Re: Elevator Turbulator tape question [Archive]

Kirk Stant

October 10th 03, 04:45 AM

Eric Greenwell > wrote in message >...

> Do you consider it an improvement to get a sharp stall, or is there some
> other effect you hope to get with "improved elevator effectiveness"?

How else can I get my 6 to snap roll!

Seriously, this was suggested by an experience 6 racer, theory is that
it allows more precise elevator control while thermaling and less
drag. Worth a try. I'm not particularly concerned about sharp
stalls, I love the 2-32 (which has a particularly sharp break for a
glider) and think all pilots should be required to fly (and stall) a
T-6 to see how a real plane flies.

I am curious about which side (top or bottom) of the horizontal stab
should have the turbulator. Intuition tells me it should be on the
bottom, to energize the airflow around the up deflected elevator.

Anyway, it might be fun to find out. Especially since it is a totally
reversible mod.

BTW, two of us flew a 220 mile XC out here in AZ today - 1400' agl
release off tow, Cu's, cloudbase at 12000'. 50 mile final glide under
a thick cirrus deck, started with 350' over a 1 knot MC (!) and made
it home with enough to do a 130 kt low pass (down a runway). Other
guy did 240, and a 65 mile final glide. He beat me, 54 mph to 52 mph.

And we were definitely NOT at 500ft at 1 mile!

Yee hah! I love this sport.

Kirk
66

Shaber CJ

October 10th 03, 02:27 PM

>and think all pilots should be required to fly (and stall) a
>T-6 to see how a real plane flies.

I have lots of time in a T-6 and I am of the opinion that newer designs are
much better. Why not train with aircraft that characteristics of the ones we
fly. The T-6 is heavy, underpowered (in my opinion) has a sharp stall break
and snaps into a spin very easy. There has been several accidents that I know
where a pilot was flying at a heavier weight than normal used a sharp break in
the base to final turn and spin in. I like flying a T-6 but cannot recommend
them for all.

Craig

Andy Durbin

October 10th 03, 02:53 PM

(Kirk Stant) wrote in message >
> I am curious about which side (top or bottom) of the horizontal stab
> should have the turbulator. Intuition tells me it should be on the
> bottom, to energize the airflow around the up deflected elevator.

Kirk,

My 28 has turb tape on top, as delivered. I don't remember if it's on
the bottom too. The measurements for all turbulator positions are in
included in the maintenance manual. If you want more info call me at
home this weekend. (Your ras email seems to be invalid).

BTW the 28 stall is very docile at the aft cg limit.

Andy (GY)

John Cochrane

October 10th 03, 08:57 PM

I would encourage a lot of caution before slapping turbulator tape on
the elevator or elsewhere upwind of control surfaces. There have been
several instances of pilots crashing or having to abandon gliders
because mylar or tape became loose on elevators. If your turbulator
tape causes separation rather than good turbulent flow, you might
follow them. Turbulator tape on the top of wings has caused really
nasty stall/spin problems. Why be a test pilot? Call the
manufacturer! SH was nice enough to provide advice on where I should
put the tape on my old discus.

John Cochrane

Kirk Stant

October 11th 03, 04:33 AM

(John Cochrane) wrote in message >...
> I would encourage a lot of caution before slapping turbulator tape on
> the elevator or elsewhere upwind of control surfaces. There have been
> several instances of pilots crashing or having to abandon gliders
> because mylar or tape became loose on elevators. If your turbulator
> tape causes separation rather than good turbulent flow, you might
> follow them. Turbulator tape on the top of wings has caused really
> nasty stall/spin problems. Why be a test pilot? Call the
> manufacturer! SH was nice enough to provide advice on where I should
> put the tape on my old discus.
>
> John Cochrane

Uh, John, I think you are confusing loose tape with correctly attached
turbulator tape. I've flown gliders with loose tape - I absolutely
agree it is scary, and checking tape is a big part of my preflight. I
check ALL the tape on my 6, everytime I fly (as part of my washing
ritual). On the other hand, turbulator tape should be relatively
benign. Yes, I know, that will look good on my tombstone. But we are
talking about increased elevator effectiveness, not decreased
effectiveness (which is what you get with loose tape). I can handle a
more sentitive elevator, if I'm looking for it.

As the ASW-27s and -28s have elevator turbulators, it probably isn't a
huge deal. But forewarned is forearmed, etc..

Anyway, I appreciate all the comments on this subject. I'm not sure I
will even try it (I'm a cheap guy and $65 for a roll of zig-zag (not
papers, even!) is a bit much) but if I do I'll be real careful and
tell you all what the results were.

C'mon, guys, compared to some of the wiring lash-up I've seen in
gliders, this is a piece of cake! And let's not even start into where
various bits of ballast are being stuck to "move the CG back a bit".
I'm sure some of you will cringe at this, but we still can fiddle
around with our gliders to find out what works best. I hope the
goddamned lawyers never catch up with us (I'll move to Iraq when that
happens), but ITS MY LIFE AND I CAN DO WHAT I WANT WITH IT, THANK YOU
VERY MUCH!

Kirk (last bastion of stupid individualism)
66

John

October 11th 03, 09:44 AM

My '6 came with turbulator tape on the bottom surface, just in front
of the elevator. 1994 C model. The stall is very gentle.

J4

Dave Martin

October 11th 03, 10:41 AM

Kirk Stant wrote

ITS MY LIFE AND I CAN DO WHAT I WANT WITH IT, THANK
YOU
VERY MUCH!
>
Kirk (last bastion of stupid individualism)
>66
>

I would defend to the end your right to do what you
want with your life.

Sadly it is not those who kill themselves that is the
problem, it is

The 'friends' left behind who have to scrape the bits
of the airfield and shovel them into a body bag. (Yes
I have done it)

The relatives who have to be comforted. (And this)

Followed by the thieving grasping lawyers who make
a fortune out of the grief left behind. (and seen
this happen)

So come on take care have a thought for the others
you will leave behind and make sure the lawyers have
to live of state benefits and our premiums stay low.

