F-16 DFLCS control law & G-onset rate

Always wondered why the F-16 has a tailhook, or how big a bigmouth F-16's mouth really is ? Find it out here !
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basher54321

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Unread post01 Nov 2019, 13:47

Patriot wrote:What would have happened if we would change the 16's AOA limit from 25.5° to say 30-35° ? What would it take? Is it only a FLCS software update or requires structural tampering as well? Would that limit change be benefitial or not? Would it improve the control/maneuver/nose-pointing_ability at low speeds (I assume yes) ?


Software changes for sure and it should allow the F-16 to reach its CL Max for good or bad. It has actually been done however you may only be able to do such a thing with Thrust Vectoring - Look at the MATV program to get some idea.
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basher54321

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Unread post01 Nov 2019, 13:58

hummingbird wrote:Also regarding the FLCS G limit's, according to the system charts for later Blocks of F-16's with DFLCS the G limit was raised to 9.3 G's (8.3 in the system) in order to allow 9.0 G's being reached very quickly at corner speed, i.e. no need to be going faster to allow the aircraft time to reach 9 G's before you drop below corner speed.




Well that guy is better than me if he can make out anything from that horrendous block diagram that he also describes as being a DFLCS for a Block 25.

The Digital FLCS only came in with Block 40/42 and was supposed to implement the same CLaws as the analogue version so outside of a Block 25 being used for testing I don't know what that is - will have to dig a bit. The reason I stated the limit was already 9.3G on the analog version is because I have it given as advise to F-16 pilots 4 years before the DFLCS even hit squadrons.
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basher54321

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Unread post02 Nov 2019, 00:04

Interestingly there was an F-16D Block 25 DFLCS Testbed:


USAF F-16D block 25 #83-1176 from the 6512th TS is flying over Edwards AFB on December 12th, 1986 during Digital Flight Control System trials, with the aircraft carrying six AGM-65 missiles, two 370-gallon fuel tanks, one ALQ-119 ECM pod and two AIM-9 missiles. [USAF photo by SSgt. Long]
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f-16adf

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Unread post02 Nov 2019, 13:47

JohnWill,

Wouldn't a non FBW, non RSS jet technically have lower G onset rates than a jet that has FBW/RSS? Possibly, because of the needed momentum necessary to over come the tail download?


Also, 2 documents on Rapid Onset Runs.
https://apps.dtic.mil/dtic/tr/fulltext/u2/a196171.pdf

https://dokumente.unibw.de/pub/bscw.cgi ... _1999_70(7)_709-12.pdf
(This link not working, so I included it down below)




They both give similar Rapid Onset Runs for the F-16, yet it seems Turkey does not include the "check six" run. All those Centrifuge training profiles: 6G for 30 sec, 8G for 15 sec, and 9G for 15 sec all have a 6G/sec onset rate. And the 9G for 15 sec test matches the video of the F-16 pilots in the centrifuge. It must be remembered, that they even before the test begins are at 1G.
https://www.youtube.com/watch?v=yeIDvevEYuw
https://www.youtube.com/watch?v=q5KxvsJApT8


F-16 G ROR.jpg








John, also in this demo from 1993 of a F-16C Block 50. At 4:45 mark of the video 9.4G appears at the lower left of the HUD, but then later goes back down to 9.0, then back to 9.4G. I thought due to the Digital FCS, Blocks 40 and beyond could not do that? Is there an error somewhere in the jets system?

https://www.youtube.com/watch?v=NqGTGVWfAoI
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F-16 G onset rates Turkish AF.pdf
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sprstdlyscottsmn

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Unread post02 Nov 2019, 19:44

Any G onset rates that vary with several factors.

Pitch Force: Rate of deflection of the pitch control surface, airflow over pitch control surface, lift curve slope of pitch control surface

Pitch rate: Pitch Force, Moment arm of pitch force, moment of inertia of the aircraft.

