On 2023-02-22, Fons Adriaensen wrote:
Is flying in a spiral not something you would do in some intentional
way?
Yes and no...
Yes and no. It can be done and has been done as a showsport. As an
aerobatics exercise. But going into a death spiral is *definitely* not
safe; anybody doing it has to be well versed in recovering from it
unless se wants to hit the ground; even sooner than se thought it would
be possible. Also, due to the somatogravic illusion which often then
sets in, and the fact that the situation can progress to a nosedive much
faster than even trained pilots often think, doing such a thing on
purpose is rather reckless. It just calls of loss of life.
There is no essential difference between a spiral and a normal turn,
except that a spiral is usually not intentional and in that case it
can become so extreme that it puts the aircraft in danger. Both spins
and spirals are done intentionally as part of pilot training.
Not *quite* true, I think. Because in a spiral you lose altitude, by
definition. In a normal bank you might not, because you can — and should
— use thrust and rudder in order to maintain it.
True, you sometimes might want to do a hard bank *and* lose altitude.
It's being done. But going into such a situation places you in a dynamic
which pilot-loop-considered is highly unstable. Especially in instrument
flying conditions, where you don't have a stable, intuitionistic
attitude reference. And since in those conditions you the pilot often
get kind of counter-intuitive signals from your middle ear and your body
as to where you are, you might even half-deterministically guide the
plane the *exact* wrong way.
In fact my favourite Swedish pilot channel, Mentour Pilot, just made a
video about this: https://www.youtube.com/watch?v=keQSpUq6Vis&t=1497s .
All (normal) aircraft have pitch stability, but few have roll
stability.
In fact almost all have perturbative roll stability as well. However,
few to none aircraft have absolute stability, in the sense that they'd
recover themselves from *all* conditions exceeding their design flight
envelope, by passive means. Actively, sure, most of today's 4th to 5th
generation fighter jets can do *unbelievable* stunts under computer
control in order to recover, automatically, as can modern passenger
jets, but a purely passive recovery from things like an upset, a stall
in any flight surface, a spiral... They're just not in the design
manual; the airplane just isn't designed — nor possibly could be
designed to be — that stable over the whole state space.
This is then why the flight control systems and warning systems try and
warn the pilot before they approach the passive stability margin. "Bank,
bank." Because after that you're wont to go into aerobatics, which
typical commercial pilots haven't trained for. (My favourite examples to
the contrary again come from that Swedish pilot of 737's, Mentour Pilot.
He inverted his plane in a simulator, and recovered from the resulting
dive. He also analyzed to the hilt an upset resulting from wake
turbulence, leading to a rapid but again recovered inversion.)
That means that if the wings are not level, there is nothing that
would make them return to level.
There in fact is, to a point. That's why we have upwards sloping wings
on aircraft with low wings, and downwards sloping on those such as the
Stratofortress. They lend a *modicum* of roll stability to an airframe,
but then they do so only within a slim margin. Beyond it, passive
stability is lost.
Which is also one of the reasons commercial pilots have trouble handling
a high, unstable bank angle. Sure, they've undergone one or two
trainings in a simulator for it, but then they almost never actually
encouter it on air. Then when it happens, panic sets in, and whoops, you
but forget about your training, you don't even get to look at your
instrumentation, as you should. Especially in instrument conditions;
whence again, 178 seconds; how it feels to do a death spiral.
https://www.youtube.com/watch?v=pc9xI4kpY4w&t=2s )
And in many cases the aircraft may very well be unstable in that axis:
if left alone, the roll angle will slowly increase.
No, it necessarily doesn't. If you're within the stable flight envelope,
it will, but you might not be, and you won't necessarily know whether
you are.
Then you now have 178 seconds to live. And you don't know it. You'll
only know it at most tens of seconds before your demise.
The "fun" thing here is that I only started minding the aerodynamics of
aircraft via this very list. About how air *really* behaves in extreme
conditions, and what it then does, around physical objects. Like
long-throw bass speakers. How it becomes turbulent and even transsonic
there.
This talk about spirals might be kind of tangential here, yes,
off-topic...but actually it might not be *fully* so.
A non-zero roll angle means that part of the lift force generated
by the wings is now sideways. That - and not the rudder - is what
makes the aircraft make a turn.
Yes, though in a bank, if you simultaneously want to hold altitude,
you'll use the rudder in the opposite direction, and because you're now
adding huge amounts of drag, you'll need to counter with increased
thrust. You'll also be flying kind of sideways, the way we often see
airoplanes land in crosswinds, only you'll be doing so at an arbitrary
bank angle towards the local gravitational gradient. That's tickling the
tail of a dragon, in a passenger airplane, whose thrust to weight ratio
is significantly below one; you cannot automatically bring up your sum
of kinetic and gravitational energy as you could in a modern fighter
plane, but must trade kinetic and gravitational energy towards each
other, considering drag, which might in high alpha situations deplete
both. Also airframe design speed and such; even if you have sufficient
total energy, you might not have a statespace trajectory from where you
are, into where you want to safely be; "it's just too late, because your
airframe cannot take the g-load in order to pull up, before you it the
ground".
These kinds of optimization problems are handled much better in modern
military airplanes. Which is why at least some of their automation,
espeically their advanced, predictive, ground proximity warning radar
systems, might in the coming years be integrated into civil aircraft.
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.
