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Old 10-19-2019, 04:29 AM   #1
heloguy
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Default Pitch Excursions while using Trimmer

So, I decided to get back in the Ka-50 to try the Georgia Oil War campaign that I never finished years ago.

My cockpit is quite different than it was back then. It used to be a Saitek X-45, with Track IR3. Now, if you care, you can check out the specs in my sig, but only because it matters for finding this potential bug, I'll mention that I currently use a Brunner CLS-E in hardware trim mode for DCS. What this allows me to do is have trim release, and coolie hat trim outside of DCS, since the FFB functionality of the Brunner is still in the early stages.

When I was setting my controls back up, I decided to go with the trim option in the special tab for 'Joystick with no spring or FFB.' In addition, I mapped the trimmer button on the cyclic within DCS to the same button. My thought was that this would allow me to use hardware trim to hold the physical stick position, as well as reset trim functions for the autopilot on the Ka-50.

Everything seems to work fine (autopilot, and the loud clicks for trim release), but I started to notice something felt weird compared to the last time I flew the shark. I remember, even with my older spring loaded joysticks, if you held the trimmer, the aircraft was pretty smooth to fly. Now, if I press the trimmer, the nose pitches down, which must by manually arrested with aft cyclic, and then pitches up when the button is released, again, only arrested with opposite cyclic.

I tested this by getting the aircraft trimmed out straight and level, and just depressing the trimmer, without moving the stick. Without fail, the nose pitched down when the trimmer button was depressed, and pitched up when the trimmer button was released.

I then unmapped the trimmer from the cyclic, and only used the hardware trim function of the Brunner stick. Everything was smooth, as I remembered. No pitch excursions whatsoever. I remapped the trim release, and sure enough, the aforementioned behavior began anew.

Again, not sure if this is a bug, as maybe the actual aircraft behaves this way. If so, I'm glad it's not something I have to pilot for real, as it would drive me nuts. If it is a bug, it would be great to get it fixed, so those that use the trim option of no spring or ffb can actually map the trimmer button.

Pretty sure I found someone who had the same problem in this post, so I don't think I'm the only one:

https://forums.eagle.ru/showpost.php...2&postcount=10

Edit: Attached a .trk. In the .trk, I start out with hardware trim only, with the trimmer button unmapped. After I get everything straight and level, I actually depressed the timmer button quite a few times to demonstrate no pitch excursions. Then, I mapped the cyclic trimmer button to the same button that operates trim release on my Brunner CLS-E. I then used the trimmer multiple times, demonstrating how the nose will pitch down when the trimmer is depressed, and pitch up when the trimmer button is released.

I created a video through Nvidia Geforce experience, and attempted to voice over (drowned out by DCS mostly), but as its over 900mb, I'm still trying to figure out how to get it posted. I think the video would be more useful, as you can see when I change settings.

Edit again: Here's the video I made to go along with the .trk:

https://youtu.be/nMy0kE_08o8
Attached Files
File Type: trk Ka-50 Trim Bug Test.trk (3.31 MB, 14 views)
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Last edited by heloguy; 10-20-2019 at 01:36 PM. Reason: terminology
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Old 10-20-2019, 06:50 AM   #2
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We discussed this issue a few weeks ago:

https://forums.eagle.ru/showthread.php?t=248239

I confirmed that it is indeed an issue:

https://forums.eagle.ru/showpost.php...5&postcount=19
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Old 10-20-2019, 01:38 PM   #3
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Quote:
Originally Posted by Ranma13 View Post
We discussed this issue a few weeks ago:

https://forums.eagle.ru/showthread.php?t=248239

I confirmed that it is indeed an issue:

https://forums.eagle.ru/showpost.php...5&postcount=19
Ranma,

Thanks for the reply, but looking through the thread linked in your post, I believe we are talking about two different issues. It may have to do with my terminology, and as such, I edited my first post to make it more clear.

I'm not using the 'Trim Reset' (Lctrl-T) function at all. I'm using the standard cyclic 'Trimmer' (T) button, as can be seen being mapped in the video I posted.

