The Hidden Troubles of the F-35 [DefenseNews]

Discuss the F-35 Lightning II
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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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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, 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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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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Unread post24 Apr 2020, 21:09

The caravan has moved on but the dogs are barking non-stop AS IF (it makes a difference except in online views perhaps).
The Pentagon will have to live with limits on F-35’s supersonic flights
24 Apr 2020 David B. Larter, Valerie Insinna, & Aaron Mehta

"WASHINGTON — An issue that risks damage to the F-35’s tail section if the aircraft needs to maintain supersonic speeds is not worth fixing and will instead be addressed by changing the operating parameters, the F-35 Joint Program Office told Defense News in a statement Friday.

The deficiency, first reported by Defense News in 2019, means that at extremely high altitudes, the U.S. Navy’s and Marine Corps’ versions of the F-35 jet can only fly at supersonic speeds for short bursts of time before there is a risk of structural damage and loss of stealth capability.

The problem may make it impossible for the Navy’s F-35C to conduct supersonic intercepts.

“This issue was closed on December 17, 2019 with no further actions and concurrence from the U.S. services,” the F-35 JPO statement read. “The [deficiency report] was closed under the category of ‘no plan to correct,’ which is used by the F-35 team when the operator value provided by a complete fix does not justify the estimated cost of that fix.

“In this case, the solution would require a lengthy development and flight testing of a material coating that can tolerate the flight environment for unlimited time while satisfying the weight and other requirements of a control surface. Instead, the issue is being addressed procedurally by imposing a time limit on high-speed flight.”

The carrier-launched "C" variant and the short-takeoff-and-vertical-landing "B" version will both be able to carry out all their missions without correcting the deficiency, the JPO said.

The potential damage from sustained high speeds would influence not only the F-35’s airframe and the low-observable coating that keeps it stealthy, but also the myriad antennas located on the back of the plane that are currently vulnerable to damage, according to documents exclusively obtained by Defense News.

The JPO had classified the issues for the "B" and "C" models as separate category 1 deficiencies, indicating in one document that the problem presents a challenge to accomplishing one of the key missions of the fighter jet. In this scale, category 1 represents the most serious type of deficiency.

While it may seem dire that an aircraft procured for flying at supersonic speeds will be unable to do so for extended periods, the F-35 may not need to do it that often.

For the F-35, as opposed to the F-22 where supersonic flight is baked into its tactics, the ability to fly supersonic is more of a “break glass in case of emergency” feature, said Bryan Clark, an analyst with the Hudson Institute and a retired naval officer. “Supersonic flight is not a big feature of the F-35,” Clark said. “It’s capable of it, but when you talk to F-35 pilots, they’ll say they’d fly supersonic in such limited times and cases that — while having the ability is nice because you never know when you are going to need to run away from something very fast — it’s just not a main feature for their tactics.”

In fact, going supersonic obviates the main advantages of the F-35, Clark said. “It sort of defeats all the main advantages of the F-35,” he explained. “It takes you out of stealthiness, it burns gas like crazy so you lose the range benefits of a single engine and larger fuel tank. When you go into afterburner, you are heating up the outside of your aircraft.” That creates all kinds of signatures that can be detected by an adversary, Clark said....

...The issue is compounded for the Navy, which must operate forward for months at a time, because any significant issues with coatings or the structure of aircraft would require a depot-level repair. And so a damaged aircraft would remain damaged until its host ship returns to home port, reducing the combat effectiveness of the air wing. [duh - every naval aircraft faces this & other issues for various reasons - sometimes replacement aircraft are flown out to the carrier]

“We might have to be operating at sea for eight months, so if you damage something on week one, guess what? It’s damaged for the rest of the deployment,” the aviator said. “And it affects your ability to evade detection by the enemy — you just degraded that asset permanently until you can get it somewhere where it can be fixed, at great expense and time.”

Other deficiencies [How GREEN is my GLOW & Tired TYRES damaging Hydraulics & Cold Battered Batteries explained]

Source: https://www.defensenews.com/air/2020/04 ... c-flights/
A4G Skyhawk: www.faaaa.asn.au/spazsinbad-a4g/ & www.youtube.com/channel/UCwqC_s6gcCVvG7NOge3qfAQ/videos?view_as=subscriber
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Unread post24 Apr 2020, 23:46

So...having missed the reality on this issue when they brought it up last summer, they attempt to double down by saying that it won’t be fixed. (Hint: It’s such a problem that it won’t be fixed. Cue the ‘putting our young men and women at grave risk’ meme).

And then they quote a (smart) CSBA guy...who was a submariner.

Humor is good for the soul on a Friday afternoon. :wink:
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Unread post25 Apr 2020, 00:13

quicksilver wrote:So...having missed the reality on this issue when they brought it up last summer, they attempt to double down by saying that it won’t be fixed. (Hint: It’s such a problem that it won’t be fixed. Cue the ‘putting our young men and women at grave risk’ meme).

And then they quote a (smart) CSBA guy...who was a submariner.

