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Will the FBW with C* law be implemented ?


cmbaviator

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normally your aircraft is symetrically loaded and assuming its not a turboprop, you only need to trim the elevators except if it has autopitch trim like the A320 and above family.

 

Why would you need to trim the aileron or rudder ? unless you have one engine failure, asymetrical load, fuel leak....

 

 

I didn't understand why the F16 pilot needed to rudder trim his f16

 

Because the FBW will not correct yaw or roll trim. Every F-16 flies a little different so it will need trim to fly straight.

Also, note on the second video I posted, the aircraft descends all the way to 17520 at around 1g and then climbs all the way to 18000 at around 1g.

 

So the pitch trim being G commanded in the F-16, meaning you trim by the amount of G you want on the aircraft, the F-16 will not maintain a specific flight path by it self unless re-trim to a different G. Hope that make since.


Edited by mvsgas
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To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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i don't understand,

 

I dont undertand why all combat aircraft are nt flight path stable with auto timming, would make maneuver easier.

 

because unlike a commercial aircraft, you are always at not constant speed, meaning that you will need to trim every time

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Because the FBW will not correct yaw or roll trim. Every F-16 flies a little different so it will need trim to fly straight.

Also, note on the second video I posted, the aircraft descends all the way to 17520 at around 1g and then climbs all the way to 18000 at around 1g.

 

So the pitch trim being G commanded in the F-16, meaning you trim by the amount of G you want on the aircraft, the F-16 will not maintain a specific flight path by it self unless re-trim to a different G. Hope that make since.

 

basically it should be similar to C* in Airbus aircraft, where it is specifically written in the manual that in pitch the control inputs are made to alter the flight path NOT to hold it.

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basically it should be similar to C* in Airbus aircraft, where it is specifically written in the manual that in pitch the control inputs are made to alter the flight path NOT to hold it.

 

yes so that means when the stick is released, the Flight path is maintained (in certain conditions of course), it wont be able to maintain perfectly the flight path in a 60° bank roll.

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after rewatching this videos, it seems quite flight path stable, the flight path vector barely moves

 

 

It's also very WIP in this video so take that with a grain of salt

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Hard to tell from that vid, pilot is actively maneuvering throughout, so it wouldn’t i’m Not sure you could tell either way. That said, it will require more trimming than the Hornet as it’s fundamentally different (used to be referred to as limiter system). Fortunately the vipers flcs and the early 2000s nasa studies are all published, so to the extent the DCS engine doesn’t impose some limitations ED should nail the flight model. Contrast with the hornet, or HBs f14, which seem to have required much more trial and error.

 

my burning question is why you think it wouldnt be replicated???

frankly to even put out this question is insulting to the efforts of the developers

 

Lol What? insulting to the developers? I don’t think you realize how simplified the flight models are. Some are better than others, but it’s still a simulation and will always be off to some extent. I love the 18, but its energy management and acceleration modeling under different drag configurations, the simplified drag models themselves, aerodynamic coupling especially at high AOA or skid turns, simplified buffeting/lack of fluttter, lack of atmospheric phenomenon like wind, wind gusts, mechanical turbulence at low agl, convective turbulence, microbursts, mountain waves are all noticeably wrong or completely missing...to name a few. No apparent longitudinal center of gravity modeling, idiot proof engines with infallible fuel feed systems, weird lack of inertia particularly on pitch axis (that is unaffected by gross weight), no wing flex, indestructible landing gear; And that’s just flight model... of one of the better modules! Relative to real world (sometimes intentionally) many of the systems, weapons, and myriad other aspects of the simulation are.

 

That’s not a criticism of DCS, it does a very good job replicating feel and focusing on the “fun stuff,” but it’s not meant to be a level D sim. It’s a game, with a lot of simplifications, so the guy’s question IMO was entirely reasonable. Much of the simplification is intentional, or due to practical or legal issues, others for gameplay. The 18 doesn’t model AOA feedback or G based deflection force due to feasibility/lack of FFB sticks, RWR/ECM are intentionally not accurately reproduced due to ITARS, jdam cryptokeys and terminal trajectory effect on air bust fragmentation patterns for a combination of reasons most likely, link-16/mids implementation more simplified than a FC3 modul... I mean, I could list a few dozen more but air defense is a great example - SAM lethality is horrendously gimped, but DCS players don’t study threat briefs, memorize SAM speeds or learn to estimate intercept times, evasion tactics, fly disciplined section formations and follow lead/wingman responsibilities, etc., so an accurate “recreation” of integrated air defenses would not be fun at all (it’s a game after all).

