horiz tail airfoil observations

I've been perusing Fred Thomas's Sailplane Design book lately. I am
curious, why do most of the modern sailplanes use a non-symetrical
airfoil for the horiz surface, and most of the "older" sailplanes use a
symetrical airfoil?

I was looking at a Krokus at the Pensacola airshow over the weekend,
made in the 80's, it is definately using somthing like the Wortman
71-150/30 while my glider, and Apis uses an airfoil that is not
symetrical.

Cheers,
Brad

With a lifting tail, the center of gravity can be moved aft for normal
flight regimes.

Jack Womack

I'm curious, though - with a lifting (i.e. positively-cambered) tail,
how do you avoid serious drag penalties and/or airflow disruption when
deflecting the elevator upwards enough to induce negative Cl (...in
other words, to have enough pitch-up control authority)? Everything
I've read points towards needing a roughly balanced Cl range for
adequate control authority in both pitch directions... I'm curious,
but a little confused...

Thanks! Take care,

--Noel


Jack wrote:
With a lifting tail, the center of gravity can be moved aft for normal
flight regimes.

Jack Womack

Modern gliders are designed so that the tailplane produces a moderate =
amount of lift in slow flight.
In steady flight, the tailplane lift coefficient may range from, say =
+0.2 for thermalling to -0.15 at VNE.
This leaves enough margin for transients, and definitely favours an =
airfoil with positive camber.

Brad wrote:
I've been perusing Fred Thomas's Sailplane Design book lately. I am
curious, why do most of the modern sailplanes use a non-symetrical
airfoil for the horiz surface, and most of the "older" sailplanes use a
symetrical airfoil?

I was looking at a Krokus at the Pensacola airshow over the weekend,
made in the 80's, it is definately using somthing like the Wortman
71-150/30 while my glider, and Apis uses an airfoil that is not
symetrical.

Is the fixed portion symmetrical but with a control surface that has a
slight undercamber on the bottom? That's what my ASH 26 elevator is
like, and I believe that is normal.

If the fixed portion is not symmetrical, is the flatter part on the top
or bottom?


--
Eric Greenwell - Washington State, USA
Change "netto" to "net" to email me directly

"Transponders in Sailplanes" on the Soaring Safety Foundation website
www.soaringsafety.org/prevention/articles.html

"A Guide to Self-launching Sailplane Operation" at www.motorglider.org

Ok, let me put on my "total newbie" outfit here...

Thinking in terms of real-world situations: In slow flight I'm sitting
in my glider, holding the stick back and keeping the angle of attack
high. I'm pulling a large Cl out of my main wing. The stick-back
condition corresponds to an upward-deflection of the trailing edge of
the elevator. So am I not generating a negative Cl with the horizontal
tail in this condition? Also, my wing airfoil still shows a Cm of
about -0.09 at this high angle of attack. Its small, but definitely
negative - so I still have a nose-down pitching moment from the wing -
therefore don't I *need* that "negative lift" (i.e. downward force) on
the tail? (I guess this all assumes the CG is ahead of the wing's
center of pressure/center of lift - but isn't that usually the case?)

Thanks, take care,

--Noel



Francisco De Almeida wrote:
Modern gliders are designed so that the tailplane produces a moderate =
amount of lift in slow flight.
In steady flight, the tailplane lift coefficient may range from, say =
+0.2 for thermalling to -0.15 at VNE.
This leaves enough margin for transients, and definitely favours an =
airfoil with positive camber.

Eric Greenwell wrote:
Is the fixed portion symmetrical but with a control surface that has a
slight undercamber on the bottom? That's what my ASH 26 elevator is
like, and I believe that is normal.

If the fixed portion is not symmetrical, is the flatter part on the top
or bottom?

I know the ASW27 has a small camber (up side down airfoil)
the 26 would most likely have it too. The Elevator under camber acts
like a servo tap, the faster you go the more up elevator you get when
flying with you hands off the stick, even if the trim spring is all
the way forward. This is a safty feature.
Udo

Is the fixed portion symmetrical but with a control surface that has a
slight undercamber on the bottom? That's what my ASH 26 elevator is
like, and I believe that is normal.

If the fixed portion is not symmetrical, is the flatter part on the top
or bottom?

the fixed portion is pretty much symetrical. the flatter part is on the
top surface

the upper part of the elevator is straight
the lower part of the elevator has a slight undercamber to it, like
what you normally see on the lower surface of a sailplane wing.

--
Eric Greenwell - Washington State, USA
Change "netto" to "net" to email me directly

"Transponders in Sailplanes" on the Soaring Safety Foundation website
www.soaringsafety.org/prevention/articles.html

"A Guide to Self-launching Sailplane Operation" at www.motorglider.org

Brad wrote:
Is the fixed portion symmetrical but with a control surface that has a
slight undercamber on the bottom? That's what my ASH 26 elevator is
like, and I believe that is normal.

If the fixed portion is not symmetrical, is the flatter part on the top
or bottom?

the fixed portion is pretty much symetrical. the flatter part is on the
top surface

This is consistent with the need to provide a down force, the usual case
for our gliders, so the airfoil is "upside down" compared to the wing.

the upper part of the elevator is straight
the lower part of the elevator has a slight undercamber to it, like
what you normally see on the lower surface of a sailplane wing.

As Udo pointed out, this is how the designer meets the requirement for
increasing "up elevator" force as speed increases. While this has a
safety advantage, the truly determined performance oriented pilot will
sometimes remove the undercamber to reduce drag. I've never wanted to do
it, because I want the safety advantage and I'm concerned the weight of
filler material might make the elevator flutter. It would take some
paperwork to make it legal, too.

--
Eric Greenwell - Washington State, USA
Change "netto" to "net" to email me directly

"Transponders in Sailplanes" on the Soaring Safety Foundation website
www.soaringsafety.org/prevention/articles.html

"A Guide to Self-launching Sailplane Operation" at www.motorglider.org

At a crazier age I tried eliminating all the undercamber in my ASW-25
elevator. This affected the pitch stability so drastically that when the
stick was released the only unknown was whether the impending loop would be
inside or outside! As pointed out, the under camber is there for pitch
stability and with passing decades the German airworthiness authority has
increased the forces. A Janus has lower pitch trim forces than a Duo Discus
for instance.


"Eric Greenwell" wrote in message
news:eFt6h.4796$T_.3143@trndny06...
Brad wrote:
Is the fixed portion symmetrical but with a control surface that has a
slight undercamber on the bottom? That's what my ASH 26 elevator is
like, and I believe that is normal.

If the fixed portion is not symmetrical, is the flatter part on the top
or bottom?

the fixed portion is pretty much symetrical. the flatter part is on the
top surface

This is consistent with the need to provide a down force, the usual case
for our gliders, so the airfoil is "upside down" compared to the wing.

the upper part of the elevator is straight
the lower part of the elevator has a slight undercamber to it, like
what you normally see on the lower surface of a sailplane wing.

As Udo pointed out, this is how the designer meets the requirement for
increasing "up elevator" force as speed increases. While this has a safety
advantage, the truly determined performance oriented pilot will sometimes
remove the undercamber to reduce drag. I've never wanted to do it, because
I want the safety advantage and I'm concerned the weight of filler
material might make the elevator flutter. It would take some paperwork to
make it legal, too.

--
Eric Greenwell - Washington State, USA
Change "netto" to "net" to email me directly

"Transponders in Sailplanes" on the Soaring Safety Foundation website
www.soaringsafety.org/prevention/articles.html

"A Guide to Self-launching Sailplane Operation" at www.motorglider.org