Question about spoilers and pitch stability

On 2/3/2013 12:00 PM, son_of_flubber wrote:
On Sunday, February 3, 2013 12:50:16 PM UTC-5, waremark wrote:
In Europe standard practice is to teach fully held off landings, touching
down just a fraction over stall speed in a 2 point attitude.

I have a question. Assume strong turbulence such that a 70 knot speed in
the pattern is needed to obtain control authority. Once the flare is
correctly executed and the glider is flying level in ground effect 6-12"
off the ground, does the glider become less vulnerable to turbulence
because it is in ground effect, and therefore would control authority be
maintained as the glider slows to stall speed?

My experience in the intermountain west of the US - based out of Boulder
(KBDU) 3 miles from the Rocky Mountain foothills - is (for discussional
purposes, ignore crosswinds for the moment) once in "the final float" with the
main wheel within inches of the ground, turbulence ceases to be a factor.

That's not to suggest I haven't been seriously concerned about it beCOMing a
factor, but the concern has never materialized. My experience includes 3
microbursts in the pattern, one of which was a crap shoot in terms of success
vs. life-endangering-crash. (I still get the shakes thinking about that
[2-pointed] one, years later.)

See below for brief discussion of round-out thoughts given the presence of
"beer-worthy" crosswinds...
- - - - - -

I understand that once the pilot is flying level 6-12" above the runway at
70 knots (at that point the plane has zero vertical velocity) that he can
very gradually lower the center wheel to the runway without 'bouncing".
The advantage of lowering the wheel to the runway is two-fold. 1)The
turbulence can no longer slam the wheel down onto the runway (it is already
there). 2)The pilot can use the wheel brake and full spoilers to stop the
glider as quickly as possible, thus minimizing runout and minimizing the
time exposed to turbulence near the ground.

If the pilot chooses to reduce speed from 70 knots to stall speed with the
wheel off the ground, he has a longer runout, the possibility of being
slammed to the pavement by a downdraft, and a longer period of time
vulnerable to unpredictable turbulence.

"What Martin Gregorie said" regarding your "longer runout" positing. In any
event, in the absence of a persistent, strong crosswind my vote favors getting
rid of the energy where the plane is generally safest, i.e. in the air.
- - - - - -

In short, I understand that there are advantages to "landing hot". I also
understand that if a pilot touches down with too much vertical velocity,
that he will 'bounce'. If the vertical velocity is low enough, you will
not 'bounce' not matter how high the horizontal velocity.

(I understand that the 'bounce' is caused by an increase in AOA, caused by
rotation around the center wheel, caused by a CG behind the center wheel,
too much vertical velocity, and therefore too much momentum pressing the
rear wheel down and increasing the AOA. And of course enough horizontal
speed such that the increased AOA causes the wheel to leave the ground.)

You seem to have a decent grasp of the dynamic issues (aero and CG-positional)
judging from the preceding brief summation. Strictly by way of FYI feedback...
- - - - - -

I want to reiterate that I'm asking a question and just stating my very
possibly fractured understanding. This topic is of great interest to me
because I'm in the process of transitioning to a glass ship and I have a
lot of training time in SGS. (There seems to be some vague correlation
between transitioning from SGS to glass and landing related PIOs.)

Knowing very little about "typical pattern conditions" at your home field, but
having most of my time in the intermountain west - which tends to be prone to
"enthusiastic pattern conditions" year 'round - maybe an observation about
landing in the presence of persistently strong crosswinds isn't unwarranted.

Next to microbursts in the pattern, howling crosswinds are my next least
favorite pattern condition to have to deal with. While they (generally) tend
to "significantly decrease" in the vertical distance descended through during
the transition from final approach attitude to hold-off attitude, they don't
always, and the thought of landing on a narrow runway with little tolerance
for being off-center, while drifting sideways due to a crosswind overpowering
the pilot's/ship's abilities to compensate for drift has always left me cold.

Under such conditions, I've tended to favor - and had my best results from so
doing - run-on landings at the lowest speed conditions permit in the absence
of "significant drift". Working from memory, my highest-ever touchdown speed
was probably around 70 knots indicated in the presence of an estimated 25-30
knot direct crosswind. I aimed for the near end of the runway, flew in ground
effect until I was essentially out of rudder, briefly used into-wind aileron
to halt developing drift, and planted the main wheel. Then - because I could -
I went to negative flap to plant the tailwheel, and used full downwind rudder
and heavy braking to minimize the into-wind arc of the main wheel. Another
pilot in an unflapped St'd Cirrus landing minutes ahead of me
independently-opted/used essentially the same technique with very similar
(also successful) results.

We both subsequently concluded held-off landing attempts would likely have
ended anywhere from "considerably less gracefully" to "genuinely ugly-ly".

Bob W.