(Andy Durbin) wrote in message . com...
Eric Greenwell wrote in message ...
I believe the explanation for this lies in the
text preceding the table:

"The C.G. position shift due to water ballast load have been included.
This is to make sure that the ASH 26 E remains within the approved
limits after the water ballast has been jettisoned."



Unfortunately it doesn't work this way for a 28 or, if it does, it
gives way too much protection.

If I load to 525kg I can only put 1.5l in the tail. That puts me at
the 525kg aft limit. If I get into weak condx and have to dump then
the following happens: The tail empties in 30 seconds and, if I
continue to dump down to 9psf, I have no tail ballast and the cg is
well forward of the 9psf aft limit. I need 2.5l in the tail for best
performance at 9psf.

This problem can only be resolved by restricting the tail dump port or
providing independent control of the tail dump valve. Both are
prohibited by the manufacturer.

I may put fixed ballast in the tail and use no tail water for 525kg.
That would reduce the problem as I would then only be short 1 litre
after dumping to 9sf.

It still doesn't explain why the mass/cg limit envelope is the shape
it is.

Andy (GY)

Ok, I did some experimenting with different pilot mass using my weight
and balance spread sheet. It includes a plot of calculated mass and
cg overlaid on the factory flight mass vs cg envelope.

I found that if I increased pilot mass to 225 pound, and forced the
525kg solution to the aft limit with fixed tail ballast, then the dry
cg pretty much hit the aft limit also. The 9 psf cg is a little
forward of the limit.

Eric was right, the flight mass and cg envelope is intended to ensure
that an “in limit” ballasted sailplane does not go aft of
the limit on dumping. The dumping cg line only lies parallel to the
envelope limit for a specific pilot weight. This may be because the
moment of the 28 ballast changes significantly as the wing tanks are
filled. (Higher pilot mass then less ballast for max GW and the less
the ballast drives the total cg).

I am much lighter than 225 pounds with parachute and use the optional
tail tank. For this combination the max GW aft limit is too
conservative.

Thanks for the feedback.


Andy (GY)