[RESOLVED] Yaw instability or SPAK is not my friend...

[Goblin]

Leatherneck et. al.

THANK YOU for making the AJS 37 Viggen! I can't remember being this excited about a flight sim release, ever. And I've been doing this for a while! :)

 

Now, back to business.

 

Some findings.

In the mission planner you can choose livery "F15 Hälsinge Flygflottilj" but the skin is a F21 Kallax bird..! Did you do this to spite me? ;)

 

Some keybindings aren't possible to map to a controller. Eject, for instance. Now, this was the same in the test pre-release. I deleted the third pre-release build before updating to 1.5.6 but the keymappings remained. Could this be the culprit, or is it a known issue?

 

The largest issue so far is yaw stability, or lack thereof, on the ground. When taking off or landing, I quickly enter PIO in Yaw. If I turn SPAK off, it's much more stable. WIP?

 

I'll keep on testing...

 

Keep up the great work! Too bad there's no Nobel prize for flightsim development, or I would nominate you guys.

Edited January 27, 2017 by Goblin

[Pocket Sized]

Perhaps the SPAK rudder inputs are also being fed to the nosewheel?

[Goblin]

Yeah, or the SPAK damping sort of fights the NWS with rudder..? I'm not sure how this is supposed to work.

[RagnarDa]

I will look into this. What sort of rudder-controller do you use, twist-stick or something else?

[RagnarDa]

Also, at what speeds do you experience the stability issue when on the ground?

[CARTOK]

Experiencing the same stability issue. Using CH rudder pedals. Issue starts from about halfway to take off speed until take off.

[alftand]

I'm having a similar issue. On the ground, any rudder input will immediately cause full deflection of the rudder, and when I release the controls it will remain in the same position. Trying to counter it results in full deflection to the other side, like the yaw curve is a step function. In the air this does not seem to happen. Using an X-55 with twist.

[Astronut]

I suspect SPAK is at least part of the rudder problems. I just ran a test sat on the runway. With SPAK on, the rudder movement was behaving in a weird manner and didn't properly center when I took my feet off the pedals. With SPAK off, the rudder behaves in a normal, smooth manner.

[Goblin]

RagnarDa,

I use Slaw Device rudderpedals.

They provide a very stable and jitter free signal.

And, there's a big difference with SPAK on and off.

 

I encounter the problem during high speed ground operations such as takeoff and landing.

Would a .trk help?

[Pocket Sized]

X-55 with twist.

 

Read the "known issues" thread.

[RagnarDa]

I suspect SPAK is at least part of the rudder problems. I just ran a test sat on the runway. With SPAK on, the rudder movement was behaving in a weird manner and didn't properly center when I took my feet off the pedals. With SPAK off, the rudder behaves in a normal, smooth manner.

Are you using a twisting stick?

 

 

 

Guys, I've looked into this and I think the problem is three-fold:

 

1) The nosewheel was turning too much. I just understood, after 4 years!, that the nosewheel is supposed to be turned in radians and not in percentages! Fixed now. This will make the nosewheel about half as sensitive.

 

2) The SPAK corrections and nosewheel turning is not synced and maybe it should be. Figuring out how to do this best.

 

3) The twisting-stick issue. One thing I liked you to try is going to Controls Settings, Axis Assign and selecting the rudder input on the stick and then Axis Tune. Set the Y Saturation to something low, like 10 or lower. See if it helps and let me know the results.

[Astronut]

I'm using an X55, but I cleared the twist axis. I use CH Pro Pedals for rudder control. I'll tweak the saturation as you asked.. I'll be right back.

[Astronut]

ok, with the rudder axis set at saturation of 10, with SPAK on the rudder seems to be acting normally now (smooth operation and it centers when you come off the pedals). Of course, if you do this then the nosewheel steering now has little authority and when SPAK is selected off, the rudder hardly moves. Is there a SPAK gain setting that is cranked up too high maybe?

[mattebubben]

It also feels like the Nose Wheel Steering is on all the time during the takeoff run where as in most other aircraft i think its automatically turned of (or atleast limited) after a certain speed is reached (as it could easily become dangerous since even slight rudder movements might cause violent maneuvers that might become difficult to control).

 

Is this how it should be? (The full Nose Wheel Steering authority being active during the entire Takeoff run).

Edited January 28, 2017 by mattebubben

[jowl]

Are you using a twisting stick?

2) The SPAK corrections and nosewheel turning is not synced and maybe it should be. Figuring out how to do this best.

 

I remember a visiting Viggen pilot at Swesim who mentioned that you would be careful not to reverse on GSA due to loss of nose-wheel dampening.

Will see if I got my notes somewhere, might even find the SFI reference.

[illern]

I have pedals that are 2 single axis.(its an Thrustmaster T500 RS wheel)

In order to have thoose pedals in DCS i need to use both axis on the same rudder input. So I put Y saturation to 50% on both and invert one axis.

