The Hidden Troubles of the F-35 [DefenseNews]

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steve2267

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Unread post13 Jan 2020, 20:16

I recalled the CL_alpha plot Spurts posted in my vapes thread the other week had an inflection right around alpha = 20°.

I shameless grabbed Spurts' plot and annotated it. I also ran across a statement somewhere in the last day -- and I cannot recall where, whether it was on here, or an article I ran across -- but it said the F-35 likes to fight at around 20° alpha. Which kind of makes sense. If the wing stalls around 20°(ish), then the pilot would be trying to max perform the wing right before the stall point, which should be right about where he get's the best "natural" CL_max, no? (Without all the Stab&Ctrl magic & greatly increased drag -- actually, wouldn't this be pretty close to the L/D_max point?)

So if you're trying to fight the jet around the corner speed and L/D_max... right before the stall point (i.e. abrupt flow separation point), but the OML, driven by VLO requirements, has abrupt wing stall issues... then, yes, I could see how Stab&Ctrl magic which is being used to handle these flow issues, could be involved here, and updates to said magic could address any issues.

CL-alpha (stall region annotated).png


How F-35 Experience Could Reduce Hurdles To Developing Fighters
07 Sep 2018 Graham Warwick
...

Requirements call for maneuverability “on par with any fourth-generation fighter,” says Canin, “which is an achievement for an aircraft with an outer mold line driven by other requirements.” The F-35 has to be able to use all the maneuverability it has. The program office called for air-to-air tracking up to stall AOA, or alpha, followed by predictable and controllable post-stall handling. “The aircraft has to be departure-resistant in any normal tactical maneuver and recover with minimal pilot input,” he says.

“At high AOA, the [aerodynamic] model is very challenging to build, so there is some augmentation outside the model to correct for errors,” says Canin, adding: “High-alpha control is all about allocation of horizontal tail power for yaw and pitch.”
...

Source: http://now.eloqua.com/es.asp?s=96691307 ... Id=13994#1 [no longer available of course] [AvWeak Original: http://aviationweek.com/combat-aircraft ... g-fighters


I find myself going back to my thoughts on that vapes thread... To "use all the maneuverability it has," I posit that LM has pushed the wing right to the edge, and those "vapes" I noted appearing simultaneously across the span of the wing -- from the "mini-lerx" (some people asked about) right out the tip -- are an indication of how LM has taken advantage of as much lift as possible across the entire wing. Spurts made a great comment that you cannot deduce a lot about the "flow field" from the vapes... that the vapes are really only highlighting a region of low pressure. So the dominant region of low pressure on the F-35 wing appears to be across the entire wing. If the flow across that entire region abruptly separates... that would tend to explain "abrupt wing stall behavior." If one wing abruptly stalls, then the other, you're going to experience asymmetric lift -- a roll.

The F-16 and F/A-18 designs do not suffer from this phenomena as greatly, because they cheat -- they depend more on their forebody strakes & LERX to generate and manage their lift as they get into high AoA / post-stall flight (F/A-18), and this is evidenced by photos / videos showing their low pressure region (i.e. "vapes") to be concentrated in the vortex street shedding coming off the strakes / LERX. The F-35, most probably due to VLO requirements, does not have LERX, well, not nearly as large -- it has the nose chine and a small "lip" running back from the leading edge of the air intake to the wing. One definitely sees vortex shedding coming off the chine/air intake lip on the F-35, but it is a much smaller region compared to the Viper/Hornet, and is small compared to the vapes that emanate from across the entire wing leading edge.
Take an F-16, stir in A-7, dollop of F-117, gob of F-22, dash of F/A-18, sprinkle with AV-8B, stir well + bake. Whaddya get? F-35.
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sprstdlyscottsmn

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Unread post13 Jan 2020, 20:46

Just a note, that Cl curve is for the F-16 in a wind tunnel, a few points on the plot HAPPENED to line up with estimations I had made independently of F-35 linear and max CL values
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steve2267

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Unread post13 Jan 2020, 20:48

Ahh, I found the quote about where the jet likes to fight -- it's in the infamous CLAW test memo. (Re-attaching it in case someone wants a copy handy.)

