737 trim

[JonathanRichardson]

Hi

We received a support e-mail asking to react to this thread. . .

The background information is that the MCP software is reacting to the instability of the flight model. It will try and keep the a/c in trim and this requires many changes small and large to achieve this. This I’m afraid is inherent in most MSFS flight models. Normally you don’t notice until you connect large motors and trim wheels and see / hear every single change of trim. But it is going on all the time when the AP is engaged, just use the CDU to monitor the raw trim values and you will see it changing, if it were not, then the AP would not be able to fly the flight model precisely. The flight model instability can be effected/improved or made worse by various parameters in the flight model, and also the way the a/c is loaded. But essentially it is inherent and there, and I doubt there is a way to completely get rid of it. To get around this as others have suggested, you need to add a section so that your trim motor/wheel will only trigger after a given amount of MSFS trim change. This is how we do it in the simulator here.

I have a feeling that if we were to change the MCP code and took out the fine trim control, you would get the a/c constantly drifting from level flight, undulating, and in turns or level changes capturing altitude or maintaining altitudes would be not be possible. Also, what might work for one very good flight model would not work for others and so forth. We looked at this a couple of years ago in detail and at that time came to the conclusions I outline here.

Regards
Jonathan Richardson
Project Magenta

[ivar hestnes]

Here are my Phidgets settings. If you decide to try that, you will have to play with the sliders to get the right trim wheel movements (adjust sensitivity). Of course, it would be a lot easier if PM fixed their PM trim offsets

Maurice

Thanks for the pictures Maurice Actually my boxes was checked all the time. Tried first yesterday to uncheck. They are now checked again. Will give it a try to move those sliders.

But one thing: I have circuit 1 on both digital outputs. And you use circuit 0. What would be the difference here? Hmmmm, I used Thomas Richters 737 program to find the offsets and circuits, but could still got it wrong LOL.



@ Jonathan: Thank you very much for jumping in with your explanation

[mauriceb]

.

But one thing: I have circuit 1 on both digital outputs. And you use circuit 0. What would be the difference here? Hmmmm, I used Thomas Richters 737 program to find the offsets and circuits, but could still got it wrong LOL.

I'm pretty sure the circuit number makes no difference. As far as offsets are concerned, all I know is that 0BC0 works for me . It's a bit tricky to get the sliders at the right values and it's just a matter of trial & error...no 'scientific' way to do that as far as I know.

Also, I have selected Invert for the signal but you may not have to (I needed to do that to drive the transistors that operate the relays)

Maurice

[mauriceb]

One more thing about that. For the calibration, I did not use the A/P. I simply used the trim switches. The idea is that when you press the trim switch up or down, there is a delay before the trim motors operate. If you get the right delay, then the A/P will also not actuate the trim motors as frequently but it will still 'hunt' a bit back & forth when you do major flight level changes although it settles down quite quickly with the right delays.

Maurice