Discus 2 Optimal CG location

On Monday, June 25, 2018 at 3:35:29 PM UTC-7, wrote:
On Tuesday, 26 June 2018 09:29:27 UTC+12, Bruce Hoult wrote:
On Monday, June 25, 2018 at 1:19:00 PM UTC-7, wrote:
I fly my 2B about 90% aft empty, but often don't add much tail water when ballasted. Often no tail water, especially in wave. Lead and a brass tail wheel helps with that.

I've found the WB a little unusual though as you need to measure the arms. They are a little different to the manual. I wondered if the wheel is supposed to be jacked somehow during weighing. Does anyone have any ideas about that?

Are you measuring with the tail on the ground? The glider should definitely be in something resembling flying attitude, though how that is defined for any particular glider is outside the scope of general W&B instructions.. The instructions for any particular glider will be in its maintenance manual.

Of course it is weighed in the correct SH specified attitude.

Then I didn't understand your "I wondered if the wheel is supposed to be jacked somehow during weighing". What does it mean?

On Tuesday, 26 June 2018 11:22:10 UTC+12, Bruce Hoult wrote:
On Monday, June 25, 2018 at 3:35:29 PM UTC-7, wrote:
On Tuesday, 26 June 2018 09:29:27 UTC+12, Bruce Hoult wrote:
On Monday, June 25, 2018 at 1:19:00 PM UTC-7, wrote:
I fly my 2B about 90% aft empty, but often don't add much tail water when ballasted. Often no tail water, especially in wave. Lead and a brass tail wheel helps with that.

I've found the WB a little unusual though as you need to measure the arms. They are a little different to the manual. I wondered if the wheel is supposed to be jacked somehow during weighing. Does anyone have any ideas about that?

Are you measuring with the tail on the ground? The glider should definitely be in something resembling flying attitude, though how that is defined for any particular glider is outside the scope of general W&B instructions. The instructions for any particular glider will be in its maintenance manual.

Of course it is weighed in the correct SH specified attitude.

Then I didn't understand your "I wondered if the wheel is supposed to be jacked somehow during weighing". What does it mean?

It is sprung, so the question is whether the wheel measurement varies depending on the extension of the landing gear, state of the springs etc. The springs were also upgraded as part of the 2001 landing gear AD.


I haven't looked into it, but my wheel measurements are slightly different from the manual.


I have a recollection of someone suggesting the main wheel should be chocked to full extension. Whether that makes a difference ...

Sooooo.....your aft heavy bits drag the CG aft.....thus may be no need for aft water ballast normally.

I am a bit below "pilot weight" in my normal glass single seaters.
The owner ballasts to his weight, up to me to keep within CG limits.
Most of this is in ASW/ASG ships.

Oh, and optimal for whom?

Some like mid range, some can deal with/like aft CG.......what is good for you may suck for another......

What is your goal?
Straight high speed cruising, tight gusty thermaling, away from stall/spin, wet, dry, etc.....

I have no clue on a Discus, but others will want to know your "idea" of optimal......

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

Thanks Michael and Christoph,

Good information.