Aft CG limit(s)

At 03:24 24 November 2003, Andy Durbin wrote:

The (ASW 28) flight manual also shows the flight mass
vs CG envelope in
graphical form on page 5.10. It shows the aft cg limit
constant at
345mm from 300kg to about 380kg. Note that 380kg is
the maximum
allowed flight mass without water ballast. The aft
limit then slopes
forward to about 315mm at 525kg.

There is also a small forward
movement of the forward cg limit as flight mass increases.


Somewhere tucked into the text of the manual, there
must be some passing reference to using this graph!

Just guesswork on my part: (1) a forward-leaning aft
CG limit line may reflect the flight CG of the glider
shifting forward with the addition of water in the
wing tanks, while (2) this shift is partially offset
by carrying water in a tail tank, and (3) this balancing
act is also reflected in the forward-leaning forward
CG limit line. (It seems to me your graph *must* contemplate
a tail tank... ballasting the wing tanks alone would
cause a forward shift in the flight CG, NOT a forward
shift in the forward CG *limit* - only the addition
of weight well aft could do that!)

In all cases, of course, (1) the unballasted minimum
seat weight must be met (otherwise, the pilot who
dumps water ballast will zip out the aft end of the
flight envelope. Not pretty.) and (2) no flying weight
may exceed maximum gross weight (I assume, as in the
good old/bad old days, that few pilots can actually
fly with full ballast... they'll bump into max gross
first.)

If my guesswork is correct, the next logical questions
about the graph are (1) what assumptions are made about
ballast distribution in wing and tail tanks and (2)
as a practical matter, what glider systems & pilot
procedures assure this distribution.

Not helping much, am I?

Judy