If you want to kill yourself go out quietly and shoot
yourself please do so. It is also easier and cheaper.

Please!

Kirk Stant

October 11th 03, 06:34 PM

(John) wrote in message >...
> My '6 came with turbulator tape on the bottom surface, just in front
> of the elevator. 1994 C model. The stall is very gentle.
>
>
> J4

That's interesting. Does your aircraft or maintenance manual have any
info on the tail tape (placement, size, etc.)? My 6 is an 87 B model,
no tape anywhere except on the rudder mylar (zigzag leading edge).

I have a hard time figuring out how increasing elevator effectiveness
would make a stall "harder", since the wing should stall at the same
angle of attack and with the same flow pattern; you may be able to get
deeper into the stall with full aft stick (maybe better spin entry)
but that should be about it.
Right now the 6's stall is so gentle as to be almost nonexistent, I
can only really get any kind of break with landing flaps, in
thermalling flap it just nods a little.

I'll check with LS/DG and see what they say about it.

Meanwhile, the rest of you guys out there relax, I appreciate your
concern but I haven't reached this ripe almost old age by doing really
stupid things. Mildly stupid things, perhaps (like paying too much
attention to RAS).

Kirk
66

Dave Martin

October 11th 03, 10:11 PM

Kirk Stant wrote

Meanwhile, the rest of you guys out there relax, I
appreciate your concern but I haven't reached this
ripe almost old age by doing really stupid things.
Mildly stupid things, perhaps (like paying too much
attention to RAS).

Kirk
66

Point taken!

Half the trouble is most gliding clubs have a box at
the gate where many members leave their brains, spend
a day on the airfield and then collect the brain on
the way home.

Sorting out .......those who know what they are doing
from
.... those who think they know what they are doing
from
.....those who know absolutely nothing but go on and
do it anyway from....... those who can't do it but
have seen somebody else do it so they will try it anyway

All goes to provides a headache for the fun police,
who can't do right for doing wrong....!

Hope the turbulator tape works.

Dave

Jack

October 12th 03, 03:52 AM

in article , Dave Martin at
wrote on 2003/10/11 4:41:

> I would defend to the end your right to do what you
> want with your life.
>
> Sadly it is not those who kill themselves that is the
> problem, it is
>
> The 'friends' left behind....
>
> The relatives....
>
> ...the thieving grasping lawyers....

> [H]ave a thought for the others....

> If you want to kill yourself go out quietly and shoot
> yourself....

Which would, of course, solve _none_ of the so-called "problems" you listed
above.

You would "defend to the death" my right to -- what? Live so that you and
others are never inconvienienced?

I know we can't expect your ilk to understand that it is that kind of
thinking that has got the world where it is today, but a little less
hypocrisy would be nice.

Jack

Bob Kuykendall

October 13th 03, 08:28 PM

Earlier, (John) wrote:

> > My '6 came with turbulator tape on the bottom surface, just in front
> > of the elevator. 1994 C model. The stall is very gentle.

To which (Kirk Stant) replied:

> That's interesting. Does your aircraft or maintenance manual have any
> info on the tail tape (placement, size, etc.)? My 6 is an 87 B model,
> no tape anywhere except on the rudder mylar (zigzag leading edge).

I've heard unconfirmed reports that the later model LS6s have a
revised tailplane with a thinner section. Again, that's just hearsay,
but it might be something to look into. Can anyone here confirm or
deny?

Thanks, and best regards to all

Bob K.

JJ Sinclair

October 14th 03, 04:04 PM

Kirk,
I agree with you that competent, current pilots, don't inadvertently stall any
ship. But your statement that you like a crisp stall, got me wondering. How do
you like a crisp stall, in the pattern, after some hard maneuvering to avoid a
mid-air? How do you like a crisp stall, when on the rocks, and get hit with a
large tail-gust?

The stall I got in my LS-7, after adding zig-zag to the horizontal stab, was
the worst stall I have ever seen in any sailplane. I mean she went near
vertical on me. Don't care to duplicate that in the above situations.

PS, I suspect the zig-zag delayed the *normal* seperation on the stab, but when
it did seperate, I lost all the down force from the horizontal stab and that
gave me the near vertical pitching moment. The wing may not have stalled at
all. Now someone please feel free to tell me just how full of Ka-ka, I am about
aerodynamics.
:>)
JJ Sinclair

Bill Daniels

October 14th 03, 09:22 PM

JJ, you are about right on. I strongly suspect that if a glider "stalls"
but retains aileron control, the wing didn't really stall - just the
horizontal tail lost its grip on the air and let the tail pop up (Nose pops
down). Normally, this is a pretty benign way to limit minimum airspeed.
Adding any device to increase up elevator authority might allow the pilot
bring the whole wing to stall with the possibility of a spin departure.

It seems to me that the only reason to increase up elevator authority would
be to improve thermaling and then only in the case of a glider whose minimum
airspeed is limited by up elevator ineffectiveness. A case might be the
G103 Twin II when the CG is near the forward limit. This limits minimum
airspeed to well above minimum sink in a steep turn.

Bill Daniels

"JJ Sinclair" > wrote in message
...
> Kirk,
> I agree with you that competent, current pilots, don't inadvertently stall
any
> ship. But your statement that you like a crisp stall, got me wondering.
How do
> you like a crisp stall, in the pattern, after some hard maneuvering to
avoid a
> mid-air? How do you like a crisp stall, when on the rocks, and get hit
with a
> large tail-gust?
>
> The stall I got in my LS-7, after adding zig-zag to the horizontal stab,
was
> the worst stall I have ever seen in any sailplane. I mean she went near
> vertical on me. Don't care to duplicate that in the above situations.
>
> PS, I suspect the zig-zag delayed the *normal* seperation on the stab, but
when
> it did seperate, I lost all the down force from the horizontal stab and
that
> gave me the near vertical pitching moment. The wing may not have stalled
at
> all. Now someone please feel free to tell me just how full of Ka-ka, I am
about
> aerodynamics.
> :>)
> JJ Sinclair