 

But anyway...

 

i don't understand,

 

I dont undertand why all combat aircraft are nt flight path stable with auto timming, would make maneuver easier.

 

because unlike a commercial aircraft, you are always at not constant speed, meaning that you will need to trim every time

 

Because almost all airplanes require frequent trimming. FBW systems like on the fa18 that will auto trim pitch to 1g (approximately) are the exception not the norm. Planes that don’t have FBW systems, even if engineered for good stability and manners, require constant trimming. I’ve trained a lot of pilots and drill into their heads to trim after every throttle/attitude change starting first flight, as most pilots instinct is to correct with stick pressure rather than trim (bad habit)

 

It’s worthwhile to understand lift vs gravity and drag vs thrust, how those forces must be in balance for straight and level flight, how they effect each other (e.g increasing lift —> increase in drag). I won’t dive into it here, but its worth reading up as it will help your flying, as understanding basic BFM requires a good understanding of aerodynamics. Increasing thrust accelerates the plane, thus increasing lift that must be trimmed. Even with no control changes all aircraft require periodic trimming as fuel is burned, decreasing weight (and drag due to decreased lift required), shifting center of gravity.

 

Asymmetric loads are a different animal. The aerodynamics are similar (but in reverse) to engine out, where asymmetric thrust throws everything out of balance. Asym loads do the same, though generally not as strong but often greater distance from COG so still quite pronounced. If that doesn’t make sense that’s ok, the take away is that an extra 1000lb on an outer pylon will shift lateral center of gravity off centerline, which will require aileron trim, increasing induced drag as the aileron extending downward changes the cord line of the wing, that in turn creates adverse yaw. Depending on the characteristics of the plane it may be significant enough for a click or two of rudder and usually a bit of elevator. That’s all 3 axis... everything affects each other.

 

Fly the f14 a bit and try to keep your VSI at 0 while accelerating or through a 6G turn for a taste of how pitch sensitive non-FBW airplanes are. The 14 will give you some lessons on induced drag, adverse yaw and control reversal as well

just a dude who probably doesn't know what he's talking about

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my burning question is why you think it wouldnt be replicated???

frankly to even put out this question is insulting to the efforts of the developers

+1

 

Btw, I don't agree with sk000tch. Even (most) Level-D sims are far less sophisticated than they should be and many of the items he's complaining about aren't simulated in such sims either.

I certainly do see wingflex, luckily flutter isn't simulated since it doesn't occur on the real F/A-18 either AFAIK, and I strongly disagree with his (incomplete?) advice concerning trim!


Edited by bbrz

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+1

 

Btw, I don't agree with sk000tch. Even (most) Level-D sims are far less sophisticated than they should be and many of the items he's complaining about aren't simulated in such sims either.

I certainly do see wingflex, luckily flutter isn't simulated since it doesn't occur on the real F/A-18 either AFAIK, and I strongly disagree with his (incomplete?) advice concerning trim!

 

+1

 

Sk000tch, are you saying your version of DCS doesn't have wind, turbulence or wing flex in the F-18? Mine does!

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I am not complaining. I think I was pretty clear about that. I was defending OP for asking an innocent question.

 

ED makes design decisions that depart from absolute realism all the time, for a variety of reasons. I tried to give examples that correspond to a few different reasons; e.g. feasibility, ITAR, fun, to emphasize why its not an insult to the devs

 

bbrz i don't know how much time you have in level D sims, or what you mean by "less sophisticated than they should be." Even older ones have 1:1 cockpits and very accurate flight/systems modeling. Ground textures, buildings, and trees might not be rendered with high poly counts but that's not really the point. Modern professional aviation is a lot of things but I have never once thought geeze I could really use some more training. D Sims are very good for getting up to speed on a new type or ongoing emergency/failure training, something we don't concern ourselves much with in DCS. DCS servers a very different purpose.

