F-16 fly by wire stall scenarios
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The F-16 stalls and is able to recover through MPO usage after abusing several limiters at once. The question – is there a theoretical FLCS algorithm/gains that allows the aircraft to perform “better”, for instance avoiding the need for MPO workaround? What tradeoffs are there?
thanks,
sh -
Well, define “better”.
In the deep stall, the only scenario requiring the MPO, the F-16 enters a stable configuration where the flight controls have almost zero delta L regardless of deflection. Keyword being almost.
If by “better” you mean “doesnt ever enter a deep stall”, that could be arranged. With some pilot input, that is ultimately the goal of the current set of limiters in use. With some minor modification to make the set of laws a little more complex, it would not be hard to add an automatic recovery procedure, where the aircraft detects an imminent deep stall and applies prompt recovery technique, pre-empting the deep stall. This would reduce pilot agency somewhat, but may increase pilot survivability in the fringe case where they GLOC into a deep stall.
If by “better” you mean “aircraft has better sustained turn performance” I would go so far as to say no. If you meant “aircraft has better high alpha nose authority” then I would hazard the guess that this would be possible, although it would increase the risk of getting the aircraft into the extremely undesirable stable configuration at 60° alpha with zero controllability (almost). Depending on how good the air data the FLCS has to work with is, and how well flight tested the aircraft is, and what the high alpha (say 30 to 50°) nose down delta L available is like, it might be possible in theory.
You will note of course that there are a lot of "might"s and "depending"s in there. This is the sort of stuff that numbers would be useful for. Perhaps a short review of NASA TP 1538 is in order?
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Hard to say, but I don’t think it would be easy to code a simple logic.
Essentially, the 60° aoa is “stable” on the f-16, so once you are there, the FLCS could try to get you out by rocking back and forth. However, iirc, the aoa probes don’t go that far, so it wouldn’t really be able to detect it except inertially, I guess… I suppose a dedicated “flat spin” switch activated by the pilot when he thinks he’s in a flat spin would work, but then you are still doing a manual action to get out of it.
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If you meant “aircraft has better high alpha nose authority” then I would hazard the guess that this would be possible, although it would increase the risk of getting the aircraft into the extremely undesirable stable configuration at 60° alpha with zero controllability (almost). Depending on how good the air data the FLCS has to work with is, and how well flight tested the aircraft is, and what the high alpha (say 30 to 50°) nose down delta L available is like, it might be possible in theory.
Does “high alpha nose authority” mean e.g. behaving “more like” high-AoA dogfighters? Of course there are airfoil differences but still.
A corollary – when dogfighting online, could one gain an advantage by using the MPO?
Naive question – with MPO enabled, can the pilot over-G the airframe just like that? Does it disable the G limiter? Or is there a more conservative algorithm still in place with MPO enabled?
With some minor modification to make the set of laws a little more complex, it would not be hard to add an automatic recovery procedure […]
I’m concerned about FLCS standing in the way of performing a recovery in the rocking-falling-down scenario. Let’s leave the need for the pilot to perform the recovery procedure himself.
You will note of course that there are a lot of "might"s and "depending"s in there. This is the sort of stuff that numbers would be useful for. Perhaps a short review of NASA TP 1538 is in order?
I can’t concentrate. Sorry if the questions are RTFM material.
Hard to say, but I don’t think it would be easy to code a simple logic.
Essentially, the 60° aoa is “stable” on the f-16, so once you are there, the FLCS could try to get you out by rocking back and forth. However, iirc, the aoa probes don’t go that far, so it wouldn’t really be able to detect it except inertially, I guess… I suppose a dedicated “flat spin” switch activated by the pilot when he thinks he’s in a flat spin would work, but then you are still doing a manual action to get out of it.Technically it’s possible to get an AoA reading by gyroscope + accelerometer combo, with no external airflow sensors. I guess a switch is always better than this hypothetical added complexity.
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That’s a good question about how the MPO actually functions. In the deep stall situation it allows the pilot to command the stabilator up and down when normally the FLCS would have 0 pilot input since AOA very high it would just command 100% nose down pilot out of the loop. The -1’s description of MPO is very brief. It’s not the T.O. for the FLCS itself. What the MPO might do is simply remove the blend-to-zero pitch command based on AOA which reduces commanded G from 9g to 1g over the range of 15-25 AOA (if enough airspeed).
“enable manual control of the horizontal tails” and “overrides the negative g limiter” and if AOA exceeds 35 degrees “overrides the AOA/g limit and allows pitch commands.”
It’s unclear if the MPO allows some 1:1 programmed stick input to tail position program or if it removes the limiters which would disallow opposite stick input. Occam’s Razor sort of thinking suggests the latter since it’s the minimum change needed to enable the rocking behavior needed to escape the deep stall trap. It’s not like you want gentle proportional control when escaping deep stall anyway. You want to rock using maximum pitch commands “bang bang” style.
