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* SoundFellas <https://soundfellas.com/>, *MediaFlake Ltd <http://mediaflake.com/>* Digital Media Services, Content, and Tools 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 > _______________________________________________ > Sursound mailing list > Sursound@music.vt.edu > https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here, > edit account or options, view archives and so on. > -------------- next part -------------- An HTML attachment was scrubbed... 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