Roll trim with asymmetric loadout in flight

[Rmnsvn]

I know it has been brought up a couple of times but I am just looking for confirmation. 

It seems very odd to me that on a FBW airplane like F16 we have to trim for bomb dropped from one wing. I've spoken to an ex block 52 pilot from Polish air force and he told me that when you drop a bomb the aircraft slightly drops wing to the heavy side and corrects itself in a few moments. Same behavior is observed in the other sim I'm being told.

So, do we expect some changes to FCLS any time soon to eliminate the need to retrim when dropping bombs or it is "correct as is" now?

Thanks in advance!

[buceador]

It seems odd to me too.  The "real" pilot would be left struggling for control in an already tense combat situation.

[Kilo]

I'd guess it depends on the F-16 version. The one we have in game does not adjust for asymmetric loadouts.

[Iron Sights]

I agree, one would think the flight control computer would compensate for this. The F-16 seems be affected the worst by an asymmetrical load out. It’s one of those things I’m not sure about when comes to the real aircraft, but maybe it’s there to add an extra challenge.

[Frederf]

I know of no version of FLCS which does this. If it always compensated for roll why would there be roll trim?

[Theodore42]

5 minutes ago, Frederf said:

I know of no version of FLCS which does this. If it always compensated for roll why would there be roll trim?

I assume in case something breaks or the aircraft gets shot up asymmetrically. But I don't remember ever having read something suggesting one or the other.

I recall reading that pilots don't use the rudder trim in the F-16.... and that pilots don't use the roll trim to correct for wind... And I also remember reading that tactically bombs are dropped from airplanes in pairs because they're still airplanes and weight imbalances are bad no matter how you trim.

On the one had I'm skeptical of information that is only coming from a Polish pilot and a modded video game form the '90s. But I doubt a real F-16 would fall out of the sky if you flipped on the autopilot in an asymmetrical state. So I'll at least assume the F-16 COULD trim for asymmetry if it was programmed to.

[Frederf]

Rudder trim is used. One can see what the airplane could have been built to do I guess by engaging the attitude autopilot. That holds a particular bank angle.

[mvsgas]

On 10/6/2019 at 6:52 AM, mvsgas said:

 

In the RL F-16, it is normal to have to trim. Here you can see the pilot trimming the rudder at 2:08

 

Not sure if it is BUG or not within DCS since I am not sure if it is modeled or not.

 

[Rmnsvn]

Thank you all for your opinions! 
It would be interesting to see some reference to one or the other but I’m struggling finding any. 
Thanks anyway!

[Deano87]

Asking on F-16.net might be good?

Edited February 26, 2022 by Deano87

[rob10]

18 hours ago, Rmnsvn said:

It seems very odd to me that on a FBW airplane like F16 we have to trim for bomb dropped from one wing. I've spoken to an ex block 52 pilot from Polish air force and he told me that when you drop a bomb the aircraft slightly drops wing to the heavy side and corrects itself in a few moments. Same behavior is observed in the other sim I'm being told.

If you're saying that you don't have to trim in the other sim, that's not true.  When you release a single bomb off one side in the other sim (the one that starts with "B") you definitely have to trim.  This is also true with the F-18 in DCS.  There was at least one other thread somewhere here with some explanation/discussion on why this isn't easily made automated.

Edited February 27, 2022 by rob10

[Frederf]

I've looked at the FLCS control law branch for the roll channel and my (limited) understanding is that it's basically an N-order differential equation with competing rates. So an upset moment say from an asymmetrical store config would be effectively dampened but not eliminated. E.g. as the roll rate gets smaller so does the gain so the roll rate is less than if the ailerons/tailerons were simply bolted neutral but not hard zero like a PID-controlled autopilot hold. So bomb falls off, roll acceleration is high and with some lag the counter rate blends in slowing it but long term it's still going to spiral until some trim is added. That's my rough understanding.

