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...