horiz tail airfoil observations

[Brad]

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

[Jack[_4_]]

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

Jack Womack

[noel.wade]

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

[Francisco De Almeida]

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]

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

[noel.wade]

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.

[Udo]

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

[Brad]

> 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

[Eric Greenwell]

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

[Karl Striedieck]

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

[Udo]

> 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

[[email protected]]

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

[Eric Greenwell]

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

[Francisco De Almeida]

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.

[noel.wade]

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

[Eric Greenwell]

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

[Bruce Greef]

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? 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.
>
>
Tilters do it with attitude. In a standard class glider you will need to hold up
elevator to achieve slow flight. AOA is controlled exclusively by elevator
pitching the entire aircraft.
On a flapped glider the angle of attack on the mainplane is controlled by
trailing edge flap deflection, which effectively changes the AOA of the
tailplane. There is one speed for each flap position where the elevator is
neutral. Often down under 45kt.
The relative centre of lift / CG position is a good question.

[Papa3]

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

[[email protected]]

Eric Greenwell wrote:
> 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

Hi Eric - I don't think this is correct. The prototype V2c I flew had a
trailing-edge tab bent down, explained to me as required for
appropriate
stick force gradient, as the elevator undersurface had no camber.
The production V2C tails added back the camber at a minor performance
penalty. Some well-known competition pilots in years back (not just
Karl)
did remove elevator camber for reduced drag, frightening the flutter
experts.

Hope that helps with the mystery,
Best Regards, Dave

PS: Jud, come out of hiding and explain it better to us
engineer-wanabees...

[J. Nieuwenhuize]

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 ;-)

[Brad]

Hi Dave,

Hey...........now that you mention it, the elevator of that Krokus had
a bent down tab along the whole trailing edge, it was about .5" wide. I
don't quite recall if the lower surface of the elevator was flat or
curved tho......................there were 6 Blue Angels screaming by
and I was slightly distracted!

Cheers,
Brad


wrote:
> Eric Greenwell wrote:
> > 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
>
> Hi Eric - I don't think this is correct. The prototype V2c I flew had a
> trailing-edge tab bent down, explained to me as required for
> appropriate
> stick force gradient, as the elevator undersurface had no camber.
> The production V2C tails added back the camber at a minor performance
> penalty. Some well-known competition pilots in years back (not just
> Karl)
> did remove elevator camber for reduced drag, frightening the flutter
> experts.
>
> Hope that helps with the mystery,
> Best Regards, Dave
>
> PS: Jud, come out of hiding and explain it better to us
> engineer-wanabees...

[mattm]

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...

[Jeremy Zawodny]

mattm wrote:
> 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...

Another data point...

I'm a 304C pilot who lost a substantial amount of ballast about a year
and a half ago. I now fly close to the rear CG limit and the ship
climbs amazingly well with a 45% bank in thermals.

I thermal around 50-52 knots dry and about 60-62 knots wet (about 20
pounds under max gross).

I was a little surprised by the difference that 50 pounds less in the
cockpit made, but I'm rather happy with the results.

Jeremy

[Papa3]

Thanks. I have to think this over a bit - it's been quite a while
since I played with these formulas :-)

Couple of questions below:

J. Nieuwenhuize wrote:
>
> 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) ...

Okay with the first point (relationship of induced drag to CL and Di)
and proportionally much smaller contribution of tail vs. wing.

>induced drag is fairly low and certainly lower than the weight penalty of a heavier tail.

Are you suggesting that a tail with a higher aspect ratio would be, by
definition, heavier or talking about the tactic of putting additional
weight in the tail to move the CG?

Good stuff.

P3

[noel.wade]

Matt -

More than likely, the issue is that your weight is closer to gross and
closer to the forward end of the CG envelope in the 304C. Both of
these things would favor a higher stalling speed and poorer steep-turn
performance (because the wings and tail are more heavily loaded in a
turn, and because the slow flight also necessitates more trim/elevator
deflection - resulting in increased drag).

Take care,

--Noel

mattm wrote:
> 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...

[J. Nieuwenhuize]

Papa3 schreef:

> >induced drag is fairly low and certainly lower than the weight penalty of a heavier tail.
>
> Are you suggesting that a tail with a higher aspect ratio would be, by
> definition, heavier or talking about the tactic of putting additional
> weight in the tail to move the CG?
>
> Good stuff.
>
> P3

Heavier construction. Heavier stabilizer means larger moment of
inertia, higher torsional stiffness of the tail... leading to maybe 4
or 6 times as much "extra" construction weight as only the extra
stabilizer weight. And you have to correct that with even more wing
surface ;-)

[Papa3]

J. Nieuwenhuize wrote:
> Papa3 schreef:
>
> > >induced drag is fairly low and certainly lower than the weight penalty of a heavier tail.
> >
> > Are you suggesting that a tail with a higher aspect ratio would be, by
> > definition, heavier or talking about the tactic of putting additional
> > weight in the tail to move the CG?
> >
> > Good stuff.
> >
> > P3
>
> Heavier construction. Heavier stabilizer means larger moment of
> inertia, higher torsional stiffness of the tail... leading to maybe 4
> or 6 times as much "extra" construction weight as only the extra
> stabilizer weight. And you have to correct that with even more wing
> surface ;-)

Okay. To summarize your comments, the induced drag created by a
stabilizer, even one operating at it's maximum (negative) Cl is
relatively insignificant to the overall system efficiency. Did I get
that right? Further, the structural considerations involved in
building a higher aspect ratio tail would more than negate any slight
decrease in drag. Also correct?