In a graveyeard spiral the same mechanism, if uncontrolled, will
increase the airspeed uncontrolledly, until the aircraft rips itself
apart. Because the craft went aside its overall stability envelope,
considered as a whole. None of its axes, individually considered,
necessarily went too far, but in whole, it still went beyond its
perturbative stability margin, and dove its nose into the ground.
This happens over and over. And it isn't correctable via passive means,
because there are even theorems which say it *cannot* overall be
corrected, passively. If you go outside of the envelope, there are full
mathematical theorems which say only active correction will suffice to
rectify *most* of the even slight upsets and pretty much of all of the
major ones. I don't remember them by name, but there are plenty of them
in aerodynamics; dozens, possibly.
Unless you watch the horizon or the attitude indicator, you will not
be aware that this is happening.
Exactly. Yes, and thank you for pointing that out. I'd too point this
out over, and over, and over. Especially in intrument flying conditions,
you should never rely on your feelings, but also on the instruments. Of
which there are two or three sets on any bigger plane. You probably
should scan them continuously even on clear skies, because even then you
might be susceptible to certain other fallacies of perception, which the
instrumentation can resolve.
As the roll angle increases, the g-force will apparently remain
vertical (relative to the aircraft) but increase as well. 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 by the way is why in fighter planes and even in Airbuses we use the
sidestick instead of a yoke: even if your hand is pinned down to a
"table", you can still apply slight sideways forces in order to control
the plane.
Though I have *no* idea how that would "fly" in an inversed 9g
manoeuvre. I mean, your hand and grip would fly just of, along with your
neck and head. They would do so even at the civilian planes' maximum g's
of something like four.
To recover:
1. Reduce power to idle.
Yes, because you're diving, and will gain airspeed.
2. Bring the wings level. This has to be done gently, to avoid even
more mechanical stress.
Yes. However, you're still in a dive at this point, and you might even
be at a speed which breaks your ailerons if you use them. You'll have to
control that as well, and the differential angle of attack of all your
control surfaces, once you start to recover.
Fons, this is precisely why I suggested such unconventional control
solutions as I did before (without having flown a plane). Because since
in a graveyard spiral you're probably already at a very high speed, you
have to slow it down while you have to bank back and yoke back. So the
(my) theory goes, if you can, you have to optimally at the same time (to
recover from a leftwards/counter-clockwise graveyard spiral), spoil your
right wing, yoke up, aileron to the right, apply airbrakes, and then
rudder left to counter the hard aileron which comes with this sort of
manoeuvre. Maybe even use a slight (1') one-sided flap to a slight
degree on the left, in order to add lift on that side and even more
drag; then the rudder might even have to be turned, because the plane
would now be tending against the wind, and flying half sideways.
Yet I think would be in a controlled flight, and since we didn't rev the
flaps or the spoilers too much, they would be usable even now. (After a
death spiral recovery, with the gain in airspeed, it's never a given any
control surface is in a condition anymore, and might go into a high
stall if used. My theory of recovery tries to distribute the load of the
recovery over as many surfaces as can be used. I don't think my theory
is too far from what NTSB, and many others, have recommended.)
3. As the wings return to level, the excessive speed will put the
aircraft into a steep climb.
Only if you're in steady flight, then. You might not be, and the airfoil
might not steady itself. After such a manoeuvre, often taking it into
slowly dying oscillation about its three separate rotational access,
coupling those into its translational axes, and finally into its forward
and downward momenta, it often goes into pilot induced oscillation (
https://en.wikipedia.org/wiki/Pilot-induced_oscillation ). Which is very
difficult to correct after a graveyard spiral, since the pilot still has
to recover, actively controlling the aircraft. Se cannot let go of the
craft for fear of the craft doing a slower nose-down, but se'll now have
to deal with induced oscillations from the airframe, say phugoid
oscillations, which can feed back into hir decision-making. (
https://en.wikipedia.org/wiki/Aircraft_dynamic_modes ).
That steep climb, if not corrected, leads to just this, and after the
drop in altitude from a spiral, even if you let it be. (
https://www.youtube.com/watch?v=rFWfrmjAQxY )
Let it happen but keep the pitch angle under control. You will regain
some of the lost altitude, and airspeed will decrease.
As I said, not necessarily. In a graveyard spiral you might already have
reached the maximum design speed of your airframe, and you might be,
beside your intentions, closer to ground than you thought you were.
Recovering from such a situation — if even possible in those 176 seconds
— doesn't quite follow your normal pilot guidelines.
4. As you approach normal airspeed, bring back power and level off.
I'd rather say, level off, of course, but proceed at something
approaching minimum speed, at a low level. Because you don't know how
badly you just damaged your airframe in recovery. You would have pulled
multiple g's, and nothing apart from fighter jets is rated for anything
like that. Especially laterally they aren't.
If you then fly a potentially damaged airframe, it's better to do it so
in lower altitude and at a lower speed, because there's 1) lower
turbulence to shake things aloof, because 2) in thicker air you'll get
more lift per mile so that less fuel keeps you up longer in case of a
recurring fault, 3) it's safer to do a crash or a "water landing" from a
lower altitude, with the lower glide slopes and lower airspeeds, and 4)
the radio coverage makes more sense in this case when flying low, at
least over high turnover airfields such we'd have in the busyest of the
US; we'd want to have it local here/there.
--
Sampo Syreeni, aka decoy - de...@iki.fi, http://decoy.iki.fi/front
+358-40-3751464, 025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2
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