Aft CG limit(s)

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)

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

(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."


snip

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

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 pounds, 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 255 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)

(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)