Ahh, engineering compromises...

I'd still be interested to see the numbers in terms of total drag on a
given elevator operating at basically neutral trim vs. max up elevator.
I guess I could sit down and do this, but it would mean pulling out
some old text books that are awfully dusty right now :-)

P3

[noel.wade]

*sigh* I had a very eloquent rant that I tried to post yesterday; but
for some reason it isn't showing up. I don't have the heart to try to
reconstruct the entire rant, so I'll summarize:

It seems there only two types of aircraft design books/articles:

1) Those that use ballpark figures and rely on historical examples of
existing designs

--OR--

2) Those designed for engineers, with accurate but very complicated
equations in Engineering notation that are indecipherable by the
layman.

Is it so hard to bridge the gap, for those of us that can't decode long
strings of Greek letters into practical terms?

I'm a computer professional, so I'd like to think I'm decent with math
- but even 3d-graphics-programming has only required a solid grasp of
algebra, trigonometry, and matrix math. The calculus and short-handed
equations in many technical articles might as well be modern art on the
page, for all I can tell. Many factors are often not defined by the
author - who assumes the reader knows what they mean; even those
targetted at "first time" designers!

In terms of this tail issue, for example, is it really too hard to put
it in terms like... "At speed ____ your design would have to pull a Cl
of ___, requiring an angle of attack of ____. With the airfoil chosen,
the coefficient of moment in this situation is ____. Applying equation
_____________ to that and the Center-of-Gravity at ___, you end up with
a total pitching force of ____. This must be counter-balanced by the
tail producing an equal and opposite amount of force. Given the wing
downwash effects and angle of incidence, the horizontal stabilizer is
flying at an angle of attack of ____. So to provide enough force, the
coefficient of lift must be ____ and/or the tail area must be ____
(assuming no elevator deflection). "

I mean, am I missing something; or can't you put it into those simple
and direct terms? I guess I've left out is the stability margin - but
that's got to be something you can factor into the above process,
right? Surely such a direct-calculation approach would require
iterative design to find the optimal solution in all flight regimes -
but even that is better for the amateur designer than an inverse
solution that cannot be solved by the average joe!

Somebody please feel free to step up and slap me if I'm way off base
here.... I've got a good wing design, a good fuselage, and a good
vertical tail; all with numbers that I can calculate and verify - but
I've been wrestling with this horizontal tail issue for a week and its
really getting to me!

Thanks, take care,

--Noel

[Udo]

noel.wade wrote:
> *sigh* I had a very eloquent rant that I tried to post yesterday; but
> for some reason it isn't showing up. I don't have the heart to try to
> reconstruct the entire rant, so I'll summarize:
>
> It seems there only two types of aircraft design books/articles:
>
> 1) Those that use ballpark figures and rely on historical examples of
> existing designs
>
> --OR--
>
> 2) Those designed for engineers, with accurate but very complicated
> equations in Engineering notation that are indecipherable by the
> layman.
>
> Is it so hard to bridge the gap, for those of us that can't decode long
> strings of Greek letters into practical terms?
>
> I'm a computer professional, so I'd like to think I'm decent with math
> - but even 3d-graphics-programming has only required a solid grasp of
> algebra, trigonometry, and matrix math. The calculus and short-handed
> equations in many technical articles might as well be modern art on the
> page, for all I can tell. Many factors are often not defined by the
> author - who assumes the reader knows what they mean; even those
> targetted at "first time" designers!
>
> In terms of this tail issue, for example, is it really too hard to put
> it in terms like... "At speed ____ your design would have to pull a Cl
> of ___, requiring an angle of attack of ____. With the airfoil chosen,
> the coefficient of moment in this situation is ____. Applying equation
> _____________ to that and the Center-of-Gravity at ___, you end up with
> a total pitching force of ____. This must be counter-balanced by the
> tail producing an equal and opposite amount of force. Given the wing
> downwash effects and angle of incidence, the horizontal stabilizer is
> flying at an angle of attack of ____. So to provide enough force, the
> coefficient of lift must be ____ and/or the tail area must be ____
> (assuming no elevator deflection). "
>
> I mean, am I missing something; or can't you put it into those simple
> and direct terms? I guess I've left out is the stability margin - but
> that's got to be something you can factor into the above process,
> right? Surely such a direct-calculation approach would require
> iterative design to find the optimal solution in all flight regimes -
> but even that is better for the amateur designer than an inverse
> solution that cannot be solved by the average joe!
>
> Somebody please feel free to step up and slap me if I'm way off base
> here.... I've got a good wing design, a good fuselage, and a good
> vertical tail; all with numbers that I can calculate and verify - but
> I've been wrestling with this horizontal tail issue for a week and its
> really getting to me!
>
> Thanks, take care,
>
> --Noel

I have wrestle with that my self.
I had no background in it when I started and still my knowledge is very
narrow. But over 25 years I have bulldoze my way through. 17 years ago
things started slowly changing for me, aside for rudimentary formulas.
With the advent of ACAD and aerodynamic software as well as the
internet, things started to fall into place. To day I use a 2D and a
3D software. Both of them are commercial programs.
Combined with subscriptions to Technical Soaring and other publications
I slowly started to make sense of it.
The results were, two projects that were limited to changing airfoils.
My new project starts from scratch. For it to be fine tuned
a rely on the 3D software, as well as what is out there on the flight
line.
Udo
PS. Go to the Glider Tech Group, a Yahoo group. I just listed a file
comparing the
DU13.7-86 vs. the FX71-150/30 for two speeds with values that are
appropriate for those speeds