Edit: From the looks of your video, we are indeed talking about the same issue. However, the thread that post is in is about a completely different issue related to the 'Trim Reset' (LCtrl-T) function, not the 'Trimmer' (T). If that's the case, I'm glad there's confirmation from another user, but it would be great if an ED rep would shed some light as to whether this is a bug being tracked, or a 'feature'.
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Last edited by heloguy; 10-20-2019 at 07:05 PM.
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Old 10-21-2019, 11:26 AM   #4
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I watched the video and looked at the track and I believe I know what the issue is. The trim is like a rubber band on the nose of the helicopter that tries to pull it back to the last-trimmed position:



What's happening is that the stick's position is in equilibrium with the AP force; the AP is trying to pull the helicopter's in one direction, but the cyclic's position is counter-acting that force. Imagine that the AP is trying to move the helicopter's nose 10 arbitrary units up, and your cyclic's position is pulling it 10 units down. The net effect is that the helicopter's nose stays where it is.

When you hold down the trimmer, the AP force is removed, but the cyclic position is still pulling the nose down 10 units, which is why it "jumps" when you hold down the trimmer but don't move the stick. You can confirm this by enabling Flight Director; it disables the AP force so with it on, holding down the trimmer without moving the cyclic should hold the nose steady.

Here's a video of me recreating the issue. I have the trim type set to "Joystick without spring or FFB" and I'm doing it on a FSSB R3, so apologies for the rough takeoff:



At 0:12 I tap the trim to set the AP attitude rather low, then pull the nose up to fly in the direction I want it to. At 0:20 I hold down the trimmer without moving the stick and you can see the nose rising. At 0:26 I tap the trim at a steeper angle this time, and at 0:36 I hold down the trimmer and the nose significantly rises up without me moving the cyclic.

Here's a video with Flight Director this time:



Tapping the trim doesn't set the AP attitude (aside from the indicator on the HUD), so holding down the trimmer later doesn't bob the nose.

Last edited by Ranma13; 10-21-2019 at 12:14 PM.
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Old 10-21-2019, 01:35 PM   #5
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The problem certainly does feel that way. It's seems as if the pitch attitude hold captures an attitude that's a couple of degrees off, in the up direction, from where the nose actually is when the trimmer button is released.
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Old 10-21-2019, 04:58 PM   #6
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I think it would be enlightening to trim the aircraft so that the AP channels don't need to deviate from the trimmed stick location to keep the attitude.

Basically a SUPER stable flight envelope BEFORE you release the trim button. I still feel that the cause is the AP's CURRENT control authority magnitude and how that gets applied to a NEW trimmed state.


To elaborate...

The air-frame will oscillate around a trimmed attitude... and the AP channels are what's responsible for keeping the nose in the rubber-band...

So... Take a for instance...

You trim to a super stable hover and note the stick position and trim offset (in a FFB setup these will be the same... and they will be very close to the trimmed position.

Now... Press trim and VERY QUICKLY move the stick to somewhere else WITHIN the trimmer's 20% control authority and release trim. The attitude of the airframe won't change much (due to inertia) by the time you release trim. Because of this badly attained trim what you will end up with is the AP channels applying up to 20% control authority to maintain the air-frame's attitude when the trim was released. There will be a large offset between where you released trim and where the stick ends up based on the AP's interest in maintaining the attitude and not the stick position.

That offset is what I think is causing the movement you're seeing. And... I'm not sure it's a bug. If it is... It may be that the logic behind the FFB mode does the wrong thing with the AP channel's trim offset.

That said... I don't fly with FFB.
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Last edited by M1Combat; 10-21-2019 at 05:09 PM.
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Old 10-22-2019, 01:50 AM   #7
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Quote:
Originally Posted by M1Combat View Post
I think it would be enlightening to trim the aircraft so that the AP channels don't need to deviate from the trimmed stick location to keep the attitude.

Basically a SUPER stable flight envelope BEFORE you release the trim button. I still feel that the cause is the AP's CURRENT control authority magnitude and how that gets applied to a NEW trimmed state.


To elaborate...

The air-frame will oscillate around a trimmed attitude... and the AP channels are what's responsible for keeping the nose in the rubber-band...

So... Take a for instance...

You trim to a super stable hover and note the stick position and trim offset (in a FFB setup these will be the same... and they will be very close to the trimmed position.

Now... Press trim and VERY QUICKLY move the stick to somewhere else WITHIN the trimmer's 20% control authority and release trim. The attitude of the airframe won't change much (due to inertia) by the time you release trim. Because of this badly attained trim what you will end up with is the AP channels applying up to 20% control authority to maintain the air-frame's attitude when the trim was released. There will be a large offset between where you released trim and where the stick ends up based on the AP's interest in maintaining the attitude and not the stick position.

