Glider Tail Stall

On Feb 20, 9:00*am, jcarlyle wrote:
Recent threads highlighted tail stalls in powered aircraft
experiencing icing. The thing that concerned me was the recovery being
exactly opposite to the wing stall that we all practice and
demonstrate, and thus have ingrained as almost automatic.

Why would the recovery be different? The tail is an inverted wing
producing a down force. You stall it by pulling back on the stick
increasing its AOA until it stalls. Releasing the back pressure
initiates a recovery - same as a wing stall.

It also
sounded like the thing that distinguished a tail stall from a wing
stall was buffet in the controls rather than in the airframe. This
distinction is pretty subtle to me, and in the heat of the moment I
wonder if I would apply the proper recovery.

It doesn't really matter. With many trainers, the buffet students are
taught to recognize as wing stall is, in fact, tail stall with a
little bit of turbulence from wing root flow separation thrown in.
Allowing the tail to stall limits up elevator authority so the wing
can never get into a full stall. Cessna 152's and SGS 2-33's are
examples.

There's a simple test for this. With the stick full back and the
glider exhibiting pre-stall buffet, apply aileron and if the glider
responds normally in roll, the wing wasn't stalled. If the wing was
stalled, the glider would probably try to spin with the application of
aileron.

Does anyone know, for a modern 40:1 glider, how violent a tail stall pitch up would be?

If the tail stalls, and the CG is within limits, the glider will pitch
nose down, not nose up, and this will help effect the recovery. If
anything, modern gliders are even more benign than older designs.

Also, if a glider has a totatally benign wing stall,
eg, non-violent wing stall break, would this imply that a tail stall would also be non-violent?

Tail stall just runs out of up elevator authority. With one
exception, tail stall is benign.

This is the exception. When the nose up moment is being produced by
something other than the elevator, the stick will be forward as the
pilot tries to limit the pitch up. In this case, the tail is
producing an up force and if the it stalls, the nose will rise further
risking a wing stall.

Two things can produce this situation. One is an aft CG and the other
is a poorly located CG hook used on a winch launch. Slab type all
moving 'stabilators' are more susceptible to this than fixed stab/
hinged elevator type tails. The fix is to be very aware of your CG
and to winch these gliders carefully.

Bumper, Toad - thanks. Sorry for my confusion about pitch direction -
you're right, it would pitch down. The recovery with a tail stall,
though, is to pull back on the stick, not push forward. That's why I'm
worried about differentiating a tail stall from a wing stall.

Bill, please see comments embedded in your reply.

Why would the recovery be different? The tail is an inverted wing
producing a down force. You stall it by pulling back on the stick
increasing its AOA until it stalls. Releasing the back pressure
initiates a recovery - same as a wing stall.

According to the videos, if the tail stalls you need to recover by
pulling back on the stick, not pushing forward on the stick as we
usually do.

It doesn't really matter. With many trainers, the buffet students are
taught to recognize as wing stall is, in fact, tail stall with a
little bit of turbulence from wing root flow separation thrown in.
Allowing the tail to stall limits up elevator authority so the wing
can never get into a full stall. Cessna 152's and SGS 2-33's are
examples.

There's a simple test for this. With the stick full back and the
glider exhibiting pre-stall buffet, apply aileron and if the glider
responds normally in roll, the wing wasn't stalled. If the wing was
stalled, the glider would probably try to spin with the application of
aileron.

I'll have to try this. I think I've never tried to move the ailerons
once I feel the pre-stall buffet - I just center the stick.

If the tail stalls, and the CG is within limits, the glider will pitch
nose down, not nose up, and this will help effect the recovery. If
anything, modern gliders are even more benign than older designs.

Why would it help recovery? The videos say the recovery for a tail
stall is to pull the stick back.

Tail stall just runs out of up elevator authority. With one
exception, tail stall is benign.

