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.

On Sunday, February 3, 2013 12:00:41 PM UTC-7, son_of_flubber wrote:
In short, I understand that there are advantages to "landing hot"....

On Sunday, February 3, 2013 2:54:33 PM UTC-5, Bill D wrote:
With a little practice, you'll find a glider can be slowed sharply with a few seconds of full airbrake while raising the nose to maintain the glide path just before entering ground effect. This can be done on short final to hit the recommended approach speed allowing the pattern itself to be flown at any speed the pilot deems safe for the gustiness.

Right, you want to dissipate the energy before entering ground effect.
I do this when high speed is called for in the pattern, but not needed for the final 20 feet of descent. For example, when there is the possibility of strong sink on downwind, and you need the extra energy to use if you hit the sink to avoid landing short.



On Sunday, February 3, 2013 3:24:02 PM UTC-5, Martin Gregorie wrote:
OTOH at The Mynd ...
That day I flew finals in my Libelle at 70kts, made a fully held-off
landing and was stopped no more than 500 ft from the threshold.

My home field is known for strong sink on final, strong crosswinds (one narrow runway), erratic surrounding topography (not flat), and in wave season, rotor in the pattern. (We like it because the wave and the ridges are very close to the airport!)

The hold-off was no more turbulent than I'd expect at home.

Granted, it can be very turbulent on downwind, but relatively calm on the ground. (After reading all the great responses) I can see that even if you have plenty of runway in front of you, you should anticipate calm and slow down before you enter ground effect. Even if there turns out to be turbulence at ground, you might hit it farther down the runway after you have necessarily slowed down. Best bet is to minimize the time spent rolling on the runway when you are exposed to possible turbulence, and also minimize the energy that you carry close to the ground should something go awry.


son_of_flubber wrote: If the pilot chooses to reduce speed from 70 knots to stall speed with the wheel off the ground, he has a longer runout,

Martin Gregorie wrote:
I'd dispute that. You land slower and can still use full air and wheel
brakes so the ground run has to be shorter.

But you've floated to the far end of the runway before you can apply the wheel brake and full spoiler. When I put the wheel on the ground at 70kts, I can sooner open full spoilers and sooner apply wheel brake (balanced with the elevator to maintain appropriate AOA). From a given point on the runway and given speed, the pilot who applies full spoiler and wheelbrake sooner will stop shorter. If you try to lose speed from 70kts to stall, before let your wheel touch, you will be far down the runway before you stop. Granted, I've screwed up if I have 70kts in ground effect (unless there is a hellish crosswind or wind gradient). The question is only how to best bring the glider to a stop once that mistake has been made.


son_of_flubber wrote: There seems to be some vague correlation between transitioning from SGS to glass and landing related PIOs.

Martin Gregorie wrote:
Could that be related to (ed. pilot) reactions during roll-out after landing(ed. ?)...

The SGS 2-33 has relatively low control authority and high stability. That makes it "easy to fly" in easy conditions. Early in my training, I transitioned from 2-33 to Blanik L-23. At the time I found the L-23 "extremely responsive and unstable" compared to the 2-33 (novice perspective . After flying the L-23 for a season, I transitioned to ASK-21 and found it another (albeit smaller) step up in responsiveness. I'm glad that I transitioned to the L-23 early before my stick and rudder habits became deeply ingrained. I'd expect that some pilots who had only flown SGS, might have PIO in a glass ship in a stressful situation, should they revert to their 2-33 formed habits and overdo control inputs. That's a possibility for me too, though I've not yet instinctively pushed the stick forward to put the skid down on a glider that does not have a skid! It could happen. I love the 2-33 for early confidence building before habits are formed, but I'm glad that I moved to the Blanik sooner rather than later. The 2-33 is just too easy to fly if you ever want to fly glass.


On Sunday, February 3, 2013 6:59:17 PM UTC-5, BobW wrote:
...(ed.ignoring) crosswinds for the moment, once in "the final float" with the
main wheel within inches of the ground, turbulence ceases to be a factor (ed.in my experience).
That's not to suggest I haven't been seriously concerned about it beCOMing a
factor, but the concern has never materialized.

It's possible to have a turbulence related incident close to the ground, but not probable. And the extra speed would only help you if the nasty gust happened to hit you before you (inevitably) slow down at the far end of the runway. The high energy will hurt you if something goes wrong. I can see that in lieu of a crosswind, and once you are sure of not landing short, that it is best to lose any excess energy before entering ground effect.


My experience includes 3 microbursts in the pattern

This might give you the willies...
http://en.wikipedia.org/wiki/File:Microburstnasa.JPG


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.

Hopefully, there are no runway lights, cliffs, trees or parked vehicles.

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 ...

...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,

So you needed the speed and consequent control authority to quickly and precisely dip and undip the upwind wing to arrest the drift, and the energy gave you the needed rudder effectiveness (before it ran out). Did the speed do anything else for you?


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.

So you planted the wheels well above stall speed, because you were forced to level the wing (to avoid ground loop) and you used the "tracking" of the wheels to arrest the drift? The tailwheel helped prevent weathervaning into the wind (a little).

Okay, I see the (only?) justified and correct use of 70kts IAS in ground effect. In lieu of a hellish crosswind, I was incorrect to enter ground effect at 70kts. Once I had made the runway in the turbulent and strong sink conditions, I should have shed the excess energy before I entered ground effect. However, once I had made the mistake of entering ground effect at 70kts, I was justified in putting the wheel down and applying full spoiler and wheel brake because that allowed me to stop with a comfortable margin of runway still in front of me; more runway than I would have had if I had floated in ground effect at 70kts until reaching stall speed. If (at 70kts) I had touched the wheel down with too much vertical speed, I might have executed the dreaded landing PIO. At best, I would have relaunched into stable ground effect and tried again.


