solved AH64 flight handling degrades/changes over time when using trim

[nrgized]

I noticed this with the initial launch patch but since I didn't use trim in the aircraft because of no reset ability I didn't look into it much when I encountered the phenomenon.

 

After commanding a number of trim inputs it appears as if something behind the scenes slowly adding up and gradually reduces the aircrafts normal flight characteristics when the cyclic is not being manipulated.

 

This is not...

• Misunderstanding of trim
• Learn to fly issue
• Change in aircraft weight
• Change in weather conditions

 

The example provided is a hover next to a building showing that the flight controls can be released and a stable hover is maintained.  After a number of trim commands coming back to the same building it requires more command inputs to maintain the hover.

Here is a video recording of the example scenario.

If you can get the issue to progress larger over time then the aircraft feels bouncy to inputs and starts to feel like the Gazelle with SAS turned off.

current_x_trim = 0.0;
current_y_trim = 0.0;

void update_trim() {
	current_x_trim = get_cyclic_x() + current_x_trim;
	current_y_trim = get_cyclic_y() + current_y_trim;
}

void trim_reset() {
	current_x_trim = 0.0;
	current_y_trim = 0.0;
}

Somewhere it feels like a residual value is being maintained that gradually its value/error manifests itself.  Also having trim forward and then setting trim to a value that is negative to center seems to trigger the issue easier.

Edited March 30, 2022 by nrgized

[bradmick]

The helicopter SAS will “saturate” over time and run out of authority. The green plus signs will tell you where the SAS is with regards to its “authority”. You can’t just trim it and leave it permanently there. As the FMC commands the SAS to respond to exterior stimuli while trying to maintain a stable hover or a desired or commanded attitude, it will use the SAS authority it has available to it and eventually run out of it. To return full SAS authority you have to recenter the SAS servo. To recenter the SAS and return it’s authority to maximum, you have to press and hold the force trim release switch for 3 to 5 seconds to recenter the SAS. Based on your description you allowed the SAS to saturate and it can no longer help you maintain a stable position.

Edited March 30, 2022 by bradmick

[nrgized]

That sounds reasonable as to what I saw. Controls felt very mushy like the Gazelle with SAS off.  Becomes a boat ride in a hover.

 

My example video while drastic so as to induce the problem is not how I fly/trim on the ordinary.  I normally encountered the issue when going from full cruise trim and essentially coming back to "centered".   Centered in this context meaning I had the cyclic centered by any means  be that manually positioning or as of today hitting the reset button.  Do that enough times and when I'd come to a hover it wasn't near how it felt when I took off.

 

Is recentering currently implemented do you know? 

 

Thanks

[Dropbear]

1 hour ago, bradmick said:

. To return full SAS authority you have to recenter the SAS servo. To recenter the SAS and return it’s authority to maximum, you have to press and hold the force trim release switch for 3 to 5 seconds to recenter the SAS. Based on your description you allowed the SAS to saturate and it can no longer help you maintain a stable position.

 

This is really good to know .. thanks for detailing this.

[cow_art]

Interesting, I noticed something similar yesterday. If SAS saturation is implemented as bradmick suggests, this could explain it. But I am not so sure if it is? If SAS saturation was implemented in DCS, I suspect the "SAS sleeve" (grey area in the controls indicator) would decrease/change over time? But this clearly is not happening in the controls indicator. 

Honestly this looks more like a bug to me. If you look at the SAS indicator for the rudder at the end of the video: The SAS desperately wants to push rudder all the way to the right, although I see no good reason for it to do so (and at this point the SAS has a lot of maneuvering room to the left within the indicated SAS sleeve). Even when the torque increases, the SAS does not try to correct the rudder to the left to counter the torque. It still insists on pushing the rudder all the way to the right.  Perhaps this is bugged or still WIP?

Edited March 31, 2022 by cow_art

[Niehorst]

1+

 

where do I find those "green plus" signs"?

Edited March 31, 2022 by Niehorst

[cow_art]

9 minutes ago, Niehorst said:

1+

 

where do I find those "green plus" signs"?

 

In the controls indicator (Right Control + Return to activate)

[bradmick]

5 hours ago, cow_art said:

Interesting, I noticed something similar yesterday. If SAS saturation is implemented as bradmick suggests, this could explain it. But I am not so sure if it is? If SAS saturation was implemented in DCS, I suspect the "SAS sleeve" (grey area in the controls indicator) would decrease/change over time? But this clearly is not happening in the controls indicator. 

Honestly this looks more like a bug to me. If you look at the SAS indicator for the rudder at the end of the video: The SAS desperately wants to push rudder all the way to the right, although I see no good reason for it to do so (and at this point the SAS has a lot of maneuvering room to the left within the indicated SAS sleeve). Even when the torque increases, the SAS does not try to correct the rudder to the left to counter the torque. It still insists on pushing the rudder all the way to the right.  Perhaps this is bugged or still WIP?

 

No, the green plus indicates the SAS position within its authority range (white boxes). If it’s bumping up against one of the sides, you need to press and hold the force trim release to recenter it. 

[cow_art]

39 minutes ago, bradmick said:

No, the green plus indicates the SAS position within its authority range (white boxes). If it’s bumping up against one of the sides, you need to press and hold the force trim release to recenter it. 

Thanks for your reply. But I still don't see how this explains what we see in the video. A few questions:

- are you absolutely sure SAS saturation is currently implemented in DCS ?