This is the exception. When the nose up moment is being produced by
something other than the elevator, the stick will be forward as the
pilot tries to limit the pitch up. In this case, the tail is
producing an up force and if the it stalls, the nose will rise further
risking a wing stall.

Two things can produce this situation. One is an aft CG and the other
is a poorly located CG hook used on a winch launch. Slab type all
moving 'stabilators' are more susceptible to this than fixed stab/
hinged elevator type tails. The fix is to be very aware of your CG
and to winch these gliders carefully.

OK, thanks - my CG is at 60%, I have a fixed stab/hinged elevator, and
I don't winch.

-John

On Feb 20, 10:15*am, jcarlyle wrote:
According to the videos, if the tail stalls you need to recover by
pulling back on the stick, not pushing forward on the stick as we
usually do.

I hate to risk adding to the obvious confususion but....

The NASA video deals with a specific case where a contamined tail
results in a uncommanded sudden downward motion of the elevator, which
in turn results in a forward stick motion, and a nose down pitch.

The situation would appear to be completely different from an
aerodynamic stall of an uncontaminated tail surface.

The use of the term "tail stall" for the icing induced pitch down
seems misleading to me since the tailplane could not be at critical
angle of attack if returning the elevator to its pre-displaced
position restores the downward tail force.

A significant difference between the two scenarios is the stick
motion associated with the event. Iced up tail - nose pitches down
as stick moves forward. Aerodynamic tail stall - nose pitches down
as stick moves aft or stay where it was.

Linking to the thread on stall awareness and recovery, the iced tail
situation that results in down elevator and uncommanded forward stick
motion may be hard to distinguish from a stick pusher event, and the
required recovery for these is exactly opposite.

Andy

Andy,

Let me reduce my original question to a specific situation. Suppose
you're flying in 60 degree weather, so no surface is contaminated with
ice. Suddenly, the nose pitches down and the stick stays where it
was.

My question is: how do you know if you've suffered a wing stall (where
the recovery is to push the stick forward), or a tail stall (where the
recovery is to pull the stick back).

-John

On Feb 20, 1:13 pm, Andy wrote:
I hate to risk adding to the obvious confususion but....

The NASA video deals with a specific case where a contamined tail
results in a uncommanded sudden downward motion of the elevator, which
in turn results in a forward stick motion, and a nose down pitch.

The situation would appear to be completely different from an
aerodynamic stall of an uncontaminated tail surface.

The use of the term "tail stall" for the icing induced pitch down
seems misleading to me since the tailplane could not be at critical
angle of attack if returning the elevator to its pre-displaced
position restores the downward tail force.

A significant difference between the two scenarios is the stick
motion associated with the event. Iced up tail - nose pitches down
as stick moves forward. Aerodynamic tail stall - nose pitches down
as stick moves aft or stay where it was.

Linking to the thread on stall awareness and recovery, the iced tail
situation that results in down elevator and uncommanded forward stick
motion may be hard to distinguish from a stick pusher event, and the
required recovery for these is exactly opposite.

Andy

On Feb 20, 12:48*pm, jcarlyle wrote:
Let me reduce my original question to a specific situation. Suppose
you're flying in 60 degree weather, so no surface is contaminated with
ice. Suddenly, the nose pitches down and the stick stays where it
was.

I don't know how to answer that without knowing more about the flight
condition and the stall margin. If the sudden pitch down was
associated with a loud noise I'd bail out!

Andy

On Feb 20, 12:11*pm, Andy wrote:

Let me reduce my original question to a specific situation. Suppose
you're flying in 60 degree weather, so no surface is contaminated with
ice. Suddenly, the nose pitches down and the stick stays where it
was.

If you are flying within the CG limits and at a reasonable speed (i.e.
within the envelope), then the main wing will stall before the tail.
Any modern aircraft will be designed to have good stability when
loaded within the CG limits and flown at reasonable speeds.