[Dragon1-1]

Unless "the other sim" means Freefalcon or the original 4.0 (where asymmetric load was not modeled, and the trim hat was so useless it was relegated to controlling the view in pre-TrackIR days), it definitely does require roll trim upon asymmetric weapons release. In fact, even flying straight with a TGP and nothing on the other cheek requires a minimum of trim in the Viper. Knobs are best for that. Now, you'd usually release in pairs to avoid that, but as a general rule, you trim the Viper in roll more than you do in pitch. Roll FLCS in the F-16 is actually pretty dumb, it doesn't do nearly as much as pitch does.

[Theodore42]

On 2/26/2022 at 1:15 PM, mvsgas said:

 

What I see in the video is a Viper pilot take off, do some cool maneuvers, and as soon as he gets straight and level his aircraft has a constant left roll input. He then adjusts his yaw trim and the Viper settles straight and level. I think the knob just wasn't set exactly correctly.

In what I read the only reason given to trim yaw is due to roll mistrim (roll trim is broken). Also since my post I read the two reasons given to use roll trim is for asymmetry and rudder mistrim. My source is the you-know-what manual.

Also, totally BALLER catch on an F-16 pilot adjusting rudder trim. Massive congrats to whoever noticed that first!

EDIT: Rudder trim is also used for asymmetric loadouts "if flight conditions change." I'll let ED figure out how to implement that.

Edited February 27, 2022 by Theodore42

[TheBigTatanka]

There's a form in one of the Greek manuals that shows you how much rudder trim to input for asymmetrical loads for takeoff. One day, when the flight model is finished, a coding wizard is going to make us a program to help us figure that rudder trim pre-takeoff.

Sent from my Pixel 3 XL using Tapatalk

[LJQCN101]

On 2/26/2022 at 6:08 PM, Rmnsvn said:

I've spoken to an ex block 52 pilot from Polish air force and he told me that when you drop a bomb the aircraft slightly drops wing to the heavy side and corrects itself in a few moments.

I would use caution when dealing with pilot quotes, as we always cross-validate our SME's comments against other sources and other SMEs, and check if they're talking about the same version of the system.

But some fundamental designs cannot change easily, such as the g-command system of the pitch FLCS (cruise gains), and the P-controller in the roll FLCS. Those doesn't change from YF-16 to F-16 block 52 in any countries based on public engineering sources (i.e. control block diagrams) and dash ones. I doubt anyone can find an example that indicates otherwise. To make changes to those fundamental designs would require a complete validation of handling qualities in the entire envelope, which is not an easy or cheap task.

One famous example is an article by an USN test pilot, who wrongly suggests that the Viper's pitch FLCS is not a g-command system (in cruise gains). Mover dismissed his article as:

 

Edited February 28, 2022 by LJQCN101

[Rmnsvn]

Alright, so trim for any asymmetry it is. Thanks all again! 

[Rakamora]

On 3/1/2022 at 3:31 PM, Rmnsvn said:

Alright, so trim for any asymmetry it is. Thanks all again! 

Roll or rudder?

[Frederf]

Both

[Rakamora]

Rgr

[Rakamora]

Did some testing with asymmetrical loadouts. First i stopped rolling action with aileron trim then I centered the ball with rudder trim. And man I gotta say she flyes alot better not slipping/skidding!

[fedaykinwolf]

For me, in the F-16 the roll trim button seems to be too sensitive. like I can never keep it flat always some roll input,

 

...I'm the furthest person from a SME, and I'm going to try more rudder trim tests this week, but it just feels like the hardest plane to trim

[ACME_WIdgets]

This is REAL pain when trying to launch 4 HARMS while in about a 30 degree climb, using HAD.  No time to trim, try to re-lock HAD target while its moving sidways - due to roll/yaw after a HARM is released.  If you don't fire the HARMs fast enough, then they all get shot down.

[Frederf]

Use the autopilot.

[Paladin1cd]

I just want them to fine tune the trim controls a bit. They are to granular right now. Gotta be.