Discus 2 Optimal CG location

Wheel will be a bit different location if spring is different strength or deformed (and/or glider is different empty weight) but correct measurement of arms and moments makes it irrelevant.

Cheers Ben

On Tuesday, 26 June 2018 15:36:52 UTC+12, Ben Coleman wrote:
Wheel will be a bit different location if spring is different strength or deformed (and/or glider is different empty weight) but correct measurement of arms and moments makes it irrelevant.

Cheers Ben

Yes, that is our conclusion too. I have heard some people chock the landing gear extension somehow.

FWIW, we have measurements of 140 and 4285. The SH manual says 128 and 4278 (128+4150).

The 12mm makes a significant difference to the CG calculation - 90% aft vs 84% aft with the factory arms.

The lesson is to be a little careful of the factory arms, even in a glider with no damage history.

The 90% aft value ties in well with Bif's value above and it does feel good for thermalling, but you quickly start wishing for less aft weight when the trim is fully forward and you still need plenty of forward stick pressure when running.

On Tuesday, 26 June 2018 16:16:40 UTC+12, wrote:
On Tuesday, 26 June 2018 15:36:52 UTC+12, Ben Coleman wrote:
Wheel will be a bit different location if spring is different strength or deformed (and/or glider is different empty weight) but correct measurement of arms and moments makes it irrelevant.

Cheers Ben

Yes, that is our conclusion too. I have heard some people chock the landing gear extension somehow.

FWIW, we have measurements of 140 and 4285. The SH manual says 128 and 4278 (128+4150).

The 12mm makes a significant difference to the CG calculation - 90% aft vs 84% aft with the factory arms.

The lesson is to be a little careful of the factory arms, even in a glider with no damage history.

The 90% aft value ties in well with Bif's value above and it does feel good for thermalling, but you quickly start wishing for less aft weight when the trim is fully forward and you still need plenty of forward stick pressure when running.

I should add that the MM uses the same arms for loaded and empty CG calculations.

When I picked up a Discus 2c from the factory in 2005 and asked for advice about the optimal C of G for performance I was told about 15% forward of the aft limit and that further after "not only is it less safe but the climb will suffer" - which surprised me at the time but I stuck to it over the years wet and dry. I guess the same will apply to the 2a and 2b as the wing profile is the same.

At 04:16 26 June 2018, wrote:
On Tuesday, 26 June 2018 15:36:52 UTC+12, Ben Coleman
wrote:
Wheel will be a bit different location if spring is different strength
or
deformed (and/or glider is different empty weight) but correct
measurement
of arms and moments makes it irrelevant.

Cheers Ben

Yes, that is our conclusion too. I have heard some people chock
the landing
gear extension somehow.

FWIW, we have measurements of 140 and 4285. The SH manual
says 128 and 4278
(128+4150).

The 12mm makes a significant difference to the CG calculation -
90% aft vs
84% aft with the factory arms.

The lesson is to be a little careful of the factory arms, even in a
glider
with no damage history.

The 90% aft value ties in well with Bif's value above and it does
feel good
for thermalling, but you quickly start wishing for less aft weight
when the
trim is fully forward and you still need plenty of forward stick
pressure
when running.


The factory set my D-2b up using the 4.4/100 aft fuselage blocking,
and the 128/4150 measurements. Mine has the 5" larger main
wheel option, as well as the tail wheel instead of the stock skid. If
these options have different arms from stock, S-H did not use them.
S-H set it up for a minimum seat load of ~190 Lbs (87 Kg) with
the empty C/G point at 681 mm (with an allowed range of 617 -
700 mm). At the time, I weighed ~210 Lbs with chute, so I was
about 20 Lbs above the minimum seat load by their calculations,
and that is where they thought it would be optimum for me to race
at. They did say that in their opinion, the D-2b did not like to be
too close to the aft limit in order to get the most out of it. I will
say that I used the handbook table to calculate the amount of tail
water to add when I filled the wing tanks. I figured that it would
put me close to the aft C/G limit. I flew it like that, and did not like
that aft C/G location at all.

RO