 

I don't really want to argue point by point but, fwiw, the F18 does have aerodynamic flutter. Its well controlled, but has arisen several times during, for example, testing new weapon systems. Under normal circumstances it will only occur at the edge of the envelop or outside %MAC limits and high AoA, nothing like the F16, but my point was more general regardless. I realize everybody thinks they're a fighter pilot but there is a whole lot involved in actually being a pilot that DCS does not attempt to simulate. That's not a bad thing, some of its not very fun. It would be nearly impossible, or at least create a huge barrier to entry, if DCS modules required the same level of proficiency as their real-world counterparts. My SAM point is a prime example.

 

And yes, deano, DCS models winds aloft, but does so as constant, and while they added the occasional bump and more recently a very artificial (but welcome) wingtip vortice effect, and a graphical effect that mimics wing flex, but they are far from accurate representations of real phenomenon. I flew A LOT this morning/afternoon, mostly aerobatic practice for Apple Turnover and playing around w/ formation flight, so I spent a lot of time staring at a wing (for formation of course, but we utilize sighting devices to judge attitude relative to horizon). We have a moderate cold front moving into warm and humid air today, which produced gusty winds down load w/ moderate low level mechanical turbulence and a temp inversion ripe for sheer (very active air). We made the decision to not use our usual box, as its near foothills and as we could see cap clouds on several mountains. We evaluated the conditions and made the no-go call, as low level acro was higher than necessary risk, diverted to a practice area with higher minimums but safer given conditions. My point is that the atmosphere is alive, its why every pilot is nearly a meteorologist... yet when was the last time you studied a METAR and made a go/no-go decision in DCS?

 

DCS doesn't try to simulate that, and that's absolutely NOT a criticism. I'd rather they focus on the "C" in DCS than atmospheric modelling personally. We've only got so much dev time and CPU cycles. But, while we like to call it a study sim, and it certainly does offer a lot to learn, it is much different than a training simulator, and certainly doesn't purport to be an accurate representation of the real world. Again, that's not a criticism - I don't wan to do CG/weight & balance calcs (or do any preflight for that matter) to determine my AoA and %MAC limits everytime stations 2, 3, 7 or 8 are loadaed when I fly the Hornet. I'm glad it lets me ripple a single 2,000lb bomb from station 2 or 8, rather than having to release simultaneously to prent violating assym load limit. I'm glad it doesn't model the limitations of the negative G baffles, or enforce prohibited maneuvers like zero G for no more than 5 seconds, no neg G below 10k, no aileron rolls more than 360°, that are prohibited in the real jet.

 

And frankly while I chuckle a little bit every time I scream across a blue water boundary at 500kts and 80ft, I'm glad I still have the fillings in my teeth and don't die from a bird strike after spending 45 minutes of my limited free time getting to that point.

 

+1

... and I strongly disagree with his (incomplete?) advice concerning trim!

 

I admittedly didn't write a textbook but what, exactly, did you disagree with? I didn't really give advice, but rather set expectations. The F16 isn't the F18. The F16 is intentionally designed to have negative static stability for improved maneuverability. If you pitch up from equilibrium position then let go it's not going to return to its original attitude like most planes, its specifically designed to not do so. What exactly it will do depends on a number of various (CG, AoA, etc), the F16 FBW system is very good, it exhibits almost no adverse yaw, the coupling/ARI systems will convert AoA to beta if you roll knife edge and hold it with rudder, producing the very cool and energy conserving effect of allowing the F16 to roll about its velocity vector rather than fuselage center. Beyond that however its primarily a limiter, preventing the pilot from killing themselves by limiting elevator authority under high AoA, automatic spin/stall prevention, etc.

 

If you really want to nerd out on this I suggest reading NASA Technical Paper 1538 "Simulator Study of Stall/post-Stall Characteristics of a Fighter AIrplane with Relaxed Longitudinal Static Stability." I'd provide a link but, well, the rules don't allow it. If you want to understand the 16's FWB system its a good place to sart.


Edited by sk000tch

just a dude who probably doesn't know what he's talking about

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1. bbrz i don't know how much time you have in level D sims, or what you mean by "less sophisticated than they should be." Even older ones have 1:1 cockpits and very accurate flight/systems modeling. D Sims are very good for getting up to speed on a new type or ongoing emergency/failure training.

