I recently subscribed to this mailing list for insightful discussions on
surround sound.

Up 'till now, the material I have received is about aviation.

Am I in the wrong place?

:-)

*Pan Athen*
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On Wed, Mar 8, 2023 at 1:03 AM Sampo Syreeni <de...@iki.fi> wrote:

> On 2023-02-22, Fons Adriaensen wrote:
>
> > And in many cases the aircraft may very well be unstable in that axis:
> > if left alone, the roll angle will slowly increase.
>
> Actually, most modern aircraft are stable in the bank axis as well. Part
> of why they have swept wings, bent wings and wingtips and the like, is
> to this regard. (Part of: most of it has to with approaching transonic
> flight. But not all.)
>
> The thing is though, and as you say below, the pilot won't feel anything
> weird when approaching a spiral. The built in stability of the airplane
> will keep everybody in their seat at 1g acceleration perpendicular to
> the floor, evenas the airplane banks to something approaching 90
> degrees, and loses all of its lift. Then it just falls, sideways.
>
> When that happens, you're in what's called a "death spiral", because
> it's extremely difficult to recover from the condition, and you
> typically don't even know you've entered one. When you do, you as a
> pilot are already in a state of spatial disorientation; you *literally*
> don't know which way is up and which down, and since the plane is by now
> basically half-way inverted, with now absolutely no lift, losing
> altitude like a falling rock, you as the pilot have very little
> possibility of correcting.
>
> *Technically*, in *theory*, you often *could* recover, if you have
> enough altitude, speed and sturdiness of airframe; even I have run it
> through in a game. But in practice, recovery from a well developed death
> spiral is mostly beyond human ability. Especially once you lose height,
> because at low altitudes, already going nose down, you can't even
> convert high air speed/energy into a corrective manoeuvre before you hit
> the terrain, and there will only be seconds to lose.
>
> This is then why the pilot flying is supposed to only look at the
> instrumentation, and why there are auditory warnings about bank angle on
> the modern jets. The Swedish commercial midsize Boeing pilot, Mentour,
> on YouTube, is first rate in explaining all of this stuff.
>
> Okay, so, finally, how would you recover from a well developed death
> spiral, presuming you realized you were in one? Well, the optimum way
> would be to use all of the airfoils at the pilot's control at the same
> time to convert kinetic and potential energy of the frame into first 1)
> orientation, and then 2) into safe height in level flight.
>
> The optimum control trajectory going there is universally wild, so that
> you can't even practice for it in a simulator. It can even be chaotic,
> in the true mathematical sense. Many of the attempts at automated
> recovery I known of literally crashed on that point; you can't do
> optimum control here, because it leads you into an unstable calculation.
> Instead, you have to have your algoritm flying off the optimum path, in
> order to keep a stability margin. (Knowing how much off the optimum path
> it should be, and what a stability margin even *is*, is to date an
> unknown as well. It's difficult to quantify.)
>
> So, how would I fly out of a death spiral, suddenly and against
> expectation fully knowing I was in one? Fully knowing which way, how
> fast, at which height, I and my aeroplane was going? Well, obviously, I
> would have to regain lift, evenas I was falling. I'd use ailerons to
> gain "level flight" evenwhile falling. While that was done, I'd yoke up,
> no matter the orientation of the airframe (assuming I wasn't downright
> inverted), in order to gain altitude and *true* level flight. I'd put
> the engines in idle and maybe spoil the airfoil, for want of
> structurally sound airspeed and the g-forces which necessarily come
> after a recovery from a spiral. Something like that.
>
> Even if I did all of that *just* right, I'd probably contact terrain.
> All on-board would be lost. Because recovering from a death spiral, once
> it's started and developed well, is pretty much an inhuman feat. It's
> almost impossible for a computer to do, as well. The many algorithms
> which have been tried out, taking control away from the human pilot,
> none of them have been shown to do any good either.
>
> Fons, this is one of the other things I follow. Amateurishly, but I
> still do.
>
> https://www.youtube.com/watch?v=b7t4IR-3mSo
> https://www.youtube.com/watch?v=JhzaogGQNFU&t=1056s
> https://www.youtube.com/watch?v=o7N0pshAC0k
>
> Now the fun thing is, as my pilot friends say, you can actually *even*
> do a full, level, aileron roll on all of the AirBus jumbojets.
> Definitely not recommended, might lose your licence, and you'd have to
> disengage a number of safety systems, then flying by eye and touch
> alone. But I'm told all of them *can* do showflying manoeuvres if need
> be. My nerd friends even claim to me, if need be, the things might be
> capable of autonomous inverted flight; I'm not too sure if any flight
> manufacturer actually ever went so far, but if one did, it surely would
> be AirBus. Damn, woman, stop it right *here*!
>
> > A non-zero roll angle means that part of the lift force generated by
> > the wings is now sideways.
>
> Now, once we've handled conventional aircrafts, let's take on fighter
> aircraft. The fifth generation ones, like the F22 and the F35. Their
> fly-by-wire flight control surface work in concert, and often in a fully
> different way. For example, instead of there being separate spoilers, in
> order to introduce a limited airfoil separation/stall, the primary
> control surfaces introduce a time-limited and measured stall in a servo
> loop. And then not, again.
>
> Those things basically fly themselves, strategically stalling some of
> the control surfaces when need be. For example, in order to
> automatically come back from a leftward death spiral, the fighter will
