2009 Lawsuit

BobW:

Assuming a condition where the C of G is out of limits, how would that
affect the flight characteristics in a way that would result in the
accident as it played out (unintentional stall-spin transforming into a
spiral dive which then wound up tight enough to generate g-forces
sufficiently high so as to break the wings)? - I'm currently going through
the reference material I have on hand as I found, to my embarrassment, that
I wasn't at all certain about the various effects of C of G on flight
characteristics.

Good on you for refreshing your CG-related book knowledge! General reply to
your 1st-sentence-question below next paragraph...


I assume that only a too far aft C of G could be a problem in this case. An
aft C of G makes a stall-spin easier to get into and can result in the spin
being unrecoverable in extreme situations correct? I would think that an
aft C of G would also make it easier to induce positive g's with the
elevator and would make the elevator control lighter and "twitchier" making
it easier to overstress the glider during the subsequent spiral. I can't
see how the C of G would lead to the spiral dive being unrecoverable
though.

You seem to've sussed out the correct (as I understand things) general answer
to your lead-in question...based on what's expressed in sentence 1 of the
immediately-preceding paragraph. Same comment applies to the second sentence
of para. 2.

As the CG is shifted aft, the aircraft stability decreases until
(definitionally) when the CG and ship/pilot neutral point coincide, the ship
is "neutrally stable." With a CG aft of the neutral point, the ship is
definitionally (stick-free) unstable. "Unstable" doesn't mean "instant
uncontrollability and certain death" but it DOES have sufficient import that
designers almost certainly define their ships' POH's aft CG limit "somewhat
forward" of the neutral point. You'd have to know the ship's designer to know
for certain how he addressed this aspect of the ship's design.

Ease of/possibly inadvertent entry to the spin, non-pilot-commanded transition
from a (generally recoverable) nose-down spin to a (sometimes not recoverable)
not-so-nose-down "flat spin", "twitchy" pitch characteristics...that about
covers the broad brush downsides to "too far aft" CG considerations.

Considering the topic of a glider transitioning from a spin to a spiral dive
greatly complicates an already complex situation (i.e. spin dynamics), and my
general response to the implicit question in para. 2's final sentence is, "The
devil is in the details/it just depends (on the ship, the air, on the system
CG, on pilot input, on LOTS of things)." There are reasons every glider POH
(of which I'm aware) limits their spin recovery verbiage "simply" to spins,
and don't consider aspects extending to botched or delayed spin recoveries. (A
line has to be drawn somewhere...)

Every pilot is free to explore those limits on their own, ideally in an
intentional manner, as opposed to doing so unintentionally. Not that I've
flown a wide variety of gliders, but I have flown or taken BFRs in 3 different
ATC-ed gliders approved for spins, spun each, and noted each had widely
varying spin characteristics and (to a lesser extent) varying recommended (and
actual) spin recovery techniques. The single-seater in that mix actually had
completely different spin characteristics in opposite spin directions...yet
was the most eager to recover "on its own" in either direction.

I've also flown 3 1st-generation high performance single seaters, none of
which I ever spun, each of which I intentionally and fairly extensively
explored departure (from controlled flight) characteristics as part of
"routine self-education." One of those 3 I know had been spun (not my
particular ship, though) during factory test flying, so I'd reason to expect
my example would spin/recover similarly...but I never wanted to put that clean
a ship that nose down out of simple (unpaid!) curiosity, given that my example
gave all sorts of aerodynamic warnings - long before departure - that if Joe
Pilot continued to do as he was, ship departure was nearing. Simply exploring
"near-departure" flight characteristics at my normal CG location satisfied my
curiosity in that particular ship. Never experienced an unexpected departure
from controlled flight in any ship (yet)...


I recall a Nimbus 4DM accident in which it was suggested that after a
certain number of turns in a spiral dive when g forces and airspeed had
built up high enough that it would be impossible to roll level and recover
- I wonder if the lawsuit was suggesting something similar about the 29? It
doesn't seem too likely that that would be the case though.

The discussion is moving into an arena well beyond what most average glider
pilots might consider "routine flight"...but that's never a bad thing, IMHO.
In any event, as complex an aerodynamic condition as is a "simple spin," I
consider the dynamics of "an extending-in-time spiral dive" well into the
nether regions of paid test flying. Simply pondering those same dynamics might
be considered an exercise in "around the campfire engineering"...caveat
emptor? Anyhow...

Clean gliders - when seriously nose down - pick up speed rapidly, and as
"structurally overbuilt" as are "pure glass" gliders and as strong as are
"post-pure-glass gliders" (i.e. those incorporating carbon/possibly-et-al
fibers), I think trying to define (say) time limits beyond which, or speeds
above which, spiral dives might be "problematically recoverable" is
essentially an 0'-beer thirty consideration, akin to pondering how many angels
can fit atop the head of a pin. Point being that spiral dives are known to be
"quickly problematic" for every type of airplane/glider construction whether
considering: 1) exclusively construction (tube-n-rag, wood, all metal,
composite), or 2) documented accident history. Whether the failure mode
initiates with the structure breaking before the pilot even has a chance to
recognize and respond, or structural failure occurs after the pilot recognizes
and responds (even if properly) but for (say) G-load reasons cannot transmit
his response from hands/feet to aerodynamic controls doesn't matter...at least
not given today's state-of-the-art in glider construction material technology.
He's still likely to die in a broken-before-the-ground ship.

I'd put practicing recoveries from developed spiral dives in the same category
as practicing departures from controlled flight in the landing pattern or
practicing playing on the freeway. :-)


Not knowing the C of G position for sure, especially in a glider which lets
you alter wing loading and tail weight to the degree a modern competition
ship [permits] would seem to be a prerequisite for flight to me.

Methinks you intended to write the preceding sentence w/o the "Not..." and if
you did, then we're in 100% agreement!!! That was my essential point in an
earlier "little Red Worry Flag" post.

Bob W.