fixed FLCS caused backflips during pitch-down motions in a negative AOA out-of-control departure

[DummyCatz]

Hi,

When flying with my squadron these days I noticed yet another newly introduced bug that is not related to inertia coupling or the yaw rate limiter. It seems like a pure pitch FLCS issue and not an aerodynamic issue.

Description of situation:  After the aircraft is departed from controlled flight, the FLCS tends to command pitch-down by driving the horizontal stab to the trailing edge down position, during pitch-down motion in negative AOA. This is quite weird since the FLCS is supposed to command nose-up in the case of a pitch-down motion, as it is receiving a negative pitch rate feedback. This looks like the FLCS deliberately pushes the nose down and increases the pitch rate while in the negative AOA, which could result in a backflip.

Testing Method: Use the MPO to force a departure and then return the MPO to NORM. Watch the horizontal stab reaction to pitch-rate and AOA. I've highlighted the issue with slow motion in the track file.

Expected: The FLCS should not try to deliberately increase the pitch rate but to reduce it.

F16 FLCS caused backflips.trk

[Zergburger]

i think the anti departure logic is overriding the pitchrate logic, if you manually take control of the pitch channel, you can get out of these departures in a few oscillations

[DummyCatz]

Let me make an update on my observations.

This bug actually appears when the aircraft is rapidly pitching down from a very high AOA, down to the normal AOA range of -5 to 25 degrees. Now the FLCS should resume normal flying logic, and in this situation, the FLCS should be given a negative pitch rate feedback, in that the aircraft is rapidly pitching down, and at the mean time the Nz is also less than 1g. As a result, the horizontal stab should start to counter the pitch-down motion.

But it doesn't. In the contrary, the stab keeps at the trailing edge down position and drives the aircraft to negative AOA. There's no sign that the stab wants to deflect trailing edge up during the continuous pitch-down motion.

Track is attached and it should be a lot more concise than the one in OP. This time I'm not using the MPO to force a negative AOA departure, but to use the inertia coupling maneuver to force a positive AOA overshoot.

F16 FLCS forcing nose down.trk

Edited June 12, 2022 by DummyCatz

[DummyCatz]

17 hours ago, Zergburger said:

i think the anti departure logic is overriding the pitchrate logic, if you manually take control of the pitch channel, you can get out of these departures in a few oscillations

I actually tried to reach out for some help from a FLCS maintainer who knows the inner logic or functions of the digital FLCS, he replied that there's no logic change in the pitch axis during a departure unless you held the MPO to OVRD position. The MPO override basically nulls the AOA feedback and also nulls the integrator, allowing the pilot to make full use of the stabilizer to rock the aircraft out of a deep stall. Otherwise the stabilizer would be kept at the full deflection state due to AOA feedback.

But the (washed-out) pitch rate feedback plus the Nz feedback is active full-time in cruise gains (gear up mode), regardless of MPO switch position. It is not the same pitch rate feedback for landing mode, but a washed-out one. It filters out the rate of change in pitch-rate and can be considered a derivative component.

[Zergburger]

im not sure what you mean by washed out in this context

BTW i also saw your report on the rudder pushing INTO the spin on a departure, seems like it may be hard to dissect the departure recovery issues with the possibility of two compounding bugs outstanding.

Edited June 12, 2022 by Zergburger

[DummyCatz]

8 hours ago, Zergburger said:

im not sure what you mean by washed out in this context

It’s a term in the control system: https://mathworks.com/help/physmod/sps/ref/washoutdiscreteorcontinuous.html

https://aviation.stackexchange.com/questions/51106/what-is-an-autopilot-washout-filter

 

8 hours ago, Zergburger said:

BTW i also saw your report on the rudder pushing INTO the spin on a departure, seems like it may be hard to dissect the departure recovery issues with the possibility of two compounding bugs outstanding.

They are not quite compounded. One is related to the rudder reaction to yaw rate, and the other is related to the stabilizer reaction to pitch rate.

Edited June 13, 2022 by DummyCatz

[Frederf]

I can think of no reason why during negative pitch rate and negative AOA beyond limits the stab should be trailing edge down.

[DummyCatz]

37 minutes ago, Frederf said:

I can think of no reason why during negative pitch rate and negative AOA beyond limits the stab should be trailing edge down.

Exactly.

[Akiazusa]

Hi

I find some clear evidence from the HAF F-16 manual(T.O. GR1F16CJ1) ,

it's at ----Section 6 Flight Characteristics----page 6-9------PITCH DEPARTURE--------Paragraph 3.

An inverted pitch departure happens at negative AOAs when the AOA significantly exceeds the negative g limiter.During the departure, the pitch axis of the FLCS commands the horizontal tails to full trailing edge UP to try to return AOA to the normal.Pitch stick commands are ineffective unless the MPO switch is used.

 

The descriptions about the negative g limiter are in the same manual,Section 1-------page 1-121---------AOA/G Limiter-----Paragraph 2.

Also descriptions about the Yaw Rate Limiter is on the same page.

 

Hope it can help.

Edited June 13, 2022 by Akiazusa

[Hummingbird]

Not sure why this is listed as "correct as is" ? As the post above says, the FLCS will be commanding the horizontal stabs full trailing edge UP, not down as is currently happening.

 

[DummyCatz]

Thanks Akiazusa san, indeed there’s some nice info in there.

 

To ED,

The post-stall characteristic and the FLCS interaction is a bit tricky by nature, so take your time. I only issue bug reports with the hope of a better Viper module as we all did, so don’t b intimidated by all the issues here. Keep up the good work. Regards.

[BIGNEWY]

I have made a report for the team to check.