 

If I´m in SPAK mode then the wheel turns correctly but the rudder missbehaves and doesn´t center.

If i´m disconnect SPAK mode then both wheel and rudder moves and center.

 

After liftoff and gear up I can use SPAK and rudder again.(it is REALLY sensitive to rudder input)

 

I cannot use any other Y saturation due to the two axis. They need to meet in the middle.

[Goblin]

Guys, I've looked into this and I think the problem is three-fold:

 

WTG!

 

2) The SPAK corrections and nosewheel turning is not synced and maybe it should be. Figuring out how to do this best.

 

Should the SPAK stabilization affect the NWS..? I don't know for sure, but I imagine that it's just a flight control damping stabilization. Will dig into the SFI.

 

By "not synced" do you mean that SPAK is affecting the NWS, but unsynced, or that SPAK just affects the rudder? If the last, is there a SPAK rudder-NWS coupling that mess things up?

[renhanxue]

I remember a visiting Viggen pilot at Swesim who mentioned that you would be careful not to reverse on GSA due to loss of nose-wheel dampening.

Will see if I got my notes somewhere, might even find the SFI reference.

I'm most definitely not a Viggen pilot, but... I don't think that's right, at least not on the AJ 37 (JA 37 may be different). As far as I can tell from the SFI the nosewheel steering is directly connected to the pedals via a simple servo. There are no artificial pedal forces, and the only dampening you get is via some purely hydraulic magic - the autopilot isn't involved at all. SFI AJS 37 del 1, kap 1, flik 8, sida 9 (page 124 in my PDF). You can't turn nose steering turning off either, the only way of losing it is by losing pressure in hydraulic system 1.

 

Goblin's speculation seems reasonable though - you have direct control of the nose wheel but SPAK is using the rudder to try to correct for your actions. Additionally, it's mentioned that the yaw dampening is strongly reinforced while the nose wheel is compressed, to improve yaw stability on the ground. SFI AJS 37 del 1, kap 1, flik 12, sida 15 (page 199 in my PDF).

 

edit: looking around though I think the advice to not reverse on GSA is indeed correct - you need the yaw dampening, but it's not connected to the nosewheel.

Edited January 28, 2017 by renhanxue

[AJaromir]

I use MFG Crosswind rudder pedals. With even supersmall input the yaw is very unstable.

[Steve Davies]

Using TM pedals and I get the same behaviour.

 

Large NWS and rudder inputs just prior rotation speed, even when I don't actually touch the pedals.

[RagnarDa]

I'm most definitely not a Viggen pilot, but... I don't think that's right, at least not on the AJ 37 (JA 37 may be different). As far as I can tell from the SFI the nosewheel steering is directly connected to the pedals via a simple servo. There are no artificial pedal forces, and the only dampening you get is via some purely hydraulic magic - the autopilot isn't involved at all. SFI AJS 37 del 1, kap 1, flik 8, sida 9 (page 124 in my PDF). You can't turn nose steering turning off either, the only way of losing it is by losing pressure in hydraulic system 1.

 

Goblin's speculation seems reasonable though - you have direct control of the nose wheel but SPAK is using the rudder to try to correct for your actions. Additionally, it's mentioned that the yaw dampening is strongly reinforced while the nose wheel is compressed, to improve yaw stability on the ground. SFI AJS 37 del 1, kap 1, flik 12, sida 15 (page 199 in my PDF).

 

edit: looking around though I think the advice to not reverse on GSA is indeed correct - you need the yaw dampening, but it's not connected to the nosewheel.

 

 

 

Although it was a while since I researched this but this is what I remember too. By "syncing" NWS and SPAK I meant maybe it should be more clever as to not fight each other.

[Rlaxoxo]

I suspect SPAK is at least part of the rudder problems. I just ran a test sat on the runway. With SPAK on, the rudder movement was behaving in a weird manner and didn't properly center when I took my feet off the pedals. With SPAK off, the rudder behaves in a normal, smooth manner.

 

Nah, Happens with SPAK OFF or ON for me

[Goblin]

I made a user curve for the rudder input, that is very weak around center, but scales to full deflection. That calmed things down.

Will be interesting to see what the steering output RagnarDa mentioned will do, as it seems like you have to keep NWS input at a minimum when rolling at high speed.

 

My curve: 0 1 3 12 32 47 60 70 80 90 100

Edited January 28, 2017 by Goblin
Curve settings

[Corrigan]

The fix Ragnar mentioned above should make the nosewheel half as sensitive (presumably linearly across the range) so that should make a pretty big difference I'd imagine.

[Goblin]

The fix Ragnar mentioned above should make the nosewheel half as sensitive (presumably linearly across the range) so that should make a pretty big difference I'd imagine.

 

Yeah, half is pretty much a big difference ;)

 

Edit: Hope that didn't sound condescending. Didn't mean it that way. :)

Edited January 28, 2017 by Goblin