My piece of code in this message was just a quick indication, to give a response to @wlbragg. I understand it was not sufficient. I will elaborate this in my next message.

@wlbragg wrote

I guess I am a bit confused then. All I really need to know is if the spiraling prop wash can be canceled, or at the least toned down, with a rudder trim tab adjustment and if that is a realistic option?

Yes, there is confusion here. (At least at my knowledge and with my current understanding, probably not taking every possible contributions into account). IMO, the rudder trim and aileron trim issues should not have been got on in this issue. These are not connected (at least at a first level) with the spiralling propwash effect, with the possibility to cancel it.. If you want, you can implement trim sliders for them, as long as they are disabled in flight, it is in accordance with reality (mechanic on the ground). However, these settings are very hard to tune without enough knowledge. On the ground by trial and error (which is close to reality) it needs many tiresome attempts. Only for a few users who know. If I had to do this in simulation, needless to say that I would fly, open the Internal Properties and do the setting here. Which means that I don't need the sliders. If I want to keep my value, I write it in -set.xml. Hence, I consider this feature far from essential. However, if it is disabled in flight, there is no cheating. Do users asked for having access to this kind of setting?

I have to admit that it is coded on my JS buttons.... Sorry for the cheating, but so comfortable...

[dany93]

My idea for canceling (or decreasing) the spiraling propwash effect:

What I would prefer is a factor added in the FDM code. c172p.xml, lines 1650 - This factor would be = 1 for the full effect, = 0 for no effect at all. Or an intermediate value for a weak effect. Below, it is <property>aero/coefficient/spiral-propwash-coeff</property> (unless you would find a shorter name, self-explaining) Set by an option with two possibilities (thank you @wlbragg or @gilbertohasnofb for adding this in the option box).

            <function name="aero/coefficient/Cnspw">
                <description>Yaw_moment_due_to_spiraling_propwash</description>
                <product>
                    <property>aero/function/qbar-propwash-psf</property>
                    <property>metrics/Sw-sqft</property>
                    <property>metrics/bw-ft</property>
                    <value>-0.05</value>
                    <property>aero/coefficient/spiral-propwash-coeff</property>
                </product>
            </function> 

The driven property is aero/coefficient/Cnspw (under /fdm/jsbsim). Unchanged. Currently (up to now), the tuning coefficient is <value>-0.05</value>. The (new) <property>aero/coefficient/spiral-propwash-coeff</property> is the one controlled by the option. Its value is normally =1 (full effect). It can be set to = 0 or whatever intermediate value for beginners.

The trim pre-setting issue is independent. Hoping that the same trim preset can be kept for both situations (which seems to be the case)

[dany93]

I don't know how we will commit this, but to make tests it's better to change the aileron trim preset (my fault, I forgot it). In c172p-set.xml (line 517) change 0.044 (current value) for 0.022, maybe 0.025 (to be tested in cruise, 2300 - 2400 RPM, 110 - 120 kt level flight). Anyway, this is with the pilot alone and it depends on load distribution.

    <controls>
        <flight>
            <aileron-trim type="double">0.022</aileron-trim>

(rudder-trim is unchanged, at 0.02)