No complaints, technical question!! Why does the F16 trim for Pitch but not for roll?

[Rhinozherous]

Hello!

As the title says, I am just curious why the F16 is able to hold 1G in terms of pitch, but it is not able to hold a commanded roll rate or bank angle? When you release a heavy weapon you have to trim left or right. When you have a TGP mounted you have to counter trim... why is the FLCS not made to counter assymetric loads or winds automaticly? It is able to hold 1G load, why not zero bank or roll?

Again, this is no complaint - I am just curious what the technical reason is behind this!

Thanks!

[Hawkeye91]

40 minutes ago, Rhinozherous said:

Hello!

As the title says, I am just curious why the F16 is able to hold 1G in terms of pitch, but it is not able to hold a commanded roll rate or bank angle? When you release a heavy weapon you have to trim left or right. When you have a TGP mounted you have to counter trim... why is the FLCS not made to counter assymetric loads or winds automaticly? It is able to hold 1G load, why not zero bank or roll?

Again, this is no complaint - I am just curious what the technical reason is behind this!

Thanks!

My guess would be there’s only a certain amount of control authority the FBW is allowed to use to maintain the current aircraft attitude. 

[Vortex225]

This video should have everything you need. 

https://youtu.be/l9NOHSWK-Q4

[Dragon1-1]

How about an answer that doesn't take 12 minutes to watch? Dunno why would it be marked as solution, I wasn't able to find the answer skipping through the video.

For the record, the reason is that the Viper's FLCS in roll is actually pretty dumb. AFAIK, it was decided not to overcomplicate things when the Viper was being developed, the designers had no idea if the relaxed stability concept will pan out, and as such, made it as simple as possible. The pitch component was a really big deal, due to relaxed stability, but roll wasn't unstable, so they didn't bother implementing a system to keep it in check.

Here, and it took much less than 12 minutes. Reading>videos.

[Frederf]

It's an interesting question which probably takes one of the original YF-16 FLCS engineer(s) or at least someone with experience in control theory. My guess is that it screwed with normal flight behavior or made some unwanted oscillatory behavior when paired with the sensor/processing/space limitations of the time. How does the F-35 FCS work? Does it incorporate this philosophy?

[Rhinozherous]

13 hours ago, Dragon1-1 said:

How about an answer that doesn't take 12 minutes to watch? Dunno why would it be marked as solution, I wasn't able to find the answer skipping through the video.

For the record, the reason is that the Viper's FLCS in roll is actually pretty dumb. AFAIK, it was decided not to overcomplicate things when the Viper was being developed, the designers had no idea if the relaxed stability concept will pan out, and as such, made it as simple as possible. The pitch component was a really big deal, due to relaxed stability, but roll wasn't unstable, so they didn't bother implementing a system to keep it in check.

Here, and it took much less than 12 minutes. Reading>videos.

Thank you very much! 

Edited July 9, 2022 by Rhinozherous

[Vortex225]

20 hours ago, Dragon1-1 said:

How about an answer that doesn't take 12 minutes to watch? Dunno why would it be marked as solution, I wasn't able to find the answer skipping through the video.

For the record, the reason is that the Viper's FLCS in roll is actually pretty dumb. AFAIK, it was decided not to overcomplicate things when the Viper was being developed, the designers had no idea if the relaxed stability concept will pan out, and as such, made it as simple as possible. The pitch component was a really big deal, due to relaxed stability, but roll wasn't unstable, so they didn't bother implementing a system to keep it in check.

Here, and it took much less than 12 minutes. Reading>videos.

Wow, wasn't expecting to find that kind of outrage here. Unfortunately, didn't have time for a longer response. I thought his video did a good job of describing pitch and roll control schemes. 

My deepest apologies for not meeting your expectations. Perhaps try to be more civil in your future online interactions. 

Edited July 9, 2022 by Vortex225

[Dragon1-1]

You were rude first, by dropping an off-topic video with a false assertion it answers the question. I'll be more civil when people start begin writing actual answers out instead of dropping a link that wastes 12 minutes of my time on not actually answering the question. FYI, the OP didn't ask for a description of the control schemes. He asked for an explanation for why they are like they are. This video did not answer the question.

I consider posting a video link without explanation rude because it wastes everyone's time, doubly so if it's not actually relevant to the thread. People are looking for a straight answer, not 12 minutes of rambling. A video is a source or an illustration, never the answer (unless the question specifically asks for a video). If the answer is in the video, type out the good bits for those who don't want to watch, or at least have the decency to link to the precise moment in which the answer is said. If you can't find such a moment, close the video and forget about it.

