Duo Dive-brakes ( Polar with spoilers extended?)

On Oct 31, 4:16 pm, Bruce wrote:
Hi Chris

Lets think it through then. Here is my very rusty attempt at physics -(I know
there are experts here maybe one will bite)

The bigger gliders tend to have higher aspect ratio wings. This means that, like
the Beetle they have a polar moment challenge. The beetle would roll easily at
speed because of a combination of aerodynamics reducing load on the suspension,
and a high centre of gravity. Add the original swing axles and you have a recipe
for landing on the roof.

In the long wings glider you have the same issue, but symmetrical on both sides,
the centre of mass of the wings is at a further distance from the roll centre of
the aircraft. It thus takes more energy to achieve a specific rate of rotation,
because you need more kinetic energy (Mass * distance** is against you because
the wings are longer AND heavier)
Think of two pendulums of equal mass, but different lengths. Then try it with
the same mass but different mass distribution (Like a metronome)
The frequency is proportional to the polar moment not the mass.

I am sure the aerodynamics experts can tell you about the relative Reynolds
numbers, but that is more a function of chord, and that is not radically
different. The taper ratio is higher in 15m than in 26m, but the tips of a given
generation seem to be of similar chord. This is where the ailerons generate the
rolling force so I assume the airfoil differences are greater than the Reynolds
number effects.

What makes it necessary to stay further ahead of a 20m wingspan glider is
inertia - stored energy. It takes longer, and / or more force to achieve the
same deflection. Then you take into account the total mass that you are trying
to deflect is greater and it gets worse.

On the other hand I understand that 47:1 (Duo x) can get addictive.

problems@gmail wrote:
J a c k wrote:
I think the Duo's airbrakes are better than many people think. The Duo is a
big heavy glider with lots of inertia. It doesn't like to change direction
quickly. That includes its behavior on sudden airbrake deployment. You
don't get a lot of sink right away.

My first reaction was that the airbrakes were weak but a little more
experience showed me that with a little patience, the brakes took effect and
produced a respectable decent rate. The Duo just makes you plan ahead a
little more than with a light single seater.

I don't understand the physics here.
Consider an analogy:
when the VW-beetle came out it had a reputation of 'turning over easily',
based on the false logic that you need less men to 'turn it over' than to
'pick up & turn over a bigger car'. Of course the forces while driving, that
tended to 'turn it over' were less for a VW, but so too were the forces that
resisted 'turn it over'.
A heavy pendulum is 'eqivalent' to a lighter pendulum.

So too for the BIG glider.
What doesn't scale up is the pilots strength.
Or is reynolds number significant ?

== Chris Glur.

Guys, if your high-school physics teacher catches you,
you're in for a dope-slap. All of yuse. Really now...

John,

Can you refer me to the SH webpage regarding the spoiler/acro issue? All I
can find on the current German version is the Duo X.

My flight manual says acrobatics are not allowed but it doesn't say anything
about "poor dive brakes" being the reason. The manual does say that the max
g loading with the spoilers deployed is reduced to 3.5. This is common for
most gliders because of the concentration of bending loads at the outboard
end of the spoilers, magnified during high g pull-ups.

Although it is purely speculation on my part, I suspect the no-acro
limitation is more a matter of the reality that poorly executed maneuvers
can lead to unintended dives, overspeed and overstress if spoilers are
deployed in a panic. I'd guess S/H is trying to stay ahead of the lawyers
rather than any structural or strength issue compared to DG.

As to your statement that the no-acro limitation means more effective speed
brakes, actual in-flight tests prove otherwise. Tom Knauff will remember our
stand-on-the-pedals dive test at the 2004 Seniors contest. Check with him.

Karl Striedieck


"John Smith" wrote in message
. ..
Karl Striedieck wrote:

As to the assertion that the DG-1000 has more effective air brakes than
the Duo, this is not so. While flying a DG-1000 I had the opportunity to
do a formation "test dive" comparison with a Duo. We (Tom Knauff in a
Duo) got in

I've never compared the two side by side. But fact is that the original
Duo is not certified for aerobatics, according to the SH homepage due to
the poor dive brakes, while the DG1000 is, as well as the new Duo X. So
yes, it seems there is a difference in air brake effectiveness.

