737 trim

[ivar hestnes]

I have an issue with my trim-function. When on autopilot, the trim is correcting every movement due to wind or whatsoever. (software and then hardware also of course). And it always trim too much, so it has to correct back again. The trimwheels almost never stop to run. Forward and back, all the time. Sometimes I have to go around because of this. Because when I use approach it trim so much that I actually miss the glideslope

If it was possible to add a delay or something to the trim it would be nice.

Anyone else has issues with this? Or improvements to share

[Michael Carter]

The 727 only does this in rough conditions. Normally, you'll only hear the trim when setting flaps or a power change, or both.

I noticed it trimming quite a bit in your video. Man, that chain drive gets on my nerves in rough weather.

[NicD]

Yes - we have the same issues. At the moment we have disabled trim wheel movement while the autopilot is connected. In the longer term we'll be programming SIOC to handle this. A few ideas here:

1) use a delay between writes so that the trim isn’t running constantly
so logic could be – once a trim command has occurred to the trim then don’t do another command until X seconds has elapsed

2) There also needs to be trend logic to eliminate constant small corrections. So don’t do a trim command if the commanded change is less than X% of trim.

[ivar hestnes]

Yes Michael, the trimwheel chain is having a really hard life. At least I have a switch to cut off the motor, but the relays still make noise though.

NicD: You definitely are spot on with your suggestions here. But I dont know how to solve it. Thats for sure. Could there be a way to configure this in the flight-model?

I tried FSX with default 737 a while ago, and if I remember correct, it was much smoother there.

My first guess is that maybe this is flight-model related. I use pmdg as the flight model in my setup. But I cant remember the trim acted like this when desktop flying earlier. Is it possible that it is the combination of pmdg and PM that cause this? hmmmm. Lots of questions

Anyone using an other 737 flight-model? Experiences?

[NicD]

Yes that's right Ivar - from my testing it's the PM AFDS that is overactive... the A/T is just too twitchy, and the movements a bit coarse. Maybe we should talk to PM about smoothing the A/T logic?

[mauriceb]

I am using Phidgets rate of change function to control my trim motors with my logic board. With that function, you can adjust how sensitive the trim wheel motion is and I don't really have a problem with that setup.
Yes, the A/P does try to trim a bit too often but it settles down quickly and it is not an issue for me.
I tried without using the rate of change function and it was a disaster since the trim wheel was moving almost constantly.

Anyway, here is my circuit and some notes I made. I used my own logic board, but you can do the same now with a Phidgets 2 relays board where the relays can be controlled by an offset. I'm sure there must be other ways to make trim function properly, but this works for me

Maurice

[ivar hestnes]

Yes, we should definitely talk to PM about this issue. Maybe they can make a solution for it. I will send a mail later today. Maybe you can do the same NicD

Thanks for explaining what you did Maurice. My trim is setup with phidgets, so I will check the "phidgets/FS rate of change" function. Thanks.

[ivar hestnes]

Hi there.

I tried to "uncheck" the use FS rate of change box. But it didnt actually help. The trimwheel now only go in one direction. But less movement on the wheel though. Still indicator moves alot, like before.

I use same offset as per your drawing Maurice.

I sent an e-mail to PM-support and a link to this thread.

[mauriceb]

Hi there.

I tried to "uncheck" the use FS rate of change box. But it didnt actually help. The trimwheel now only go in one direction. But less movement on the wheel though. Still indicator moves alot, like before.

I use same offset as per your drawing Maurice.

I sent an e-mail to PM-support and a link to this thread.

What do you mean by 'uncheck? That setting should be checked in both inputs. I will send you screenshots of mt settings as soon as I can.

Maurice

[mauriceb]

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