Kirk Stant

October 15th 03, 12:13 AM

(JJ Sinclair) wrote in message >...
> Kirk,
> I agree with you that competent, current pilots, don't inadvertently stall any
> ship. But your statement that you like a crisp stall, got me wondering. How do
> you like a crisp stall, in the pattern, after some hard maneuvering to avoid a
> mid-air? How do you like a crisp stall, when on the rocks, and get hit with a
> large tail-gust?
>
> The stall I got in my LS-7, after adding zig-zag to the horizontal stab, was
> the worst stall I have ever seen in any sailplane. I mean she went near
> vertical on me. Don't care to duplicate that in the above situations.
>
> PS, I suspect the zig-zag delayed the *normal* seperation on the stab, but when
> it did seperate, I lost all the down force from the horizontal stab and that
> gave me the near vertical pitching moment. The wing may not have stalled at
> all. Now someone please feel free to tell me just how full of Ka-ka, I am about
> aerodynamics.
> :>)
> JJ Sinclair

JJ,

I like planes that have enough control authority to stall, not those
limited to just mushing along at a high sink rate. I absolutely do
not believe that it is safer to have a high sink rate while turning
base to final because all the stall symptoms are hidden - a stall
break will provide the cue to unload and recover. Anyway, the whole
point is to not stall unless you want to, whether you are mushing
along with the stick full aft, or recovering from a g-break; either
way for a while there you are a passenger - and as a pilot you
shouldn't get in that situation. And yes, I do practice stalls, and
fly fast steep patterns, and believe in spin training, and practice
getting slow in a steep thermalling turn, etc. I guess the difference
is I like control of the plane in pitch all the way into and back out
of a stall. Might be due to liking acro...and mildly disliking mushy
POS's like 2-33s, etc. On the other hand, I do like ASK-21s, and they
will not stall at normal CGs (except inverted, of course...). So
there are always exceptions.

If turbulator tape resulted in your tail stalling, that is a different
matter altogether. I have a hard time understanding it, especially
since other similar configuration glass have turbulated tails - but if
I go ahead and try it I will definitely check it out. So thanks for
the heads up!

Of course, there is a lot of difference between a 6 and a 7, so who
knows what will happen!

Kirk

Andy Blackburn

October 15th 03, 04:46 AM

I'm not aware of any glider that stalls from a sharp
break stall on the horizontal stab, though many of
us have experienced running out of elevator authority
- just try any combination of pushing the c.g. forward
or increasing the bank angle in a turn to make this
real (I have, and it's annoying in a thermal, though
not particularly dangerous unless taken to extremes).

Turbulator tape trips the boundary layer from laminar
to turbulent and should allow the elevator to create
lift at higher, rathern than lower angles of attack,
so unless you change the wing or tail incidence it's
hard to imagine that it would lead to the tail stalling
first. What likely happens is it gives you enough
additional authority to get the wing to really stall,
rather than mush around just past max Cl angle of attack.

I won't say whether this is good or bad per se.

9B

At 20:30 14 October 2003, Bill Daniels wrote:
>JJ, you are about right on. I strongly suspect that
>if a glider 'stalls'
>but retains aileron control, the wing didn't really
>stall - just the
>horizontal tail lost its grip on the air and let the
>tail pop up (Nose pops
>down). Normally, this is a pretty benign way to limit
>minimum airspeed.
>Adding any device to increase up elevator authority
>might allow the pilot
>bring the whole wing to stall with the possibility
>of a spin departure.
>
>It seems to me that the only reason to increase up
>elevator authority would
>be to improve thermaling and then only in the case
>of a glider whose minimum
>airspeed is limited by up elevator ineffectiveness.
> A case might be the
>G103 Twin II when the CG is near the forward limit.
> This limits minimum
>airspeed to well above minimum sink in a steep turn.
>
>Bill Daniels
>
>'JJ Sinclair' wrote in message
...
>> Kirk,
>> I agree with you that competent, current pilots, don't
>>inadvertently stall
>any
>> ship. But your statement that you like a crisp stall,
>>got me wondering.
>How do
>> you like a crisp stall, in the pattern, after some
>>hard maneuvering to
>avoid a
>> mid-air? How do you like a crisp stall, when on the
>>rocks, and get hit
>with a
>> large tail-gust?
>>
>> The stall I got in my LS-7, after adding zig-zag to
>>the horizontal stab,
>was
>> the worst stall I have ever seen in any sailplane.
>>I mean she went near
>> vertical on me. Don't care to duplicate that in the
>>above situations.
>>
>> PS, I suspect the zig-zag delayed the *normal* seperation
>>on the stab, but
>when
>> it did seperate, I lost all the down force from the
>>horizontal stab and
>that
>> gave me the near vertical pitching moment. The wing
>>may not have stalled
>at
>> all. Now someone please feel free to tell me just
>>how full of Ka-ka, I am
>about
>> aerodynamics.
>> :>)
>> JJ Sinclair
>
>

Robert Ehrlich

October 15th 03, 11:18 AM

JJ Sinclair wrote:
>
> Kirk,
> I agree with you that competent, current pilots, don't inadvertently stall any
> ship. But your statement that you like a crisp stall, got me wondering. How do
> you like a crisp stall, in the pattern, after some hard maneuvering to avoid a
> mid-air? How do you like a crisp stall, when on the rocks, and get hit with a
> large tail-gust?
>
> The stall I got in my LS-7, after adding zig-zag to the horizontal stab, was
> the worst stall I have ever seen in any sailplane. I mean she went near
> vertical on me. Don't care to duplicate that in the above situations.
>
> PS, I suspect the zig-zag delayed the *normal* seperation on the stab, but when
> it did seperate, I lost all the down force from the horizontal stab and that
> gave me the near vertical pitching moment. The wing may not have stalled at
> all. Now someone please feel free to tell me just how full of Ka-ka, I am about
> aerodynamics.
> :>)
> JJ Sinclair

