Elevator Turbulator tape question

"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

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.

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.

"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

In article et,
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)

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

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.

On Thu, 16 Oct 2003 16:31:46 +0000, Robert Ehrlich
wrote:

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.

This is common for all aircraft. You want to minimize drag at a
certain point of the envelope (best L/D, optimum cruise speed), so you
design your aircraft that the induced drag is 0 at this speed - and
induced drag = 0 means Cl = 0.


Bye
Andreas

On 16 Oct 2003 14:24:10 GMT, Ray Lovinggood
wrote:

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.

You probaly cannot get any warning before you stall an all-flying
tail, because you cannot feel the airflow over the tail becoming
turbulent before it separates. Stick forward lowers the AoA
immediately, therefore stopping the stall immediately.

The position of turbulator tape depends on the airfoil - if you know
where the laminar airflow comes turbulent (measured in percent of
airfoil depth), put it a little forward of this point ("a little" will
need some experiments) if you want to gain a little performance - but
I doubt that you will feel any difference.




Bye
Andreas

On 17 Oct 2003 00:48:22 -0700, (Phoebus_810)
wrote:

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.


Ask he

Fiberglas-Technik Rudolf Lindner GmbH & Co. KG,
Steige 3, D - 88487 Walpertshofen
Tel: 07353/2243
Fax: 07353/3096
E-Mail:

http://www.ltb-lindner.com/index.html


They are the ones who are taking care of the Phoebus. The owner,
Helmut Lindner, is the son of Rudi Lindner (who designed the Phoebus).


Bye
Andreas