horiz tail airfoil observations

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

Hey Karl when you were totally banana's tell us the story of your 15M
ASW17 at 200knots thru the start gate.

Regards

Al



Karl Striedieck wrote:
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

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,
In this case the elevator and the shape is not just for safety but
also to maximize the performance, the airfoil was design as a
complete working unit. If there is a compromise it must be very small.
If you fly with the most optimum C of G there is very little elevator
deflection for the normal climb and speed range in a steady state and
if there is, let say -2 to + 2 deg of defection, I can tell you there
is no measurable drag penalty.
Udo

Udo wrote:
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,
In this case the elevator and the shape is not just for safety but
also to maximize the performance, the airfoil was design as a
complete working unit. If there is a compromise it must be very small.
If you fly with the most optimum C of G there is very little elevator
deflection for the normal climb and speed range in a steady state and
if there is, let say -2 to + 2 deg of defection, I can tell you there
is no measurable drag penalty.

As I understand it, the drag penalty is not from the elevator deflection
(some of which would be required anyway), but because the airfoil is not
optimum for the lift (down force) it is producing; i.e., the undercamber
is on the side of the airfoil producing lift. There is always some drag
from the elevator, even with the control surface undeflected, because of
the lift (down force) it is producing.

--
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 22:36 14 November 2006, Noel.Wade wrote:
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?

Not necessarily. Remember that the angle of attack
of the tailplane increases at the same time that the
angle of attack of the wing increases (in fact the
downwash from the wing affects slightly the alpha on
the tailplane but this can be disregarded for the present
discussion).

While the CL in steady flight for the wing ranges from
about +1.50 to +0.30, the CL range for the tailplane
is only about +0.20 to -0.15 (I am assuming an unflapped
glider, for flapped gliders it is even less). The CL
variation at the tailplane is thus only a fourth to
a fifth of the wing's CL variation.

In this light it is no longer surprising that the elevator
in steady flight is counterintuitively deflected to
cancel most of the lift (negative or positive) that
the tailplane would produce in response to the changes
in angle of attack.

This situation regarding elevator deflection vs lift,
and the slight inefficiency it entails, is the reason
why all-flying tailplanes were popular with designers
in the seventies, until they gave up due to the difficulty
in ensuring nice handling and stability.

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?)

No, for stability all that is required is that the
CG is ahead of the COMBINED centers of pressure of
wing + tail. For modern sailplanes, even the foremost
CG position is still behind the wing's center of pressure
(except maybe for some dedicated aerobatic types, I
don't know).

Now why do designers wish to have the tail producing
some upward lift at slow speed? Mainly because the
spanwise lift on the wing dips a little in the vicinity
of the fuselage. A bit of lift from the tailplane helps
to smooth out this irregularity and leaves a more efficient
wake behind the sailplane.

Spamcans like Cessnas do have tails producing downward
force all the time. Gliders cannot allow themselves
such wastefulness!

Regarding the shape of the elevator itself, Udo already
wrote everything there is to say.

Good questions Noel.

Wonderful explanation, Francisco - thanks for taking the time to write
all of that out!

I still scratch my head as to why the Thomas book recommends such a
large Cl range for the horizontal tail, though. His example for a 15m
ship with some pretty common dimensions winds up with a tail Cl range
of around 0.67 to -0.73 at a static stability margin of -0.05 (pg. 136
to 139 of the Thomas book).

And regarding the positive lift on the tail: Your explanation makes
sense in light of the (basic) modelling I've done of spanwise lift
distribution... However the wing airfoil still exhibits a negative
(i.e. nose down) pitching moment. So something needs to counteract
that force - especially because positive lift from the tail would
amplify the nose-down trend. Are you saying that the CG is
sufficiently far aft that it provides the "counterbalancing force", to
put it in layman's terms? I hate to keep repeating his name (but his
book is the most comprehensive one that I've read) - however, Thomas
talks about "aft CG" a lot, and in his measurements you never see
anything further aft than about 50% of the MAC. And as a result of all
of this, doesn't a positive-lifting tail then limit your forward CG
position?

Thanks again, take care,

--Noel
P.S. My R/C gliders were so much easier - just move the battery (CG)
around until the plane was pitch-neutral with 0 tail trim! :-P

noel.wade wrote:

And regarding the positive lift on the tail: Your explanation makes
sense in light of the (basic) modelling I've done of spanwise lift
distribution... However the wing airfoil still exhibits a negative
(i.e. nose down) pitching moment. So something needs to counteract
that force - especially because positive lift from the tail would
amplify the nose-down trend. Are you saying that the CG is
sufficiently far aft that it provides the "counterbalancing force", to
put it in layman's terms? I hate to keep repeating his name (but his
book is the most comprehensive one that I've read) - however, Thomas
talks about "aft CG" a lot, and in his measurements you never see
anything further aft than about 50% of the MAC. And as a result of all
of this, doesn't a positive-lifting tail then limit your forward CG
position?

