Uncommanded trim inputs?

[MediumRareBaku]

Using 1.3 in 11.41 with experimental flight model on and beta Gizmo selected.

Basically, when trying to hand fly, autopilot fully off, I have uncommanded pitch trim that does not stop. I've double checked my inputs and do not believe I have any axis assignments or conflicting controls that are causing this. It seems to stop if I put pitch up or pitch down commands with the yoke, but resumes when in neutral.

I didn't know the -300s had MCAS

Edited May 24, 2020 by MediumRareBaku

[Litjan]

Hi - there are some modes when the 737-300 will use trim by itself - not very much unlike the infamous MCAS!

There is a "mach trim" system at very high speeds, that adds some trimming to offset the center-of-lift movement at higher Mach numbers

You are probably seeing the "speed trim" system in action that works when flaps are not up AND N1 > 70% (iirc), so you will see it mostly after take-off.

It automatically adds trimming to maintain the current airspeed - so if your nose rises and the speed falls, the plane will trim "nose down" to avoid speed going too low. Most pilots get confused about this, because when they push down on the yoke to accelerate after take-off, the plane will trim "nose up" to get back to the initial speed! Pushing the yoke against this will cut out the trim, but letting go will enable it again!

Trimming with the trim switches will "reset" the target speed to the current airspeed. Obviously raising the flaps will also help - or reducing thrust (for level flight, for example).

Let me know if this is what you are observing?

Cheers, Jan

 

[MediumRareBaku]

On 5/24/2020 at 1:48 AM, Litjan said:

Hi - there are some modes when the 737-300 will use trim by itself - not very much unlike the infamous MCAS!

There is a "mach trim" system at very high speeds, that adds some trimming to offset the center-of-lift movement at higher Mach numbers

You are probably seeing the "speed trim" system in action that works when flaps are not up AND N1 > 70% (iirc), so you will see it mostly after take-off.

It automatically adds trimming to maintain the current airspeed - so if your nose rises and the speed falls, the plane will trim "nose down" to avoid speed going too low. Most pilots get confused about this, because when they push down on the yoke to accelerate after take-off, the plane will trim "nose up" to get back to the initial speed! Pushing the yoke against this will cut out the trim, but letting go will enable it again!

Trimming with the trim switches will "reset" the target speed to the current airspeed. Obviously raising the flaps will also help - or reducing thrust (for level flight, for example).

Let me know if this is what you are observing?

Cheers, Jan

 

So, speed trim matches what I'm seeing based on when I'm seeing it, but either I wasn't paying attention pre 1.30 when I was flying the 733 or something is different. Is this a new addition? I haven't flown it since the first half of 2019 so perhaps its just bad memory. It makes that whole first phase of flight very awkward for me now in a way I don't remember.

Based on your description then, as I take off and am in the process of cleaning up and accelerating, speed trim is going to be putting in nose up trim as I accelerate because it wants me to maintain the old speed. What is the purpose of this, since I'm in a flight phase based on first holding my post-takeoff pitch, and then lowering the nose to accelerate to flying speeds? Why would the plane want me to keep my same speed until I've cleaned up, which in theory I would never be able to do if it trimmed me at the same speed?

I've got my dad's old FCOM from the 733/734, so I'm also going to read up on this on my end to try and better understand it.

Edited May 27, 2020 by MediumRareBaku

[Litjan]

Hi again,

a (very) brief google search netted this:

http://www.b737.org.uk/flightcontrols.htm

Searching with google again, netted this:

https://www.youtube.com/watch?v=AXr0PExNqBA

Posted November 2014, I will help you with the math: ca. 5.5 years ago

The purpose of this system is much the same as the new MCAS system - it was required for certification, because at high thrust levels you could get into the situation where the plane would slow down - but the nose won´t drop (fast enough), as the lessening elevator forces are not able to compensate the (more or less static) pitch up moment from thrust in underwing mounted engines. So they put in a system that "amplified" the natural pitch stability that you would want in any aircraft (plane gets faster -> nose goes up, plane gets slower -> nose goes down).

You are not alone - it confused the heck out of many real pilots (that did not read and understand the book word for word). In the real plane you trim almost all the time - so the effect of this system would only really show when someone has poor piloting technique (prolonged flight with constant yoke input needed).

Cheers, Jan

 

[MediumRareBaku]

That video is helpful, Jan. I'll practice with it some more today to see if it will make sense in real time now.

[Litjan]

Yeah, once you know why the plane is fighting you (and more importantly how to stop it) it´s not so bad anymore ;-)

Cheers, Jan