Manual trim in F-16

[SnakeHandler]

Viper, no, the torgue caused by the rotating engine isn't strong enough to feel. At least to me. I'm talking about guys over-Ging and torguing the airframe.

[Raptor_claw]

The roll rate trim still seemed sensitive to asymmetric loads more than I would have thought, but the Viper was less sensitive to uncommanded roll when dropping a big egg off of one side than anything I ever flew and dropped bombs from.

Not sure this applies to what you are calling "sensitive", but...
There is a fundamental difference between the pitch (longitudinal) axis and the roll/yaw (lat/dir) axes that relates back to trim operation. The pitch axis control laws are what is called "proportional plus integral", the key word here being "integral". This just indicates there is an integrator in the design. In effect, the integrator works like an "auto-trim" system. It basically serves the same purpose as a traditional trim wheel, it's just that its driven by the FLCS (constantly) rather than directly by the pilot. Any trim that the pilot dials is just added to the command from the stick. If the pilot dials in 1g worth of trim and lets go of the stick, the FLCS sees a 2g command (the nominal 1g, plus the 1g from the trim system). It treats this exactly the same as it would a simple 2g command from the stick.
By contrast, the roll and yaw axes do not have embedded integrator elements. If there are lateral and/or directional asymetries (aerodynamic or mass), there is no embedded "auto-trim" at work within the FLCS, and the pilot has to perform that function (by using roll stick and/or yaw pedal and/or using the trim system).

OTOH, when my LEF folded up, the FLCS tried to help me and I did not go into an uncontrolled roll or yaw. I still had to trim all the way and hold fifteen or sixteen pounds of pressure, but I am fairly sure the FLCS control laws were not designed to work properly with such a drastic "modification" to the aero properties the jet and I were experiencing.

There is very limited logic for failure reconfiguration in the F-16 CLAW. (What it does have is mostly related to air data system failures). The CLAW does not know if surfaces are failed or damaged, it pretty much continues to command all surfaces as if they are working all the time. I can't say with certainty how much work was done back in the original F-16 design specifically for LEF failure modes (before my time). I can tell you that "newer" aircraft are designed to be flyable (and landable) with the LEFs in "salute mode". That is, one failed straight "up" (perpendicular to the wing, leading-edge up) and the other locked full down. (And yes, they run models in the wind-tunnel like that to get data).

[Raptor_claw]

Viper, no, the torgue caused by the rotating engine isn't strong enough to feel. At least to me.

Yeah, the rotation of the engine is a pretty insignificant effect for "normal" flying around. It can have a bit of an effect during very dynamic conditions due to the gyroscopic effect. (I'm sure everyone has done the deal with the rotating fan (or wheel, or whatever) where you can feel an unexpected force when you move it around.) Same thing with a rotating engine. For the F-16 the most notable effect is a bit of a yaw moment that is generated by the combination of engine rotation and high pitch rates. The pitch rate has to be pretty high for the yaw to amount to anything at all, so you'd be most likely to see it during pitch rocking, in which case it would almost always be swamped by other, stronger moments (aerodynamic, inertial coupling, etc).
That's a long way of saying, no, it's not really a factor.

[Obi_Offiah]

Not sure this applies to what you are calling "sensitive", but...
There is a fundamental difference between the pitch (longitudinal) axis and the roll/yaw (lat/dir) axes that relates back to trim operation. The pitch axis control laws are what is called "proportional plus integral", the key word here being "integral". This just indicates there is an integrator in the design. In effect, the integrator works like an "auto-trim" system. It basically serves the same purpose as a traditional trim wheel, it's just that its driven by the FLCS (constantly) rather than directly by the pilot. Any trim that the pilot dials is just added to the command from the stick. If the pilot dials in 1g worth of trim and lets go of the stick, the FLCS sees a 2g command (the nominal 1g, plus the 1g from the trim system). It treats this exactly the same as it would a simple 2g command from the stick.
By contrast, the roll and yaw axes do not have embedded integrator elements. If there are lateral and/or directional asymetries (aerodynamic or mass), there is no embedded "auto-trim" at work within the FLCS, and the pilot has to perform that function (by using roll stick and/or yaw pedal and/or using the trim system).