Back to the original thread... Shedding excess speed before entering ground effect reduces the chance of landing PIO. Reducing vertical speed at touchdown reduces the chance of landing PIO. Holding off until reaching stall speed rules out the possibility of landing PIO. Should you screw up and "bounce"(once), your first goal is to return the glider to level flight, then land by correctly "holding off" (in lieu of a devilish crosswind). If you are running out of runway at that point, you might gradually put the wheel down before you reach stall speed, then open full spoilers and wheelbrake to stop, hopefully soon enough.


Thank you for your responses. It's very helpful to think this through sitting by the fire on a stormy day in winter. Plus it's fun to re-imagine those hairy landings from last summer.

On 2/4/2013 10:44 AM, son_of_flubber wrote:
Major snip

On Sunday, February 3, 2013 6:59:17 PM UTC-5, BobW wrote:
Snip

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
...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,

So you needed the speed and consequent control authority to quickly and
precisely dip and undip the upwind wing to arrest the drift, and the energy
gave you the needed rudder effectiveness (before it ran out). Did the
speed do anything else for you?
We're likely talking nuance here, but the ways I think speed assists Joe Pilot
in the presence of a strong/steady crosswind a a) reducing vector angles (&
hence fuselage to runway angle at touchdown); b) enhancing control authority
prior to touchdown.

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.

So you planted the wheels well above stall speed, because you were forced
to level the wing (to avoid ground loop) and you used the "tracking" of the
wheels to arrest the drift?

Yep to all. It was a 15 meter bird, and the thought of wingtip-to-ground
contact above stalling speed terrifies me. Plant the main wheel before
drift/wingtip-to-ground contact become issues, establish the best total
plane-to-ground contact(s) possible ASAP, use controls as much as you
dare/must, brake enthusiastically.


Back to the original thread... Shedding excess speed before entering ground
effect reduces the chance of landing PIO.
Yup.

Reducing vertical speed at
touchdown reduces the chance of landing PIO.
Yup.

Holding off until reaching
stall speed rules out the possibility of landing PIO.
Yup...insufficient energy to do anything but bounce - not "flying bounce".

Should you screw up
and "bounce"(once), your first goal is to return the glider to level
flight, then land by correctly "holding off" (in lieu of a devilish
crosswind).

(Presuming a "flying bounce"...) Yup. Stay away from trying to "fine tune"
rapid stick inputs in some new-to-you way in the event of a "flying bounce"
(which likely WILL alarm you first time it happens). Lots have tried the "fine
tuned/rapid stick motions" approach...with scant success. Locking the stick in
pitch during the arc of your flying bounce and waiting for ground arrival is
pretty much guaranteed to be a better choice.

If you are running out of runway at that point, you might
gradually put the wheel down before you reach stall speed, then open full
spoilers and wheelbrake to stop, hopefully soon enough.

Yup...but if this intentional "fine tuned" forward stick motion is botched
(easy to do in the absence of thought beforehand), the next arrival will
generally be even harder in the vertical velocity sense. A plan beforehand is
good!

Back to botched flare/drifting touchdowns in the presence of strong/persistent
crosswinds briefly...

One of my worst arrivals happened when I ballooned a flare (too high a
speed...the X-wind made me nervous), began drifting "way beyond my comfort
level", ballooned the 2nd flare (all the while downwind drift velocity
increasing), and dropped in from about 3 feet. It was UGLY. Amazingly it (same
15 meter ship) survived without damage, thanks largely to this happening on an
empty WW-II ramp (nothing to hit...well, other than the ramp, I mean [wry
chuckle]), but both main and tail tires squealed throughout most of the
resultant ground loop into the wind. Can't remember if squealing rubber or the
downwind wingtip skid dragging was louder. The lack of ship damage had zero to
do with pilot skill. I'd well over 1,000 hours in the ship at the time. The
worst of it was I attempted the 2-point landing (rather than a run-on one,
which is easy to do in that ship) only because it was a large, empty ramp.
What WAS I thinking?!?

At least I had the sense to do it with no peanut gallery...

Never again attempted a 2-point landing in the presence of a strong X-wind.

Bob W.

On Mon, 04 Feb 2013 09:44:08 -0800, son_of_flubber wrote:

That day I flew finals in my Libelle at 70kts, made a fully held-off
landing and was stopped no more than 500 ft from the threshold.

To clarify: I meant 500ft into the field, not 500 ft from the far end.
The headwind component combined with landing uphill stopped me quite a
lot shorter than I expected considering the Libelle's famously weak
airbrakes. I don't recall using the wheelbrakes much if at all.

Martin Gregorie wrote:
I'd dispute that. You land slower and can still use full air and wheel
brakes so the ground run has to be shorter.

But you've floated to the far end of the runway before you can apply the
wheel brake and full spoiler.

If I saw that I was running out of space ahead and the end of the field
looked unpleasant I'd want the wheel on the floor as a preliminary to an
intentional ground-loop - the fastest way to stop. That involves putting
the upwind tip on the ground as a pivot and using forward stick to get
the tail off the ground so the glider swings round easier. It also
minimizes the chances of snapping the tailboom due to side loads from the
tail wheel hitting grass clumps etc. I haven't had to do that yet but a
friend did, landing in a very small field with a stone wall at the far
end.


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