Pitch/Roll Control Problem

[Tiger-II]

Hi,

 

Before I get flamed, no this is NOT my flight controls!

 

I'll try to describe what is happening:

 

Pitch and roll channels exhibit the same behavior in flight.

 

I'll make a steady control input (e.g. 1/4 deflection left) which should yield a constant* roll rate.

 

What is actually happening is the roll rate is developing, then the ailerons are instantaneously returning to neutral, then they are moving back to the stick position.

 

Rinse and repeat the entire flight. Same for the pitch channel.

 

This aircraft has FBW only in pitch, so why the ailerons are acting weird, I don't know.

 

It is also occurring in pitch, and you can see it as a developing pitch rate that inexplicably stops/starts the entire time you hold a stick input.

 

After landing, the elevators were wobbling around and slightly nose-down position despite the aircraft being stationary.

 

This makes it nearly impossible to fly smoothly, and forget dog fighting. AAR was nearly impossible until I realized there was a parasitic control input somewhere causing the aircraft to fly unstable.

 

This is a pretty serious problem and prevents accurate/precise flying.

 

* assuming all things remain equal.

Edited February 20, 2021 by Tiger-II

[LJQCN101]

This is pretty normal behavior.

 

The roll axis uses a mixture of CAS and mechanical control. The CAS translates stick input to roll rate command. Roll rate feedback is included to make sure a fixed stick position equals to a certain roll rate. (But it's not perfect since it's just proportional control). This will result in a relatively linear roll rate response as you deflect the stick.

 

There're three stages that the CAS is working to provide such an roll rate response:

 

1. When you initially deflect the stick, the current roll rate of the aircraft is much lower than the CAS commanded value, so CAS deflects the aileron (up to 5 degrees max limit). At the same time the mechanical control deflects the aileron too, so what you'll see initially is that the aileron deflects a lot more than you stick asks.

 

2. Considering roll inertia, roll rate sensor delay, and the limited authority of CAS, the roll rate response of the aircraft tends to overshoot when roll rate is rapidly reaching the CAS commanded value. When this happens, the CAS tends to deflect the aileron in the opposite direction to stop the roll rate from further increasing, hence what you saw. (The ailerons returning to neutral.)

 

3. Since the roll axis is statically stable, you'll need positive aileron deflection to hold the roll rate. When the current roll rate of the aircraft reached CAS commanded value, the CAS commands aileron to move back to hold the roll rate.

Edited February 21, 2021 by LJQCN101

[LJQCN101]

The pitch axis is statically unstable in certain AOA regions and subsonic regions. Your stick commands G or load factor in up and away flight, so what you'll see is that the elevator initially deflects to increase load factor. When approaching the commanded load factor, the elevator just returns to neutral to hold the load factor. This behavior is shown in the FBW demonstration video: 

 

 

After landing, the elevators were wobbling around because the AOA and pitch-rate feedback is still active. It wobbles as in real life, but not as crazily as in DCS F-16.

Edited February 21, 2021 by LJQCN101

[Tiger-II]

Hi,

 

Thanks for the detailed reply.

 

The problem is that the roll and pitch rates are highly unstable - you can't roll or pitch continuously without the roll and pitch varying to such an extent it upsets the steady-state flight path.

 

To put it another way - a stable +3.0 g pull becomes 3.0 g +/- 0.5 g, or a constant roll rate of 30 deg/sec, becomes 30 deg/sec +/- 10 deg/sec.

 

Make sense?

 

This is not correct, and it didn't do this previously.

[LJQCN101]

Could you upload a short video illustrating the issue, with control indicator (ctrl+enter) turned on? I'd like to know the flight conditions you were in.

[RichardG]

Something is wrong with this after last update. Roll and pitch are spiking. Its the plane/module, not the control interface. 

[LJQCN101]

I haven't been able to reproduce and nothing changed regarding FCS. Are you flying through wake turbulence of other aircraft?

 

(Or if the cockpit is too hot or too cold, the virtual stick would jerk by itself indicating the pilot is uncomfortable.)

Edited March 8, 2021 by LJQCN101

[Alesia_Aisela]

This really sounds like the behavior I see when the pilot is to hot/cold.  The major visual cue is that the stick will shake/tremble.  If the pilot is to cold you can hear faint teeth chattering as well.