CL Max and FLCS

So I see from CFD that the CL Max for the F-16 is quite high, but that the FLCS keeps it lower at the speed for max turn (the plateau). But I also see that the true CL Max occures above the FLCS limit of 25.5 deg. Why is the F-16 so limited to a lower CL Max? It would have a substantially faster and tighter turn with the full lift it can generate. yes I am aware that it would increase drag and deceleration, but that is just a fact of flight. It seems to me that the F-16 in the only aircraft that limits itself. Are there S/C concerns above 25.5 deg AOA or 15 deg at higher speeds? Just trying to expand my knowlege and understanding. I know there are a few people here in the know.

If you look at the graph of Angle of Attack vs, Load Factor (sometimes called "g" ) limit for the F-16, you will see that the AoA limit for Category I or II external stores is 15 degrees at 9g and 25.5 degrees at 1g. And you are right, there are stability and control issues above that limit, usually related to max command rolls while at the high AoA. You can see that most contemporary fighters have two vertical tails, while the F-16 has one. The F-16 designers looked at two tails, but decided (with USAF concurrence) that two tails did not provide enough benefit to justify the added weight, cost, drag, and structural problems of two tails.

So, as it is, the F-16 can compete and usually win against anything short of an F-22 in a WVR fight, but at low speeds, it loses some of its advantages.

Any F-16 pilot can further explain how they overcome the low speed disadvantages.

I should mention that Category III stores (air to ground) are limited to approximately 15 degrees AoA at all g levels (for the same reasons).

Salute!

It all comes down to maintaining energy. We had the highest sustained turn rate for a 360 of anything in the air in 1979.

Sure, we could have pulled 9 gees at 20 deg AoA and a bit slower, same for 25 deg. Meanwhile we are losing 20 knots per second and having our brains splattered all over the HUD as the gomer strafes us. So the ramp from 15 deg to 25 deg was there to prevent that. We lost some nose pointing ability that the "normal" jets had, but we generally didn't have to sweat it.

We could also move the stick anyplace we wanted and not depart. We could roll really nice at 25 deg AoA and not have to sweat keeping the stick from moving a mm left or right and then using rudder to roll (ask Double Ugly folks about this).

One thing to look at is acceleration toward the center of the turn compared to speed. THAT is what determines your turn radius. And we could pull 9 friggin' gees at about 350 knots and hold it forever when below about 10K. NOBODY could touch that in those days and for some years afterwards.

I initially whined about the FLCS limits until I flew the thing. Made me a believer in a few seconds.

That's my story, and I'm stickin' to it,

Thanks, Gums. Your explanation should mean a lot more then the technical hoo-hah I posted.

Every jet has advantages and disadvantages. The F-16 is no different. She just has more advantages than disadvantages. Don't fight in the limited areas but if you have to... then the F-16 does just fine against other fighters. She also accelerates better than anything out there other than the F-22.

Generally getting slow in a 1v1 fight is okay if and only if there is just one other jet out there to fight against. The F-16 excels at 1v1's and will do just fine slow. But if there is more than one bad guy... don't get slow. The F-16 with the FLCS does a great job not getting slow.

The Viper lets it's pilot concentrate on fighting his opponent and not fighting to maintain control of his plane at the same time.

Gums wrote:We could also move the stick anyplace we wanted and not depart.

I think that Gums nailed this one. Yes, other airplanes can reach higher AOA. But there is a world of difference between having a greater theoretical turn capability, and actually being able to use it in a combat situation.

Thanks again Gums.

johnwill wrote:If you look at the graph of Angle of Attack vs, Load Factor (sometimes called "g" ) limit for the F-16, you will see that the AoA limit for Category I or II external stores is 15 degrees at 9g and 25.5 degrees at 1g. [...] I should mention that Category III stores (air to ground) are limited to approximately 15 degrees AoA at all g levels (for the same reasons).

Just 2 notes:

1)CAT III limits the aircraft at 16-18 degrees AoA (new vipers).
2) well... what's that CAT II? (perhaps old vipers?)

Cheers...

JpoLqr,

You're correct on the AoA limiter. I recall it was slightly variable, so called it approximately 15 degrees to CMA.

As you know, there is no Cat II switch position (never has been), but there are Cat II loadings. When a loading is Cat II, the Cat I switch position is used, but the pilot must observe one extra maneuver limit. Cat I loadings are permitted to fly full command 360 degree rolls, but Cat II is limited to full command 180 degree rolls. Cat III has reduced roll rate and AoA capability (automatic) and is limited to 180 degree max command rolls (pilot observed).

Roughly speaking, Cat I is air to air without 370 tanks, Cat II is air to air with 370 tanks, and Cat III is air to ground. There are exceptions found in the -1 stores limitations chart.

All of this information is extremely old (1979 - 81), which was the last time I worked F-16. So if things are different now, blame it on my old age.

Thanks for the info johnwill!

Modern vipers have completely "forgot" CAT II!
For maximum command roll, the limit of bank angle change is 360 degrees, even for CAT III. On the other hand, the maximum roll rate command for CAT III is reduced about 40% off that available in CAT I (by the FLCS limiter).

Cheers...

Thanks for all your replies, especially yours Gums. I guess turning 20deg/sec for 10 seconds is better than starting at 30 deg/sec and slowing to 14 deg/sec after half a turn. I didnt think about how keeping the alpha down gives you more roll controll too.

JpoLqr,
Glad to hear they lifted the limit on bank angle change. Back in the Stone Age, the limit was conservative and was needed (possibly) at low airspeed (under 200 kt.). I think there was also a 200 kt min airspeed limit for hard maneuvering in Cat III.

Salute!

I gave an example about sustained turn rate, but maybe not all the story.

Instantaneous turn rate was much higher than 15 or 20 degrees per second.

Look at some film from an adversary took when he managed to sneak into six and we had to do a "break turn", and tell me how we could do the "bat turn". After FLCS control laws kicked in to keep us below bad aero limits, we settled into that nice sustained turn rate you see on the charts.

One performance consideration for selecting the YF-16 over the YF-17 was sustained turn rate. The 17 (and later Hornet) could turn better for about a second, then they had to ease off and go around the turn watching the YF-16 continue to sustain that great turn rate and NOT LOSE ENERGY doing it.

Then there was transonic acceleration. Oh yeah, then there was range and fuel burn rate ( Hornet burned gas like the Eagle, even though it was smaller and lighter),

Gotta go,

Gums sends ....

Interesting conversation once again! With all that experience from the early days we can actually see the whole viper evolution, from YF-16 up to block52+.

P.S. johnwill, that 200K limitation is still here, even stricter. The CAT-III jet cannot be operated at speeds below 200K, except for T/O & landing.

Cheers...

Salute!

We used the "switch" to simulate something like a Flogger when doing BFM/ACM lessons. AoA versus gee limit was interesting.

OTOH, once you drop the eggs in a real Cat III config, flip that stoopid switch and have a real Viper!

I have heard the rumor that the Navy Aggressors had several FLCS control laws they could use to simulate other jets. Anybody know about this rumor?

Gums sends ....

I dont know about that specifically but I have heard that Lockmart and NASA can change the FLCS in it to simulate a variety of airplanes performance, same thing one a broader scale. Gums, I have read that the newer Floggers could outclimb Vipers. Arent Vipers the newer of the two designs? Any info on that?