Elevator Turbulator tape question

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.

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