Duo Dive-brakes ( Polar with spoilers extended?)

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.

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.

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:
The original Duo (can't speak for the new X model with the drag flap)
definitely requires more attention to energy management when landing than
your garden variety 750 pound glider. Any excess coming over the trees won't
go away by diving with full spoilers. This is a consequence of increased
weight and relatively smaller spoilers and is common to all "large" gliders.

The problem can be magnified by improperly adjusted spoilers. It is possible
that owners who have readjusted their wheel brakes have inadvertently
reduced the travel of the spoilers. Also, the Duo spoilers are heavier to
extend than smaller gliders and it takes extra muscle to hold them fully
open during the approach and flare. And as the airspeed decreases they get
heavier because the dynamics of airflow don't help hold them open.

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
tight formation with me on the wing and at 65 knots pushed over while
deploying full spoilers. The two ships stayed exactly even in a descent of
500 plus feet.

Many US pilots I've noticed landing at various contest sites like to stick
the glider on the ground at speeds 20 and 30 knots above stall and then roll
thousands of feet to their trailers. This is poor preparation for the time
precise energy management is needed to get into a small outlanding field. As
the size and weight of the gliders increase the problem of stopping
magnifies, so unless you are flying a Sparrowhawk or 1-26 you should make
every landing a tail dragger touch down at an intended spot.

Karl Striedieck

Great info on the comparison, and excellent advice on the landings. I
couldn't agree more. Pick a spot and grade yourself on each landing as
to how close you came to sticking that spot.

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...

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.

I have some 300 hours in a Duo, and my experience is
that the Duo's brakes are more effective the faster
you fly, compared with other gliders. Hence at the
80 knots used in the test that Tom carried out I would
expect that the brakes would work well. It is at a
55 knot approach speed that the brakes are not very
effective. My technique is that if I need to lose height,
I put the speed up to at least 65 knots. This means
a longer 'float', if the speeed is not reduced, but
in general when doing a field landing the higher speed
would mean that the approach could be made much closer
to any hedge or other obstruction. This is a fundamentally
different philosophy, from the suggestion that approaches
in the Duo must be made at a constant slow speed. At 01:42 02 November 2007, Karl Striedieck wrote:John,The Duo Tom Knauff was flying when we did our full
boards dive off was a turbo version and thus heavier than a non-motorized
ship. The DG-1000 I was flying had heavier pilots, so the payload was about
the same. We agreed to this test before task opening on a day we had some
extra time, because this notion that the Duo had inferior speed brakes had been
floating around for some time and I wanted to see if it held any water.
I joined up on Tom's right wing, less than a span away, and he pushed over,
deploying full brakes and pushing the speed up to 80 knots. In this stabilized
condition I had full brakes deployed in the 1000 and did not fall back
as one might expect of a ship with better braking.As to the matter of tail weights, wheel brakes and
gear configuration there are compromises with both approaches (long and short
main gear). The old Duo's short gear was simpler, lighter and farther aft.
This makes for easier ground handling but requires a nose wheel to handle
max braking. The longer gear of the Duo X and DG 1000 keeps the gear doors
cleaner, but is more likely to go on the nose if the brake is good.Speaking of wheel brake effectiveness, I've spent more
time adjusting, bleeding, modifying and cursing the Duo brake than
all other maintenance matters combined. Maybe my expectations are too high
after years of flawless performance from Schleicher's Cleveland disc brake
systems.Karl Striedieck'John Smith' wrote in message h... 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.

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.


No, that was another to whom I was replying. Use care with editing, please.


Jack