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Probably a dumb question but can someone explain to me why, after stalling, with no lift in the surfaces, the stabilizers have enough control authority (in mpo) to pitch as up and down in order to get to the desired vertical dive to restore lift?
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That’s a good question about how the MPO actually functions. In the deep stall situation it allows the pilot to command the stabilator up and down when normally the FLCS would have 0 pilot input since AOA very high it would just command 100% nose down pilot out of the loop. The -1’s description of MPO is very brief. It’s not the T.O. for the FLCS itself. What the MPO might do is simply remove the blend-to-zero pitch command based on AOA which reduces commanded G from 9g to 1g over the range of 15-25 AOA (if enough airspeed).
“enable manual control of the horizontal tails” and “overrides the negative g limiter” and if AOA exceeds 35 degrees “overrides the AOA/g limit and allows pitch commands.”
It’s unclear if the MPO allows some 1:1 programmed stick input to tail position program or if it removes the limiters which would disallow opposite stick input. Occam’s Razor sort of thinking suggests the latter since it’s the minimum change needed to enable the rocking behavior needed to escape the deep stall trap. It’s not like you want gentle proportional control when escaping deep stall anyway. You want to rock using maximum pitch commands “bang bang” style.
MPO :
- disable the neg G limiter independantly of the AOA
- above 29 deg, puts the FLCS in WOW gains :
=> gain of x2 in pitch gain command
=> by pass the elevator command proportional + integral
=>change the filter law of the pitchrate loopback
Basically
=> it disables the neg G limiter whatever the AOA
=> above 29 deg, gives direct control to the elevators with stick -
Is “WOW gains” TO&L or standby? Because you mention pitch rate feedback I assume TO&L. Sort of weird since >29AOA in TO&L that’s a blend of AOA and pitch rate (is there any pitch rate still left in the blend at that AOA?). Double pitch gain compared to normal TO&L or TO&L is 2x compared to cruise? By “bypass elevator P+I” so it’s just a P-controller or bypassing the normal elevator command to instead be a P+I controller?
Yoda, stall isn’t zero lift it’s just beyond maximum lift. Even a sheet of cardboard provides lift at 45 degrees, just inelegantly and inefficient. Stall isn’t “all hope is lost” but more “your best lift is behind you.”
According to the chart page 11 of Code One magazine, Semper Viper Deep Stalls / Departures by Dryden the pitch moment at stick full aft-neutral forward is distinct in the range -60 through +60 AOA (for some CG). That means stick placement actually has a pitch input distinct from other stick placements. The unfortunate thing is that there is a small region ~+50AOA where all stick placements result in positive pitch moment (similar at ~-55 but negative). By 70AOA all pitch moments are negative and there’s no stable AOA beyond that.
So at this point where all possible stick inputs yield a pitch increase there’s no hope. You want to pitch up as much as you can to as high an AOA as you can and then command the most nose-down pitch rate possible (which is full forward stick, the lines never cross). Then hold that minimum pitch rate as it goes slightly positive until you return to a region where full forward stick gives negative pitch moment and recovery is assured. >65to70 AOA or <-60to-70 AOA the pitch moment lines collapse to each other and the airplane is gunna do whatever it’s gunna do regardless of input. The pilot’s control is negligible outside of these values. But the pitch moments are highly restorative as well so the airplane should be naturally driven back to that control zone pretty rapidly.
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Is “WOW gains” TO&L or standby? Because you mention pitch rate feedback I assume TO&L. Sort of weird since >29AOA in TO&L that’s a blend of AOA and pitch rate (is there any pitch rate still left in the blend at that AOA?). Double pitch gain compared to normal TO&L or TO&L is 2x compared to cruise? By “bypass elevator P+I” so it’s just a P-controller or bypassing the normal elevator command to instead be a P+I controller?
Yoda, stall isn’t zero lift it’s just beyond maximum lift. Even a sheet of cardboard provides lift at 45 degrees, just inelegantly and inefficient. Stall isn’t “all hope is lost” but more “your best lift is behind you.”
According to the chart page 11 of Code One magazine, Semper Viper Deep Stalls / Departures by Dryden the pitch moment at stick full aft-neutral forward is distinct in the range -60 through +60 AOA (for some CG). That means stick placement actually has a pitch input distinct from other stick placements. The unfortunate thing is that there is a small region ~+50AOA where all stick placements result in positive pitch moment (similar at ~-55 but negative). By 70AOA all pitch moments are negative and there’s no stable AOA beyond that.