> make its right back control surface stall, losing lift momentarily, and
> so roll the aircraft as well as make it pitch up, lose airspeed because
> of the increased drag, and so gain an amount of lift.
>
> Fons, I like this kind of analysis. Optimum control, and where its
> limits lay. This stuff is *fun*!
>
> > That - and not the rudder - is what makes the aircraft make a turn.
>
> The rudder is also a fun turn — pun intended. It's even "fun" in the
> death spiral recovery manoeuvre. Because you cannot efficiently come out
> of a spiral using only ailerons. While coming down, in order to rectify
> the spiral, you actually have to apply a lot of rudder as well.
> Otherwise you'll end up a whole lot more down, with disastrous airspeed.
>
> > The vertical component of lift is reduced, and a pitch-stable aircraft
> > will just by itself increase its airspeed to restore it. It can do
> > that only by going down at that same time.
>
> True.
>
> > Unless you watch the horizon or the attitude indicator, you will
> > not be aware that this is happening.
>
> True. Whence the 178 seconds above. Also, "spatial disorientation in
> aviation". This Youtube channel of mine, "Mentour", has done quite a
> number of features on just this thing. He's a commercial pilot, and even
> has access to flight simulators. See above even for him inverting his
> native 737 in one.
>
> > As the roll angle increases, the g-force will apparently remain
> > vertical (relative to the aircraft) but increase as well.
>
> Actually the g-force does not increase at all. That's why the death
> spiral is so nasty: you don't feel anything at *all*, evenwhile you're
> going nose down into the ground.
>
> Much of that is because of the intrinsic stability of the aircraft.
> Because the stability means the craft wants to stay at 1g towards the
> floor. While it stays that way — no matter its actual attitude — you
> won't feel anything off even if the thing is inverted in a barrel roll —
> a nice and harmless aerobatic movement — or in a death spiral — with
> at most two seconds to die.
>
> > And at some point you will notice that you are pinned down in your
> > seat and unable to move - you are effectively in a centrifuge, way too
> > fast, going down, and the g-forces will be so high that they can break
> > up the aircraft.
>
> This only happens once you gained too much airspeed and try to recover
> by pulling up on the yoke. True, if you're already there, not much can
> be done to recover. But at least don't then pull up the yoke too fast in
> order to break the airframe. At max do something like a "gentle" 5g
> curve, and if you then manage to not crash into the terrain, level off
> and apply some spoiling.
>
> Ah, you too think about this. Hmm. 8)
>
> > To recover: > > 1. Reduce power to idle.
>
> Preferably as soon as you know you're losing altitude. Because you'll be
> trading potential energy for kinetic energy/speed from the get go. This
> is also why I mentioned fighter jets and dog fighting from the get go:
> that energy count-down (or up) is how dogfighting has been counted from
> the start. It's how dogfights are won, and the energy management is also
> how planes are either crashed or landed safely.
>
> >> 2. Bring the wings level. This has to be done gently, to avoid even
> >> more mechanical stress.
>
> Yes. However, this is difficult to do once you went into spatial
> disorientation, your synthetic horizon is at something like 120 degrees,
> and you descend at a about a five kilometres per minute, from an
> altitude of, say, a generous ten thousand feet. Within a thick cloud
> cover, with all of your instruments yelling at you at the same time.
>
> >> 3. As the wings return to level, the excessive speed will put the
> >> aircraft into a steep climb.
>
> What is "level", here? In a death spiral, the optimum recovery will take
> you through a route where you'll *definitely* not be level. Your nose
> will be looking down, at an airspeed which is *way* over your craft's
> design limits. That will also take place well after you can laterally,
> in ailerons, balance the aircraft; as such, even a very little take on
> the ailerons, or the rudder, the yoke, would immediately either stall
> some control surface, or made better, tear each of them apart. And you
> don't really know what is "level" hear, either; even your
> instrumentation is probably fucked up already; believe you me, no
> inertial thingy ever survives the kind of vibration an aircraft induces
> on itself when put into a multiple g's acceleration, combined with a
> wide stall.
>
> >> Let it happen but keep the pitch angle under control.
>
> Exactly so. "Let it happen." Many of the worst accidents on record have
> happened because pilots fought their planes, instead of "going with the
> flow" which a plane, designed to be statically stable from the start,
> would have done by itself. For example, (ya'll, prolly not Fons) take a
> look at: https://en.wikipedia.org/wiki/Pilot-induced_oscillation .
>
> >> You will regain some of the lost altitude, and airspeed will
> >> decrease.
>
> Recovery from a near miss death spiral is still more involved. Because
> you might have to operate the aircraft at structural load, and do a
> recovery from a prolonged stall over all of the airframe. You might
> actually have to "fly" your airframe over a minute in a full stall over
> every part of it, and then try to regain aerodynamic control. "After
> sinking, flying, and shaking like a rock from a cannon."
>
> It can be done. But nobody teaches you how to do this, and in fact, I
> don't know of *one* algorithm which has flown this route.
>
> > 4. As you approach normal airspeed, bring back power and level off.
>
> That should be obvious, then. It's that third stage before "Profit"
> which always slights the eye. ;)
>
> > Ciao,
>
> Moro.
> --
> Sampo Syreeni, aka decoy - de...@iki.fi, http://decoy.iki.fi/front
> +358-40-3751464 <http://decoy.iki.fi/front+358-40-3751464>, 025E D175
> ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2
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