- the white boxes are the range within which the SAS can currently move the controls, right? So if SAS saturation happens, why doesn't the white box change shape (or its position relative to the white diamond)? In other words: when the SAS authority range gets smaller I would expect the box to shrink.

- In OP's video around 3:25 you see the helicopter is in a hover and the green indicator for the rudder is bumping on the right edge of the SAS box. It probably wants to got further right at that point but it can't because the authority box ends there. A few moments later, torque increases from 60% to 80%. Shouldn't the SAS try to counter that by going left on the rudder? The white box indicates that it in fact could go left (the white box looks like the SAS has a lot of authority to go to the left, it just can't go any further to the right).   But it does not.

That leads me to believe that something is fishy. Or I am severely misunderstanding something and would be grateful if you could clear up my confusion. Thanks!

Edited March 31, 2022 by cow_art

[Raptor9]

7 minutes ago, cow_art said:

- are you absolutely sure SAS saturation is currently implemented in DCS ?

- the white boxes are the range within which the SAS can currently move the controls, right? So if SAS saturation happens, why doesn't the white box change shape (or its position relative to the white diamond)? In other words: when the SAS authority range gets smaller I would expect the box to shrink.

First question: yes it is. The only thing missing is a visual advisory message "SAS SATURATED" on the EUFD and an audio tone when this happens.

Second question: the white boxes don't move or change shape with SAS movement. They represent the SAS authority relative to the flight controls that the SAS can operate within. Therefore the white boxes will be linked to wherever the flight controls are. The SAS authority range doesn't get larger or smaller, the SAS position is saturated when it is at the limit of the available authority range and cannot apply any additional input to the servos.

The logic of how the SAS operates under various modes and conditions is still WIP, but the authority range within which they can operate is not.

Edited March 31, 2022 by Raptor9

[cow_art]

1 minute ago, Raptor9 said:

First question: yes it is. The only thing missing is a visual advisory message "SAS SATURATED" on the EUFD and an audio tone when this happens.

Second question: the white boxes don't move or change shape with SAS movement. They represent the SAS authority relative to the flight controls that the SAS can operate within. Therefore the white boxes will be linked to wherever the flight controls are. The SAS authority range doesn't get larger or smaller, the SAS position is saturated when it is at the limit of the available authority range and cannot apply any additional input to the servos.

Great, thanks for clearing that up for me

[bradmick]

Think of the SAS as operating on a -1 to 1 range, it stays fixed, it’s range of authority doesn’t change, however the SAS actuator is position within that authority range does.

Imagine you’re at a perfectly stationary with no wind, and you’re the perfect pilot, the SAS has no reason to move, it’ll remain perfectly fixed at the center. Now imagine a wind from the right starts. The SAS is going to apply input in the direction of the wind, this is the green plus. It will move further right, essentially applying right cyclic to the flight controls until it runs out of travel. It has zero ability to come back to its zero point on its own and will “stick” in this position. This is the point at which you the pilot interrupts the force trim and command the SAS sleeve to center. 
 

During this action the SAS should still provide stability to the aircraft, depending on conditions it can take 3-5 seconds to recenter because it’s still trying to provide a stable platform.

Edited March 31, 2022 by bradmick

[cow_art]

That makes a lot of sense, thank you!

So how do you deal with this in reality? Are you waiting for the "SAS SATURATED" message to pop up, and then recenter the SAS sleeve when it happens? Or do you do it periodically so you don't have the SAS Saturated message pop up at an inconvenient time?

[Goofeyfoot]

I saw this interesting post. Here’s what’s happening to me. I do get the warning that says that the SAS is saturated. Then I get all this beeping noise. I’m trying to dive down towards a bunch of trucks when this happens, typically.

So what am I supposed to do to stop the beeping?

Thanks. 

[Floyd1212]

On 3/31/2022 at 8:59 AM, cow_art said:

So how do you deal with this in reality?

I believe in reality the pilot presses and holds the Force Trim Release for the entire time they are moving the cyclic around, so essentially preventing it from getting saturated in the first place.  It would only get saturated if you had the scenario above with the gust of wind while they had their hand off the cyclic. 

This is very different from how I think most of us are flying this bird, by tapping the trim button on occasion when we need to take our hand off the stick, or get into a stable hover. 

[realtrance]

Where is the Firce Trum Release setting in the controller settings menu?

[shagrat]

Trim hat up. It should be under Cyclic.

[realtrance]

Thanks!Where is the Firce Trum Release setting in the controller settings menu?

[TylerPL666]

I spend more than 100h in Apache before April update, and even more after and Im 100% positive that earlier SAS was working great, and right after he started yealing and beeping at me. Right now I prefair to reset trim with holding it for 5sec in perfect center (during slowing down) and push left antitorqe pedal all the time cause I do not trust hoover trim anymore. Enebling hoover mode and holding trim until SAS stop fighting and relising inputs staying in one place is ok, but trying turn little left or right is almost immposible and start engine failure warning when trying to avoid crash. For some time was trying to fly all the time with forcetrim hold, and let go only when flying straight, but even then rudder SAS starting to bounce. Recently I even saw that my collective SAS isnt "match" my throttle (X56) without any cureve/saturation/deadzone - so why its adding throttle even I do not turn any hold mode? And none of that problems happend on relise. Any other pilot with same amount of time spend in Ah-64 will agree. Hoovering with Gazzele its a peace of cake in compare with Apache now.

Maybe I do not understand something - is there any way to "just fly" without any velocity/altitude/hoover autopilot? (FMC Relise isnt an option cause beeping/warning all the time). 

Edited June 8, 2022 by TylerPL666