Thermal Core Tip Stalling

There have been several incidents and accidents over
the last few years when gliders, especially big span
ones, have suddenly spun or spiral dived when entering
or manoevring in strong narrow thermals. I am therefore
wondering if pushing a wing down into a strong thermal
core could cause a momentary tip stall? Wing going
down plus air coming up could cause the stalling angle
of attack to be exceeded.

Derek Copeland

Derek,

You may want to review some videos on my website. I am continuing
flight testing to determine the minimum slip angles and control inputs
to produce spin entries in several different ships. I expect, however,
that an open class glider would be especially subject to the condition
I've described.

http://ocglider.com/Grob_spin_avis.htm

I would prefer that you not share this link with the group. You may
distribute it to others who may backchannel you on this subject. That
will keep interest within my bandwidth limits.

Thanks

Chris O'Callaghan




Derek Copeland wrote:
There have been several incidents and accidents over
the last few years when gliders, especially big span
ones, have suddenly spun or spiral dived when entering
or manoevring in strong narrow thermals. I am therefore
wondering if pushing a wing down into a strong thermal
core could cause a momentary tip stall? Wing going
down plus air coming up could cause the stalling angle
of attack to be exceeded.

Derek Copeland

wrote:
I would prefer that you not share this link with the group. You may
distribute it to others who may backchannel you on this subject. That
will keep interest within my bandwidth limits.

Too late, gotta watch which reply button you use. I found out about
this years ago when I botched a practice slip in a G103B, and have since
demonstrated it to a number of doubters, including a few CFIGs (I think
I posted about it in r.a.s. a couple of times, too). The most I've been
able to get is one full turn before it degrades into a spiral...

Marc

On 30 Jan 2006 19:48:03 -0800, wrote:

Derek,

You may want to review some videos on my website. I am continuing
flight testing to determine the minimum slip angles and control inputs
to produce spin entries in several different ships. I expect, however,
that an open class glider would be especially subject to the condition
I've described.

http://ocglider.com/Grob_spin_avis.htm

I would prefer that you not share this link with the group. You may
distribute it to others who may backchannel you on this subject. That
will keep interest within my bandwidth limits.

Thanks

Chris O'Callaghan


I confess I went and looked at your videos. Very interesting indeed.

I have some difficulty understanding what the yaw string is
indicating. It appears the glider falls off in the direction of the
pre-stall bank. I would have expected a slipping stall/spin to fall
off in the direction of the applied rudder instead. In the case of a
slipping stall I would have thought this would be opposite the
direction of the pre-stall bank. Learn something evey day.

Jim wrote:
I have some difficulty understanding what the yaw string is
indicating. It appears the glider falls off in the direction of the
pre-stall bank. I would have expected a slipping stall/spin to fall
off in the direction of the applied rudder instead. In the case of a
slipping stall I would have thought this would be opposite the
direction of the pre-stall bank. Learn something evey day.

I can't speak for Chris, but I can tell you what works for me. Get into
a steep stable slip, say left wing down, full right rudder. Now,
imagine that you don't like what you're seeing, and "recover" by kicking
and holding full left rudder, followed shortly by moving the stick all
of the way to the back right corner. In the G103s and K21s I've tried
this in, the nose will yaw (quickly) to the left, left wing will drop,
nose falls through, and you'll spin for part or all of a turn, then the
speed starts increasing and you end up in a spiral...

Marc

On Tue, 31 Jan 2006 10:56:28 -0800, Marc Ramsey
wrote:

Jim wrote:
I have some difficulty understanding what the yaw string is
indicating. It appears the glider falls off in the direction of the
pre-stall bank. I would have expected a slipping stall/spin to fall
off in the direction of the applied rudder instead. In the case of a
slipping stall I would have thought this would be opposite the
direction of the pre-stall bank. Learn something evey day.

I can't speak for Chris, but I can tell you what works for me. Get into
a steep stable slip, say left wing down, full right rudder. Now,
imagine that you don't like what you're seeing, and "recover" by kicking
and holding full left rudder, followed shortly by moving the stick all
of the way to the back right corner. In the G103s and K21s I've tried
this in, the nose will yaw (quickly) to the left, left wing will drop,
nose falls through, and you'll spin for part or all of a turn, then the
speed starts increasing and you end up in a spiral...

Marc

Yes, I can certainly visualize that! It sounds like one would be
going from a full-rudder left slip into a full-rudder left skid, for
which the glider oblidges the pilot with a spin out the bottom. Whew.

No need for the wing to move down. Just moving the aileron down can
take the wing tip AOA past Cl-max into stall, especially if there is an
upward flow component at the tip relative to the rest of the wing.

BTW, it's not just the tip, you can stall the whole wing too. The SGS
2-33 is prone to do this when entering a big strong core. I suspect it
happens more often in big slow gliders because the same updraft causes
a larger change in AOA due to the slow speed, and the inertia slows the
pich down from longitudinal stability.

Hi Marc,

of the way to the back right corner. In the G103s and K21s I've tried
this in, the nose will yaw (quickly) to the left, left wing will drop,

I'll try this as soon as I get the chance!

Greetings from Zuerich and have a good season start!
-Gerhard
--
Gerhard Wesp
Zuerich, Switzerland
+41 (0)76 505 1149 (mobile)
+41 (0)44 668 1878 (office)
+41 (0)44 200 1818 (office fax)