At what Angle of Attack is F16when buffet occur ?

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saberrider

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Unread post12 Nov 2017, 17:38

At what AoA in low speed regimes pre stall buffet occur ?
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basher54321

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Unread post14 Nov 2017, 20:07

You mean like a physical shaking of the aircraft, that other aircraft types would have experienced?

Thought for the most part you couldn't reach that AoA due to the limiter so would not see the shaking - maybe sub 150 kts at high AoA perhaps but not certain?
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Unread post15 Nov 2017, 07:37

basher54321 wrote:You mean like a physical shaking of the aircraft, that other aircraft types would have experienced?

Thought for the most part you couldn't reach that AoA due to the limiter so would not see the shaking - maybe sub 150 kts at high AoA perhaps but not certain?

But , I believed till now about buffet of the F16 wings will occur early on comparative to another 4gen. fighter (F18,Mig 29 )due to span of the wings.
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Unread post15 Nov 2017, 10:57

So physical movement of the wings? - do you have any examples?
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vilters

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Unread post16 Nov 2017, 17:51

At low speed (down to 100kts), there is no buffet to speak of.

But at higher speed and high G load you feel (and hear) the air "unsticking" and hammering the airframe.

The wings don't shake a lot.
It is mostly the air stalling and unstalling on the airframe.
But depending what is on them, the wingtips do "vibrate" under the pressure/vortexes/stalling/unstalling.
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Gums

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Unread post16 Nov 2017, 19:45

Salute!

Right, Vilters.

Need Roscoe here, but the LEF's use mach and such to determine their deflection. And it's those doofers that dramatically reduce "buffet" that we encountered in some earlier planes.

I think I recall a nibble or two, but nothing like I saw in the Sluf or Voodoo. Never flew the Double Ugly, but those guys say it was a strong buffet.

My first backseat ride was an ACM duel with a Top Gun A-4. I was amazed when we got down to 130 or 140 KIAS and no buffet, but plenty of turn capability. It wasn't fair. One turn and the A-4 was doing the last ditch maneuver as we gunned him.

GD did a super job on that aspect of the design. I also add that at 15 or 20 deg AoA in the Double Ugly or Sluf would have you trying hard to read the shaking instruments.

Gums recalls...
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rheonomic

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Unread post17 Nov 2017, 04:33

Gums wrote:Need Roscoe here, but the LEF's use mach and such to determine their deflection.


Up and away it's a function of Mach, AOA, and (maybe) pitch rate if memory serves.
u = (CB)⁻¹(cvdt_des - CAx)
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saberrider

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Unread post17 Nov 2017, 15:47

rheonomic wrote:
Gums wrote:Need Roscoe here, but the LEF's use mach and such to determine their deflection.


Up and away it's a function of Mach, AOA, and (maybe) pitch rate if memory serves.

OK, but at what AoA start to shake ? 18 degrees or less in
high speed ,because in my Sim is early @ 14. -16.But read about 27degrees for the lower ones?And everyone talking here about 30 degrees like so is easy to maintains at lower speed but I couldn't turn a full radius if pulled back at corner speed.
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Unread post17 Nov 2017, 21:21

Salute!

Actually, any noticeable buffet prolly happens around 15 deg AoA when the gee starts to decrease on the limiter function. AT that point you have zero LEF deflection.

Guess you will have to get the flight test data from the JTF from 1974, huh?

Gums sends...
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saberrider

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Unread post17 Nov 2017, 22:52

Thanks Gums, for taking the time to answer first hand , but in CAT 1 i suppose buffet to be at higher angle if not you'd have a bumping ride every time you pull and stay in that position of angle.By the way you telling me that at 15 degrees /9G 's LEF's are not fully down position?Woww!!!I believed that the max 9G come with full 25* deployment of the LEF's. and around 7*down stabilator.
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Unread post18 Nov 2017, 00:25

Salute!

Go find the AoA/gee limiter function. I can take a photo, but here's what is is like.

At "x" speed you can pull 9 gees, so about 360 knots IAS or a little above. Your AoA will eventually reach 15 deg, but you can pull the 9 gees at 4 or 5 or 10 deg AoA.

As you slow down, the function goes from 15 deg AoA at 9 gees to 1 gee at 25 or 26 deg AoA. Books might say 27 deg, but what's a degree or so among friends. That means that once you hit the max AoA you are at 1 gee max command and most likely actual gee.

Now, if you wish to see a deep stall, then keep the nose high at 80 deg pitch and unload. Once below 100 knots or so you will lose pitch authority and you are at the mercy of your trajectory. If the FLCS is commanded full nose down you will like likely get into the deep stall. In my only close call, I relaxed the stick and was looking over my shoulder. Slow speed horn came on and I just let the jet do its thing. A nice tail slide with all the lights on and the thing flipped over and soon had good speed.

I would not allow a sim to make a big thing about buffet, as there was and is little to none buffet in the VIper regardless of loadout and such. If you wish to jitter the HUD display, then start at 15 deg AoA and wash it out by 25 deg. Not any gee input, only AoA. And all that is just for looks, as the real plane didn't anything like that.

Gums sends...

P.S. See if your sim can duplicate my deep stall entry procedure and get back to us.
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vilters

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Unread post18 Nov 2017, 00:53

Amaai Gums. . . . 100 Kts IAS with the nose 80° high? => Gonna be a wild ride home. Change in underwear at arrival.. LOL.

And Gums is right :"G" capability reduces with increasing AOA. At max AOA, you are out of options and 1G flight is all what's possible.

Something before max AOA, the FLCS takes over the rudder control completely, and at max AOA it removes all pilots inputs until back under max AOA.

=> The only input possible is by using the MPO switch to pump the thing out of a deep stall. (works both upright and inverted)
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saberrider

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Unread post18 Nov 2017, 02:01

So no more rudder deflection at AOA max . limiter .Only MPO to work with for dropping the nose . But if the damn thing enter in the spin ? Maybe MPO not enough to work just pitch .Rudder override function is an good things to add .Read about no aileons inputs in stall.Thanks to all for answer me.
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Unread post18 Nov 2017, 02:51

Salute!

c'mon Saber!!

Go read the FLCS control laws and look at the graphs. Sheesh. I am ure you can find them someplace.

The FLCS cuts out manual pitch and roll and rudder above "X' sensed AoA. It then provides "anti-spin" rudder/yaw control. Unfortunately, if you are about 40 deg AoA you have no down pitch authority ( a basic aero feature of the jet), but you have "positive" authority. But you can't use it because FLCS has cut you out. Hence, the MPO and we could pull back and et the nose up, then rock the sucker outta the deep stall.

PLZ look at all the previous posts about all this. Sheesh.

Gums sends...
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rheonomic

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Unread post18 Nov 2017, 16:38

PDF is kind of shitty but the block diagrams are in this: F-16 Simulator for Man-in-the-Loop Testing of Aircraft Control Systems (SIMTACS)

Might be too hard to make out though.

NASA TP 2857 has better quality lat/d block diagrams.

Image

Also: https://i.imgur.com/nLkcuTR.png

Have fun, I guess.
u = (CB)⁻¹(cvdt_des - CAx)
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