Aft CG limit(s)

[Andy Durbin]

Recent threads that discussed handling and performance have referred
to the aft cg limit, perhaps implying that there is a fixed aft limit.

For the ASW-19b and the ASW-28, the gliders I have most recent
experience in, the aft cg limit moves forward as the gross weight
increases. I assume this is true for other glider types but I don't
have data available.

For the ASW-28 the manufacturer recommends an optimum cg range of
300-310mm aft of datum but does not seem to mention that the aft limit
of this "optimum" range is behind the aft cg limit at max gross
weight.

I have found that a cg position of 320mm aft of datum gives best climb
performance at 9psf loading without compromising the high speed
performance. That cg is at the aft limit for 9psf but well forward of
the 345mm "dry" aft limit. (9 pounds per square foot is a loading
limit for USA standard class contests)

The low weight cg range for the 28 is 227-345mm aft of datum for those
that want to consider percentages.

So how many of you know if the glider you fly has a weight dependent
aft cg limit?

Andy (GY)

[JJ Sinclair]

Andy,
The CG limits are firm for any weight of your ship. Its up to you to add trim
weights to bring the CG back to desired with a heavy pilot. I set up my ships
for ideal CG without wing water. When I load water, I must add weight to the
tail to compensate for the forward CG shift caused by the wing water. Many
ships will have a tail water tank to accomplish this.

That is, unless I'm flying the Genesis 2, then I must add water to my nose
water tank, because the wing tanks are aft of the empty CG and wing water
causes the CG to shift aft, so I must compinsate with forward weight in the
form of nose tank water.
JJ Sinclair

[Judy Ruprecht]

At 15:12 23 November 2003, Andy Durbin wrote:
>For the ASW-19b and the ASW-28, the gliders I have
>most recent
>experience in, the aft cg limit moves forward as the
>gross weight
>increases. I assume this is true for other glider
>types but I don't
>have data available.

Is it possible you're mixing apples & oranges? The
July 1977 ASW 20 handbook Flight manual p.18 lists
a firm *in flight* cg range of 240 mm (9.45') to 360
mm (14.17') aft of datum, with the datum being the
leading edge of the wing at the wing root rib, excluding
the fillet of the wing/fuselage fairing.

Assuming similar wording in the '19 & '28 manuals,
I think you're confusing this with a table shown elsewhere
in the manual, entitled 'Empty Weight c of g positions
and limits.' Here, it does appear the minimum cockpit
load increases (and aft cg shifts forward) at higher
weights. Please note, however, that the x axis represent
the EMPTY CG and the Y axis represents EMPTY weight.


Per wording in the '20 manual at p. 30a, this table
is to be used when tail ballast requires a re-calculation
of the minimum cockpit load. This re-calculation is
needed because the *flight* cg must be within the *flight*
limits.

Clear as mud, huh?

Judy

[Andy Durbin]

Judy Ruprecht > wrote in message >...
>
> Is it possible you're mixing apples & oranges?

The text of section 2.5 of the ASW-28 flight manual gives a center of
gravity range for flight of 222mm to 345mm. It does not relate this
limit to flight mass.

The 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. I believe
this diagram clearly shows a flight aft cg limit that is dependent on
mass.

I re-checked the ASW-19 manual and I did jump to the wrong conclusion
there. The data I looked at relate to empty mass cg as suggested by
Judy.

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.

Thanks

Andy (GY)

[Øyvind Moe]

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

There is no weight dependant envelope of cg for the ASW27

Section 2.7 state the foremost limit is 0.210 m and the aftmost limit is
0.320 m.

In the non-approved part of the Performance section, point 5.3.2.3 there is
a discussion of the optimal cg position.

Øyvind Moe

[Eric Greenwell]

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

[Judy Ruprecht]

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

[Nyal Williams]

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'.
>
>--
>-----
>Replace 'SPAM' with 'charter' to email me directly
>
>Eric Greenwell
>Washington State
>USA
>
>

[Judy Ruprecht]

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

[Bob Korves]

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]

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)

[Bob Korves]

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]

(Andy Durbin) wrote in message >...
> 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]

(Andy Durbin) wrote in message >...
> 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)