Semper Lightning: F-35 Flight Control System09 Dec 2015 Dan “Dog” Canin [LM F-35 Test Pilot Pax River]"...Generally, the F-35 tries to keep sideslip near zero, but in some cases it intentionally creates adverse or proverse yaw as necessary to control roll and yaw rates. We’ll talk about the use of pedals at high AOA in a later article [never appeared], but, for general flying around, the best coordination we’ll get is with
our feet on the floor [older hand flown miljets flown this way - in my era at least - 1960s-70s]....
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But Wait, There’s More!FBW does more than just stabilize the airplane and clean up its response. It determines the very nature of the response itself. That response can be programmed to be whatever we want, as a function of the airplane’s configuration, speed, or whether it’s in the air or on the ground. For example, if we make a lateral stick input in CTOL mode, we get a roll rate. But in jetborne mode, we get a bank angle. At high speed, a pitch stick input commands a normal acceleration (“g”); at low speed with the gear up it commands a pitch rate; at low speed with the gear down, it commands an AOA; and in the hover, it commands a rate of climb or descent....
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Protecting Us From OurselvesThe control limiters in the F-35 – love them or hate them – are there to help. They not only make the airplane safer, but also more effective, by allowing us to fly aggressively without worrying about breaking something or losing control.
But flying the F-35 is not completely carefree. The control engineers had to give us some rope in a few places, since doing otherwise would have compromised capability and possibly even safety. So it’s important for us to understand what’s protected and what isn’t....
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Rolling and yawing – so-called “asymmetric maneuvering” (maneuvering using lateral stick or pedal inputs) – is another story.
If we don’t pull more than 80 percent of the positive NzW limit or push to less than negative 1g, we can roll and yaw to our heart’s content. But if we push or pull more than that, we have to abide by a pilot-observed limit of 25 degree/second. (Stick your hand out in front of you and roll it through 90 degrees while counting to three potatoes. Yup, it’s slow.) I know what you’re thinking: “How do I know when I’m more than 0.8NzW?” You don’t – unless you’re good at mentally dividing the basic flight design gross weight (BFDGW) by your current gross weight and multiplying it by 0.8 times the basic g-limit for the airplane. (If you can do that, continuously, you’re probably in the wrong line of work.) And, “Why 25 deg/sec?” Because that’s the loads folks’ definition of “zero”: if you’re rolling less than 25 deg/sec, they consider that not rolling, so symmetric limits apply.
But, mostly, you’re thinking: “What’s with the pilot-observed limit? Why couldn’t the control engineers just protect us with CLAW?” The reason is that the analysis and the design work to handle every asymmetric input, under every flight condition and loading, would be prohibitive. And if they put the 25 deg/sec limit into CLAW, it would be tactically restrictive and possibly unsafe. So they picked the middle ground of telling us not to roll too much while we’re on the g-limiter.
So what happens if we make a big roll input at 0.9 NzW? First of all, the CLAW folks haven’t completely abandoned us: 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. But there’s no guarantee that the unload will be quick enough to prevent an overload.
Does that mean we can break the airplane by pulling and rolling? Not really. The pilot-observed limits were decreed to make sure the airframe delivers its contractually specified life. If we exceed them, the wings won’t fall off, but we might reduce some of that life. The bottom line: If you’re on the g-limiter and want to roll, back off a little, then roll. This will not only keep you within the rules, it will give you a better roll rate in the bargain. If you can’t back off – because, say, you’re trying not to hit the ground, or trying not to get shot (and I don’t mean by your buddy during BFM) – then do what you need to do! The worst thing that will happen is that you’ll trip an OVER G advisory or an overload HRC,
[5] and have to explain your heroic act to the maintenance officer when you return. Presumably, the maneuver will be worth the airframe life you expend....
...The F-35 is an inherently unstable airplane, required to handle a wide range of CG. Its control surfaces are sized to meet the requirements of both maneuverability and low observability. As a result, the combinations of body rates, AOAs, CGs, Machs, and weapon bay door positions that define the controllable envelope of the F-35 are extremely complex – and the boundaries of that envelope are reflected, with all that complexity, in CLAW. If the control engineers opened up the limiters and gave us, instead, “rules of thumb” to maintain control – ones that we had a fighting chance of remembering – the rules would most likely be so restrictive that we’d give up more than we gained. Could we evolve to that in the future? Sure, if we decide it’s a positive trade. As the control engineers hate to hear us say, “It’s only software.”...
...The second article will address how FBW works in the F-35, and why it was designed that way." [NOT SEEN SO FAR!]
"[5] An OVER G advisory will trip if you exceed the book symmetric or asymmetric maneuvering limits by more than 0.5g. For the purposes of this ICAW, the airplane defines as “asymmetric” any roll rate over 50 deg/sec, so there’s a 25 deg/sec buffer there as well. So if you stick to the flight manual roll rate limit, you should never see this ICAW. What you might trip, though, is an “overload” HRC, which has a much more sophisticated algorithm behind it and will only trip when you’ve exceeded an actual limit on some component of the structure. CLAW should in all cases prevent actual overload to failure, but during rolling maneuvers it may allow one of these indications to trip, requiring a maintenance inspection."
Source: http://www.codeonemagazine.com/article.html?item_id=187 
Posted: 17 Nov 2016, 02:29
Interface
Here's a relevant CLASSIC thread! 