I am not an expert in aerodynamics, but I don't agree with your above interpretation.
It implies that just below the stall your elevator was producing a down force,
near the maximum possible or rather the (negative) lift coefficient was near its maximum
possible value and this maximum value was reached when you lost control, before
the wing reached its maximum (positive) lift coefficient. It seems to me that it
is an error to believe that more down force on the tail plane is needed for obtaining
an higher nose up attitude. As airfoils commonly used in sailplanes are unstable,
more nose up attitudes tend to self amplify when you consider only the forces
on the wing and the tail plane has to counter this by a lower down force (or a higher
up force, depending on position of the CG). This is not in contradiction with the
fact that you need aft stick in order to keep a higher nose up attitude.
This change of attitude also change the angle of attack on the tail plane by an
amount exceeding the variation of force needed and the back stick has to compensate for this.
For these reasons I think that when you come closer to the stalling angle of attack
for the wing, the tailplane on the contrary is far from its stalling angle of attack.
When the wing reaches its stalling angle, a further increase in angle of attack
will lower the lift coefficient, thus increasing again the angle of attack due
to the glider beginning to fall and so on. This also increases the angle of attack on the
tailplane which produces an upward force, despite the full aft stick, and this
produces the pitching moment that makes the nose drop. It would be logic to think
that in this case, the increased efficiency of the the tail plane due to the added
turbulator increases the pitching moment and brings the glider more quickly
closer to a vertical dive.

JJ Sinclair

October 15th 03, 03:18 PM

I find this a very interesting subject (LS stab stalls) I have a lot of time
in the LS-6 and found no stab stall problems at all. The LS-7 was the same
until I put zig-zag on, then got an abrupt nasty stall. I only have about 50
hours in the LS-8, but I could feel and hear stab separation a good 5 knots
above wing stall when flying with 9 lb wg loading. My CG on all 3 was about 85%
of allowable range.

All three use a thin stab section and none come with zig-zag on the stab. If
I'm getting separation (nervous stick) a good 5 knots above wing stall, doesn't
that mean I'm getting close to stalling the stab? If I put zig-zag on the stab
and delay the stall, won't I be masking the tell-tail sings of stab stall?
JJ Sinclair

JJ Sinclair

October 15th 03, 04:28 PM

I find this a very interesting subject (LS stab stalls) I have a lot of time in
the 6 & 7. I found NO stab stall problem at all in my LS-6. The LS-7 was the
same until I put zig-zag on, then I got an abrupt, nasty stall. I only have
about 50 hours in the LS-8, but I could feel and hear stab separation, a good 5
knots above wing stall, when flying at 9 lb wg loading. The CG on all 3 was
about 85% of allowable range.

All 3 use a thin section and none of my ships came with zig-zag on the stab. If
I'm getting separation (nervous stick) a good 5 knots above wing stall, doesn't
that mean I'm getting close to stalling the stab? If I put zig-zag on the stab
and delay the stall, won't I be masking the tell-tail signs of stab stall? Are
we investigating something that LS already knows?
JJ Sinclair

Robert Ehrlich

October 16th 03, 11:49 AM

JJ Sinclair wrote:
>
> I find this a very interesting subject (LS stab stalls) I have a lot of time in
> the 6 & 7. I found NO stab stall problem at all in my LS-6. The LS-7 was the
> same until I put zig-zag on, then I got an abrupt, nasty stall. I only have
> about 50 hours in the LS-8, but I could feel and hear stab separation, a good 5
> knots above wing stall, when flying at 9 lb wg loading. The CG on all 3 was
> about 85% of allowable range.
>
> All 3 use a thin section and none of my ships came with zig-zag on the stab. If
> I'm getting separation (nervous stick) a good 5 knots above wing stall, doesn't
> that mean I'm getting close to stalling the stab? If I put zig-zag on the stab
> and delay the stall, won't I be masking the tell-tail signs of stab stall? Are
> we investigating something that LS already knows?
> JJ Sinclair

How do you know that your feeling and hearing comes from the stab
and not the wing ? I can't believe the stab is stalling before the
wing, for the reasons I tried to explain in my previous post. This
happens only on canard stabs. But separaion beginning on the wing
before full stal is much more likely and this can also cause nervous
stick.

Martin Gregorie

October 16th 03, 12:54 PM

On Thu, 16 Oct 2003 10:49:22 +0000, Robert Ehrlich
> wrote:

>JJ Sinclair wrote:
>>
>> I find this a very interesting subject (LS stab stalls) I have a lot of time in
>> the 6 & 7. I found NO stab stall problem at all in my LS-6. The LS-7 was the
>> same until I put zig-zag on, then I got an abrupt, nasty stall. I only have
>> about 50 hours in the LS-8, but I could feel and hear stab separation, a good 5
>> knots above wing stall, when flying at 9 lb wg loading. The CG on all 3 was
>> about 85% of allowable range.
>>
>> All 3 use a thin section and none of my ships came with zig-zag on the stab. If
>> I'm getting separation (nervous stick) a good 5 knots above wing stall, doesn't
>> that mean I'm getting close to stalling the stab? If I put zig-zag on the stab
>> and delay the stall, won't I be masking the tell-tail signs of stab stall? Are
>> we investigating something that LS already knows?
>> JJ Sinclair
>
>How do you know that your feeling and hearing comes from the stab
>and not the wing ? I can't believe the stab is stalling before the
>wing, for the reasons I tried to explain in my previous post. This
>happens only on canard stabs. But separaion beginning on the wing
>before full stal is much more likely and this can also cause nervous
>stick.

I'd expect major flow separation under the tailplane at the elevator
hinge under these conditions (low airspeed, fully deflected elevator).
If the tailplane doesn't actually stall, I'd at least expect a sudden
reduction in elevator effectiveness as the flow separated. That might
be all that's needed to give the fairly gentle pitch down we all
associate with a typical glider stall.

Ob. question: what have I missed?

