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

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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Replace 'SPAM' with 'charter' to email me directly

Eric Greenwell
Washington State
USA

At 19:06 24 November 2003, I stuck my foot in my mouth
by saying:

(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!)

That last phrase should have read, 'only the addition
of weight well aft could make the forward CG limit
LINE shift forward.' (That's because the LINE represents
the CG limit - a fixed value - being affected by the
distribution of weight in wing & tail tanks.)

Apologetically,

Judy

Our DuoDiscus also has changes in the allowable forward and rearward CG
limits based on mass, looking at the graph in the maintenance manual (page
6.7). It even shows a different rear limit at heavy weights with ballast
tank installed (not to say filled). Then, in the flight manual, page 2.7,
it says that the limits are simply 45 mm to 250 mm aft of datum. The two
clearly disagree. I agree with you, Eric, that aerodynamically speaking the
limits should not change with mass. The DuoDiscus manual goes further,
stating in regards to fin ballast:

....water ballast may be carried in the fin tank to compensate for the
nose-heavy moment of
-water ballast in main wing panels and/or
-loads in the aft seat (6.2.6)

Pilots wishing to fly with the center of gravity close to the aft limit, may
compensate the nose-heavy moment of loads on the _AFT SEAT_ with the aid of
the diagram... (6.2.6)

Compensation of masses exceeding the placarded minimum front seat load is
not allowed. (6.2.6)
---
A lower (than minimum) front seat load must be compensated by ballast...
(2.7) (the rear seat occupant may not be factored in, although clearly well
forward of the CG)

There are more examples but this post is already too long.

I can see that a fat boy in the front seat and a full load of water in the
tail, although weighing the same as with a skinny guy in front, no tail
ballast, and a rear pilot, would have more rotational inertia, possibly
becoming a spin recovery problem. That makes sense right up until you see
that 200 liters of ballast are allowed well out in the wings!

Not allowing one to be in CG limits with a below minimum pilot in the front
seat and a rear pilot helping to make the difference has nothing to do with
inertia and could only be that Schempp-Hirth has discovered American style
product liability. I understand trying to prevent mistakes but the pilots
that will make the mistakes won't read the manual anyway -- too damned
complicated! The DuoDiscus buyers end up being test pilots after all...
-Bob Korves









"Eric Greenwell" wrote in message
...
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".

--
-----
Replace "SPAM" with "charter" to email me directly

Eric Greenwell
Washington State
USA