Glider Tail Stall

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. 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.

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

I don't fly in icing situations, I don't have flaps, and control seal
checking is part of my preflight, so this is probably all academic.
Still, I'd like to know...

-John

"jcarlyle" wrote in message
...
snip

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

-John

John,

Since the tail is providing down force in normal flight, if it stalls due to
icing the tail will lose downward "lift". the tail will then go up causing a
nose pitch *down*. I doubt that benign normal wing stall behavior would have
any effect on how violent a tailplane stall might be.

bumper

The aircraft pitches DOWN during a tail stall. The video link posted
earlier provides a pretty good overview.

I don't think that a benign wing stall indicates a benign tail stall.
It's about the seperation characteristics of the tail airfoil
( changed by ice ) not the wing airfoil.

Todd

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, 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

On Feb 20, 9:15*am, jcarlyle wrote:
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.

What? I would love to see you provide evidence to back this up. The
buffet is primarily caused by the wing stalling (i.e. causing airflow
separation) near the root of the wing, and the ugly airflow from this
striking the aft fuselage and tail as it goes by.

A tail is an upside-down wing in a conventional aircraft (canard
aircraft are the exception) - therefore raising the angle of attack of
the wing *decreases* the angle of attack of the tail.

NOTE: There is a difference between a tail stall and an aircraft who's
tail is sized and limited in elevator travel to prevent the pilot from
really getting a good full stall with a break. My DG-300 is an
example of this - I can get a stall break if I perform an accelerated
stall; but if I start in slow flight and gradually increase the stick
pressure, the aircraft doesn't exhibit a true "stall break" - it just
buffets a lot and develops a good rate of descent.

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.

Uhhh, no not necessarily. You are ignoring the effects of wing taper,
twist, and span-wise distribution of lift. Wings can (and ARE)
designed to stall near the wing root first, and then progress outward
along the span. So you can get "stall buffet" and still have aileron
control because the outboard parts ofthe wings are not stalled (they
are flying at a lower effective angle of attack and therefore still
have clean airflow over part or all of the wing chord). It has
nothing to do with the tail stalling!

Oh, and before you do that test, please remember that applying aileron
input will make one of the ailerons move _down_ - thus increasing the
angle of attack on that part of the wing. If you've got a partially-
stalled wing and you increase the angle of attack on the outboard
(unstalled) part of it, you may stall the whole wing. Meanwhile, you
are _decreasing_ the angle of attack on the wing that has the aileron
deployed upwards. Thus that wing will likely _not_ completely stall.
Everyone knows what happens when one wing stalls and the other doesn't
completely stall, right? Spin time! This is why we teach people not
to use much (or any) aileron input when feeling the pre-stall buffet.
Again, this has nothing to do with the tail stalling.

--Noel