On Jan 31, 7:23*am, "
wrote:
On Jan 30, 11:35*am, T8 wrote:



On Jan 30, 8:36*am, "

wrote:
The pilot could have also pulled back on the stick, which might have
restored the balance of lift, drag, and weight, and arrested the high
sink rate. *

No way. *He'd have gone in (very) hard on the tail.

It's probably possible to land a 15m glider in negative flap, but
you'd need a final approach speed of *70 kts to do it. *From the
description of this incident, it sounds like the pilot was closer to
50 kts and certainly under 55.

You need to keep in mind that flaps change wing incidence, tail
incidence (relative to wing), and especially max CL. *All at once.
This pilot made two mistakes (flaps not locked, too slow too high -
accident would not have happened if he hadn't had to close the
spoilers!) and a very clutch response that saved his ass.

Uncommanded flap changes in close proximity to the ground or other
aircraft are life threatening. *You need procedures and control locks
that absolutely prevent this.

My $0.02.

-Evan Ludeman (15m guy)

Evan,

I did not mean to imply that pulling back on the stick was the
solution to the problem. *Note I used the word "might". *I agree with
you that at a relitively slow speed, and neg flaps, that there would
not be enough up elevator authority to completely arrest the high sink
rate. *You would reduce the sink but probably not enough.

But, you picked up on the minor point of my post, not the major point
which is the fact that the original post contains the misconception
that the retraction of the flaps caused the glider to "stall", and
that a redeploying the flaps caused a "stall recovery"..........

BTW, landing tail first is not necessarily a bad thing, but landing
tail first with a high sink rate is a bad thing.

I did witness a PIK 20 make a succesful landing with it flaps stuck in
the full neg position...(no spoilers on this model). *The approach
speed was relatively fast, but no super fast. *The landing took up
quite a bit of runway, but really was uneventful! *(this was not a
"sudden" retraction of the flaps however).

Cookie

I agree that the LS-6 didn't stall. I like tail first landings also.

My point was that the -6 was likely slow enough that it simply
couldn't fly in negative flap and certainly couldn't flare. Looking
up some old charts and extrapolating a bit, I can guesstimate that a
reasonable Cl max with flaps down would be 1.3 - 1.4 and perhaps as
much as 0.9 with flaps negative (by extrapolation -- the chart doesn't
go there because it's not normally of interest!). That's a big
difference. Means -- roughly -- that the minimum flying speed with
flaps up is 22% higher than with flaps down. If you are somewhere in
that speed range and the flaps retract, you sink, regardless of what
you do with the stick. That's consistent with what th LS-6 pilot
reported. A stabilized approach in negative flap is obviously
different - you approach faster because you must to keep flying.

Now if I take my 22% number -- for whatever good that might be -- and
adjust a 52 kt approach speed I get 63 knots. Fair enough. That
makes sense with your observation of the PIK, fast, but not blazingly
so. 70 may be overkill.

regards,
Evan Ludeman / T8