That offset is what I think is causing the movement you're seeing. And... I'm not sure it's a bug. If it is... It may be that the logic behind the FFB mode does the wrong thing with the AP channel's trim offset.

That said... I don't fly with FFB.

If this oscillation based on pressing and releasing the trimmer is normal for the ka-50, then it would definitely be a real pain to fly. No helo I’ve ever flown IRL has had this issue.


My ffb stick doesn’t utilize the in-game ffb functions. It’s entirely hardware based. That said, Ranma isn’t using a ffb stick. I’m about 99% sure this is a bug in the module.
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Last edited by heloguy; 10-22-2019 at 02:52 PM.
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Old 10-22-2019, 03:24 AM   #8
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I took another look and I agree, it does look like a bug, and the same bug as the trim reset one. When you press trim down without moving the stick, the aircraft's nose will dip down, and when you release it, the aircraft's nose will pop back up to the same position as it was in when you first pressed it down. As far as I understand the Ka-50's AP system, this is not supposed to happen.
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Old 10-22-2019, 04:51 PM   #9
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I don't think either of you are understanding what I'm trying to say. No offense meant... maybe I'm not explaining it well.

All I'm asking is that you test with a very small trim offset (When the AP channels are exerting very small amounts of control authority to maintain trimmed attitude) and with a large trim offset (when the AP channels need to exert somewhere near their maximum control authority to maintain air frame attitude). This will be valuable information.

I'm just seeing both of you perform a minor test that doesn't prove anything and certainly doesn't prove any "cause" for what you're seeing.

I'm not saying "there is no issue here... move along...".

I'm saying "Test, test, Test. We'll find a bug IF it's there AND we'll have a good bit of info about what settings, procedures and even environmental factors affect it"

Until you do that... NOBODY will be convinced . If you want someone to be convinced... you need to do the leg work . That's all .


Also...

The oscillation I'm talking about is not what you're talking about. I'm talking about any free body's natural tendency to oscillate around an equilibrium. It's basically the concept that no motorcycle or aircraft EVER can go straight. EVER. It doesn't happen. That's the oscillation I'm referencing. Not what happens here when you re-trim the KA-50. I'm going to stop trying to make the point though because I don't think that either of you have the background knowledge to get the reference. Please don't take offense..

That said... I'll try to find an explanation and I'll post it here.
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Old 10-22-2019, 05:15 PM   #10
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Here... this is true for all unstable bodies... Motorcycles are one of these as well. The oscillation is never 0.

https://en.wikipedia.org/wiki/Flight...-wing_aircraft)

Quote:
Short-period pitch oscillation
A short input (in control systems terminology an impulse) in pitch (generally via the elevator in a standard configuration fixed-wing aircraft) will generally lead to overshoots about the trimmed condition. The transition is characterized by a damped simple harmonic motion about the new trim. There is very little change in the trajectory over the time it takes for the oscillation to damp out.

Generally this oscillation is high frequency (hence short period) and is damped over a period of a few seconds. A real-world example would involve a pilot selecting a new climb attitude, for example 5° nose up from the original attitude. A short, sharp pull back on the control column may be used, and will generally lead to oscillations about the new trim condition. If the oscillations are poorly damped the aircraft will take a long period of time to settle at the new condition, potentially leading to Pilot-induced oscillation. If the short period mode is unstable it will generally be impossible for the pilot to safely control the aircraft for any period of time.

This damped harmonic motion is called the short period pitch oscillation, it arises from the tendency of a stable aircraft to point in the general direction of flight. It is very similar in nature to the weathercock mode of missile or rocket configurations. The motion involves mainly the pitch attitude {\displaystyle \theta }\theta (theta) and incidence {\displaystyle \alpha }\alpha (alpha). The direction of the velocity vector, relative to inertial axes is {\displaystyle \theta -\alpha }\theta -\alpha . The velocity vector is:

Sooo...


If you catch the oscillation (by releasing trim) at a place where some of the 20% control authority in the AP channel is used... Then re-trim... We don't know what the shark's computer (and by that I mean the selected control logic ini the options really...) is doing WITH THAT EXERTED CONTROL AUTHORITY.

We need to.
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