The Hidden Troubles of the F-35 [DefenseNews]

[mixelflick]

Nevertheless, you accuse me of being a liar. In fact you went so far as to capitalize it for the whole world to see (UTTER LIES). Saying that I'm spreading that (supposed) lie to help make my points. That's a pretty strong accusation to make, knowing you can do so behind an anonymous username.

Ok, while I still stand by the logic of what I said in my previous post, I admit that I used some harsh words towards you, namely and specially the ones that you're referring in the part that I quoted above (I should have used other words).

As such I apologize for the words that I used above.

Apology accepted.

Now lets just forget this and move forward..

[ricnunes]

Apology accepted.

Now lets just forget this and move forward..

Thanks and roger that.

[steve2267]

The forum was a bit quiet today, so like an idiot I googled "F-35 news" and went to see if there were any new stories. What did I find but National Bullsh*t re-running some old, regurgitated trash. And, like an idiot, I read it. It referred back to the June 2019 hit piece by little Miss Aircraft Designer extraordinarie, Ms Insinna, which also referenced the piece by Larter:

When US Navy and Marine F-35 pilots most need performance, the aircraft becomes erratic
DefenseNews By: David B. Larter June 12, 2019

WASHINGTON — The U.S. Navy’s and Marine Corps’ F-35s become unpredictable to handle when executing the kind of extreme maneuvers a pilot would use in a dogfight or while avoiding a missile, according to documents exclusively obtained by Defense News.

Specifically, the Marine short-takeoff-and-vertical-landing variant and the Navy’s carrier-launched version become difficult to control when the aircraft is operating above a 20-degree angle of attack, ...

Pilots reported the aircraft experiencing unpredictable changes in pitch, as well as erratic yaw and rolling motions. ...

A Lockheed Martin executive told Defense News in a statement that he expects the issue to be resolved or downgraded soon as a result of software fixes.

“We’ve implemented an update to the flight control system that is planned for integration in the third quarter of this year — and we expect this item to be resolved or downgraded,” said Greg Ulmer, Lockheed Martin vice president and general manager of the company’s F-35 program.

...

In a deficiency report from the fleet, aviators said the issue "will cause modal confusion, prevent precise lift vector control, and prevent repeatable air-to-air combat techniques, resulting in mid-air collisions during training, controlled flight into terrain, and aircraft loss during combat engagements with adversary aircraft and missiles," according to the documents.

“Fleet pilots agreed it is very difficult to max perform the aircraft” in those circumstances, the document notes.

...

“It has random oscillations, pitch and yaw issues above [its] 20-[degree angle of attack]," the aviator said.

...
https://www.defensenews.com/smr/hidden-troubles-f35/2019/06/12/when-us-navy-and-marine-f-35-pilots-most-need-performance-the-aircraft-becomes-erratic/

I just re-read through this entire thread, including the last few pages of troll baiting & bickering to make sure I didn't miss anything.

Here's my question:

Has anyone read any followup reports or actual journalistic reporting (and not this yellow journalism) or corresponded with anyone who knows what the h*ll

• experiencing unpredictable changes in pitch
• erratic yaw and rolling motions
• modal confusion
• random oscillations, pitch and yaw issues above 20° alpha

mean?

Never did find a clarification on what modal confusion is. Maybe a Bee pilot got confused and though he had inadvertently pushed the magic Mode 4 button? (Like that would happen.)

These uber reporters do not even give an inkling where in the flight envelope this occurs. Was it at low altitude only? At all airspeeds? Or only within a certain airspeed range? (20° alpha probably ain't happening @ 500kts, for example.)

Are these issues just "buffet" being blown way out of proportion? Or is this another name for transonic rolloff? (But again... don't think you're going to be above 20° alpha in the transonic region and still have your wings attached.) I can't decide if this was just sensational BS by non-pilots that don't know what buffet is? Or is this something altogether different than buffet? (Frankly, I'm unclear on how some FLCS CLAW changes are going to fix buffeting.) If CLAW changes made the issue go away, does that imply the "issues" were CLAW-induced from the beginning?

