Glider Tail Stall

On Feb 20, 10:15*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.

I think the others made the essential point that the video subject was
a special situation where ice on the tail leading edge led to a
separation bubble on the lower surface with flow re-attaching aft of
the hinge line. The re-attached flow was sucking the elevator down
(nose down) so the pilots were advised to pull back to counter this
force to retain control of their aircraft.

The violent pitch down was due to this effect. The video also talked
about non-linear elevator effects which could lead to PIO's

As for the tail/wing stall discussion, unless you've flown a glider
with yarn tufts on the wing and tail, this is hard to visualize.

Yes, of course, the wing is designed to stall at the root first. In
fact, unless there is a very effective root fillet, there will be some
root flow separation visible at any AOA.

In every case I know of, the yarn tufts show full flow separation on
both upper and lower surfaces of the tail when as nose 'breaks'
suggesting a wing 'stall' to the pilot. At the same time the wing
tufts show attached flow over the majority of the wing which is why
the ailerons work as expected. You can say the tail just loses
effectiveness or you can call it a tail stall - the distinction isn't
really important.

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, 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

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.

On Feb 21, 4:15*am, Jim White wrote:
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

Jim,

Was the gust vertical ? and in a downward direction ? That's what I
would think was necessary to produce a tail stall. A downward gust
would exceed the stall angle of attack on the tail and produce a nose
down pitch.

Of course a strong gust can produce pitch excursions without causing
tail stall specifically.


On knowing how gliders really work, I think that some knowledge of
aerodynamics is mandatory to be a safe pilot, but some is optional.

For example:

Mandatory knowledge:
HOW lift relates to angle of attack (especially stall).

Optional knowledge:
WHY airfoils work the way they do.

Mandatory knowledge allows a pilot to correctly fly the aircraft.
Optional might help support that, but isn't critical to the flight.
Being an aerospace engineer, I hear pilots explain a lot of flight
physics completely wrong, but they are competent and safe pilots.
Conversely, I hear some pilots explain the physics correctly, but
would not allow a loved one to fly with them.

IMHO all the aerodynamics that I pilot needs to understand is in
"Stick and Rudder" by Wolfgang Langwiesche.

Todd
3S

At 15:49 21 February 2009, bildan wrote:
On Feb 21, 2:15=A0am, 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.


I know my glider's VNE, Va, and stall speed.
I know its G limits, CofG limits, flap speed limits.
I know that it is cleared for loops and spins but not inverted flight.
I know that stall speed will increase by 1.5 at 60degrees of bank.
I know that contamination (rain, ice, or bugs) on the leading edge spoils
the magic and increases stall speed by an undefined amount.
Oh, by the way, I have been taught about high speed stalls.

What other aerodynamics stuff do I really need to know?

If I stay within the flight envelope and concentrate on my flying,
lookout, and xc tactics I reckon I should be safe without actually knowing
how it actually works.

It is a bit like met. I can't do the maths but I can read a tephi.

Jim

In message
,
bildan writes
snip
If you want to
significantly enhance your safety, yes, it is necessary.

It is? If something goes wrong I'll be too busy trying to aviate my way
to safety to think about the aerodynamics. Hopefully they have informed
the training I have received (which is what I successfully used to
resolve in the only significant incident I've been in so far) , but the
training is devised by much better and more experienced pilots than
myself.

--
Surfer!
Email to: ramwater at uk2 dot net