[Frederf]

Explaining how the F-16 works does not explain why it wasn't designed in a different way.

[Cab]

14 minutes ago, Dragon1-1 said:

You were rude first, by dropping an off-topic video with a false assertion it answers the question. I'll be more civil when people start begin writing actual answers out instead of dropping a link that wastes 12 minutes of my time on not actually answering the question. FYI, the OP didn't ask for a description of the control schemes. He asked for an explanation for why they are like they are. This video did not answer the question.

I consider posting a video link without explanation rude because it wastes everyone's time, doubly so if it's not actually relevant to the thread. People are looking for a straight answer, not 12 minutes of rambling. A video is a source or an illustration, never the answer (unless the question specifically asks for a video). If the answer is in the video, type out the good bits for those who don't want to watch, or at least have the decency to link to the precise moment in which the answer is said. If you can't find such a moment, close the video and forget about it.

 

[Dragon1-1]

TL;DW, link to the relevant timestamp next time.  

[Fuggzy]

2 hours ago, Dragon1-1 said:

You were rude first, by dropping an off-topic video with a false assertion it answers the question. I'll be more civil when people start begin writing actual answers out instead of dropping a link that wastes 12 minutes of my time on not actually answering the question. FYI, the OP didn't ask for a description of the control schemes. He asked for an explanation for why they are like they are. This video did not answer the question.

I consider posting a video link without explanation rude because it wastes everyone's time, doubly so if it's not actually relevant to the thread. People are looking for a straight answer, not 12 minutes of rambling. A video is a source or an illustration, never the answer (unless the question specifically asks for a video). If the answer is in the video, type out the good bits for those who don't want to watch, or at least have the decency to link to the precise moment in which the answer is said. If you can't find such a moment, close the video and forget about it.

If it's ok with you, Dragon 1-1 sir, I'd like to plagiarize portions of this for use everywhere that I frequent, sir.

Edited July 9, 2022 by Fuggzy

[Dragon1-1]

Preach it, I hope others will openly push back against it instead of quietly thinking "not another damn video...". Forums are for writing and reading, video links are an extra. 

[toilet2000]

On 7/8/2022 at 4:26 PM, Frederf said:

It's an interesting question which probably takes one of the original YF-16 FLCS engineer(s) or at least someone with experience in control theory. My guess is that it screwed with normal flight behavior or made some unwanted oscillatory behavior when paired with the sensor/processing/space limitations of the time. How does the F-35 FCS work? Does it incorporate this philosophy?

Since the question was somewhat already answered, I'll use some of my limited control theory experience to speculate on the reasons. I would think the same as you, but I'll add more possible details:

Auto-trim means the control loop requires at least an integrator to get rid of the steady-state error (so that it actually auto-trims). Those are always used cautiously, as they can definitely deteriorate the transient response. From my limited research on the subject, the roll FLCS doesn't contain any feedback loop. This is generally done because as someone else said, the roll is stable but adding a control loop could lead to instability, especially so if for some reason there's an unforeseen delay in the control loop (phase margin) or a big perturbation (gain margin). An integrator makes it simply much more likely to happen as it removes 90 degrees of phase meaning you're already half-way there.

All this to say, it would have drastically complexified the roll control system and made it possibly unstable, whereas it was stable to begin with (which was not the case on the pitch axis). This adds a ton of complexity to the stability analysis, especially given complex maneuvering as is the case for military fighters, and the F-16 was one of the first aircraft with FBW controls. There's generally a saying in control theory that anything straying away from the natural response of a system should not be done except if needed. IMHO this is the case for the roll-axis in the F-16.

Edited July 11, 2022 by toilet2000

[alexscott292]

Interesting implications for IMC.  As the system always attempts to maintain 1g, a slow uncommanded roll will be hard to feel and the FLCS will busily maintain 1g all the way into an unrecoverable situation.

[Lee1]

Mine trims in both pitch and roll. Very necessary when you drop a 2000lb bomb from one side.

 

[Machalot]

On 7/11/2022 at 2:33 PM, Lee1 said:

Mine trims in both pitch and roll. Very necessary when you drop a 2000lb bomb from one side.

 

What do you mean yours trims in roll? Do you have a different Viper model than everyone else? 