I'm fairly certain above 20° alpha, the F-35 is definitely experiencing flow separation. What with the VLO requirements precluding aero tricks like leading edge fences or sawteeth etc, the aircraft OML design is hard pressed to fix separation locations to the same geometric location at all times, let alone at the same location from port to starboard, or that the flow separates off the port and starboard identically. Hence I can easily see how flow separating randomly from the port to the starboard could lead to not only buffet, but to "random" roll oscillations. BUT... the OML of the F-35B is pretty close to the F-35A... why didn't the -A have similar issues? (Could the fairing over the gun create enough flow asymmetry that it actually ameliorates the issue?) Of course, I could be all wet too.

Anyway, has anyone read or seen any further elucidation of just what the h*ll these cub aero reporters were getting all huffy about?

[spazsinbad]

Maybe 'modal confusion' is similar to 'model confusion'? Pilots don't know what F-35 MODEL they's in?

[quicksilver]

“Anyway, has anyone read or seen any further elucidation of just what the h*ll these cub aero reporters were getting all huffy about?“

Were I interested in the matter, I would go to the LM AIAA papers.

[Gums]

Salute!

Interesting "opinions", I feel. I did not see the original stuff back in June.

When I get a fellow aviator repeat the heresay I pull out my iPhone and show him the Paris demo. Typical reaction is "duuuhhhhh, errrrr, planes aren't supposed to be able to fly like that"

Gums sends...

P.S. My first official F-22 demo I saw was back in 2007 at the Ft Lauderdale all-service bash, complete with a performance by the Birds. Reaction amongst our august, seasoned, combat veteran pilots of my vintage was the same. "How do dey do dat?"

[quicksilver]

steve, help us understand your concern given that we are now 7 months beyond the reporting which included statements from both USG and LM leadership that the issues were already resolved, or on a pathway to resolution or downgrade.

My limited understanding of the claws is that there is some kind of transition in the control laws to a different logic (my word) that occurs once the jet gets above a certain aoa. We have spoken in recent threads about how the claws in many aircraft can sometimes ‘over-protect’ the pilot (or aircraft, depending how one looks at it) from exceeding a variety of different limits. Those limits are sometimes not placard limits but rather are other kinds of limits that are put in place in the claws to prevent an exceedance for other things discovered in flight test like, e.g. — loads on specific structure, or a sideslip rate that combined with a certain alpha might be predictive of a departure from controlled flight, etc. And, sometimes what a test pilot finds is that the ‘over-protection’ can be adjusted a bit to make sure the pilot isn’t just along for the ride in circumstances where ones proximity to other jets creates its own danger. In other words, the jet can be unnecessarily out of the pilot’s control because the claws won’t respond to certain control inputs in a meaningful way. IIRC, early in the life of the Viper there were some CFIT mishaps where control law limits were a contributing factor. These things get resolved in on-going engineering and flight test.

I’ve pointed out before that in spite of all we humans think we know, the ‘imperfect’ and the ‘unknown’ always accompany technological advancement.

[spazsinbad]

'QS' comment above is reflected by the LM Test Pilot Dan Canin article repeated now a few times on this forum explaining.

Semper Lightning: F-35 Flight Control System 09 Dec 2015 Dan “Dog” Canin

http://www.codeonemagazine.com/article.html?item_id=187

[steve2267]

QS,

Not sure I'd describe my inquiry as "concern," rather more like "curiousity." Given the absolute sh*t nature of the reporting, I could note tell what the "journos" were writing about.

• experiencing unpredictable changes in pitch
• erratic yaw and rolling motions
• modal confusion
• random oscillations, pitch and yaw issues above 20° alpha

Does anyone know what "modal confusion" is -- other than an attempt to get a different callsign? (Maybe someone really wanted "hammer", but would not under any condition be given that, so they figured they'd try for "MC"?)

"Unpredictable changes in pitch" and "erratic yaw and rolling motions" sounds like it could be the buffet "Dolbe" Hanche described in some of his Norwegian blog articles. But "random oscillations, pitch and yaw issues" leaves me scratching my head trying to understand what is being described. Depending on how flow is shedding off of different parts of the aircraft, and its frequency... I suppose you could get some oscillations, especially if it somehow coupled with the CLAW somehow.