A propensity for the tail to stall in these conditions would be
similar to what would happen when an aircraft is loaded aft of the CG
limits and stalls (look up "deep stall"; and once you understand it
you will never load an aircraft aft of the recommended CG limit
_ever_). I've seen this happen with experimental R/C aircraft that I
was testing (as a hobbyist, not as a professional).

Of course, flying outside of the CG limits and/or pulling high-G
aerobatics (wherein you might induce sudden and extreme AOA changes in
the flying surfaces) are another situation entirely...

The beauty is if you're not fying an aircraft with contaminated flying
surfaces, both a tail tall and a main wing stall should have roughly
the same recovery procedu Allow the aircraft to regain flying
speed and reduce the angle-of-attack (once air is flowing normally
over the aircraft, you're fine), and then return to a normal pitch-and-
bank attitude. The only difference is in recognizing what you need to
do in order to regain flying speed. If you're holding the stick back
and the airspeed needle is down around zero, stop holding the stick
back! If you're holding the stick forward and the ASI is resting on
its stop, relax the forward pressure!

The vast majority of the time, bringing the controls closer to neutral
is the right thing to do - mostly because of the built-in stability of
nearly all aircraft produced in the last 60 years. But again, this is
true only when the aircraft is loaded within the CG limits.

Kirk - I really lament the fact that so few pilots truly understand
what the air is doing as it flows over their aircraft. Seeing the
diagram in a pilot handbook and memorizing answers for the FAA test is
just not good enough. SO MANY "pilot-error" accidents could be
avoided, if only people had a better grasp of the cause-and-effect
relationship between the cockpit controls, the flying surfaces (as the
controls are moved), and the airflow over the aircraft. Being able to
understand "if I do X, then Y will happen" (in terms of the airflow
and the aerodynamics) goes a long way towards knowing how to get out
of trouble, or avoid it altogether! Without this understanding, I
think a lot of pilots don't really know what risks they're taking as
they blithely fly along, and they don't understand how their safety-
margin is changing as they take different actions in the cockpit.

*sigh* OK, rant over... :-P

--Noel

On Feb 20, 2:48*pm, jcarlyle wrote:
Andy,

Let me reduce my original question to a specific situation. Suppose
you're flying in 60 degree weather, so no surface is contaminated with
ice. Suddenly, the nose pitches down and the stick stays where it
was.

My question is: how do you know if you've suffered a wing stall (where
the recovery is to push the stick forward), or a tail stall (where the
recovery is to pull the stick back).

-John

I don't know how you would encounter a tail stall in a glider without
ice buildup or some severe damage to the tail. It's just not a
realistic scenario. In other words, don't worry about it.

Todd
3S

At 20:47 20 February 2009, wrote:

I don't know how you would encounter a tail stall in a glider without
ice buildup or some severe damage to the tail. It's just not a
realistic scenario. In other words, don't worry about it.

Todd
3S

I am pretty sure I have encountered a tail stall due to a gust experienced
in straight and level flight in a K18 at 50kts. I was alarmed to find
myself suddenly pointing at the ground! Normal recovery worked. Thankfully
has never happened again.

I know how gliders work...magic. Is it really necessary to know how stuff
works to fly a glider? Design and build one yes, but to fly one?

Jim

On Feb 21, 2:15*am, Jim White
I know how gliders work...magic. Is it really necessary to know how stuff
works to fly a glider? Design and build one yes, but to fly one?

Jim

Define "necessary".

If you find minimal competence acceptable, chancing that you will
never encounter a situation beyond your skill, you might get away with
an ignorance of aerodynamics.

But, if you want to achieve anything beyond that, If you want to
significantly enhance your safety, yes, it is necessary.

Jim,

Thanks for sharing your story. What I'm most gratified to learn is
that normal recovery techniques worked.

-John

On Feb 21, 4:15 am, Jim White wrote:
I am pretty sure I have encountered a tail stall due to a gust experienced
in straight and level flight in a K18 at 50kts. I was alarmed to find
myself suddenly pointing at the ground! Normal recovery worked.