Marc Ramsey wrote:
My opinion is the opposite, Duo spoilers likely do an adequate of
spoiling lift, what they don't do is produce much drag.

The irony is wonderful: pilots are ocasionally chastised for saying
"spoilers" when what they "really mean" is (Schmepp-Hirth) airbrakes.
And now we have a Schmepp-Hirth glider equipped with those airbrakes
that really deserve to be called "spoilers"!

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

Hi Karl

The Duo passed the same JAR - now EASA certification that the DG1000 etc. did.
That means they have to limit the speed to below Vne in a relatively shallow
dive. (30 degrees)
So you are right, they will exhibit very similar performance in a dive test. It
is possible the rearward location on the duo results in the airbrake becoming
less effective in the flare, but I doubt it.

The no aerobatics certification appears to be simply a liability limitation.
Apparently the Duo will loop and spin as well as any other high performance two
seater. Which is to say, what's the point - the glider can do it, but if you
want to do aerobatics, get a different aircraft.
The DG method of shortening the wingspan makes some sense, because it improves
the aerobatic handling. But there are compromises.

Still enjoying your Duo?

Bruce

Karl Striedieck wrote:
John,

Can you refer me to the SH webpage regarding the spoiler/acro issue? All I
can find on the current German version is the Duo X.

My flight manual says acrobatics are not allowed but it doesn't say anything
about "poor dive brakes" being the reason. The manual does say that the max
g loading with the spoilers deployed is reduced to 3.5. This is common for
most gliders because of the concentration of bending loads at the outboard
end of the spoilers, magnified during high g pull-ups.

Although it is purely speculation on my part, I suspect the no-acro
limitation is more a matter of the reality that poorly executed maneuvers
can lead to unintended dives, overspeed and overstress if spoilers are
deployed in a panic. I'd guess S/H is trying to stay ahead of the lawyers
rather than any structural or strength issue compared to DG.

As to your statement that the no-acro limitation means more effective speed
brakes, actual in-flight tests prove otherwise. Tom Knauff will remember our
stand-on-the-pedals dive test at the 2004 Seniors contest. Check with him.

Karl Striedieck


"John Smith" wrote in message
. ..
Karl Striedieck wrote:

As to the assertion that the DG-1000 has more effective air brakes than
the Duo, this is not so. While flying a DG-1000 I had the opportunity to
do a formation "test dive" comparison with a Duo. We (Tom Knauff in a
Duo) got in
I've never compared the two side by side. But fact is that the original
Duo is not certified for aerobatics, according to the SH homepage due to
the poor dive brakes, while the DG1000 is, as well as the new Duo X. So
yes, it seems there is a difference in air brake effectiveness.

Bruce wrote:

The Duo passed the same JAR - now EASA certification that the DG1000
etc. did.
That means they have to limit the speed to below Vne in a relatively
shallow dive. (30 degrees)
....
The no aerobatics certification appears to be simply a liability
limitation.

No. JAR 22 requires 30 degrees for all liders, but 45 degrees to be
certified for aerobatics and cloud flying.

Karl Striedieck wrote:

Can you refer me to the SH webpage regarding the spoiler/acro issue? All I
can find on the current German version is the Duo X.

http://www.schempp-hirth.com/index.php?id=130&L=1

"Duo Discus XL becomes certified for simple aerobatics!
The improved effectiveness of the airbrake system makes it now possible
for us to apply the approval for simple aerobatics including spinning."

John,

Thanks for the steer. I'd appreciate some other information if you have the
time.

Is the 20 meter DG-1000 authorized for acro? If it is I'm curious about the
reason. Both ships were designed to meet JAR standards regarding strength
and dive brake performance.

Although the 20 meter DG-1000 and the original Duo have identical speed
brake (spoiler) affectivity, I would love to have a Duo X when it comes to
off field landings, as it is much better than the other two. With 95% of my
flying being in a contest environment the exposure to "rural visitations,"
as Gren Siebels called them, is high. However, the extra $45K added to the
price tag by the sagging Dollar since I bought mine cancels that dream.