Maybe the question I am trying to ask, and doing a poor job, is: "are these idiot reporters describing buffet / making a big todo out of buffeting?" Or was there something else going on?

But if it was buffet... I don't see CLAW fixing buffet. I am guessing you can't move the stabs or any other control surfaces fast enough (nor would you want to?) to try to damp out buffet like that.

If it was not buffet, the engineer in me is curious what it was.

It would appear the story "died" and went away because the CLAW fixes LM put into place corrected any issues identified by testing.

Again, as a recovering aero engineer, am curious what was "corrected" or was a "non-event" in the first place. It's really hard to tell with the level of reporting. (I think I may have "missed" this story when it came out -- oh... I may have been on up in Canada paddling the boundary waters on the Boy Scout Atikokan High Adventure with my son during this time. That would explain why I didn't see it until now.)

[steve2267]

Saw Spaz' post in a different thread... viewtopic.php?p=433023#p433023

The article he quotes there mentions the F-35 (as well as other designs) having "abrupt wing stall" characteristics. Yeah, I could see how if the wings abruptly stall that's going to have an unexpected pitch characteristic. And if one wings abruptly stalls before the other... that would tend to induce roll... and roll-yaw coupling could give you yaw. So... it's beginning to sound like issues dealing with how the Stab & Ctrl wizards were dealing with the onset of stall through post-stall flying qualities.

[quicksilver]

Modal confusion... e.g. — I make a control input that would normally produce a roll but I get a pitch response instead.

Our expectations of how a flying machine should/will respond to our inputs is very much a consequence of our experience flying the machines we have flown before. Even with experience in different types there is still a learning process that goes on in each new type. In short, they are all different — sometimes by greater or lesser degrees — but different nonetheless. Mastery of each requires not just technical knowledge but repeated exposure that develops a ‘feel’ for predictable response; one is trying to get to the point where one intuitively and instantaneously understands, “...when I do this, I get (...the jet will do) that,” or, “...if I want to do maneuver X, I have to do input Y first.“.

Sometimes the learning can be quite stark, even for those trained to understand and identify the differences in very technical terms.

[sprstdlyscottsmn]

If this is the old Transonic Roll Off issue...

" However, TRO “has evolved into a non-factor,” says Nelson, who likens the effect to a momentary “tug” on one shoulder harness. “You have to pull high-g to even find it.” The roll-off phenomena exhibits itself as “less than 10 deg./sec. for a fraction of a second. We have been looking for a task it affects and we can’t find one.”

is the quote I have on it

[sprstdlyscottsmn]

Modal confusion... e.g. — I make a control input that would normally produce a roll but I get a pitch response instead.

That sounds like the pilots feedback from the old "F-16 dogfight" CLAW test. It was trying to blend from low AoA laws to high AoA laws at 20-25 degrees when teh test pilot said it should be upped to 30-35 degrees.

[quicksilver]

'QS' comment above is reflected by the LM Test Pilot Dan Canin article repeated now a few times on this forum explaining.

Semper Lightning: F-35 Flight Control System 09 Dec 2015 Dan “Dog” Canin

http://www.codeonemagazine.com/article.html?item_id=187

Great recall spaz. A great example of a product that helps pilots understand how and why F-35 is/may be different than the machine they flew before.

[steve2267]

If this is the old Transonic Roll Off issue...

" However, TRO “has evolved into a non-factor,” says Nelson, who likens the effect to a momentary “tug” on one shoulder harness. “You have to pull high-g to even find it.” The roll-off phenomena exhibits itself as “less than 10 deg./sec. for a fraction of a second. We have been looking for a task it affects and we can’t find one.”

is the quote I have on it

I suspect this is not the old TRO issue... I don't think the jet will be up around 20°+ alpha up in the transonic region. It sounds like they are trying to fight the jet (i.e. fly @ 20° and fight other aircraft) up around 20° alpha, which sounds like just below natural wing stall AoA... that is around L/D_max or corner speed.