Elevator trim question

[Wiqvist]

I asked this in another thread but my brain mixed up the question so I guess no one understood what I realy asked for.

I am not sure if my elevator trim is correctly set up. When I am at external view and set the elevator trim so the rear wing is totaly flat, then shows the trim indicator inside the cockpit that the elevator trim is down by 20%. And if I set it so the indicator shows 0%, the trim point up when I look at it. Is this correct?

In the C172 so is the indicator showing 0% when the trim is flat, so therefor have I become a bit unsure if this is correct on the C182 model.

Thank you in advance for Your answers.

[lonewulf47]

I cannot reproduce your finding on my end. I made a test by setting the trim tab visually to zero (=flush with the elevator panel) and eded up with the trim indicator inside the cockpit neatly showing take-off position. Just to make sure: Have you run the latest updater http://a2asimulations.com/forum/viewtop ... 20&t=41279 ?

[Wiqvist]

Thank you for Your reply.
Yes I got the update. Then it must be something on my set up. I have tried to calibrate it but it does not help(I use Saitek's trim Wheel). When I set it as on your picture so the marking is on the Take off position(use TrackIr so I look at the right Level), so is it about 5% down, and the trim on the wing point upwards. I am not sure of how I should fix this, except for a New installation.

[Oracle427]

Sounds like we have a parallax error. When you say you are using TrackIR, are you moving your head down to be eye to eye with the marking for the trim TO index? If so this is not realistic as there is no easy way to bend down that way without being a contortionist or by standing outside the aircraft and looking inside with your head resting on the seat.

If you look down from above as in Oskar's picture, you should have the trim in the ballpark area.

Note that TO trim is a ballpark setting and I believe that having it set a small amount (like a 2-3mm gap at the trailing edge) above the elevator is the typical position when aligned with the index.

[AKar]

Yeah, when I last checked, mine matched Oskar's.

And yes, I consider any value in GA planes as ballpark. If I know that I'm nose-heavy, I trim the plane a bit nose-up for takeoff, vice versa if tail-heavy (not really an issue with Skylane too often). I usually climb at 100 kts or so initially, so I prefer nose-down trim settings.

Exact TO trim settings are more or less ballpark values all the way up to the airliners, which is something that seems to be wildly misunderstood in tube flyer simulation circles; while the W&B calculation may give you a setting down to two decimals, the exact CoG is always somewhat of an estimate, as exactly precise load weight distribution is generally somewhat unknown (and quite insignificant practically in that degree of accuracy). That kind of reasons are why I never want to use my flight planner to load my plane or to link with my weather program - doing all this separately just adds to the realism, with some inaccuracy in between them.

-Esa

[lonewulf47]

Thank you for Your reply.
Yes I got the update. Then it must be something on my set up. I have tried to calibrate it but it does not help(I use Saitek's trim Wheel). When I set it as on your picture so the marking is on the Take off position(use TrackIT so I look at the right Level), so is it about 5% down, and the trim on the wing point upwards. I am not sure of how I should fix this, except for a New installation.

There's no way you can alter this. This is a fixed setting (btw, I'm using the SAITKE Trim Wheel also). However as has been mentioned, TrackIR is often used in a way that eliminates parallax error and that's not the idea behind. The Trim Indicator should be reading approx. zero (=Take-Off Triangle) when looking from the Pilots eyepoint. By saying "approx." it should also become obvious that this is not a 100% accurate setting. It vastly depends on the rigging of the A/Cs flight controls. You can have different positions of the trim indicator within the same fleet of A/C. The trim indicator is more an approximation than an accuratey scaled indicator.

[joediamond]

I get the same result as the OP. This is after the latest update. With the trim indicator popup showing 0% the tab is slightly up. It flies just fine, better than before the update. Before the update it would fly itself off the runway without any back pressure, after the update it does not.

The patch notes for the update mentioned the trim being adjusted so I believe everything is just fine.

[Wiqvist]

Thank you all for trying to help me with this. It has nothing to do with TrackIr. I just mentioned that when I tried to explain what I was asking. As Joediamond, so is it showing up like the same for me, and if I in that view set the trim so the wing is flat, the indicator says it is down by 20%.

It does not affect my flying, I set it to what feels best and gives me best trim, not what number say. I just wonder if it is correct, that the trim should point upwards when the indicator say 0%. On the C172 so is the wing flat on 0% but on the Skylane so is it not flat. I for sure understand if they are different, just wonder if they are? Or if it is my calibration or something not correct. It is for sure not a problem, just like to have it correct.

[Caldemeyn]

Can you show us a picture of your indicator and elevator trim tab ?

[AKar]

I must add to my earlier comment, that I haven't checked it since last update. I checked the change log, and apparently the trim was affected. So I don't know how it's now.

Off the main topic, but adding to Oskar's note on how the trim indicators may vary between the planes in the fleet: a neat trick with many GA airplanes that are generally flown with about the same CoGs is marking the control yoke positions for known conditions (cruise, approach....) to the yoke's tube using marker. Works within a few knots indicated in many Cessnas, I'm told!

-Esa

[joediamond]

Exact TO trim settings are more or less ballpark values all the way up to the airliners

You got that right! We always joke about it at work because on the 737 our takeoff close out info gives us a trim setting with two decimal places, such as 5.25. It's pretty pointless when the index mark on the trim indicator is a good .5 units wide.

[AKar]

It's pretty pointless when the index mark on the trim indicator is a good .5 units wide.

That's another issue with planes having mechanical indicators. Another is the simple question; let's say you fly a lightly loaded 738 with 30 bags @ 16.5 kgs each (t: 0.5 tonnes) in downstairs, all loaded to cpt3 (main rear cargo) - what is their moment arm? Well, some ground crew may load them all the way in to the compartment (against the front wall), others may distribute them somewhat evenly along the compartment floor area, while others may stack them as close as possible to the door. While the length of the compartment is significant (over 5 meters?), the W&B calculation does not differentiate these scenarios.

Just as another example of variables involved.

-Esa

[lonewulf47]

Just as another example of variables involved.

And, btw, just think about using standard weights for passengers in load calculations or - even worse - wrong assignments for passengers. I once had my A/C loaded with a full passenger load among which were 80 children, all sitting on economy, but erroneously checked in as adults. Guess how much mistrim that caused. As there were roughly 4000 kgs missing in the eco section the A/C was nose-heavy outside it's envelope but nevertheless able to fly. So I was lucky enough that we survived...