So at this point where all possible stick inputs yield a pitch increase there’s no hope. You want to pitch up as much as you can to as high an AOA as you can and then command the most nose-down pitch rate possible (which is full forward stick, the lines never cross). Then hold that minimum pitch rate as it goes slightly positive until you return to a region where full forward stick gives negative pitch moment and recovery is assured. >65to70 AOA or <-60to-70 AOA the pitch moment lines collapse to each other and the airplane is gunna do whatever it’s gunna do regardless of input. The pilot’s control is negligible outside of these values. But the pitch moments are highly restorative as well so the airplane should be naturally driven back to that control zone pretty rapidly.
Cool, thnx Frederf, very informative! There is always more to learn. That’s what i love about this hoby of ours!
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Is “WOW gains” TO&L or standby? Because you mention pitch rate feedback I assume TO&L. Sort of weird since >29AOA in TO&L that’s a blend of AOA and pitch rate (is there any pitch rate still left in the blend at that AOA?). Double pitch gain compared to normal TO&L or TO&L is 2x compared to cruise? By “bypass elevator P+I” so it’s just a P-controller or bypassing the normal elevator command to instead be a P+I controller?
Yoda, stall isn’t zero lift it’s just beyond maximum lift. Even a sheet of cardboard provides lift at 45 degrees, just inelegantly and inefficient. Stall isn’t “all hope is lost” but more “your best lift is behind you.”
According to the chart page 11 of Code One magazine, Semper Viper Deep Stalls / Departures by Dryden the pitch moment at stick full aft-neutral forward is distinct in the range -60 through +60 AOA (for some CG). That means stick placement actually has a pitch input distinct from other stick placements. The unfortunate thing is that there is a small region ~+50AOA where all stick placements result in positive pitch moment (similar at ~-55 but negative). By 70AOA all pitch moments are negative and there’s no stable AOA beyond that.
So at this point where all possible stick inputs yield a pitch increase there’s no hope. You want to pitch up as much as you can to as high an AOA as you can and then command the most nose-down pitch rate possible (which is full forward stick, the lines never cross). Then hold that minimum pitch rate as it goes slightly positive until you return to a region where full forward stick gives negative pitch moment and recovery is assured. >65to70 AOA or <-60to-70 AOA the pitch moment lines collapse to each other and the airplane is gunna do whatever it’s gunna do regardless of input. The pilot’s control is negligible outside of these values. But the pitch moments are highly restorative as well so the airplane should be naturally driven back to that control zone pretty rapidly.
“WOW gains” AKA “UPRIGHT PITCH ROCK GAIN” is not Landing Gain, this is Weight On Wheel gain
this is the gain you have on ground with WOW activated
you can check that on ground you have full control of your elevator directly
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@Master:
Cool, thnx Frederf, very informative! There is always more to learn. That’s what i love about this hoby of ours!
if you read FLCS article available in the article section of benchmarksim site, you will have much more information about the deep stall and FLCS behavior
https://www.benchmarksims.org/forum/attachment.php?attachmentid=1963&d=1267814283
it is always a bit disapointing to see people refering to some public simplified information like Code One magazine when BMS provides much more detailed explanations
i wonder if people have read our articles ?
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I admit I haven’t more than a skim. The Code One article is just one I knew and has the pretty chart and narrates more fluidly than any technical type document. The problem is Ctrl-F “WOW gain” or “pitch rock gain” in any document I’ve tried is I get zero results. Either this name is a loose nickname for something else I can read or I don’t have anything which references it.
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I admit I haven’t more than a skim. The Code One article is just one I knew and has the pretty chart and narrates more fluidly than any technical type document. The problem is Ctrl-F “WOW gain” or “pitch rock gain” in any document I’ve tried is I get zero results. Either this name is a loose nickname for something else I can read or I don’t have anything which references it.
This is straight from engineering document;)
The information you have is totally simplified or not accurate compared to the real
Not many people know that wow gains are different than LG gains , I suppose most pilots do not even know as this is not mentioned in any Pilots manuals
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I’d rather not open another thread on a similarly technical subject.
Question – why are the EEGS funnel and the FPM visibly delayed? A modern-day puny ATMEL chip in an Arduino integrates orientation data hundreds of times per second. I understand that military aircraft electronics need be EM-resistant and robust, but still. Or is displaying the funnel on the collimated HUD the bottleneck in the real aircraft?
MPO :[…]
Basically=> it disables the neg G limiter whatever the AOA
=> above 29 deg, gives direct control to the elevators with stickThat direct stick control is interesting. Despite different aerodynamic characteristics, it could possibly allow for more high-AoA gun kills. Bleeds energy, sure, but normally one has to appease the AoA limiter no matter what.