I had a discussion about this sometime ago with Johnwill. Its really interesting because while the roll channel lacks an integrator, it still is a closed-loop system, it will somehow manage to compensate for turbulence and will 'check' a roll as soon as the pilot stops applying lateral force. Its funny because despite not having an integrator it is still a feedback system, so the lack of an integrator somehow prevents it from 'seeing/feeling' the lateral imbalance.

Obi

[johnwill]

Obi,
Raptor claw is much more knowledgable about the FCS, so believe what he says. My knowledge is mostly about interaction between FCS and the structure.

Raptor Claw,
An example of the FCS not knowing if a surface failed occured during F-16XL No. 1 flight test. During a full stick roll at .8/20k, the right elevon horn failed. After everything settled down, the airplane flew normally with no flight control or hydraulic warning lights, Since the actuator was still strokeing normally, everything appeared normal.

There was some co-lateral damage - vertical tail tip over the rudder was lost, as was the right AIM-9 and half the launcher.

[Obi_Offiah]

Obi,
Raptor claw is much more knowledgable about the FCS, so believe what he says. My knowledge is mostly about interaction between FCS and the structure.

Hi John, I do believe what Raptor claw posted, I just find the whole roll thing a little confusing.

BTW, the new business jet from Dassault, the Falcon 7x (the first FBW biz jet apparently) does have an integrator in the roll channel and auto-trims for roll. I wonder how Dassault managed to maintain the roll response?

Obi

[MVSGas]

Folks have to understand that this machine "ain't like any machine" you ever flew before unless you came from Nighthawks, Raptors, Lightning Deuces, the Space Shuttle, and prolly some stuff at Groom Lake.

Hey Gums, do you really believe the F-16 FBW is that revolutionary anymore? I mean, when it was first introduce yeah but, now a day?
With anything from B-2 to SU-35 (the new one not the old SU-35) I thought the same FLCC/DFLCC set up, or similar set up is more common now, or at least I think so. Does the supper bug still have the cable back up?
Most of this aircraft must have similar trim set up, or so I assume.

[Gums]

Trim and control laws for the Viper

Salute

I am not gonna get into a pissing contest with Raptor Claw.

I have the basic documents that GD provided us when we had to check out the first pilots. A lotta stuff we used was verbal word-of-mouth from the Test folks, but I had the responsibility to use something "better". So I worked with the GD folks best I could at the time - say seventy-nine and early eighty.

I may try to get these things on the net in PDF format, and I am sure John-boy could cash in some green stamps with folks at the factory in Ft Worth to do the same.

I do not know what the digital folks did to convert from an electronic analog system. But I do know many of the "functions" that were implemented in the analog system I flew for about five years.

Try looking up Laplace transforms.

http://en.wikipedia.org/wiki/Laplace_transform

When the Earth was still cooling, we had to use the original op-amps to do this stuff, GASP. They were the size of shoe-boxes. By the late seventies, they were "chips".

The original YF and production models used those "hard-wired" doofers to connect our stick and trim to the control surfaces. And you know what? The stuff worked.

There were more op-amps in the pitch rate part of the system than anything.

There were also plenty of "integrator" modules for roll, besides pitch, but not as many.

The pitch functions used eight gees ['normal' with respect to the longitudinal axis] for max command on the jet, not nine. So flying along with zero trim was a one gee feel on your butt. Pull full back and you commanded eight gees from FLCS, but felt nine gees. BFD.

I agree about the "trim wheel" adding to the pilot command. When I did my demo's for the students, we started at the basic command, then rolled the wheel all the way back, with no stick input.

When I talk about asymmetric loads, I only have about two thousand five hundred hours of carrying stuff on the wings and dropping the stuff before I flew the Viper. The Viper did not react like all those other planes when we dropped stuff. It was very stable in roll, and we did, in fact, find the need to use stick and trim when pulling off tgt.

I am glad to hear that FLCS programmers are implementing stuff to account for battle damage and folded-up LEF's.