Humor is good for the soul on a Friday afternoon. :wink:



I just love documents that we can't read which are excerpted or shown to anonymous or
unqualified people for analysis. All of which results in a "new" story that confirms that
JPO didn't do what they said they weren't going to do.
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Unread post25 Apr 2020, 03:20

Anyone heard anything regarding this:

https://www.defensenews.com/air/2020/04 ... c-flights/

Looks like at certain altitudes, the stealth coating on the B and C models is damaged when going supersonic for an extended period of time.

Zero mention of the A having an issue.
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Unread post25 Apr 2020, 03:34

commisar wrote:Anyone heard anything regarding this:
https://www.defensenews.com/air/2020/04 ... c-flights/
Looks like at certain altitudes, the stealth coating on the B and C models is damaged when going supersonic for an extended period of time. Zero mention of the A having an issue.

Posted above today and posted earlier from DEFneuz - just a usual repeat from INSINNER - a speciality of de house.

GOto Start of this thread and beyond: viewtopic.php?f=22&t=55673 one of 13 DEAFiciencies: 12 Jun 2019
" • Supersonic flight in excess of Mach 1.2 can cause structural damage and blistering to the stealth coating of the F-35B and F-35C."
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Unread post25 Apr 2020, 03:44

commisar wrote:Anyone heard anything regarding this:

It's talked about in the original article that this thread is based on; a single F-35B and a single F-35C, back in somewhere around 2013 or so, in a single flight each, experienced damage to the skin of their tails when using their afterburners for extended periods in order to fly at supersonic speeds and high altitudes, near the edge of their flight envelope. The test team tried to replicate this issue, but to date no other F-35 has ever received that damage.

Because Lockheed and the JPO don't know exactly why the damage occurred (but didn't occur in later tests), they can't guarantee it won't ever happen again, and so it was made into a pair of Category 1 deficiencies (one for the B, one for the C) and a time limit on afterburner usage above certain Mach speeds has been put into place for pilots to abide by when combat doesn't necessitate it (there's no software limited preventing pilots from exceeding time limits if they feel it's necessary). F-35s have also since (back in like 2014 or 2015) received an upgrade to their stealth coating on those tail surfaces that face the afterburner plume, but again, they can't guarantee that this

Neither Lockheed nor the JPO are aware of any materials available today that would meet the stealth and thermal requirements needed to sufficiently guarantee that no harm would come from extended afterburner use, and so ultimately they've decided that it's not worth spending potentially hundreds of millions, or possibly billions of dollars on a wild goose chase for a material that might not yet be feasible.
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Unread post25 Apr 2020, 04:48

Dragon029 wrote:
commisar wrote:Anyone heard anything regarding this:

It's talked about in the original article that this thread is based on; a single F-35B and a single F-35C, back in somewhere around 2013 or so, in a single flight each, experienced damage to the skin of their tails when using their afterburners for extended periods in order to fly at supersonic speeds and high altitudes, near the edge of their flight envelope. The test team tried to replicate this issue, but to date no other F-35 has ever received that damage.

Because Lockheed and the JPO don't know exactly why the damage occurred (but didn't occur in later tests), they can't guarantee it won't ever happen again, and so it was made into a pair of Category 1 deficiencies (one for the B, one for the C) and a time limit on afterburner usage above certain Mach speeds has been put into place for pilots to abide by when combat doesn't necessitate it (there's no software limited preventing pilots from exceeding time limits if they feel it's necessary). F-35s have also since (back in like 2014 or 2015) received an upgrade to their stealth coating on those tail surfaces that face the afterburner plume, but again, they can't guarantee that this

Neither Lockheed nor the JPO are aware of any materials available today that would meet the stealth and thermal requirements needed to sufficiently guarantee that no harm would come from extended afterburner use, and so ultimately they've decided that it's not worth spending potentially hundreds of millions, or possibly billions of dollars on a wild goose chase for a material that might not yet be feasible.

It was one incident apiece for the B/C. It occurred ~50,000' and above M1.2. Changes were made to the jets, and they were unable to recreate the phenomenon in further tests. The phenomenon has never been seen in fleet jets.
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Unread post25 Apr 2020, 11:29

For the F-35, as opposed to the F-22 where supersonic flight is baked into its tactics, the ability to fly supersonic is more of a “break glass in case of emergency” feature, said Bryan Clark, an analyst with the Hudson Institute and a retired naval officer. “Supersonic flight is not a big feature of the F-35,” Clark said. “It’s capable of it, but when you talk to F-35 pilots, they’ll say they’d fly supersonic in such limited times and cases that — while having the ability is nice because you never know when you are going to need to run away from something very fast — it’s just not a main feature for their tactics.”

In fact, going supersonic obviates the main advantages of the F-35, Clark said. “It sort of defeats all the main advantages of the F-35,” he explained. “It takes you out of stealthiness, it burns gas like crazy so you lose the range benefits of a single engine and larger fuel tank. When you go into afterburner, you are heating up the outside of your aircraft.” That creates all kinds of signatures that can be detected by an adversary, Clark said....


Clark has been dismissed in these comments as a “submariner” and as an “unqualified” person. I am also an unqualified person so I have to ask where does his analysis break down? To me it seems a reasonable assessment.
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