Karl Striedieck


"John Smith" wrote in message
...
Karl Striedieck wrote:

Can you refer me to the SH webpage regarding the spoiler/acro issue? All
I can find on the current German version is the Duo X.

http://www.schempp-hirth.com/index.php?id=130&L=1

"Duo Discus XL becomes certified for simple aerobatics!
The improved effectiveness of the airbrake system makes it now possible
for us to apply the approval for simple aerobatics including spinning."

Karl Striedieck wrote:

Is the 20 meter DG-1000 authorized for acro?

With 20m it's authorized for "basic" acro, which means Loops, Turns and
erect Spins. No rolls and no negative g.

With 18m it's authorized for full aerobatics.

If it is I'm curious about the
reason. Both ships were designed to meet JAR standards regarding strength
and dive brake performance.

Because the dive brakes are *not* of equal strenght. I've never compared
side by side, but the DG1000 definitely allows for a much more sloppy
approach. (Not that I would advocate sloppy flying!) I've read that you
compared them and think both are the same, I definitely don't share your
opinion.

Just for interest - lifted from the SH website news
section 25/10/2007:

'Duo Discus XL becomes certified for simple aerobatics!

The improved effectiveness of the airbrake system makes
it now possible for us to apply the approval for simple
aerobatics including spinning.'

John Galloway




At 14:01 01 November 2007, John Smith wrote:
Karl Striedieck wrote:

Is the 20 meter DG-1000 authorized for acro?

With 20m it's authorized for 'basic' acro, which means
Loops, Turns and
erect Spins. No rolls and no negative g.

With 18m it's authorized for full aerobatics.

If it is I'm curious about the
reason. Both ships were designed to meet JAR standards
regarding strength
and dive brake performance.

Because the dive brakes are *not* of equal strenght.
I've never compared
side by side, but the DG1000 definitely allows for
a much more sloppy
approach. (Not that I would advocate sloppy flying!)
I've read that you
compared them and think both are the same, I definitely
don't share your
opinion.

On Nov 1, 6:56 am, John Smith wrote:
Karl Striedieck wrote:
Is the 20 meter DG-1000 authorized for acro?

With 20m it's authorized for "basic" acro, which means Loops, Turns and
erect Spins. No rolls and no negative g.

With 18m it's authorized for full aerobatics.

If it is I'm curious about the
reason. Both ships were designed to meet JAR standards regarding strength
and dive brake performance.

Because the dive brakes are *not* of equal strenght. I've never compared
side by side, but the DG1000 definitely allows for a much more sloppy
approach. (Not that I would advocate sloppy flying!) I've read that you
compared them and think both are the same, I definitely don't share your
opinion.

Karl's side by side measurement is fascinating, but not what I would
expect.

I've got about 40-50 hours each in DG-1000S and Duo Discus and the
DG-1000S seems much more tolerant to sloppy handling on approach,
seems to wash off energy much more effectively with spoilers than the
Duo Discus and have less run out in ground effect. So why is this,
more effective drag (not lift spoiling) vs. speed in the DG-1000S? I
can't explain it but I definitely believe it is true. Couple this with
a more forgiving undercarriage and landings in the DG-1000S seem much
more tolerant of sloppiness than the Duo. This is not a slam against
the Duo, I like both gliders. I'll give the Duo the benefit in
handling, lighter aileron forces and very nice slow speed behavior as
it floats around a thermal (you can hear that inner wing rumble and
she just floats around).

BTW out of date now with the Duo-X but Karl did write up a comparison
of the Duo and DG-1000S in the June 2003 SSA Soaring magazine.

Since this topic is already all over the place - one thing I see in
the Duo-X collateral is Schemp Hirth still promote the light tail
weight and therefore easier ground handling (I'm sure aimed at the
DG-1000S). I wish they'd actually make the tail a little heavier to
help reduce those little tail raising surprises under brake - not that
I've ever done this but I've seen others do it :-) I suspect that is
a major change in moving the U/C more forward, but with adding the U/C
springs, and later stretched cockpit this is one thing I'd hoped they
would have tweaked as well. Anybody with Duo-X experience - is the
tail at least a little heavier?

Regards


Darryl