F-35A High Angle of Attack Operational Maneuvers
Author unknown 14 January 2015

Test Aircraft: AF-2, Test 715, Flight 505, Configuration 10-001B (Clean Wing), 0.1-v12.006 (R33.1)

...

High Angle of Attack Blended Region

The flying qualities in the blended region (20-26 degrees AOA) were not intuitive or favorable. This was especially frustrating because as the sortie progressed it was apparent that the aircraft fought best at the lower end of this alpha whether turning or established in a tree/scissors; so the lateral/directional control was often unpredictable. This flight seemed to be especially effective in revealing this flaw because in most tests the AOA is readily apparent (or targeted) and, therefore, the response is expected. However, during a dynamic fight, where attention is focused on the bandit rather than the specific AOA, the lateral/directional response was often confusing. There were multiple times where a roll rate was expected yet not achieved or a body-axis yaw rate was expected and beta resulted. In other cases, the response changed during the maneuver as the AOA blended into this region.

During a tree, the anti-spin logic engaged as a direct result of this unpredictability. The F35 had gained a 3/9 advantage and the pilot desired to maneuver behind the bandit. A full rudder input had no result initially, but after a few seconds the jet began to maneuver simultaneously to the command being abandoned and replaced with stick input. Once the delayed result appeared from the initial rudder input, the rudder was promptly re-input to encourage the aircraft to continue. A fantastic yaw rate followed, only to be spoiled by the anti-spin logic. The anti-spin logic was surprisingly pronounced. As has been experienced on other high AOA missions, there is ample control authority for arresting yaw rate. Whereas rudder inputs often feel sluggish/gradual or delayed, the anti-spin logic is immediate, abrupt, and forceful. Perhaps some of the available authority may be given to the pilot while still preventing departure.

In retrospect, a seemingly valuable improvement would be to adjust the blended region to at least 30° AOA. There are two reasons. The first is to ensure predictability. Since this aircraft seemed to fight best near 20°, controls should not be blended near this region. The pilot is not consciously at "high" AOA at 25° but at 40°, an affirmative decision was made to be there. The second is purely geometric and also aids in predictability. Geometrically, at 26° the aircraft is still relatively "shallow" so it's still intuitive that a roll stick would result in a stability axis roll and a rudder would result in yaw. Mathematically, an even blend (50%) would occur at 30 degrees (sin30°) and this seems to match the "seat of the pants" feeling for the pilot as well.

...


Note: the CLAW test reportedly took place around 18-22,000' MSL, and starting airspeeds ranged from 380kts - 440kts.

Ya know... the more I re-read this old portion of the CLAW test... I would not be surprised at all if the "journos" were simply regurgitating old tripe from the CLAW test, but this time focusing on the "modal confusion" aspect, and unexpected aircraft responses. Combined with the fact that the aircraft is moving through aerodynamic stall / separating flow, potentially transonic shocks dancing across the upper wing surfaces / and into post-stall flight characteristics... Applying Occam's Razor, poor journalistic practices / rehashing old stories would seem to be the most likely explanation -- a whole lot of todo about nothing (or problems already solved and put to bed with CLAW updates).
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F-35 High AoA Maneuvers.pdf
F-35A High Angle of Attack Operational Manuevers, 14 Jan 2015.
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Take an F-16, stir in A-7, dollop of F-117, gob of F-22, dash of F/A-18, sprinkle with AV-8B, stir well + bake. Whaddya get? F-35.
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steve2267

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Unread post13 Jan 2020, 20:56

sprstdlyscottsmn wrote:Just a note, that Cl curve is for the F-16 in a wind tunnel, a few points on the plot HAPPENED to line up with estimations I had made independently of F-35 linear and max CL values


Spurts, thanks for that clarification.

