Strange aileron behaviour when stationary with SPAK enabled

I noticed after I landed and came to a stop that the ailerons were "quivering". Turning of SPAK stops it but turning it back on the quivering returns.

Probably related to that, outer elevons move when pressing wheelbrakes, if the SPAK is on.

You can set a deadzone on your pedals in the DCS axis settings and see if there's still quivering.

Probably related to that, outer elevons move when pressing wheelbrakes, if the SPAK is on.

I'm fairly certain I wasn't pressing the wheelbrakes when I saw it.

You can set a deadzone on your pedals in the DCS axis settings and see if there's still quivering.

I don't use pedals.

I noticed after I landed and came to a stop that the ailerons were "quivering". Turning of SPAK stops it but turning it back on the quivering returns.

Same here. Tried different settings and also deadzones. Same behaviour when SPAK is on...

When airspeed is zero SPAK will command full deflection to try to compensate for any disturbance or stick movement, as the gains are effectively set to infinity.

When airspeed is zero SPAK will command full deflection to try to compensate for any disturbance or stick movement, as the gains are effectively set to infinity.

Where did you read this?

In any case, I don't think that's what's going on. (And my ground control authority has nothing like infinite gain).

There's this weird pulsating of the engine while at Ground Idle of a few % RPM. My elevons are moving up and down in time with that.

In any case, I don't think that's what's going on. (And my ground control authority has nothing like infinite gain).

 

There's this weird pulsating of the engine while at Ground Idle of a few % RPM. My elevons are moving up and down in time with that.

The SPAK gains change with airspeed, the very slight disturbance from the engine pulsing (also a bug that's being looked into) is enough to make the elevons visibly move because the gains are very high when stationary.

I didn't read this from a manual or anything, it's just how these control systems work.

Agree, same problem. They kinda do the same when landing

Do they move in pitch or in roll?

I've seen it in pitch at least. Outer elevons.

Ok, then I don't understand why they would move if the ac isn't moving, as far as I understand there are accelerometers and moment gyros that after been compensated for Mach nr, speed and altitude give signals to the servos of the outer elevens and the rudder.

Would really a small increase in rpm have that effect on the accelerometers?

On the other hand, if the rpm make the nose wheel strut to compress the moment gyros might give a signal and if it's so that the gain in 0 speed is great, then this could happen?

On the other hand, if the rpm make the nose wheel strut to compress the moment gyros might give a signal and if it's so that the gain in 0 speed is great, then this could happen?

Exactly.

The elevons are moving a lot because the control system knows at such low airspeeds lots of deflection is needed for any correction.

Maybe in the real plane the gains stop increasing below a certain airspeed to prevent this? It would make sense, and I believe it's what most aircraft do (ie the F-16 doesn't flail it's control surfaces when it hits a bump in the taxiway, but they do move a bit)

Which function of SPAK do you argue does this, then? It does not, in my understanding, attempt to keep the aircraft trimmed (below Mach 0.92 or something like that).

Which function of SPAK do you argue does this, then? It does not, in my understanding, attempt to keep the aircraft trimmed (below Mach 0.92 or something like that).

Accurate rate damping requires variable gains AFAIK. We'd need a dev to confirm though

Sure, but what is it damping? My impression is that it damps commanded inputs (sensed by reading stick input electronically), not just any movement of the aircraft.

Strange aileron behaviour when stationary with SPAK enabled

My first impression was as you wrote Corrigan, but I think it's more to it, it uses the gyros and accelerometers to dampen outside forces as well, to make the ac fly "clean".

The jaw gyro is one example, it counteract the yaw instability and dampens yaw movement. When on ground the rate of degrees that the rudder moves is increased and when the nose wheel is lifted the rate of degrees return to normal.

^this is what I think, I don't know that it so.

Sure, but what is it damping? My impression is that it damps commanded inputs (sensed by reading stick input electronically), not just any movement of the aircraft.

With SPAK off, the plane will tend to oscillate in pitch and yaw after maneuvering due to the aircraft's inherent lack of stability. SPAK damps out these oscillations.

With SPAK off, the plane will tend to oscillate in pitch and yaw after maneuvering due to the aircraft's inherent lack of stability. SPAK damps out these oscillations.

Alright, fair enough, after reading the SFI once more I can kinda see how this might be that.

My first impression was as you wrote Corrigan, but I think it's more to it, it uses the gyros and accelerometers to dampen outside forces as well, to make the ac fly "clean".

 

The jaw gyro is one example, it counteract the yaw instability and dampens yaw movement. When on ground the rate of degrees that the rudder moves is increased and when the nose wheel is lifted the rate of degrees return to normal.

 

^this is what I think, I don't know that it so.

I'm pretty sure this is a bug. The logic might be fundamentally sound, but having watched my aircraft for a few minutes, I really don't think the elevons should be deflecting maximally up and down at random in ground idle.

I don't think it's correct as it is right now either, but, well... looking at video clips of Gustav 52 the outer elevons do seem kinda twitchy on the ground, at times. Just not to this extent, not at all.

Strange aileron behaviour when stationary with SPAK enabled

I don't think it's correct as it is right now either, but, well... looking at video clips of Gustav 52 the outer elevons do seem kinda twitchy on the ground, at times. Just not to this extent, not at all.

Yes, I watched a video with G52 https://youtu.be/4qvnvv5c-Xs and when he stop at 6:38, the outer elevons do move up a bit and then down to match the inner elevons.

Where did you read this?

If it's a closed loop feedback system he is right, it will just want to go to one extreme or the other really quick, as any errors (difference between the commanded position and the actual position and the desired position) will build up exponentially, but since the control surfaces can only move at a certain speed and no more, they will oscillate over a short range of movement.

In any case, I don't think that's what's going on. (And my ground control authority has nothing like infinite gain).

 

There's this weird pulsating of the engine while at Ground Idle of a few % RPM. My elevons are moving up and down in time with that.

The gain doesn't have to be infinite, only much higher than the control system can respond to, and it will oscillate.

Strange aileron behaviour when stationary with SPAK enabled

Once the Viggen has tasted flight, it desperately tries to go back; flapping its ailerons to once again enter the heavens.