Aha! Now it all makes sense. It seems to me that the
cg range should be fixed for a given airfoil no matter
the weight. Providing for staying within the aft
range after dumping ballast has to be the reason for
the shift described and this should be true for all
gliders.


At 18:00 24 November 2003, Eric Greenwell wrote:
Andy Durbin wrote:

I don’t know why the aft limit moves forward with
increasing
mass for the ASW-28. Could it be that Schleicher found
the stall/spin
recovery characteristics unacceptable at max GW at
the dry aft limit.

Would a 27 owner please say if that glider weight/cg
envelope also
shows a variable aft limit.

The Take-off Mass vs In-flight CG range diagram for
my ASH 26 E shows
the CG range (aft limit AND forward limit) changing
with mass. Above 490
kg, the allowable aft limit moves forward; below 480
kg, the forward
limit moves rearward. 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.'

Apparently (as an example), if you have the CG at the
chart's
_unballasted_ aft limit with full ballast, it will
move behind the aft
limit when you dump the ballast.

I'm guessing that on strictly aerodynamic considerations,
the CG range
would not depend on the mass. This would seem sensible,
based on the
text of section 2.7:

'2.7 Center of Gravity

The limits of the C.G. are as follows:

forward limit 290 mm aft of BP
aft limit 410 mm aft of BP'

BP means 'reference datum'.

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Eric Greenwell
Washington State
USA