I will note that companies seem to like to stick with something they have used in the past / that has worked well for them. To wit, while I have not found any confirmation in open source literature, it seems reasonable to assume (dangerous!) that the F-35 airfoil may be the same as what the F-16 used, OR very similar. Additionally, although not identical, and with the Viper's trailing edge being perpendicular to the fuselage, the wing planform of the F-35 is not drastically different than the F-16. So I would not be surprised to learn that Cl curve in that plot for the F-16, is similar to the F-35. Specifically, that there is some sort of inflection in the 20-26° range. Maybe the LM guys and gals squeezed a few more degrees (or tenths of a degree) out of the Cl & CL curves before separation occurs. But I doubt they found an extra 5° alpha. FWIW.
Take an F-16, stir in A-7, dollop of F-117, gob of F-22, dash of F/A-18, sprinkle with AV-8B, stir well + bake. Whaddya get? F-35.
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Unread post13 Jan 2020, 21:01

The fight described in the article is slow speed scissors stuff, nowhere near transonic speeds.

This, described in the Canin article, is what was happening wrt creating modal confusion in the fight described in the other article — “As g increases, the roll rate is reduced, and, if we’re commanding more than 50 deg/sec, the airplane unloads to get us back within the 0.8NzW limit.”

The comment about fighting better at 20a was (in my view) more about getting predictable response to control inputs because once you went higher they got confusing modal responses (command ‘roll’, get ‘pitch’ response first).

They wanted more alpha, the jet was plenty capable of giving more, but the non-modal (my word) response was a consequence and thus they preferred not to go there (into and above the blend).
Last edited by quicksilver on 13 Jan 2020, 21:11, edited 2 times in total.
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steve2267

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Unread post13 Jan 2020, 21:08

quicksilver wrote:The fight described in the article is slow speed scissors stuff, nowhere near transonic speeds.

This, described in the Canin article, is what was happening wrt creating modal confusion in the fight described in the other article — “As g increases, the roll rate is reduced, and, if we’re commanding more than 50 deg/sec, the airplane unloads to get us back within the 0.8NzW limit.”


Too many "articles" and I'm becoming confused about just what we're discussing.

I can see how the quote you make about roll rate being reduced at high G, as introducing "modal confusion." But I also see "modal confusion" in the CLAW test report which was occurring at scissoring speeds (what -- 150-250kts?), that is, command roll, but get yaw (or roll & yaw), or command pitch but get nada etc.

So whether it be when scissoring, or trying to turn & burn up around corner speed or higher... I see that we are basically discussing flying qualities that are a result of the CLAW logic. Ho hum. Far far far from the "world is ending" as the journos would have the reading public believe.
Take an F-16, stir in A-7, dollop of F-117, gob of F-22, dash of F/A-18, sprinkle with AV-8B, stir well + bake. Whaddya get? F-35.
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Unread post13 Jan 2020, 21:29

steve2267 wrote:
quicksilver wrote:The fight described in the article is slow speed scissors stuff, nowhere near transonic speeds.

This, described in the Canin article, is what was happening wrt creating modal confusion in the fight described in the other article — “As g increases, the roll rate is reduced, and, if we’re commanding more than 50 deg/sec, the airplane unloads to get us back within the 0.8NzW limit.”


Too many "articles" and I'm becoming confused about just what we're discussing.

I can see how the quote you make about roll rate being reduced at high G, as introducing "modal confusion." But I also see "modal confusion" in the CLAW test report which was occurring at scissoring speeds (what -- 150-250kts?), that is, command roll, but get yaw (or roll & yaw), or command pitch but get nada etc.

So whether it be when scissoring, or trying to turn & burn up around corner speed or higher... I see that we are basically discussing flying qualities that are a result of the CLAW logic. Ho hum. Far far far from the "world is ending" as the journos would have the reading public believe.


And alotta this stuff has already been tweaked (it’s now 2020...) — for example, tripping anti-spin logic (in 2015) vs the high yaw rates Hanche (or perhaps the Venable/Heritage article) refers to and that we also see as a matter of routine in the airshow demos.

Because we live in the digital age, flight test has known more and learned more (i.e. ‘discovery’) about the F-35 than any fighter aircraft ever.
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