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Duo Dive-brakes ( Polar with spoilers extended?)



 
 
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  #1  
Old October 31st 07, 07:59 AM posted to rec.aviation.soaring
problems@gmail
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Posts: 2
Default Duo Dive-brakes ( Polar with spoilers extended?)

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.

  #2  
Old October 31st 07, 03:31 PM posted to rec.aviation.soaring
Bill Daniels
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Posts: 687
Default Duo Dive-brakes ( Polar with spoilers extended?)


problems@gmail wrote in message ...
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.


OK, the first post above wasn't too clear and, since it is just speculation,
may not even be correct. Maybe, what the pilot 'feels' when typical air
brakes are deployed is the sudden onset of deceleration. The increase in
sink isn't great enough to feel.

The Duo is heavy, clean and accelerates like a rocket when the nose is down.
The airbrakes don't reduce this much so the deceleration is smaller than
lighter gliders giving the first impression that the brakes are weak. The
rate of sink with air brakes extended is within the normal range gicving a
~7:1 glide which will get you down but won't do a lot to control airspeed.
You still have to control airspeed with pitch.

Bill Daniels


  #3  
Old October 31st 07, 03:38 PM posted to rec.aviation.soaring
Andreas Maurer
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Posts: 345
Default Duo Dive-brakes ( Polar with spoilers extended?)

On Wed, 31 Oct 2007 08:31:55 -0600, "Bill Daniels"
bildan@comcast-dot-net wrote:

The Duo is heavy, clean and accelerates like a rocket when the nose is down.
The airbrakes don't reduce this much so the deceleration is smaller than
lighter gliders giving the first impression that the brakes are weak.


The strength of the dive brakes can easily be measured by the time it
takes to bleed off excessive speed during the flare.

Let's face it, the Duo flares forever if you are too fast. Other ships
(DG-505, DG-1000, Janus) decelerate much quicker.

Clear case: Weak airbrakes.



Bye
Andreas
  #4  
Old October 31st 07, 04:36 PM posted to rec.aviation.soaring
BB
external usenet poster
 
Posts: 140
Default Duo Dive-brakes ( Polar with spoilers extended?)

I think the Duo's airbrakes are better than many people think. The Duo is a
big heavy glider with lots of inertia.


I don't understand the physics here.


Like other duo pilots, I notice that the glide angle in a stabilized,
on-the-triangle glide with spoilers open is decently steep, but that
the duo does not slow down quickly if you use spoilers to stop excess
speed, especially in ground effect.

Here's a theory -- or at least the physical possibility of this
observation. Spoilers, true to their name, do two things -- they add
drag and they spoil lift. Since lift = weight, "spoiling lift" really
that induced drag is raised because of the discontinuous lift
distribution at the spoiler position.

Now, it makes sense that the duo spoiler configuration is more
oriented to the production of induced rather than profile drag. The
spoilers are wide (spanwise) but not very deep (up and down). The span
is bigger than your 15 meter glider, so there is less induced drag to
start with, and increasing it is more effective in reducing glide
angle.

If this were the case, we'd expect the spoilers to be less effective
overall at higher speeds, and especially so in ground effect, which
reduces induced drag, but to produce a good glide angle at a
relatively slow approach speed, out of ground effect. This seems to be
about what we observe. This also explains why spoilers on all big
gliders seem less effective at "high parasitic drag" maneuvers.

The bottom line remains, if you fly a duo, accurate speed control in
the pattern is essential. And yes, even if the stabilized glide angle
is the same as other gliders, the spoilers are "less effective"
because bleeding off excess speed is one of the desirable "effects" of
spoilers.

John Cochrane

  #5  
Old October 31st 07, 08:58 PM posted to rec.aviation.soaring
JJ Sinclair
external usenet poster
 
Posts: 388
Default Duo Dive-brakes ( Polar with spoilers extended?)

On Oct 31, 8:36 am, BB wrote:
I think the Duo's airbrakes are better than many people think. The Duo is a
big heavy glider with lots of inertia.

I don't understand the physics here.


Like other duo pilots, I notice that the glide angle in a stabilized,
on-the-triangle glide with spoilers open is decently steep, but that
the duo does not slow down quickly if you use spoilers to stop excess
speed, especially in ground effect.

Here's a theory -- or at least the physical possibility of this
observation. Spoilers, true to their name, do two things -- they add
drag and they spoil lift. Since lift = weight, "spoiling lift" really
that induced drag is raised because of the discontinuous lift
distribution at the spoiler position.

Now, it makes sense that the duo spoiler configuration is more
oriented to the production of induced rather than profile drag. The
spoilers are wide (spanwise) but not very deep (up and down). The span
is bigger than your 15 meter glider, so there is less induced drag to
start with, and increasing it is more effective in reducing glide
angle.

If this were the case, we'd expect the spoilers to be less effective

One thing I haven't seen in this discussion is the fact that the Duo's
spoilers are not located right behind the spar. They are located quite
a bit farther aft and thus produce mostly drag and not as much spoiled
lift. I liken them to the H-301 Libelle spoilers that produced mostly
dray and therefore had to be opened earlier and left out longer to get
the same result as, say the H-201 Libelle which had true spoilers
located at the high point of the airfoil.
JJ


overall at higher speeds, and especially so in ground effect, which
reduces induced drag, but to produce a good glide angle at a
relatively slow approach speed, out of ground effect. This seems to be
about what we observe. This also explains why spoilers on all big
gliders seem less effective at "high parasitic drag" maneuvers.

The bottom line remains, if you fly a duo, accurate speed control in
the pattern is essential. And yes, even if the stabilized glide angle
is the same as other gliders, the spoilers are "less effective"
because bleeding off excess speed is one of the desirable "effects" of
spoilers.

John Cochrane



  #6  
Old October 31st 07, 09:16 PM posted to rec.aviation.soaring
Bruce
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Posts: 174
Default Duo Dive-brakes ( Polar with spoilers extended?)

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.

  #7  
Old October 31st 07, 09:38 PM posted to rec.aviation.soaring
Marc Ramsey
external usenet poster
 
Posts: 207
Default Duo Dive-brakes ( Polar with spoilers extended?)

JJ Sinclair wrote:
One thing I haven't seen in this discussion is the fact that the Duo's
spoilers are not located right behind the spar. They are located quite
a bit farther aft and thus produce mostly drag and not as much spoiled
lift. I liken them to the H-301 Libelle spoilers that produced mostly
dray and therefore had to be opened earlier and left out longer to get
the same result as, say the H-201 Libelle which had true spoilers
located at the high point of the airfoil.


My opinion is the opposite, Duo spoilers likely do an adequate of
spoiling lift, what they don't do is produce much drag. By comparison,
the trailing edge dive brakes on my Ventus caused an increase in lift
through much (if not all) of their travel, while they were producing a
tremendous amount of drag. I found it a lot easier to make steep short
landings in the Ventus than in the Duo...

Marc
  #8  
Old November 1st 07, 01:29 AM posted to rec.aviation.soaring
Karl Striedieck
external usenet poster
 
Posts: 71
Default Duo Dive-brakes ( Polar with spoilers extended?)

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

"Bruce" wrote in message
...
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.



  #9  
Old November 1st 07, 01:41 AM posted to rec.aviation.soaring
John Smith
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Posts: 256
Default Duo Dive-brakes ( Polar with spoilers extended?)

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.
  #10  
Old November 1st 07, 02:23 AM posted to rec.aviation.soaring
Gary Emerson
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Posts: 152
Default Duo Dive-brakes ( Polar with spoilers extended?)

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.
 




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