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Robert Ehrlich

October 16th 03, 01:59 PM

Martin Gregorie wrote:
>
> I'd expect major flow separation under the tailplane at the elevator
> hinge under these conditions (low airspeed, fully deflected elevator).
> If the tailplane doesn't actually stall, I'd at least expect a sudden
> reduction in elevator effectiveness as the flow separated. That might
> be all that's needed to give the fairly gentle pitch down we all
> associate with a typical glider stall.
>

Low airspeed is not a factor, stalling is related to angle of attack,
not to airspeed. For the wing there is a relationship between angle
of attack and airspeed due to the fact that the lift must remain equal to
the weight, so lower airspeed implies higher lift coeefficent, i.e.
higher angle of attack, as long as increasinng the angle of attack
increases the lift coeefficent, i.e. up to stall angle. But nothing
like this exists concerning the tail plane.

Unless the stops on the elevator allows an execessive excursion,
there should be no separation at the hinge. Even in this case
this should occur before the nose up attitude is obtained, at the
moment when the elevator is deflected in order to obtain the
rotation providing this nose up attitude because at this moment the
(negative) angle of attack is higher (in absolute value) than when
the nose up attitude is obtained.

The pitch down associated with a typical glider stall is not due
to stalling or separation at the elevator, but on the contrary
to the fact that the wing is stalled and not the elevator, both
having positive angles of attack. Only on canards the elvvator+
stabiliser has a higher incidence than the main wing and so stalls
before the main wing, and this is well known to completeley avoid
wing stalls. On aircratfs with conventional tailplanes the main
wing stalls before the tailplane and the ensuing pitch down
decreases the angle of attack on both the main wing
and the tailplane and should avoid stall on the tailplane as
well as recover from stall on the main wing.

JJ Sinclair

October 16th 03, 02:14 PM

Robert wrote>>>>>>>>>>>>>>>>>>>>
>How do you know that your feeling and hearing comes from the stab
>and not the wing ?

In the LS-8, I can feel it in the stick a good 5 knots above stall. I don't
believe it is separated air coming from the wing, because the T-tail is just
too high to allow that. I have felt the same thing about 3 knots above stall in
a 301 libelle which has a low tail and it was *dirty air* coming from the
wing.

BTW, I only felt the nervous stick with a load of water and I wasn't trying to
stall the ship, It happened when I was thermalling.
JJ Sinclair

Ray Lovinggood

October 16th 03, 03:24 PM

Where should I put turbulator tape on the all-flying
stabilator on my LS-1d? Right now, there is a the
head of a big 'ol hex-head bolt, safety pin, and peg
(to anchor the safety pin) that sticks out in the breeze
on the upper surface of the stabilator.
(This is asked with tongue in cheek)

By the way, when it stalls, just a very quick foreward
movement on the stick gets it going again. No warning
before it stalls.

Ray Lovinggood
Carrboro, North Carolina, USA

At 13:54 16 October 2003, Jj Sinclair wrote:
>Robert wrote>>>>>>>>>>>>>>>>>>>>
>>How do you know that your feeling and hearing comes
>>from the stab
>>and not the wing ?
>
>In the LS-8, I can feel it in the stick a good 5 knots
>above stall. I don't
>believe it is separated air coming from the wing, because
>the T-tail is just
>too high to allow that. I have felt the same thing
>about 3 knots above stall in
>a 301 libelle which has a low tail and it was *dirty
>air* coming from the
>wing.
>
> BTW, I only felt the nervous stick with a load of
>water and I wasn't trying to
>stall the ship, It happened when I was thermalling.
>JJ Sinclair
>

JJ Sinclair

October 16th 03, 03:54 PM

Robert,
If the stab isn't stalling, can you explain the sharp stall in the LS-7 after
adding zig-zag and the elevator buffet in the LS-8, a good 5 knots above wing
stall?

Could it be this LS-8 wasn't set with the proper declanage? With all the
sloppy workmanship we are seeing these days, it wouldn't surprise me, a bit.
:>(
JJ Sinclair

Eric Greenwell

October 16th 03, 04:23 PM

In article >,
says...
> Robert wrote>>>>>>>>>>>>>>>>>>>>
> >How do you know that your feeling and hearing comes from the stab
> >and not the wing ?
>
> In the LS-8, I can feel it in the stick a good 5 knots above stall. I don't
> believe it is separated air coming from the wing, because the T-tail is just
> too high to allow that. I have felt the same thing about 3 knots above stall in
> a 301 libelle which has a low tail and it was *dirty air* coming from the
> wing.
>
> BTW, I only felt the nervous stick with a load of water and I wasn't trying to
> stall the ship, It happened when I was thermalling.
> JJ Sinclair

If it happened while thermalling, this suggests it isn't the elevator
stalling. Here's why:

While circling, the elevator's angle of attack (AOA) is greater than
the wing's AOA, because of the differing airflow directions.

This greater AOA tends to increase the upward force on the elevator
(or reduce it's downward force), which is why it is more difficult to
stall a glider in a turn.

Or, if we think of the elevator as an "upside down" wing that is
producing lift downward (pushing the tail down), it's AOA is
_reduced_.

With a lower AOA, it's not going to stall in a turn if it can't do it
in straight ahead flight.

Question: with water, was the CG kept in the same place as without
water, or did it move forward?

--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

Martin Gregorie

October 16th 03, 05:12 PM

On Thu, 16 Oct 2003 12:59:05 +0000, Robert Ehrlich
> wrote:

>Low airspeed is not a factor, stalling is related to angle of attack,
>not to airspeed.
>
Yes, of course, but airspeed does affect the energy in the boundary
layer and that in turn does control where (and if) the airflow
separates from the surface.