G onset rate: Pitch rate, lift curve slope of wing/body, airflow over wing/body, weight.

A FBW system can ensure that the optimum pitch control surface position and wing position are obtained for a consistent and predictable pitch performance. A non-FBW, but hydraulically boosted, flight control system may allow the pitch control device to exceed its peak position on the lift curve slope resulting in a stalled pitch surface, reducing available pitch moment and increasing drag. Non boosted (e.g. cable-driven) systems require the pilot to physically fight the airflow over the pitch control surfaces.
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johnwill

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Unread post02 Nov 2019, 21:09

Spurts, your list of factors is fine for stable non-RSS airplanes, but for airplanes like F-16, the big driver for pitch acceleration and g onset is the lift center of pressure being forward of the CG. In a subsonic pull up or turn, the 1g tail load is up. When the pull is commanded, the tail very briefly will have an incremental trailing edge up movement (down load), enough to start a positive pitch rate and increase AoA and lift. That increased lift forward of the CG is what drives rapidly increased AoA, lift, and g. The tail is then moved slightly leading edge up, with more up load, modulating AoA, lift, and g to prevent unstable divergence. That up tail load is what reduces wing and aft fuselage load, AoA, and drag compared to a stable airplane.

I know you already know this, it is mainly to help others understand it.
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Unread post02 Nov 2019, 23:39

f-16adf wrote: At 4:45 mark of the video 9.4G appears at the lower left of the HUD, but then later goes back down to 9.0, then back to 9.4G. I thought due to the Digital FCS, Blocks 40 and beyond could not do that? Is there an error somewhere in the jets system?


I was thinking that was just down to video editing - as in they might have had to stick in a different bit of footage (the one with 9.0) to fill it out.
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Unread post03 Nov 2019, 02:33

As far as I've been told now by two current pilots the F-16C Blk.50/52 (& MLU) DFLCS allows for more than 9 G's (9.3 is what the charts I've seen so far indicate) in the system in order to allow 9.0 G's to be rapidly reached at corner speed before the aircraft slows down too much. Overshooting 9 G's also apparently happens routinely, and 10 G's has been seen, which as John points out is also a combination of where the sensors are located.
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basher54321

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Unread post03 Nov 2019, 11:58

If that is the case then it doesn't sound like much changed at all - an over G inspection was apparently required at 105% e.g. 9.5G+ (symmetric) despite uncertainty over the figure being genuine. If they are seeing less than 11.2G as an F-16N pilot reported in the past then that might be considered progress perhaps.
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Unread post03 Nov 2019, 21:34

Just a side simple question.. this always puzzled me. Why specifically F-16 AOA limit is set to 25°? Would 35 or more degrees of AOA avilability be worse to have?
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Unread post03 Nov 2019, 23:48

One reason being that the F-16 loses a lot of its directional stability between thirty and fifty degrees AoA where most of the vertical tail is blanked out by the fuselage.

Therefore when they did the MATV program they used Thrust Vectoring to provide the needed directional stability meaning it could then go to much higher AoA. It demonstrated something like 85 degrees AoA stable and over 110 degrees AoA transient and could do similar tricks to the YF-22 like the J Turn, helicopter, cobra etc.

Other jets have similar AoA limits like the Typhoon (already mentioned I think) probably for similar reasons.

There must be a lot on the MATV if you go looking - this is not the thread.
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Patriot

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Unread post04 Nov 2019, 00:49

Thanks Basher.

Inusfficient directional stability reserve. Gotchya. So I automatically assume that if the Viper would be a 2-tail design and have more vertical surface to stable itself the muzzle for AOA would of been probably a lot less tight..

Does 2-tail alone are enough means to make the airframe free from AOA restrictions - like i.e. Hornet ?
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Unread post04 Nov 2019, 00:56

Got a couple of comments on this thread. First and foremost, what difference does it make what the G-onset rate is, or the AOA limit, or any of the other BS discussions that go on here. How fast will it go? The fastest jet is obviously the best. How many G can it pull? The highest G is obviously the best. How much AOA can it do? The higher AOA is obviously the best. How high can it fly? the higher flyer is obviously better. YGBSM.