I think I had it backwards before - according to Thomas, the stabilizer
must provide upward lift when the wing is operating at a high lift
coefficient (like thermalling), and a downward load at a low coefficient
of lift (like cruising). This is on page 133 of my edition, in the
"Longitudinal trim in unaccelerated flight" portion.

--
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

noel.wade wrote:

P.S. My R/C gliders were so much easier - just move the battery (CG)
around until the plane was pitch-neutral with 0 tail trim! :-P

Not quite along the quantitative lines we've been following, but it is
pretty amazing to see how little the average pilot pays attention to cg
and its effect on performance in their common flight attitudes. Ask
some of the stockier pilots in your club to "trim for 55kts" (or some
similar, reasonable speed) and leave the trim there. On landing,
take a look at the elevator. I would wager at least one in two is at
or near full up deflection. Assuming this puts them at the outside
edge of the Cl ranges discussed, that's an awful lot of downforce
being produced (1/2RhoV2ClS IIRC). Aside from the "negative lift",
what's the typical induced drag that goes along with this? I'm
assuming it's pretty high given the relatively low aspect ratio,
especially of older models...

P3

Papa3 schreef:
Not quite along the quantitative lines we've been following, but it is
pretty amazing to see how little the average pilot pays attention to cg
and its effect on performance in their common flight attitudes. Ask
some of the stockier pilots in your club to "trim for 55kts" (or some
similar, reasonable speed) and leave the trim there. On landing,
take a look at the elevator. I would wager at least one in two is at
or near full up deflection. Assuming this puts them at the outside
edge of the Cl ranges discussed, that's an awful lot of downforce
being produced (1/2RhoV2ClS IIRC). Aside from the "negative lift",
what's the typical induced drag that goes along with this? I'm
assuming it's pretty high given the relatively low aspect ratio,
especially of older models...

P3

Not quite correct actually; induced drag is proportional to the squared
lift coefficient and inversely proportional to the aspect ratio.
Knowing that the lift coefficient of your stabilizer is always lower
(main wing stalls first) induced drag is fairly low and certainly lower
than the weight penalty of a heavier tail. Also bear in mind that while
thermalling a glider you're flying at a relatively moderate angle of
attack, not at stall speed. (At the Discus for example you're flying
about 30% above stall speed in a thermal) This is different in
landing...

Nevertheless I usually fly at the back end of the cg-range; mainly
because of the difficulty to achieve "natural" ballast ;-)

J. Nieuwenhuize wrote:
Papa3 schreef:
Not quite along the quantitative lines we've been following, but it is
pretty amazing to see how little the average pilot pays attention to cg
and its effect on performance in their common flight attitudes. Ask
some of the stockier pilots in your club to "trim for 55kts" (or some
similar, reasonable speed) and leave the trim there. On landing,
take a look at the elevator. I would wager at least one in two is at
or near full up deflection. Assuming this puts them at the outside
edge of the Cl ranges discussed, that's an awful lot of downforce
being produced (1/2RhoV2ClS IIRC). Aside from the "negative lift",
what's the typical induced drag that goes along with this? I'm
assuming it's pretty high given the relatively low aspect ratio,
especially of older models...

P3

Not quite correct actually; induced drag is proportional to the squared
lift coefficient and inversely proportional to the aspect ratio.
Knowing that the lift coefficient of your stabilizer is always lower
(main wing stalls first) induced drag is fairly low and certainly lower
than the weight penalty of a heavier tail. Also bear in mind that while
thermalling a glider you're flying at a relatively moderate angle of
attack, not at stall speed. (At the Discus for example you're flying
about 30% above stall speed in a thermal) This is different in
landing...

Nevertheless I usually fly at the back end of the cg-range; mainly
because of the difficulty to achieve "natural" ballast ;-)

Aha! Now things make much more sense!

Being one of those "stockier" types I find a fairly different
experience in the 304C that some of us fly. For me, full back trim
results in about 50 kts (nominal landing speed), and thermalling beyond
about 30 degrees of bank seems to massively increase the sink rate.
However, in an L33 full back trim flies about 5 kts slower and it loves
steep banks with me. Other pilots in the 304 (at the rearward end of
the CG range) report performance much more like I get with the L33. It
must be that the elevator design is different...