That's why turbulation works - it energises the BL, thus preventing
flow separation.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Bill Daniels

October 16th 03, 05:14 PM

"Eric Greenwell" > wrote in message
.. .
> In article >,
> says...
> > Robert wrote>>>>>>>>>>>>>>>>>>>>
> > >How do you know that your feeling and hearing comes from the stab
> > >and not the wing ?
> >
> > In the LS-8, I can feel it in the stick a good 5 knots above stall. I
don't
> > believe it is separated air coming from the wing, because the T-tail is
just
> > too high to allow that. I have felt the same thing about 3 knots above
stall in
> > a 301 libelle which has a low tail and it was *dirty air* coming from
the
> > wing.
> >
> > BTW, I only felt the nervous stick with a load of water and I wasn't
trying to
> > stall the ship, It happened when I was thermalling.
> > JJ Sinclair
>
> If it happened while thermalling, this suggests it isn't the elevator
> stalling. Here's why:
>
> While circling, the elevator's angle of attack (AOA) is greater than
> the wing's AOA, because of the differing airflow directions.
>
> This greater AOA tends to increase the upward force on the elevator
> (or reduce it's downward force), which is why it is more difficult to
> stall a glider in a turn.
>
> Or, if we think of the elevator as an "upside down" wing that is
> producing lift downward (pushing the tail down), it's AOA is
> _reduced_.
>
> With a lower AOA, it's not going to stall in a turn if it can't do it
> in straight ahead flight.
>
> Question: with water, was the CG kept in the same place as without
> water, or did it move forward?
>
> --
> !Replace DECIMAL.POINT in my e-mail address with just a . to reply
> directly
>
> Eric Greenwell
> Richland, WA (USA)

Eric, I need to jump in here on JJ's side. I have experienced exactly what
he is describing and interpreted it the same way.

The horizontal tail (high or low mounted) is operating in the wings near
field flow or, in this case the wings downwash. Even if the incidence of
the wing and tail are the same, the tail will be at a larger negative angle
of attack, relative to its local flow, than the wing is at a positive angle
of attack. In the case of a "T" tail, the horizontal will not be in the
turbulent wake of the wing since that turbulence is embedded in the wings
downwash which tends to depart downwards and back from the wing.

Since the low aspect ratio tail in not an efficient "wing", it must operate
at a larger negative AOA to produce sufficient downforce to balance a
forward CG.

In a thermaling turn, the negative AOA of the tail must be increased still
further to balance the centrifugal force acting on the CG while maintaining
a low AS. It is not unreasonable to think that, at some point, the tail
will reach its negative stalling AOA while the wing is still below its
stalling AOA, resulting in the nose dropping and the AS increasing.
(Obviously, as the CG is moved aft, the need for downforce diminishes.)

JJ's "nervous" elevator is more likely to be the airflow separating and
re-attaching to the lower surface of the tail than an effect of the
turbulent wake of the wing. If, as suggested, adding turbulator tape to the
underside of the horizontal tail allows it to develop greater downforce
before stalling, the wing can be brought to a greater AOA and perhaps a wing
stall.

The counter argument that suggests that the horizontal tail is flying at a
positive angle of attack when the glider is flown near minimum airspeed must
assume that the pitching moment of the wing produces an nose-up pitching
moment that exceeds the nose-down moment of the CG acting ahead of the wings
center of lift - OR that the CG is placed aft of the center of lift. Both
of these conditions would produce serious static pitch instability which
would not pass JAR 22 certification standards.

I must conclude that, for normal CG locations, the horizontal tail flies at
a negative AOA relative to its local flow and that this negative AOA
increases as the airspeed diminishes. Further, that the horizontal tail
negative AOA can, and often does, reach its stalling AOA at the minimum
sustainable airspeed while the wing flies just below its stalling AOA. This
condition produces very benign "stall" characteristics.

Bill Daniels

Robert Ehrlich

October 16th 03, 05:31 PM

JJ Sinclair wrote:
>
> Robert wrote>>>>>>>>>>>>>>>>>>>>
> >How do you know that your feeling and hearing comes from the stab
> >and not the wing ?
>
> In the LS-8, I can feel it in the stick a good 5 knots above stall. I don't
> believe it is separated air coming from the wing, because the T-tail is just
> too high to allow that. I have felt the same thing about 3 knots above stall in
> a 301 libelle which has a low tail and it was *dirty air* coming from the
> wing.
>
> BTW, I only felt the nervous stick with a load of water and I wasn't trying to
> stall the ship, It happened when I was thermalling.
> JJ Sinclair

Anyway I find easier to believe that the T-tail is in dirty air (not necessairly
separarted, the wake of the wing and/or fuselage may be suffcient to cause
this nervous stick) than to believe the tail plane is stalling at a negative
angle of attack. Most airfoils reach their stalling angle near 18 degrees, as
you are just above stall speed the angle of attack is just below this value,
so the difference between the (positive) angle of attack of the wing and the
(negative according to your views) angle of attack on the tail plane should be
nearly the double, i.e. 36 degrees, which seems geometrically impossible. And
this would imply that at this attitude you need a down force on the tail plane
that is exceeding its maximum capabilities, which is in contradiction with the
fact that, due to the instability of the main wing, you are at the attitude
that needs the lowest down force, if even the force needed is downwards.

Regarding this last point, I heard something interesting during the instructor
course I attended during last September, from the instructor that had my group
in charge and who is a pilot with a huge experience of more than 10000 hours
and a long experience in teaching to future instructors the way sailplanes are
flying. He said that there is a very common believing that tail planes are always
providing a down force, but this is not true, especially in recent high performance
sailplanes (I am not sure to rememeber but I believe he cited LS). He said that
one of the points where the manufacturers are trying to improve L/D, i.e. reduce
drag is by trying to have a neutral tail plane near the best L/D speed and
corresponding angle of attack, because this eliminates the induced drag due
to the tailplane lift. This implies, due to the instability of the main wing,
that the tailplane provides a down force at wing angles of attack lower than this
(neutral) angle (and so at higher speeds) and an up force at higher angles of attack,
i.e. at lower speeds.

Robert Ehrlich

October 16th 03, 05:50 PM

Bill Daniels wrote:
> ...
> The counter argument that suggests that the horizontal tail is flying at a
> positive angle of attack when the glider is flown near minimum airspeed must
> assume that the pitching moment of the wing produces an nose-up pitching
> moment that exceeds the nose-down moment of the CG acting ahead of the wings
> center of lift - OR that the CG is placed aft of the center of lift. Both
> of these conditions would produce serious static pitch instability which
> would not pass JAR 22 certification standards.
> ...

