Rudder trim vs dihedral effect

[Eseem]

With the heavy spring even at the lowest setting on my Saitek pedals, I was all giddy about being able to trim out the dihedral effect in this airplane and keep my feet comfy while having all roll tendencies cancelled out. However, while the rudder trim does affect coordination/slip, it does not in my version do any difference on the dihedral effect and linked roll tendency. Unless the rudder trim works in a mystic way beyond my comprehension, I'd say this is a bug in the aircrafts physics, as the dihedral effect seems very much correct when using the rudder itself, regardless of trim setting. Am I the only one experiencing this? Or do I completely misunderstand the workings of the 182 rudder trim?

[Oracle427]

IRL, I've never found an significant affect on roll when using the 182 rudder trim if I set the trim to keep the airplane coordinated. Usually the trim is left just a hair right of center in cruise.

If the plane wants to roll I need to apply a little pressure on the yoke to cancel it out. I have experimented with using the trim to correct a slight tendency to roll and found that it just ends up increasing drag from getting uncoordinated and then bleeding off airspeed before correcting the roll.

I never found the 182, or the 172 for that matter, to be dynamically stable in the roll axis. It will happily roll off to one side if you don't stop it.

[Eseem]

Interesting! But the rudder trim should cause the same aerodynamic effects as stepping accordingly on a pedal, yes?

Sent from my SM-G920F using Tapatalk

[Oracle427]

Yes, it does, as it moves the rudder, but stepping on the pedal doesn't really seem to trigger much of a roll unless you step on it somewhat firmly. You would need to be uncoordinated for that to happen.

[Eseem]

I don't have any engine, but gliders definately rolls with rudder input. The a2a 182 seems to do so too, in a proportionally less effective manner, as the stubby wings and a relatively lower alpha (we usually operate in the bottom of our speed range, best glide etc., higher alpha -> more pronounced dihedral effect) would suggest. My quest for balancing everything is simply based on the fact that on my PC, I use a springloaded stick with an orgeo that requires quite the deadzone. If I had a stick or yoke able to simulate the proper feeling of flight controls, the matter would be very different indeed. (I don't like autopilots, I like trimming)

[mallcott]

Interesting! But the rudder trim should cause the same aerodynamic effects as stepping accordingly on a pedal, yes?

Sent from my SM-G920F using Tapatalk

Not necessarily. Look at the relative size of rudder and trim tab. the total available force is tiny compared to the rudder itself, plus the trim tab is attached TO the rudder and thus not in a fixed relationship with the airflow. It does not act directly upon the airframe... but rather on the control surface on the airplane.

In real life the trim tab is regarded as a `force multiplier and de-multiplier` for the surface it acts upon and the control that moves it, NOT as a control surface, although there have been instances where it has been used as such in extremis.

I think you have a fundamantal misconception of the effect of a trim surface, it's action and reaction. For example, the same tab can act in either a servo, or anti-servo effect, depending on how the remainder of the surfaces and aerodynamics act and what it is called upon to compensate for...

The masking effect in your simulator is caused because it too treats the trim as a `lesser control surface`. Which it isn't. Not A2A's problem but a necessity for a simulator where 99.73% of the user base possess centring-spring rather than force-feedback controllers and where the mix of other factors do not properly replicate the real world - induced drag, friction, non-linear control surface response with speed or angle-of-attack and a myriad of other factors.

If you want to balance the aircraft in the sim, you assure that offset weights are matched, moderate power is used and a modest amount of trim is added to complete the balancing act. Any additional modification to that balance MUST be added via control surfaces or else those errors in aerodynamics are simply magnified and exacerbated.

It's why your sim and addons cost a few bucks rather than a few hundred thousand.
The marginal extra cost and dedication to applied techniques that function outside the core sim marks the A2A range out as better than most, but even though it squawks like a duck, flaps its wings like a duck and lands on water like a duck it's still a fake duck - albeit one more convincing than the moose that would like you to THINK it's a duck...

[Oracle427]

There is no trim tab on a 182T. The rudder trim acts on some internal assembly that moves the rudder control surface. Net result is largely the same as moving the trim surface moves the control surface that ultimately produces the effect desired by the pilot.

