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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johnwill

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Unread post04 Nov 2019, 02:56

My knowledge of the flight control system is mostly limited to how it affects structural loads. All I know about why the AoA limiter is xx degrees is what knowledgable people have told me. An important reason is to help the airplane maintain energy during hard maneuvering. Lower AoA means lower drag, less speed loss, maintaining energy. Don't argue with me about if that was the right choice. It's both ignorance and apathy for me - I don't know and I don't care.
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hummingbird

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Unread post04 Nov 2019, 20:42

I honestly don't see where anyone made this a comparison thread? The F-4, F-15 & F-14 were only mentioned to give examples of jets without any FLCS keeping them within safe limits.

It's obvious the fact that the F-16 has an FLCS automatically keeping the aircraft within structural limits is a great advantage as the pilot then doesn't have to constantly watch his G's or be careful about janking the stick too hard. As an F-15 pilot once told me "A Viper driver can just pull on the stick and he gets 9 G's. I constantly have to watch myself not to pull too hard or too little", which probably a big reason as to why the Viper usually dominates the Eagle in high speed fights, the Viper simply gets to 9 G's quicker than any Eagle pilot is safely & reliably capable/willing of doing him/her-self in an aircraft without a FLCS.

With all that said the actual topic of this thread is that the real F-16's (D)FLCS allows for a very high G-onset rate regardless of the limiter, higher than what we're seeing in the current early access version in the DCS flight simulator. Up to 10 G/s is what evidence seems to suggest the real aircraft is achieving, which is the same as in the EF Typhoon, and plenty enough for in terms of what any pilot would want.
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hummingbird

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Unread post06 Nov 2019, 12:58

A video of the Gripen to show that it also has a very high G-onset rate, definitely a lot higher than 6 G/s:

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Patriot

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Unread post06 Nov 2019, 13:47

How about here? This g-meter seems to be far more acurate. The Gripen's one above looks to me like a post production video add on, rather that an actual feedback from the aircraft FLCS.

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f-16adf

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Unread post06 Nov 2019, 14:18

Gripen onset rate according to this:

viewtopic.php?f=30&t=1029

"Turn performance: 9 G sustained, G onset rate at least 6 G/s (1-9 G in 1.2 s), min -3 G, 20+ deg/s sustained, 30 deg/s instantaneous"

I think this poster (Ola) got this number from Hilgren, (circa 2002ish). And all his above figures are seemingly correct.

If the jet is at 9G in approx 1.2 sec that means it is about 6.7G/sec, since the jet is already at 1G in normal flight. Hence, at 1 sec he would be at 7.7G. I posted that Gripen video earlier in this thread. And at 5:35 he starts his turn. He is already starting the turn at 1G, (which we all experience). So if we are going to keep on this route, he hits 9G, not before 1 second, but ~1.16 sec. Divide that out and that is approx 6.8G/sec. give or take. It certainly is not 9 or 10G/sec.



And as far as the EF is concerned. I have seen info from 3 sources, and they all give different figures. One says 8/9, the other 10, and the last says 12G/sec. So which is it? And all are from the UK I believe. And does this matter? The Typhoon is a monster and basically outperforms all the teen series with ease.

And time all the F-16 turns to 9G going by the HUD, see what you get-
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Unread post08 Nov 2019, 21:48

Maximum g-onset can have serious physiological impacts. I didn't take time to look through the mil-specs (I'm not even sure there are guidelines there) but I can tell you that for F-16, F-22, etc, the max onsets the FLCS allows are set by pilot feedback (during design), not by aircraft control power or structural loads concerns. As a general rule, I would expect that the 8/9 range is probably typical. I would be very surprised if any pilot would accept 12G/sec. If the aircraft is allowed to generate that, I would expect the pilot would get into the habit of self-regulating how quickly he applies aft stick...
f-16adf wrote:And as far as the EF is concerned...One says 8/9, the other 10, and the last says 12G/sec. So which is it? And all are from the UK I believe. And does this matter? The Typhoon is a monster and basically outperforms all the teen series with ease.
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Unread post09 Nov 2019, 02:19

8 or 8.5 seems much more believable than 10 or 12. The Gripen number is from Saab. I also have another doc that gives a similar figure for that jet. Quick question, if the F-16 can do more, say 7 or 8G/sec why then does centrifuge training have all their ROR's with a 6ish G onset? Wouldn't that seem rather unrealistic? Also, on the first page of this thread I posted a quote from an F-16 Colonel (from the book Viper Force, 56th Fighter Wing-To Fly and Fight the F-16), who said at high altitude it was 3G/sec, and second only to the Raptor. Does that seem correct to you?

I tried to have my brother ask some of his squadron buddies but nearly all the prior F-16 guys moved to different units or retired. So any info on this matter would be great.
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Unread post09 Nov 2019, 03:06

My 8-9 number was kind of a wag, it might be a bit lower than that, but definitely 6 would be on the low side. I'm not really too familiar with the capabilities that centrifuges have, it may be 6 is the most they can physically generate. I'm honestly a little surprised they can get that high. I assume the only way they have to increase G is to increase spin rate, and there would be limits as to how quickly they could do that. The other option would be to rapidly shorten the arm (i.e. reduce the diameter of the circle) at a constant rotation speed - but don't think they are built that way.