Here is the point where I think there is a confusion. What do you call "center
of lift" ? The pitch stability needs only that the neutral point is behind the
CG, the neutral point is the location of the increment of lift provided by
all surfaces (wing and tail plane) when there is an increment in AOA. If you
call "center of lift" the point where you can reduce the lifting forces on the
wing only to a single vector (this is implied by what you say concerning
the pitching moment created by weight and lift) this is a different point which is
moving forward when the AOA increases and may be ahead of the CG at high
AOA, while the neutral point is always behind it. The confusion is both
betweeen an incremental force and its actual value and by the force provided
by the wing only and by wing + tail plane.

Bill Daniels

October 16th 03, 07:46 PM

"Robert Ehrlich" > wrote in message
...
> Bill Daniels wrote:
> > ...
> > The counter argument that suggests that the horizontal tail is flying at
a
> > positive angle of attack when the glider is flown near minimum airspeed
must
> > assume that the pitching moment of the wing produces an nose-up pitching
> > moment that exceeds the nose-down moment of the CG acting ahead of the
wings
> > center of lift - OR that the CG is placed aft of the center of lift.
Both
> > of these conditions would produce serious static pitch instability which
> > would not pass JAR 22 certification standards.
> > ...
>
>
> Here is the point where I think there is a confusion. What do you call
"center
> of lift" ? The pitch stability needs only that the neutral point is behind
the
> CG, the neutral point is the location of the increment of lift provided by
> all surfaces (wing and tail plane) when there is an increment in AOA. If
you
> call "center of lift" the point where you can reduce the lifting forces on
the
> wing only to a single vector (this is implied by what you say concerning
> the pitching moment created by weight and lift) this is a different point
which is
> moving forward when the AOA increases and may be ahead of the CG at high
> AOA, while the neutral point is always behind it. The confusion is both
> betweeen an incremental force and its actual value and by the force
provided
> by the wing only and by wing + tail plane.

You can approach the issue with the center of lift of the wing only and
address the tail separately or deal with the aircraft as a whole and talk
about neutral point. Both avenues will arrive at the same conclusions if
done properly. Perhaps it's my very obsolete training, but I still prefer
to deal with each surface separately.

However, if the wings center of lift ever gets forward of the CG, you do not
have positive static stability, regardless of any other factor. In this
case any reduction in airspeed will require that the stick be moved forward
to counter increasing tail heaviness - clearly an unacceptable situation.

I agree that designers want to reduce trim drag to the minimum by trying to
make the tail fly at a zero angle of attack. To do this, the CG must be
moved aft and static stability sacrificed so there are limits to this
approach. I would disagree that this should occur at L/D max. Gliders
spend little time at L/D max and the best overall contribution to
performance would be to have the tail at zero AOA at high speed. In fact,
most gliders will exhibit lower pitch stability at high speed and some will
even show a tendency to "tuck" (nose down) if the stick is released,
indicating that the neutral point in the static stability curve occurs at
high speed and suggesting that the tail is near a zero AOA.

Any glider I have ever flown (with the exception of some deliberate aft CG
flight tests) will have the stick positioned further aft at low speeds than
at high speeds indicating that the tail moves toward greater negative AOA as
the glider slows. (I've actually mounted protractors on the stick to prove
this to myself.) For any given trim setting, back pressure on the stick is
needed to reduce airspeed and forward pressure is needed to increase
airspeed - which is essentially the definition of static stability.

Bill Daniels

Eric Greenwell

October 16th 03, 08:25 PM

In article t>,
says...

> > If it [stick shaking] happened while thermalling, this suggests it isn't the elevator
> > stalling. Here's why:
> >
> > While circling, the elevator's angle of attack (AOA) is greater than
> > the wing's AOA, because of the differing airflow directions.
> >
> > This greater AOA tends to increase the upward force on the elevator
> > (or reduce it's downward force), which is why it is more difficult to
> > stall a glider in a turn.
> >
> > Or, if we think of the elevator as an "upside down" wing that is
> > producing lift downward (pushing the tail down), it's AOA is
> > _reduced_.
> >
> > With a lower AOA, it's not going to stall in a turn if it can't do it
> > in straight ahead flight.
> >
> > Question: with water, was the CG kept in the same place as without
> > water, or did it move forward?
> >
>
> Eric, I need to jump in here on JJ's side. I have experienced exactly what
> he is describing and interpreted it the same way.
>
> The horizontal tail (high or low mounted) is operating in the wings near
> field flow or, in this case the wings downwash. Even if the incidence of
> the wing and tail are the same, the tail will be at a larger negative angle
> of attack, relative to its local flow, than the wing is at a positive angle
> of attack. In the case of a "T" tail, the horizontal will not be in the
> turbulent wake of the wing since that turbulence is embedded in the wings
> downwash which tends to depart downwards and back from the wing.

Agreed, though I would say the elevator operates at a lower AOA than
the wing, due to the incidence and the local flow. I like to measure
the AOA the same way at both surfaces.

> Since the low aspect ratio tail in not an efficient "wing", it must operate
> at a larger negative AOA to produce sufficient downforce to balance a
> forward CG.

Not agreed.

First, the tail can not control it's angle of attack independently of
wing, at least for the flapped elevators we are talking about on the
LS7. An all-moving elevator does change it's AOA, of course.

Second, the elevator is a flapped surface, and can vary its lift
coefficient, which is how it adjusts it's force, rather than AOA.

Third, the actual force is also dependent on the surface area, and the
torque it produces also depends on the tail boom length. The
efficiency of the surface is relevant only to the choice of tail boom
length, surface size, and airfoil. These are set by the designer.

> In a thermaling turn, the negative AOA of the tail must be increased still
> further to balance the centrifugal force acting on the CG while maintaining
> a low AS.
> It is not unreasonable to think that, at some point, the tail
> will reach its negative stalling AOA while the wing is still below its
> stalling AOA, resulting in the nose dropping and the AS increasing.
> (Obviously, as the CG is moved aft, the need for downforce diminishes.)