 

2..the F18 does have aerodynamic flutter...it will only occur at the edge of the envelop

 

3. yet when was the last time you studied a METAR and made a go/no-go decision in DCS?

 

4.I admittedly didn't write a textbook but what, exactly, did you disagree with? I didn't really give advice, but rather set expectations.

 

5. If you want to understand the 16's FWB system its a good place to sart

1. More than half the total hours you have IRL, I've flown Level-D sims ;)

It's new to me that Level-D sims have very accurate flight modelling. I don't think that I need to quote e.g. the MCAS drama. Most Level-D sims are rather bad at the edge of the performance envelope.

As you have written, they are good for type ratings and emergency procedures, but they are much more procedure trainers than highly accurate flight sims.

 

2. You have a link or example when flutter occurs on the F/A-18 during normal ops 'at the edge of the envelope'?

 

3. A go/no-go decision in DCS doesn't make much sense if you are designing a mission where everyone is staying on ground ;)

That said, DCS does a remarkable realistic job when e.g. flying an approach in bad weather conditions with a a low ceiling, turbulence and rain, better than every other sim IMO.

 

4. I disagree that it is a 'bad habit' to use stick pressure instead of trim. IMO you have to use stick pressure first to achieve the required attitude, then you trim away the stick pressure.

 

5. I know the F-16 FBW, thanx. The point I'm trying to make is, that of all the 'missing effects' I severely doubt that the basic F-16 behavior will be one of them.

Btw, I don't know how you can determine if wingflex on the DCS F/A-18 isn't correctly simulated and what precise effect it has in the real F/A-18.

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"During full-scale development of the F/A-18A/B Hornet, it was discovered that when carrying heavy stores on the outboard wing pylons and AIM-9 missiles on the wing tips, the aircraft experiences an “unacceptable 5.0 - 6.0Hz oscillation at low altitude and high speed”. From "Higher-Order Spectral Analysis of F-18 Flight Flutter Data," Walter A. Silva NASA Langley Research Center, Hampton, Virginia, 23681

 

Similarly, "Robust Flutter Margins of an F/A-18 AIrcraft from Aeroelastic Flight Data," Journal of Guidance, Control, and Dynamics Vol. 20, No. 3 May-June 1997 includes a discussion of computational methods for predicting flutter, including nominal flight envelop plotting against flutter margin.

 

Many more, I'd post links, but I am on a short leash around here.

 

As to your other question, never know who you're talking to around here but in this case it doesn't take a hornet driver to know that flex is proportional to wing loading.

 

I'm not really trying to get into an argument, if you feel constant elevator is superior to trimming for pitch, then by all means do so. You'd get docked on a checkride, but this is a sim so who cares. I'll leave the sim question alone as well other than to note that the FARs on training device qualification are voluminous and specify every detail down to accuracy of control deflection force and travel, exact placement of gauges, switches, the accuracy of the friggin seats, the sounds, and myriad other criteria, but DCS is probably more accurate :doh:

 

Anyway, who cares. OP isn't around to stick up for anymore, and I'd rather play than argue on forums.

just a dude who probably doesn't know what he's talking about

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Interesting reply. Lot's of unrelated text but you didn't answer a single question (and I suggest you re-read what I wrote about trimming).

 

Have a nice day :)


Edited by bbrz

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Interesting reply. Lot's of unrelated text but you didn't answer a single question (and I suggest you re-read what I wrote about trimming).

 

Have a nice day :)

 

You asked a single question... about flutter, to which I answered with two peer review papers discussing the topic.

 

FWIW, I am not sure we even disagree about trim... I don’t think anybody was advocating using trim in lieu of stick/yoke. The issue was students holding elevator in deflection during, for example, climb, rather than trimming to pitch

just a dude who probably doesn't know what he's talking about

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You asked a single question... about flutter

No, that was not the exact question. Read again. (btw. the F/A-18 in DCS is a C model, not an A, B or SRA)

 

Didn't see any reply about the effect of wingflex either.


Edited by bbrz

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2. You have a link or example when flutter occurs on the F/A-18 during normal ops 'at the edge of the envelope'?

