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.

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...