It is unreasonable to think this. We all know that it is easier to
stall a glider in straight flight than in circling flight. Most
gliders can't be stalled after reaching a certain bank angle, commonly
30-40 degrees. The reason: reduced elevator authority because the
relative airflow is different at the wing and the tail.

In straight flight, the air hits the elevator at the same AOA that it
does at the wing. In circling flight, the air hits the elevator at a
greater AOA than it does at the wing. This greater AOA reduces the
down force available. This greater AOA (what you would call a lesser
negative AOA) means the elevator is further from stalling than in
straight flight.

> JJ's "nervous" elevator is more likely to be the airflow separating and
> re-attaching to the lower surface of the tail than an effect of the
> turbulent wake of the wing.

This is still possible, but not because it _delays_ the stall of the
surface by keeping the flow attached better, but because makes it the
flow separate sooner.

> If, as suggested, adding turbulator tape to the
> underside of the horizontal tail allows it to develop greater downforce
> before stalling, the wing can be brought to a greater AOA and perhaps a wing
> stall.

Yes, IF...

snip

> I must conclude that, for normal CG locations, the horizontal tail flies at
> a negative AOA relative to its local flow

Agreed.

> and that this negative AOA
> increases as the airspeed diminishes.

Maybe, maybe not. In any case, the elevator must be deflected upwards
to increase the lift coefficient of the surface, as this AOA change
isn't sufficient.

> Further, that the horizontal tail
> negative AOA can, and often does, reach its stalling AOA at the minimum
> sustainable airspeed while the wing flies just below its stalling AOA. This
> condition produces very benign "stall" characteristics.

If this were true, it'd be amazing. For example, if it were the
elevator controlling the stall behavior, why is a glider so much
harder (more back stick required) to stall with a forward CG than a
rearward CG? In a forward CG, you use more back stick to get the nose
up, which is when the elevator should be most likely to stall. This
suggests the pitch angle (relative to the fuselage) of the forward CG
glider would be less at "stall" than with a rearward CG, yet is the
same.

The stall behavior of our gliders is controlled by the wing: airfoil
shape (often varying from root to tip), wing twist, winglets, and
planform. The benign behavior of the newer gliders (say, last 20
years) is a tribute to the airfoil designers ability to get improved
performance AND better stall behavior, coupled with the glider's
designers careful balancing of the other factors.

--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

Phoebus_810

October 17th 03, 08:48 AM

Dave Martin > wrote in message >...
> Kirk Stant wrote
>
> Meanwhile, the rest of you guys out there relax, I
> appreciate your concern but I haven't reached this
> ripe almost old age by doing really stupid things.
> Mildly stupid things, perhaps (like paying too much
> attention to RAS).
>
> Kirk
> 66
>
> Point taken!
>
> Half the trouble is most gliding clubs have a box at
> the gate where many members leave their brains, spend
> a day on the airfield and then collect the brain on
> the way home.
>

> Sorting out .......those who know what they are doing
> from
> ... those who think they know what they are doing
> from
> ....those who know absolutely nothing but go on and
> do it anyway from....... those who can't do it but
> have seen somebody else do it so they will try it anyway
>
>
> All goes to provides a headache for the fun police,
> who can't do right for doing wrong....!
>
> Hope the turbulator tape works.
>
> Dave

Hello all
I have a Phoebus C with all-flying tail. It has a very thin profile. I
had contakt with a german pilot that "had heard somewhere" that you
could improve the plane by attaching turbulators on the elevator.
Because the plane has some odd stall habbits ( compared to modern
planes), I wouldn´t like to try it out whitout to here from somebody
with experience with it.
So my question is: do anyone of you know where I could get first hand
information in the matter.

Matts

Robert Ehrlich

October 17th 03, 10:57 AM

Bill Daniels wrote:
> ...
> However, if the wings center of lift ever gets forward of the CG, you do not
> have positive static stability, regardless of any other factor. In this
> case any reduction in airspeed will require that the stick be moved forward
> to counter increasing tail heaviness - clearly an unacceptable situation.
> ...

Not necessarily. With a reduction in airspeed there is an increase in AOA
of the wing, so necesarily an increase in AOA of the tail plane. If its
surface and lever arm are sufficient, this will override the increased
pitching (up) moment due to the forward move of the center of lift. This
is the condition for stability and it can clearly be met by a sufficient
high product of tail surface by lever arm. If this occurs, as the pitching
down moment due to the tail override the pitching up moment due to the wing,
in order to obtain this reduction in airspeed, you have to lower this tail
moment by moving the stick back. This is what happens on all stable
aircrafts.

>...
> Any glider I have ever flown (with the exception of some deliberate aft CG
> flight tests) will have the stick positioned further aft at low speeds than
> at high speeds indicating that the tail moves toward greater negative AOA as
> the glider slows. (I've actually mounted protractors on the stick to prove
> this to myself.) For any given trim setting, back pressure on the stick is
> needed to reduce airspeed and forward pressure is needed to increase
> airspeed - which is essentially the definition of static stability.
>

I completely agree that the stick moves aft when speed becomes lower and
vice-versa, but this doesn't imply that the tail has a greater negative AOA.
The change in AOA is due to both the stick action and the global change of
attitude of the aircraft relativeley to the airstream due to the need of
increasing the wing AOA. With the airfoils usually used in sailplanes, the
second effect overrides the first one. Again I agree these moves of the stick
associated with the changes in speed and AOA are essentially the definition
of static stability, but this doen't imply that, assuming that the position
of the CG is such that you start with a tailplane at negative AOA, slowing
the glider needs a more negative AOA. If this were true, this would mean that
in order to keep the wing at a higher AOA, you need a higher pitching up moment,
i.e. the wing alone tends to revert to its previous AOA, i.e. is stable by
itself. This is not the case with the cambered airfoils usually used in sailplanes.