I have 2750 hours in the F-16 and I have no idea what the G onset rate is. All I can tell you is it gets to pain in a hurry and gets grey and dark really fast. Any faster is beyond human capability. But not every jet is doing the same performance at the same G. So, again, WHOGAS

AOA limits....I flew the F-4 Phantom II fighter...Bomber...mostly bomber for 179 hours, after 7 years of flying the F-16. I used to tell people the F-4 represented decades of engineering designed to make a supersonic jet feel like a P-51. The whole feel system was BS. After flying the F-16 side stick it was clear to me there was no reason to have the complex bellows and trim system in the F-4. I came home frequently from an F-4 wresting...BFM....experience with a throbbing arm. When the system failed you were in a very precarious position either unable to move the stick or balancing on a pin head. The lack of a limiter IS NOT an advantage. I flew against Tomcats....forget about it. Thank God I went back to the F-16. I flew without concern about limits....G or AOA, for 21+ years. I would not give you a dime to fly the Tomcat, F-15, or Phantom instead.

To Basher, my understanding of the AOA limiter is different. I have posted and referred to the Joe Bill article on high AOA in the F-16. The problem as I understand it is more that the Jet loves AOA in the 35+ region and actually has a nose up tendency up through 45 AOA. That is the reason for deep stall. The strakes and CG result in pitch up moment in that region that the horizontal tails cannot overcome. When you get in a deep stall the recovery procedure is to INCREASE AOA using the MPO to get into a nose down pitching region and try to rate through the 35-45 AOA range back into the <25 AOA. So the limiter was designed to prevent transition into this region. Not to prevent yaw issues, although they occur at higher altitudes and speeds. As I have articulated earlier, this is actually an advantage in my mind as the Viper driver does not have to worry about getting into a slow speed high AOA death spiral....something very easy to do in the Mighty Phantom II. I also never worried, like Tom Cruise, about hitting jet wash and winding up in a spinning monstrosity. BS I know, but the Tomcat was not a jet for carefree outperformance of other 3-4th Gen jets. It was a high speed, high altitude, long range interceptor. What the Navy needed but not a phone booth killer as many wish to portray it.
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basher54321

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Unread post04 Nov 2019, 01:22

Hello jb

The OP is trying to ascertain the accuracy of an F-16 flight model in a computer game called DCS.

The explanation (as One of the reasons) I have given re AoA is pretty much word for word how Joe Sweeney (one of the test pilots on the MATV ) describes it. He is surely talking about stability in the Yaw axis do you not think. Joe Bills article has the jet settling into deep stall around 50 - 60 degrees AoA IIRC.
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Unread post04 Nov 2019, 01:44

I'm aware what the OP was trying to do. I was responding to the thread drift into G and AOA limiters and all the other drivel on this site trying to place one jet over another based on criteria X, Y, Z. Which seems to be a common form of drift, most often used to propose the Tomcat...pick your jet, is better than another jet. Total BS.

I don't know Joe Sweeney so I would never try to contradict a test pilot but I can tell you from experience that deep stall occurs at low speed well below 50 AOA and HAL has control of the jet anyway well below 35. Yaw departures were only an issue in roll coupling situations usually at higher altitudes, medium speeds, with a directionally unstable configuration (B/D model with CL tank for example). High AO low speed departures led to deep stall, not yaw departure, even when yaw was present during departure, except in an inverted (negative AOA) departure where the yaw rate limiter was not active. Maybe Johnwill can chime in but yaw stability was not a high AOA concern transmitted to me nor encountered by me....and yes, I have departed an F-16 and rocked it out. No yaw concerns. Maybe with asymmetric stores or, in the case to the MATV, while doing extreme transients in yaw or very low or nonexistent speed while thrust vectoring.
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