I do have some time in gliders too and there is a significant roll coupling due to I assume is the bigger wingspan and lift induced due to the relative increase in AOA by the outer wing.

While in cruise you can step very firmly on the rudder of the 182 and the plane will yaw quite a bit to one side, bleed airspeed and roll a tiny bit. If you let go of the pedal it will roll right back and quickly stabilize more or less where it was before after 1 or 2 small oscillations. It doesn't even want to dutch roll.

I'm not following why you need a large deadzone with your stick. I use a very basic setup myself and have zero deadzone and max sensitivity. That allows one to introduce the very small inputsa necessary to maintain control. Trimming does not allow hands free flight for more than a few seconds. One must constantly make minute, imperceptible control pressures to maintain level flight. The A2A sim is much better in this regard to most FSX aircraft. The only way to relieve this workload, just as IRL is to turn on the autopilot. IMHO, adding in any deadzone only increases the workload of the sim pilot. It would be like trying to fly the real airplane with loosely rigged control cables. It would be a nightmare!

[Nick - A2A]

Interesting discussion. I think I understand the point that the OP is making. In the A2A version of the 182T, it's possible to use rather small movements of the rudder pedals to 'pick up' a low wing. For instance, even with the aileron axis disconnected, it's quite easy to use just rudder inputs to counteract the aircraft's natural left-turning tendencies and make a wings level climb out after take-off (assuming calm weather anyway). Similarly if we trim the aircraft for hands-off, level flight we can let go of the yoke completely, and just use gentle pressure on the rudder pedals to keep the wings level and the thing flying straight.

However, using just rudder trim to try and accomplish the same goal seems to be rather more difficult. That's because the roll response to yaw produced by the rudder trim seems less pronounced than the roll response to the same amount of yaw from direct rudder input. There is still a roll response to rudder trim input, but for any given amount of yaw, it's much smaller and perhaps less immediate too.

As Oracle says, the rudder trim system moves the rudder directly (via the steering bungee) so this seems a little odd. To be honest, in the past I've only really noticed it when I've attempted to use the rudder trim during climb out and found it wasn't really possible to 'trim out' the control forces effectively. Quite a heavy right foot was still required throughout the climb.

I guess it's possible that this simulates limitations of the rudder trim system in the real 182, and that the bungee-connected series of cogs and chains mean that the whole assembly is a bit 'sloppy' in use.

However, to reiterate, the response to rudder input in the A2A 162 is quite different to this:

While in cruise you can step very firmly on the rudder of the 182 and the plane will yaw quite a bit to one side, bleed airspeed and roll a tiny bit.

In the sim version, very firm pressure on the rudder will result in a significant roll and opposite rudder pressure would then be needed to correct that roll. Perhaps A2A 'beefed-up' the roll response to rudder inputs to better deal with the left-turning tendencies of the aircraft at high power settings?

Cheers,
Nick

[Oracle427]

I should qualify that statement by saying that stepping hard on the rudder does not equate to anywhere near full deflection. I'm not sure how far I moved the rudder pedal, but it takes a lot of pressure to move it around while in cruise. Soooo, in the sim we can easily apply full deflection with 5lbs of force whereas in the real airplane it might take over 50lbs of force to deflect halfway.

I can't comment on the difference between the trim behavior and direct rudder input in the sim.

[mallcott]

There is no trim tab on a 182T. The rudder trim acts on some internal assembly that moves the rudder control surface. Net result is largely the same as moving the trim surface moves the control surface that ultimately produces the effect desired by the pilot.

Visually you're correct. As a real world description you are also correct.

As a simulator you are wrong...

Even A2A modeling has the trim operating as a tab. It's how it is defined in the simulator because there is no force variation, so the virtual surface if present MUST act as a secondary control surface. You are dealing with the laws of digital creation here, not aerodynamics.

I gave the duck analogy for good reason. What you SEE is not what you get. You get what MS and LM give you. And they don't even model servo v. anti-servo trim tabs very well.

