Aft CG limit(s)

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".

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Eric Greenwell
Washington State
USA