 

I don’t see anything specific to C variant... what’s next specific lot? A to C didn’t involve aerodynamic changes; radar, avionics, and armament don’t change oscillations (unless you’re adding heavy armament to wingtips or something). The LEX fences on every Hornet variant are a fix for the stabilizer flutter caused by the LEX vortices. Despite the fences at high AOA and low speed can still be dangerous:

 

MCAS is a questionable example btw, pretty unique scenario. Regardless, all sims break down at edges of envelop. I fly competitive acro and I’ve never flown a sim that even comes close to accurately modeling any maneuvers involving complex or asymmetric stall like basic inside snap roll, and forget any gyroscopic stuff like lomcevaks or any tumbling.

just a dude who probably doesn't know what he's talking about

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Guys, I got a question. To clarify, I'm not contesting anyone post: bbrz and sk000tch; You guys are having your own conversation and I not arguing any of your points. I am simply curious about something.

 

Does anyone think the NASA paper 1538, written in 1979 has any relevance to the DCS F-16? Specially considering the aircraft has change drastically since then and the study did not used real aircraft, only simulated aircraft.

 

I am just curious, Not looking for anything else. I don't have a secret agenda. Just curious if anyone thinks that paper has any relevance.


Edited by mvsgas
spelling

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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No kidding on the off topic....

 

To answer your question though that paper came out right after f-16 came into service, and is frequently cited by every academic paper that follows. It’s old, but while avionics have changed significantly, aerodynamics are the same now as they were then (our understanding of control laws have improved, largely due to this type of work, but the physics are the same). The paper is hundreds of pages of analysis and data re of FLCS. Between that and subsequent papers, detailed equations and matrices that govern the multi-input -> multi-output FBW details (down to specific control surface angle as a function of stick pressure and AOA), where coupling and limiters are activated, are all publicly available. Later articles get into detail about thrust, which is often difficult to model as the thrust generated at various throttle positions/fuel flow/inlet temps and different altitudes is hard to come by. Drag is the same way, you can find tables of ram drag values for the turbine... that kind of info isn’t available for other planes (even the a-10).

 

Again I don’t k ow if it’s because of the novelty of a low longitudinal stability FBW design (in 1976 at least), the widespread adoption/number produced or just its age, but the result is a ton of very detailed information in the public domain. That takes a lot of the guess work out of the modeling process for ED, ensures we should get a high fidelity simulation, and avoids the problem of utilizing documents that are available online but really shouldn’t be when creating the module.

just a dude who probably doesn't know what he's talking about

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So even tho the paper did not used real aircraft, and the aircraft they use to simulate had different horizontal stabilizers. Even tho the aircraft simulated had a different Flight control computer with different logic and different limiter. Considering the flight control computer and many components that send information to the computer and how the computer use that information has change. Even tho the aircraft have different weight and balance, different engines, the angle of the flight control can travel is different, the paper is still relevant to you.

 

 

Ok thanks.

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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I’m just a pilot/engineer, not a dev, so I probably have no idea what I am talking about. I did come across this while looking for some info to answer your question:

 

In a paper/post discussing the flight model of a popular f-16 simulator, the dev stated “I will try to explain you as simply as possible what were the challenges of the F-16 development, and the way the real engineers (and consequently BMS) have developed and improved the FLCS ... Please note that this is mostly based on the NASA Technical Paper 1538 (TP 1538) Simulator Study of Stall / Post Stall Characteristics of a Fighter Airplane with Relaxed Longitudinal Static Stability”

just a dude who probably doesn't know what he's talking about

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  • 1 month later...
Hi guys,

 

i was wondering if the F16 will have FBW and more specifically the C* law.

 

I think the F16 was the first aircraft to have the FBW C* law than would be used and modified by Airbus for their A320 and above and later on byt BOEING but with the C*U law.

 

 

From what i understand, is that the C* law is base on pitch rate demand at low speed ang G demand on high speed. Besides it is fligh path stable, so for instance, if you fly at 200 kt IAS level and accelerate to 500 kt IAS, you should remain level, maintain path in turns..., in a conventional aircraft ( Direct law) you would pitch up or down if you decelerate.

 

http://www.icas.org/ICAS_ARCHIVE/ICAS2012/PAPERS/605.PDF

 

No because the F-16 has its own FBW code and what you describe would hinder its performance.:music_whistling:

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