Elevator response behavior question with IRIS yoke

[peternorth]

Hello,

I have a question about how the a2a c172 responds to elevator changes with the IRIS dynamic yoke. I am using Prepar3d v3.2 For this example, assume the following. FSUIPC has a value of -16,000 for full back deflection, +16,000 for full forward defelction, and 0 for neutral.

Most spring yokes when you pull or push on the yoke, the springs return it back to neutral, so the elevator value rests at 0 in cruise.

However, the IRIS dynamic yoke has the ability to stay where it is after trimming through magnetic induction. So for example, if in level flight I have the yoke pushed forward and I have a value of +3,000 for the elevator, I'm still able to trim the plane to keep it level, even though the game has the yoke registered as being pushed forward a little, and the pressure in the yoke goes away, like in real life.

My observations have been, that when I want to change altitudes, or apply a small amount of back pressure when I bank, I feel the elevator response is much more drastic than in a real c172 when I am trimmed and my yoke is not at neutral. The VSI goes very quickly to around 1,000feet/min, and the artificial gyro quickly goes to 10 or 15 degrees with only a little bit of pressure.

My question is, is this software modeled to always treat the elevator response as a delta from 0? A change from +3,000, but trimmed, to +4,000, the game "feels" like it wants to make an elevator change of+4000 instead of +1000, according to the behavior described in the above paragraph. It would make sense since most yokes are spring based and naturally return to 0. This is a problem though for the IRIS yoke if this is true.

[AKar]

Yes, the FSX trim system "assumes" the stick is released to mid-position for zero forces. The trim system on the other hand is somewhat akin to a trimmable horizontal stabilizer. There is a third-party addon, the FS-Force, but I'm not entirely convinced how it works with custom flight models that are designed to deal with the regular sim behavior and to give accurate performance at specific situations. I've only done some quick tests quite a while back, so can't really comment too much about it.

-Esa

[peternorth]

I am using FSforce, but I dont believe there is any code in the plugin for resetting the elevator value to 0 as I trim. I can try asking in their forums. Anyone have any suggestions for workarounds? Or would this have to be coded in by the team? I added in my first post that I'm using prepar3d, and not FSX.

[AKar]

Yes, I think so too as it is basically the whole point of FS Force to not reset the elevator to zero deflection when trimmed to zero forces.

If I understand the phenomenon you're describing correctly, it seems to be an inherent limitation to the simulation. As you note, the flight models in FSX are based around zero deflection trimmed position. You can find some material that suggests the advanced flight models of A2A products are tuned to more or less match the control movements from trimmed position in actual airplanes. The discrepancy will come from the fact that basically the control movements from significantly deflected (but zero force) position in real airplane at given AoA+CoG are transposed over zero displacement elevator position in the FSX. I haven't measured this quantitatively, but I'm under impression the flight models of more advanced addons are significantly tuned from default behavior to retain this behavior, and also believable total control authorities, at various speed ranges for instance. This necessitates non-linear elevator effect on the pitch response. Effectively, a given amount of 'pull' starting from zero deflection is not equal to the same amount of 'pull' from a displaced starting point. The default simulator behavior is much more linear in this respect: essentially the elevator's effect seems to be pretty much invariant regardless of trim setting, which basically just displaces the entire elevator's range of authority, in somewhat similar way a trimmable horizontal stabilizer would do.

In that sense, when I was figuring out how to set up my sim, I concluded that FS Force is more or less incompatible with perhaps the most of more advanced addons.

To retain a proper support for both FFB and spring-centering controllers in airplane simulations that model trim tabs (or a trim system adjusting the centering springs as in some airplanes) would take an approach quite different from the one used in FSX platform: essentially the flight model would have to be created bottom up for correct control effects throughout the deflection range at all AoA+CoGs, with zero force position 'floating' as it is in real airplane. So basically it would start from an assumption of FFB. With spring-centering controllers, the stick positions would then be mapped from the zero force flight control position, so even with centering stick, the hands-off won't necessarily center the flight control but release it to the stick-free position determined by the flight dynamics. In the sims I have, some use such an approach, namely Condor and DCS. It is noteworthy that both have great FFB support (while I agree that the FFB sticks I've tried aren't too good), and yet retain proper handling with centering sticks. On FSX platform, that would require quite extensive rework of the flight dynamics, and to my understanding would necessitate a use of completely custom interpretation of control inputs.

-Esa

[peternorth]

Esa,

Thank you for the very detailed response. I quite enjoyed reading it. Your observations on the simulator's behavior being linear in displacement on the elevator does seem to match what I'm seeing as well, and I may just have to accept this as a limitation. FSX and P3d may not have been designed with realistic force feedback forces as a priority.

Thanks again.

[AKar]

Peter,

Yes, the FSX/P3D "assumptions" on the flight dynamics do result in issues with FFB, but not only with it. The situation is actually a bit complicated, as it appears to me.

The platform behavior is fundamentally inaccurate in this respect, and it results in various handling issues even without force feedback. For instance, as the trim function actually increases the control authority, it is often possible to perform curious maneuvers, or to hold the airplane steady at speeds or CoGs that would be impossible in reality. Now, when it comes to high-end addons, the developers work around these issues. A great example is A2A Piper Cherokee, a simulation of a real airplane with well-documented lack of nose-up elevator authority at low speeds (and especially during landing flare where it is exacerbated by ground effect). This is very well simulated in A2A Cherokee, where it is actually possible to run out of elevator even when trimmed to slow speed. Still, the handling must remain credible with clean wing, in cruise regime or during takeoff too. This no doubt appears to require some 'bending' of the FSX default dynamics to be possible. Nevertheless, for the hand-crafted flight dynamics to remain as designers intended them, the simulation apparently depends on the FSX "default" flight dynamics working as they are supposed to.

How the FS Force works, it appears to disable the default FSX trim system altogether, and to replace it with an FFB command to displace the controller. It should work very well with default airplanes and with ones that are very similar. The issue comes up, however, when the hand-crafted flight dynamics are actually based on the assumption of the default trim behavior. When the default trim behavior is not in place, as it is not when disabled by FS Force, I've got little doubt (even after just very little testing) that the matched behavior is adversely affected.

A comparable case would be that of ground friction corrections: many addons use "tricks" to achieve realistic taxi and breakaway power settings for instance. As a simplistic example, in many of those, the tricks to achieve them seem to include increasing the idle thrust values. Now, if we 'corrected' the friction values to realistic using an appropriate module, the elevated idle thrust would remain, resulting in issues elsewhere. If you now reduced the idle thrust values back to the realistic values, you'd need to re-work your drag model too, in turn adjusted to off-set your increased idle thrust.

When something is bent to match, it is modestly difficult to re-straighten it again.

-Esa

[Scott - A2A]

Peter,

I will take another look at the elevator response using your situation and see what we can do.

Scott.

[Scott - A2A]

Peter,

I don't think your setup will be perfect in the sim, largely because of the lack of force feedback. The reality is, the elevator is the elevator, regardless of the trim, as all the trim is doing is putting force and moving the elevator which is, in turn, moving the airplane.

What you are feeling mostly in the trim on a real airplane is the force change on your hands. Since we don't have force feedback, having the trim physically move the elevator in a linear fashion isn't a good compromise.

What we do is to simulate these forces in Accu-Sim, since this is how your mind perceives the elevator position in the cockpit, by force.

So, moving the trim in Accu-Sim does move the elevator with more wind force, and you can offset / neutralize this by pushing the elevator in the opposite direction.

I hope this makes sense,
Scott.