[Eseem]

Trying to revive this, could anyone on the 182 team give their 10 cents on the rudder trim's functionality in the sim? Thanks

[mallcott]

Trying to revive this, could anyone on the 182 team give their 10 cents on the rudder trim's functionality in the sim? Thanks

So we are clear, what was it in the foregoing that you dont understand? The rudder trim functions as... a rudder trim.

[AKar]

Interesting discussion and findings indeed, I've completely missed this discussion!

Simulation of trims in FSX/P3D is something I've always found to be...less than optimal at least. The pitch trim acts as if it was a rough approximation of trimmable horizontal stabilizer instead of simulating a proper trim tab. The trims of other axes seem to work by a similar analogy, instead of properly working through the main control surfaces, displacing them in the process like real trim tabs, or direct trims as well, do. To me, it appears that this way of simulation engine managing the trims independently from the actual control surfaces can create situations like this, where the rudder trim functions differently to the actual rudder in cases where it obviously should not.

-Esa

[mallcott]

Interesting discussion and findings indeed, I've completely missed this discussion!

Simulation of trims in FSX/P3D is something I've always found to be...less than optimal at least. The pitch trim acts as if it was a rough approximation of trimmable horizontal stabilizer instead of simulating a proper trim tab. The trims of other axes seem to work by a similar analogy, instead of properly working through the main control surfaces, displacing them in the process like real trim tabs, or direct trims as well, do. To me, it appears that this way of simulation engine managing the trims independently from the actual control surfaces can create situations like this, where the rudder trim functions differently to the actual rudder in cases where it obviously should not.

-Esa

Correct. As explained earlier the compromise is caused because of the spring-loaded rather than force-managed devices simmers mostly use to control the virtual control surfaces - it therefore makes little sense to mimic an actual trim functionality that does not correlate with real-world desktop devices to correspond to real-world actual aircraft control devices. For example the total `throw` of a desktop joystick is usually only a percentage of the real world device. Yokes likewise. FS Force makes a reasonable stab at taking things to the next level, but of course that only works with force feedback - in itself a terribly faux representation of the forces acting...

I don't think its possible to alter trim function in the sims (FSX or P3D) to any significant extent, so for the purposes of simming the trim functions as a separate control surface, not as a force-multiplier as it does in real life. I've never seen an `external` flight model from A2A, Flight1, Majestic, PMDG, Real Air, or any of the `really real` brand leaders that even attempts that - because they know that ultimately it's the virtual controller that is the ultimate limitation.

In real life `running out of trim` is a symptom of not having enough authority in the trim system. Yet for the purposes of certification the aircraft control surface will (almost) always be able to overcome the trim effect with sufficient force applied. I say `almost` because things can get a bit scary at the margins of the flight envelope and CG location with some aircraft, some require the sort of force that only Schwarzenegger in his prime could dish out, and of course older aircraft may not have been subject to the same strict certification so can be a bit contrary.

The 182 is an example of an airplane that sits on the `safe` side of that regime, but not by an awful lot hence the need to add a lot of trim for approach and landing.

For this reason it is inadvisable to make changes to the aircraft .cfg or FDE sections without first ensuring you have a `100/100`representation of the relationship between virtual controller and its desktop counterpart. The non-linear relationship is there for a reason. It's usually the reason half-baked simpletons make sweeping changes to the trim settings in the aircraft .cfg without understanding what it actually MEANS.

[Nick - A2A]

So we are clear, what was it in the foregoing that you dont understand? The rudder trim functions as... a rudder trim.

The OP's remarks relate to the fact that yaw-roll coupling from using the rudder is markedly different from yaw-roll coupling from using the rudder trim. Given that the trim control moves the rudder directly, this is odd wouldn't you say?

I agree that the shortcoming of the trim system in FSX/P3D are probably insurmountable with the usual sort of hardware. It's something we've discussed here a few times in the past.

However, in this case I wonder if there's an 'added' roll response to rudder input which has been deliberately applied in the FDE; perhaps to aid behaviour such as slipping or countering torque/p-factor roll on take-off. In any case, rudder trim can be used to yaw the A2A 182 without the same roll response that comes from using the rudder itself. (This is something which is easiest to demonstrate if a hardware axis is used for rudder trim.)

Nick