My BFM skill is close to zero. Could any of you try e.g. scissoring with MPO turned at the right moment?
The pitch control still remains sigmoidal I presume?
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I’d rather not open another thread on a similarly technical subject.
Question – why are the EEGS funnel and the FPM visibly delayed? A modern-day puny ATMEL chip in an Arduino integrates orientation data hundreds of times per second. I understand that military aircraft electronics need be EM-resistant and robust, but still. Or is displaying the funnel on the collimated HUD the bottleneck in the real aircraft?
That direct stick control is interesting. Despite different aerodynamic characteristics, it could possibly allow for more high-AoA gun kills. Bleeds energy, sure, but normally one has to appease the AoA limiter no matter what.
My BFM skill is close to zero. Could any of you try e.g. scissoring with MPO turned at the right moment?
The pitch control still remains sigmoidal I presume?
Yes but as wow is only triggered when MPO AND AoA > 29 you can’t trigger it on purpose since the AOA is limited at 25 deg
Pay attention that negG limited is desactivated by MPO indépendante of AOA
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@Master:
Probably a dumb question but can someone explain to me why, after stalling, with no lift in the surfaces, the stabilizers have enough control authority (in mpo) to pitch as up and down in order to get to the desired vertical dive to restore lift?
It’s more because of the wing lift oscillating. The wing is lifting and stalling periodically, and the stabs are just countering the resulting pitching moment due lift. The stabs may or may not be doing the same thing, but because going to MPO increases the stab gain (deflection limits, more directly) and the stab’s local AOA is not “fixed” within the stall state a pilot can use them to get into phase with the oscillation and “rock” the jet out of it’s stalled condition - which is actually a stable condition. MPO provides the pilot with the ability to have some leverage over the forcing function in the oscillation, but it doesn’t really give the stabs any more “authority” - if there was a true in creas in control authority the pilot could just put the stick forward and bring the nose down.
And it’s the AOA limiter that “helps” get/keep the nose hung up…if you’ve spent a bunch of time flying a sim or trainer that doesn’t have an AOA limiter it’s really easy to spot AOA hangs (nose slow to return to the horizon) in BMS that are attributable to the AOA limiter - I’ve discussed this with actual pilots that have flown F-16s and they just nod in the affirmative and are surprised that BMS behaves in accord.
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Here’s a dumb question. If FLCS is g-command system and untrimmed pitch is 1.0g is pitch inherently divergent over time if pitch starts 0.00001°? Presumably pitch >0 will have some nose up pitch rate if load factor 1.0 is maintained. I haven’t noticed BMS F-16 “drifting up” at small pitches and wondered why it isn’t doing what a simple understanding of g-command 1g trim would suggest.
Or is this somehow solved because untrimmed pitch is actually “0g radial acceleration” where “0g” results in 1g load factor level fight?
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This is another “it depends” question…in that the F-16 was (if I recall…) the first modern fighter to be designed to be inherently statically unstable, and to be made controllable via fly by wire FLCS. As far as I know the F-16 is predominantly classically controlled in that it limits and presumably trims to AOA and not G (but I’d have to know more, really…I only say this because I find the BMS Viper easy to over-G when loaded up if I don’t watch it), but statically unstable when others may only have “relaxed” static stability - even marginal - in the interest of being more maneuverable.
Pitch angle at 1G couple, really…in that if you pitch up and trim to maintain 1G (no climb) you need to bleed speed to maintain altitude as you approach Clmax, past the critical AOA, and enter stall. But in the F-16 the AOA limiter will do it’s best to prevent you from getting to or past critical AOA (independent of G, I think…but I could be wrong about that)…and it’s around here that you can start to observe AOA hang-up/hesitation if you try and put the nose down quickly with just forward stick. This is part of what the HART maneuver is illustrating. But there’s more about the jet to find out than just that…this…
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I’m not talking about aerodynamic stability, just FCS control law behavior well within normal flight parameters.
If you set 1 degree FPA and let go of the stick trimmed 1g you are commanding 1g where straight-line flight is cosine(1)= 0.99984769515639123915701155881391 g. The difference between commanded g and straight line g should produce a slow FPM rise over time.
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it is always a bit disapointing to see people refering to some public simplified information like Code One magazine when BMS provides much more detailed explanations
i wonder if people have read our articles ?
So many times, over and over. Its very interesting stuff!
I’m not talking about aerodynamic stability, just FCS control law behavior well within normal flight parameters.
If you set 1 degree FPA and let go of the stick trimmed 1g you are commanding 1g where straight-line flight is cosine(1)= 0.99984769515639123915701155881391 g. The difference between commanded g and straight line g should produce a slow FPM rise over time.
Well, it does - this is demonstrated in AHC sorties too, for 50 and 60 degree climbs.