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Discus 2 Optimal CG location



 
 
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  #21  
Old June 26th 18, 06:04 PM posted to rec.aviation.soaring
Michael Opitz
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Posts: 230
Default Discus 2 Optimal CG location

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




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  #22  
Old June 26th 18, 08:19 PM posted to rec.aviation.soaring
[email protected]
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Posts: 51
Default Discus 2 Optimal CG location

Hi Dave,

good practice is to trim a standard class glider at a speed of 120-130kph with elevator in neutral position without water ballast. This keeps elevator deflection low in the relevant speed ranges. It usually results in a mid-range to slightly aft C/G position at your take-off weight.

Such a setup can easily be achieved with a few test flights and the help of a folding rule, adding ballast as required. You will need to stay within allowed C/G ranges, of course.

I'm flying an LS8, but what I heard from Discus 2 pilots at various competitions is in line with what RO said. The D2 has a reputation of rather poor handling with aft C/G locations.

Best regards,
Christoph
  #23  
Old June 27th 18, 03:55 AM posted to rec.aviation.soaring
Dave Springford
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Posts: 251
Default Discus 2 Optimal CG location

Thanks Michael and Christoph,

Good information.
 




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