Force Trim Release

19 May 2022 10:34 #4654 by Graeme
Force Trim Release was created by Graeme
Hello, I’ve just updated to the latest version of the EC135, and I’m really enjoying it. The Sandel instruments look very good.I’m experiencing some odd behaviour using force trim release. Flying with a typical spring centered joystick, straight and level, I press force trim release preparing for a change in attitude and immediately get a roll or pitch input, despite being properly trimmed.I’d expect auto-trim to remove all the forces so that when pressing the force trim release to manouver there is no change with a neutral stick? Am I misunderstanding, or is there a setting I need to change for a spring centered joystick?I’ve tried the other way, making a stick input to roll the aircraft (holding against the trim), then pressing the force trim, but in that case the aircraft “snaps” into a much harder turn. 

Obviously it can be flown with A.TRIM off, but that doesn’t seem like correct behaviour.  Thanks for any help you can provide.

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21 May 2022 14:30 #4655 by rotorsim
Replied by rotorsim on topic Force Trim Release
If I understand correctly, is this about flying on autopilot?

The Force Trim button has fundamentally different functions when flying with or without the autopilot.
Without AP or in AP SAS mode it is the Force Trim function.
With AP, the button is used as an override option. In A.TRIM mode, for example, you can use it to change the reference flight attitude. Press and hold FTR, steer the helicopter into the desired attitude, then release FTR. Immediately afterwards you let the stick go back to its centering. The AP then fixes the attitude set when the button was released.

Releasing the stick to center very quickly after releasing the button is important, otherwise we don't know if your stick movement is meant to be an intentional override.

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23 May 2022 01:16 #4659 by Graeme
Replied by Graeme on topic Force Trim Release
Sorry, I don't think I've explained it correctly. The issue with me is with A.TRIM. I set an attitude and the aircraft will hold it, but the virtual stick position does not move to accomodate the aircraft state. The "neutral force point" on the virtual stick is only set at one moment, and not kept in alignment with what is required when the AP is disconnected. Essentially, the AP is not keeping the aircraft in trim. I've tried to show it in a video here in the hope it makes sense? streamable.com/tvq3v7

Again, apologies for the confusion, I'm just a fixed wing pilot wanting to learn helicopters more! 

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23 May 2022 07:47 #4660 by rotorsim
Replied by rotorsim on topic Force Trim Release
You are trying to combine two different overrides. 

Possibility 1: Change attitude permanently with FTR. To return to the old attitude, FTR must be used again (also Beep trim can be used). 

Possibility 2: Temporary override by moving the stick out of its centered position. The override is ended by returning the stick to the centered position.

The combination is not possible. In the video, you initiate a temporary override but don't finish it. Instead to this you begin a permanent change with FTR. It is then no longer clear which references are now the correct ones.

In the real machine this works because the stick is not simply centered in its physical center, but where the control input actually needs to be.

So: With FTR, the reference flight attitude is changed. By simply moving the stick while maintaining the reference, a curve can be flown, for example.

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23 May 2022 16:50 #4661 by Graeme
Replied by Graeme on topic Force Trim Release
OK Thanks, That explains the issue using force trim with control defection in A.TRIM - the real aircraft can, the sim can't because of control issues. I'd expect it would be possible to detect if in A.TRIM, control deflection held and force trim pressed - it could simply "hold" the virtual stick position and wait for the hardware to recenter just like it does in AP SAS Mode? I'm sure other helicopters work this way.

What about the second issue where disconnecting A.TRIM after a speed change will leave the aircraft substantially out of trim? Is this something that can be resolved? I think A.TRIM should maintain the trim state continuously not just memorise a trim state at the point of engagement?

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23 May 2022 17:46 #4662 by rotorsim
Replied by rotorsim on topic Force Trim Release

[...] it could simply "hold" the virtual stick position and wait for the hardware to recenter just like it does in AP SAS Mode? [...]


 

This would work great if it weren't for the temporary override feature directly from the stick.

What about the second issue where disconnecting A.TRIM after a speed change will leave the aircraft substantially out of trim? Is this something that can be resolved?


 

Most users were overwhelmed with regaining control when the AP was turned off. That's why we decided a while ago to slowly reduce the trim down to 0 when turning off the AP. You then have to make adjustments, but you have full control travel again.

You can switch this off yourself with the entry
[AFCS]
ResetTrimAtDisengage=0

in the ec135v5.ini file (see www.rotorsim.de/ec135v5/users-guide?start=12). 

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