I can't verify the 3g/sec onset at altitude, but it is natural that the max G-onset you can generate reduces as you go higher. You are at a lower dynamic pressure (~calibrated airspeed) for a given Mach, so you need a proportionally higher change in AOA to get the same change in G that you would get down low. To generate the larger AOA change in the same time (to get the same Nz onset), you would need higher pitch rates. If you let pitch rates get too high, though, you are risking overshoots of the G limiter, because those higher rates take longer to arrest or reverse. So, as usual, it's all a tradeoff.
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Unread post09 Nov 2019, 03:30

Thanks for the info. I think the one at Wright-Pat goes above 10. I could've had him ask, he was there this past summer.
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Unread post12 Nov 2019, 01:20

The centrifuges used for F-16 profiles are usually 10+ G/s, same for EF centrifuges.

As for the EF being a monster, in terms of maneuvering performance it for sure is, but is not what we're discussing here, we're talking about control limitation set by the designers, and there is no reason to believe the EF would have been given a higher G-onset rate in its FLCS than the F-16, as like Raptor mentioned it is about the max rate the pilot can take & really needs vs not overshooting the AoA/G limit by too much.

Btw 8-9 G/s doesn't sound unrealistic to me either, but I wouldn't doubt 10+ G/s either based on what I've now read several F-16 pilots have written me, as the specs of the centrifuges used for their training usually read 10 G/s, and they are always said to be slower than the real thing.

That said the G-onset of the F-16 in DCS lies around 5-6 G/s atm it seems, and that is definitely too low. Also hitting 9 G takes quite long in DCS as the onset rate slows down to a crawl at 8 G. In BMS the onset rate is noticably higher and it doesn't slow down so drastically and 9+ G is quickly reached. As a result the BMS F-16 feels quite a bit more agile in pitch.
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Unread post12 Nov 2019, 03:01

When I was assigned to the USAF Safety Center, F-16 G-LOC mishaps were one of the big issues. 9G capability is a challenge for the pilots to maintain blood flow and positive partial pressure of oxygen supply to the brain and eyeballs.

In conventional positive stability aircraft with slower G onset rates, the pilot sees vision tunneling, greyout, and vision blackout progressively as the G loads reduce the blood pressure and partial pressure of oxygen supply to the head. With these clues, the pilot can back off the G loads, or improve his L1/M1 straining manuever to increase the blood pressure to the brain to prevent Loss of Consciousness.

The brain and the eyes have several seconds of residual oxygen within their tissues to keep functioning without the blood supply. But if you shut off the blood supply before that residual oxygen is used up with the rapid G-onset of the unstable aircraft such as the F-16, the pilot can blow past his physiological capability to maintain that oxygen supply to the brain without the visual clues. Pull to 9 gs, everything appears perfect, then the oxygen supply is used up and it is lights out without warning.

The rest of the problem is that once the blood flow is restored, it takes several seconds before the pilot wakes up and then another 20-30 seconds before they regain their wits. When they passed out, they let go of the stick and the F-16 returns to 1 G flight. Except they were in a 9 g turn (probably in full AB), and 1 G flight becomes a descending, rapidly accelerating spiral toward the earth. Most of the time the pilot was unable to regain control of the aircraft before it impacted the ground at a very high rate of knots.
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Unread post12 Nov 2019, 04:07

Yeah, the onset rate of the F-16 killed a few pilots IIRC. It was worlds beyond anything seen until then.
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hummingbird

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Unread post12 Nov 2019, 11:33

As far as I can tell the F-16's (D)FLCS must allow for 10 G/s, otherwise there would be no point in profiles with such a high G-onset rate in the USAF training facilities. Or that pilots keep saying the onset rates are slower in the centrifuges than in the real airplane.

Measuring times to G on HUD tapes is probably not going to get us far in terms of assessing max onset rate, it will only show us what is at the very least possible, as the G-onset rate also depends on the pressure the pilot applies to the stick. Hence some videos show faster onset rates than others for the same plane, i.e. because the pilots are pulling at different pressure.

Btw F-16adf, keep in mind that a straight horizontal turn which starts with a roll actually will start either at or very close to 0 G in the horizontal plane in which the turn will then take place. I'm not saying the turns you measured started like this, but just want you to keep it in mind.
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Unread post12 Nov 2019, 13:36

Well, on this one i'm going to have to disagree. I have flown many upon many hard turns (near the Utility Category limit) in Cessna 172's and Cherokee Archer's and upon rolling into the turn I have never experienced zero G. (A Coordinated turn ie. keeping the ball centered with rudder) When I have experienced zero or negative G ---is pulling the yoke back for a few moments and then pushing it forward.

Regarding the Gripen video, https://www.youtube.com/watch?v=eXaPfUs6sQw at about 5:33 his G meter is at 1G and stays positive all throughout turn commencement. His turn to hit 9G takes about 1.16 Seconds nearly matching what the Swedish poster said on this forum about its onset rate. viewtopic.php?f=30&t=1029
The jet simply never hits 9G at 1 second or before.

Now as far as the F-16 is concerned, and as what the Colonel said in the article I posted: 3G at high altitude (and only bettered by the Raptor). Dropping down from 30-35,000ft to SL, is the question here. I simply am not going to believe unless told by an actual Pilot that you are going to gain approx. 7G/sec in onset rate from that altitude loss.
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Unread post13 Nov 2019, 04:25

hummingbird wrote:Btw F-16adf, keep in mind that a straight horizontal turn which starts with a roll actually will start either at or very close to 0 G in the horizontal plane in which the turn will then take place. I'm not saying the turns you measured started like this, but just want you to keep it in mind.


Not true. In an F-16, if the airplane is trimmed for 1g in level flight and a pure roll command (no change in pitch command) is applied, the airplane will roll, but the g command and actual g is still 1g, not 0g. If you roll 90 deg or if you roll 180 deg or 360 deg, the airplane will still be at 1g.
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