My LEF failure did not "advise" FLCS. The FLCS just tried to maintain my trimmed condition, which was zero roll rate. It saved my skinny butt, I am here to tell you. So I trimmed full aileron the opposite way without thinking about it. Was way I flew other planes for twenty years. I also cranked in some rudder trim, but found out later that it would have bad results once I had a higher AoA in the flare. BFD..It worked for me.

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I am not independently wealthy, but with a little help I think we could get some of these old GD pubs converted to PDF and put them here on the best damned aviation website there is.

I am reluctant to send them anywhere, as they are treasures for me, personally. But there has to be some more out there, huh John-boy?

Gums sends ...

[_Viper_]

Does that mean no matter where the F-16's nose is pointing or even if the plane is upside-down and/or descending the FLCS tries to keep it going forward

No, I mean forward as in "into the wind", 1g, no roll rate. But Gums and Snakehandler are much better equipped to explain it.

Ok then maybe I should wait their opinions and not to ask anything stupid before that ;P

[johnwill]

Gums, I'd love to see all the old pubs here, so long as it's legal. Maybe someone in your neighborhood has a scanner. If you can scan them, email them to me, and I'll convert them to .pdf and post them on F-16.net.

I can check with some of the kids still toiling away if there are specific docs you think others would like to see. I have an FCS block diagram (Blk 30) but not sure it is legal to post.

I heard about your LEF problem right after it happened. Everyone at GD greatly admired your airmanship. Two questions I'd like to ask, what broke and why didn't the asymmetry brake lock the LEF?

[Guysmiley]

I heard about your LEF problem right after it happened. Everyone at GD greatly admired your airmanship. Two questions I'd like to ask, what broke and why didn't the asymmetry brake lock the LEF?

Gums actually did an interview with this site where it was discussed, great reading! http://www.f-16.net/interviews_article28.html. I don't know enough about the construction, if the torque tube slips completely off the drive motor's splines will it pivot freely?

[johnwill]

Thanks for the information. There is a system called the asymmetry brake which monitors the position of the left and right LEF, and if it's working right it will lock both surfaces if their positions differ by 6 degrees. That was the basis for my question to Gums about the asymmetry brake. If the brake fails, the LEF can rotate freely if that coupling slips off the motor splines, except for friction and drag in the powered hinges.

[Raptor_claw]

I am not gonna get into a pissing contest with Raptor Claw.

I guess I'm unsure what to make of that comment, if anything. It certainly wasn't my intent to be in any way confrontational, or to imply that you were incorrect in any part of your post(s). If I did, I apologize.

Moving on...

I do not know what the digital folks did to convert from an electronic analog system. But I do know many of the "functions" that were implemented in the analog system I flew for about five years.

You've heard the old joke "How do I get to Carnegie hall?". Well, the conversion to the digital system had a lot of the same: practice, practice, practice. It was a painstaking process and involved more that a little trial and error. The target for those first conversions was simply to make it fly exactly the same as analog. No improvements or added capability (at first) - just make it match. Obviously that goal was never completely met, but it got pretty close.

When the Earth was still cooling, we had to use the original op-amps to do this stuff, GASP. They were the size of shoe-boxes. By the late seventies, they were "chips".

Ever get a chance to see the B-58 computer? (I can't even remember what they called it, exactly.). I've never seen one myself, but I heard alot about it. Big ol box full of gears, pulleys, etc.

There were also plenty of "integrator" modules for roll, besides pitch, but not as many.

While I'm not familiar with the in's and out's of the analog implementation (i.e. the hardware stuff) this does make sense. While there are no classic integrators (that is, 1/s elements) in the roll or yaw logic, there are plenty of dynamic elements. There is a lag filter on the roll stick path, there's a lead-lag filter in yaw, there are high frequency (structural coupling) filters, etc. I assume the integrator modules you refer to are parts of the hardware buildup to create those filters.

I agree about the "trim wheel" adding to the pilot command. When I did my demo's for the students, we started at the basic command, then rolled the wheel all the way back, with no stick input.

As I recall the authority limit on the trim path is something like +/-2.4 G's, which would be incremental. So, full throw should put you at about +3.5 or -1.5 G total (with no stick input).

...asymmetric loads... The Viper did not react like all those other planes when we dropped stuff. It was very stable in roll, and we did, in fact, find the need to use stick and trim when pulling off tgt.