Edited July 13, 2022 by Machalot

[Bremspropeller]

On 7/11/2022 at 9:13 PM, toilet2000 said:

From my limited research on the subject, the roll FLCS doesn't contain any feedback loop.

There is a Roll Rate Limiter in Cruise Gains whose job is keeping the aircraft going pointy end forward ("to help prevent roll-coupled departures"). It schedules away roll rate with increasing AoA, slowing airspeed or TE-down horizontal tail deflection. Roll authority is also taken away when stomping the rudder ("large total rudder deflection"). In T/O and Landing Gains, the Roll rate is reduced to a fixed value which is not dependant on AoA.

Not a feedback loop, but it is fed back and controlled.

[BuzzU]

Yes, the Viper has roll trim. Here's another long video to annoy Dragon that shows the proper way to bomb and trim in the Viper. It's not a trim and forget method.

 

 

Edited July 13, 2022 by BuzzU

[Cab]

On 7/9/2022 at 11:22 AM, Dragon1-1 said:

TL;DW, link to the relevant timestamp next time.  

It was in the original link, but I guess it didn't carry over.

Kinda ruined it, I guess. 

[toilet2000]

2 hours ago, Bremspropeller said:

There is a Roll Rate Limiter in Cruise Gains whose job is keeping the aircraft going pointy end forward ("to help prevent roll-coupled departures"). It schedules away roll rate with increasing AoA, slowing airspeed or TE-down horizontal tail deflection. Roll authority is also taken away when stomping the rudder ("large total rudder deflection"). In T/O and Landing Gains, the Roll rate is reduced to a fixed value which is not dependant on AoA.

Not a feedback loop, but it is fed back and controlled.

Thanks a lot for the info! I figured there was some control logic that would apply here, but couldn't find any relevant info after a brief search. As you said it's indeed not a "feedback loop" as would generally possibly lead to unstable behavior, even though there is some information fed back, so AFAIK my speculation seems to still be appropriate, but feel free to correct what I said if you have more info or knowledge!

 

2 hours ago, BuzzU said:

Yes, the Viper has roll trim. Here's another long video to annoy Dragon that shows the proper way to bomb and trim in the Viper. It's not a trim and forget method.

 

 

 

We're not talking about being able to trim. We're talking about auto-trim. The F-16 FLCS auto-trims in pitch, meaning you don't have to trim in pitch generally for the aircraft to fly at 1G.

[BuzzU]

It gives us something to do.

Edited July 13, 2022 by BuzzU

[Bremspropeller]

39 minutes ago, toilet2000 said:

Thanks a lot for the info! I figured there was some control logic that would apply here, but couldn't find any relevant info after a brief search. As you said it's indeed not a "feedback loop" as would generally possibly lead to unstable behavior, even though there is some information fed back, so AFAIK my speculation seems to still be appropriate, but feel free to correct what I said if you have more info or knowledge!

I'm pretty much with you on this. There's very little transparancy in "the book" as to what the nominal limits actually are.

Some more info - Cruise Gains RRL activation:

- AoA 15° and above

- airspeed below 250kts (I suppose its KCAS)

- horizontal tail more than 5° TE down

- total rudder command more than 20° (that is pilot + FLCS command, such as ARI)

- combination of hori.z tail more than 15° TE down and AoA above 22° (which is funny as both are within separate activation boundaries mentioned above)

In CAT III the max achievable roll rate is about 40% of CAT I, plus additional constraints in terms of AoA, airspeed horizontal tail position and total rudder-deflection apply (they are not disclosed).

Landing Gains gives you only abou half the roll rate of Cruise Gains - no matter of airspeed, AoA or tail position.

Above 35° AoA, roll-command is taken away from the pilot completely and antispin control-inputs will be commanded.

 

That's about all I could find.

 

 

[RuskyV]

That’s really interesting, it does highlight how the current beta version differs quite a bit in role compared to those numbers. Pretty sure we don’t currently get a 40% reduction in role for CAT3 or landing gains. .

[toilet2000]

12 hours ago, RuskyV said:

That’s really interesting, it does highlight how the current beta version differs quite a bit in role compared to those numbers. Pretty sure we don’t currently get a 40% reduction in role for CAT3 or landing gains. .

Don't forget that it's the max attainable roll rate, not the roll rate in every condition. Not sure how accurate DCS is on that part, but it's something to consider when testing that. Loading up your F-16 with stores even in CAT I will most likely lower the roll rate you can reach, meaning it's going to close the gap between CAT I and III.