fixed Pitch instability/oscillation

[Hummingbird]

Decided to test the pitch stability today in terms of the aircraft's ability to return to 1.0 G flight after a positive or negative pitch command and then releasing the stick. What I found was that after commanding positive or negative G, and then releasing/centering the stick, the aircraft will oscillate in pitch around the 1.0 G attitude that it needs to return to by up to +- 0.8 G. 

In other words if you e.g. pull 4-6 G and quickly release/center the stick (zero pitch command), the FLCS will attempt to reduce G's back down to 1.0 too rapidly by applying too much horizontal stab trailing edge down, resulting in the aircraft undershooting 1.0 G (by as much as -0.8 G, i.e. it hits 0.2 G) before getting back up to 1.0 G.

Here's a video recording, as well as track file of it occuring:

 

F16FLCS1Govercorrect.trk

 

Seems to be a FLCS command filter issue for when you decrease pitch/G commands by a big amount in either direction. 

 

Edited June 26, 2022 by Hummingbird

[BIGNEWY]

Thanks for the PM, 

we will continue to investigate, but we have not seen any evidence to support this issue currently. 

[DummyCatz]

I can't think of any hard proof right now, maybe it's necessary to ask the pilots. But I've been told by the FLCS maintainer that the pitch FLCS, especially the g-command system of an F-16 is a first-order system. The response, and in this case the g-response, of such system should follow a first-order response curve, like below:

 

There're several typical flight control designs that deploys a first-order system, such as the Jas-39. You can find the exact same flight tested g-response curve in the paper CAREFREE MANOEUVRING AND AUTOMATIC RETURN TO NORMAL FLIGHT ENVELOPE JAS 39 GRIPEN  https://www.sciencedirect.com/science/article/pii/S1474667015332274

What is shown in your video and in DCS is seems to be a second-order oscillatory response.

Just my thoughts.

Edited June 29, 2022 by DummyCatz

[NineLine]

Just an update, we have 2 SME's reviewing the FM, once we have info we will share. 

[RuskyV]

The first observation 

I have noticed a short but strong pull up will create a small oscillation where by the jet seems to then look to stabilise (understanding it looks to maintain 1g) this drags the FPM back down ever so slightly after the initial increase in AOA from pitching up. 
 

The second observation 

Is regarding pitch during take off and landing phase’s where you pull to hold a set pitch attitude,  it seem in both take off and landing cases I often find I’m holding quite a bit of back pressure to initially rotate or aero brake. The behaviour gives the impression the jet is sucking to the ground and makes balancing the jet in these transition phases of flight a bit of a "work out".

 

I have built up a fair bit of time in the viper module so I’m coming at this from a more seasoned prospective, however willing to still learn.
 

 

 

[dutchili]

First observation: well noted. The center of mass is behind the mid point, that is why 

Second observation: when the gear is down, controls are less sensitive.

[Spurts]

Second point, Center of mass is in front of the landing gear so any weight on wheels is going to pull the nose down.

[coolneko]

4 hours ago, RuskyV said:

The first observation 

I have noticed a short but strong pull up will create a small oscillation where by the jet seems to then look to stabilise (understanding it looks to maintain 1g) this drags the FPM back down ever so slightly after the initial increase in AOA from pitching up. 
 

The second observation 

Is regarding pitch during take off and landing phase’s where you pull to hold a set pitch attitude,  it seem in both take off and landing cases I often find I’m holding quite a bit of back pressure to initially rotate or aero brake. The behaviour gives the impression the jet is sucking to the ground and makes balancing the jet in these transition phases of flight a bit of a "work out".

I think the current implementation of the FLCS is overshooting / undershooting 1g when you let go of the stick after commanding g. Even a small amount of force will have the FLCS drop to 0.8 or so g as it tries to find 1g. Pulling back hard and then letting go I've seen it go down to 0.5 g then oscillate back to 1g. Not sure if this is correct, but in the other viper sim no matter how ham fisted you are with the stick the fpm doesn't wobble around and the FLCS smoothly transitions to 1g.

 

I may be wrong but I think during landing and takeoff (maybe just weight on wheels?) the FLCS blends commanding g and pitch with stick input, instead of just commanding g. That's why it takes a bit more pressure to point the nose up.

[RuskyV]

53 minutes ago, coolneko said:

I think the current implementation of the FLCS is overshooting / undershooting 1g when you let go of the stick after commanding g

This is how it feels to me but I’m not qualified to say it’s right or wrong so I’m treading a little carefully, I’ve also noticed the same tendency in extreme cases of +g, maybe you see the same?

2 hours ago, Spurts said:

Second point, Center of mass is in front of the landing gear so any weight on wheels is going to pull the nose down.

I can see how this would be more prominent on take off as you have full fuel onboard, but when landing shouldn’t the behaviour be slightly less based on a lighter fuel load? I know the tank cross feed is controlled automatically balancing out fuel, however under a certain weight is there any manual intervention needed? I can’t remember where I read it but somewhere in a manual it said there might be times where this might need to be adjusted manually.

just looking to expand my understanding.

[DummyCatz]

5 hours ago, RuskyV said:

The first observation 

I have noticed a short but strong pull up will create a small oscillation where by the jet seems to then look to stabilise (understanding it looks to maintain 1g) this drags the FPM back down ever so slightly after the initial increase in AOA from pitching up. 

It's a matter of whether the g-response shaped by FLCS should follow a first-order response curve or a second-order response curve. Explained here:

 

[Spurts]

7 minutes ago, RuskyV said:

 

I can see how this would be more prominent on take off as you have full fuel onboard, but when landing shouldn’t the behaviour be slightly less based on a lighter fuel load? I know the tank cross feed is controlled automatically balancing out fuel, however under a certain weight is there any manual intervention needed? I can’t remember where I read it but somewhere in a manual it said there might be times where this might need to be adjusted manually.

just looking to expand my understanding.

when you are landing it takes very little pull to get the nose up for braking initially.  As you slow down and lift decreases you get more weight on wheels causing the nose down forces to increase while also decreasing the effectiveness of the horizontal tail in holding the nose up.  These two effects cause an increase in pull needed to hold the nose up and get stronger as you slow more and more.

[DummyCatz]

5 hours ago, RuskyV said:

The second observation 

Is regarding pitch during take off and landing phase’s where you pull to hold a set pitch attitude,  it seem in both take off and landing cases I often find I’m holding quite a bit of back pressure to initially rotate or aero brake. The behaviour gives the impression the jet is sucking to the ground and makes balancing the jet in these transition phases of flight a bit of a "work out".

 

I have built up a fair bit of time in the viper module so I’m coming at this from a more seasoned prospective, however willing to still learn.

In take off and landing gains the FLCS is a pitch-rate command system below 10 degs AOA, and will hold zero pitch-rate hands off. When above 10 deg AOA, the FLCS tries to push the nose down as the AOA feedback takes part in, making the system a blended pitch-rate and AOA command system. There's some nice info over at the f-16.net forum: https://www.f-16.net/forum/viewtopic.php?f=21&t=53703

Edited September 1, 2022 by DummyCatz