That makes sense. The roll axis is tuned pretty tightly. Part of the benefit of that lag on the roll stick I mentioned. It allows the inner loop (disturbance rejection) gains to be a little higher. Also, the need for inputs when pulling off tgt is to be expected. (I'm sure you're familiar with the phenomenon, but others may not have thought about it). Let's say you've got an unbalanced 2000 lb store out on a wing, let's say 6 feet from the a/c centerline. At 1g that would mean the aircraft would have to create 12000 ft-lbs of rolling moment to balance. Now let's say you load up to 6g's - that store effectively "weighs" 12000 lbs, and the moment needed to balance goes up 6x as well. However much roll surface was in before - you're gonna need more. The design for the "newer" fighter has some logic to account for that change. Whatever trim the pilot has in is automatically scaled up or down as load factor changes to attempt to balance the effect. It's not a perfect system, of course, but it definitely does help.

[Raptor_claw]

Hi John, I do believe what Raptor claw posted, I just find the whole roll thing a little confusing.

BTW, the new business jet from Dassault, the Falcon 7x (the first FBW biz jet apparently) does have an integrator in the roll channel and auto-trims for roll. I wonder how Dassault managed to maintain the roll response?

Obi

Not sure I can address your confusion, exactly. It is just a classical closed loop system with gains designed for a desired roll mode.
The one bit of an oddity with the F-16 is the lag filter on the roll command path. It uses one time constant for commands "coming in" and a different (faster) one for commands "going out" (stick releases or reversals). That implementation is definitely a player in stopping rolls quickly (i.e. containing bank angle overshoots.) (Apparently, the analog hardware implementation for that bit of logic was a real challenge.)
As to the 7x, that is a bit curious. It may be that it's more of an outer-loop kind of function, more like an autopilot. Or, maybe they are just taking the hit on getting a classical response to pilot inputs. Hard to tell.

[Gums]

FLCS control laws and trim

Yeah, good points Rap.

I am gonna spend the bucks and we'll all have some of these historic documents from the seventies for all to read.

Tap, tap, tap ....... janitors? I'll pay the bucks and send them to someplace or other via FTP or whatever when I get them done.

The analog flow diagram and the text explanation is just exactly what you describe, Rap. A lag for inputs from the stick, and a lot less lag when releasing pressure on the stick.

From a pilot viewpoint, this seems good for "tracking" when using the gun. A slower input correction, but a quick reaction when you get the pipper where you want it. Let Snake-deuce help here.

Even with the digital FLCS computer, the operation of the ISA's comes into play. My flow charts show limits to the max commands to the ISA's that are based upon physical movement, and seem to be related to the physical capabilities of the ISA's and actual control surfaces.

From what Rap is describing, it looks like the FLCS software dweebs modeled the analog functions and control laws best they could.

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When I talk about dropping eggs from one wing and not the other, I am talking about using aileron and rudder for the pullout. The extra weight on the opposite wing certainly requires some aileron and rudder. This is especially true for bent wings versus straight wings that I had in the Dragonfly. The gees make that asymmetrical loading exactly as Rap describes.

The Viper roll command laws make this easier, as they are trying to keep the commanded roll rate what you had trimmed for. Nevertheless, they had their limits, so some opposite stick was required once dropping and pulling.

The older jets had the effect of more weight on one wing, plus the drag of that other bomb. So we had to use rudder and aileron to get a decent pull after dropping. The more gees we pulled made the problem worse. So the secret was to use lottsa rudder and aileron while getting the nose up, then relax holding back stick. Keeping the stick back made things worse. If you reached the aero limits of your jet, then you could not keep from rolling into the "heavy" wing.

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For all the non-pilots here, look at the Birds versus the Bugs at an airshow.

The Viper does a roll and flat-ass "parks". The Bugs have a slight "wobble" when the jet tries to stop rolling. Some of this is due to the big control stick versus the side-stick the Viper has. Some is related to momentum that the pilot is trying to compensate for.

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I'll do my best to get these old documents available, as they are a part of history.

To fly the electric jet when nothing in the air was completely controlled by electrons was really neat.

Gums sends ...