Optimum CG Range

On 15 Dec, 15:42, Andy wrote:
On Dec 15, 6:52*am, wrote:

Gives adequate pitch authority to pull to max lift coefficient, thus
tightest turn. From my experience, this is usually about 75-80% aft in
manufacturer's approved range.
UH

It's not quite that simple though is it?

For the ASW-28, and probably other modern gliders, the "manufacturer's
approved CG range" is dependent on the glider mass. * Again for the
28, a cg position of 75-80 of approved range at min weight (315-321 mm
aft of root leading edge) will be behind the approved aft CG limit at
max gross wt (306mm).

I used to think that the change in aft cg limit with increasing mass
was to protect for the case where the tail tank fails to dump. *If
that is true then ASW 28 built without the optional tail tank would
not have the variable aft limit. *Do they?

Comments or other explanations of the variable aft limit?

Hank - Where is your 28 CG at max gross or at the max weight you fly
at if lower?

Andy (GY)

Are you sure you are reading the manual right? I own a 27 and the aft
limit remains the same. Waibel argued that it was by design that the
CofG moves forward when adding ballast and that this automatically
made for more efficient high speed flight when flying with high wing
loading. He even stated that the fin ballast tank was unnecessary. It
is possible that the practical aft limit for CofG position when
ballasted is well forward of the position and aft limit when empty for
this reason.

Also, if you have a tail tank then it might be wise to ensure that
filling the tail tank only keeps the C of G within limits if there is
any possibility of it not emptying when you dump ballast.

I don't have a 28 manual to look at, have you got one in electronic
form?

On the subject generally. I would recommend flying the glider (within
manufacturers limits) with a CofG that you find best suits your style
and ability. This can be achieved by experimentation. As I understand
it, moving the CofG back improves efficiency at low speeds and in
thermals by reducing the necessity for the tailplane to produce
downwards lift (and drag) in those phases of flight. In extremis it
allows sufficient elevator authority to fly near the stall in this
configuration. The downside is the reduced stability in pitch which
could lead to less efficient handling and pilot induced losses.

Jim

On Tue, 15 Dec 2009 09:56:22 -0800 (PST), jimboffin
wrote:


Are you sure you are reading the manual right? I own a 27 and the aft
limit remains the same. Waibel argued that it was by design that the
CofG moves forward when adding ballast and that this automatically
made for more efficient high speed flight when flying with high wing
loading. He even stated that the fin ballast tank was unnecessary. It
is possible that the practical aft limit for CofG position when
ballasted is well forward of the position and aft limit when empty for
this reason.

Unfortunately he neglected the fact that especially the 27 with its
tiny horizontal tail is usually flown with very high wing loading,
hence especially the 27B really benefits from its tail ballast tank.


On the subject generally. I would recommend flying the glider (within
manufacturers limits) with a CofG that you find best suits your style
and ability. This can be achieved by experimentation.

100% agree.

As I understand
it, moving the CofG back improves efficiency at low speeds and in
thermals by reducing the necessity for the tailplane to produce
downwards lift (and drag) in those phases of flight.

In the 27 you can really notice this - flying a 27 with a forward CG
badly affects its climb performance, yet increases its perfomance the
more the fster you fly.



Bye
Andreas

On Dec 15, 10:56*am, jimboffin wrote:
Are you sure you are reading the manual right?

Don't need to read it. The allowable mass/cg envelope is depicted
graphically on page 5.10 of the flight handbook. I only have the
German version on hand, the English translation is in the glider. The
graph shows an aft limit of 345mm from 300Kg to 380kg, then reducing
linearly to 322 mm at about 460 kg then linearly to about 306 mm at
525 kg max gross.

The envelope also shows that the forward limit moves forward above
380kg but the shift is much smaller than for the aft limit. It also
clearly shows that the aft end of the recommended cg range of
300-310mm falls outside the aft cg limit at max gross.

Hank - what is on page 5.10 of your 28 flight handbook? If it is a
mass/cg envelope what is the allowable CG at 525kg? I'm surprised
that 2 28's both with tail tanks would have different limits. Mine is
serial 28048.


Andy (GY)

On Dec 15, 11:14*pm, Andy wrote:
On Dec 15, 10:56*am, jimboffin wrote:

Are you sure you are reading the manual right?

Don't need to read it. *The allowable mass/cg envelope is depicted
graphically on page 5.10 of the flight handbook. *I only have the
German version on hand, the English translation is in the glider. The
graph shows an aft limit of 345mm from 300Kg to 380kg, then reducing
linearly to 322 mm at about 460 kg then linearly to about 306 mm at
525 kg max gross.

The envelope also shows that the forward limit moves forward above
380kg but the shift is much smaller than for the aft limit. *It also
clearly shows that the aft end of the recommended cg range of
300-310mm falls outside the aft cg limit at max gross.

Hank - what is on page 5.10 of your 28 flight handbook? *If it is a
mass/cg envelope what is the allowable CG at 525kg? *I'm surprised
that 2 28's both with tail tanks would have different limits. *Mine is
serial 28048.

Andy (GY)

OK oops- I have to admit not paying large amount of attention to
variable CG since I don't remember the last time I flew above about
480kg. Easterner ya know. I suspect this limitation relates to Waibel
trying to ensure not going out the back if mains dump and tail
doesn't. Gotta admit it doesn't interest me enough to do the math.
Review does show that empirically through flight experience, I've
pretty much confirmed that the factory suggested range works well.
UH

One more little tid-bid. I have found when flying a flapped ship and
my CG was in the correct position for best performance, I never
touched the trim knob! I'm flying a standard class bird now, so this
doesn't apply, but in the H-301, ASW-20, Nimbus-3, LS-6 and ASH-25 it
did. Moving the flaps did all the speed changes and moving the trim
was not necessary. How's that check with you flapped guys?
JJ

On Dec 16, 9:31*am, JJ Sinclair wrote:
One more little tid-bid. I have found when flying a flapped ship and
my CG was in the correct position for best performance, I never
touched the trim knob! *I'm flying a standard class bird now, so this
doesn't apply, but in the H-301, ASW-20, Nimbus-3, LS-6 and ASH-25 it
did. Moving the flaps did all the speed changes and moving the *trim
was not necessary. How's that check with you flapped guys?
JJ

Dissabled the trimmer completely on '20 (tape around trigger). Nice to
feel elevator feedback. '27 doesn't seem to like this as much though
have reduced trimming force. When CG is far enough back on '27 to "fly
the flaps" , it doesn't seem to fly as nice, at least for me. That
said, in CG position I like, the stick doesn't move much.
'28- always trim-trim trim- good reason not to circle.
UH

My old LS-3 was set up so the stick position was about the same at min
sink (straight ahead, not turning), best L/D, and--from memory--80
kts. Took a tape measure with me a few times and measured the distance
between the stick and instrument panel. That's probably a better
indication of drag caused by control deflection (if that's what you're
worried about) than trim position. I flew it mostly using the flaps,
with a little elevator to initiate the pull up and push over if I was
in a hurry. The push over at the top of a steep zoomie by going full
negative flaps, then popping them down into thermaling position as I
turned was a wonderfully smooth feeling. Thermaling itself was
whatever combination of stick and flap that worked. The end result was
the CG on the aft limit (dry). Performed really well but was pitch
sensitive in gusty thermals. I was told that was an issue with that
airfoild, shared by other gliders that used it (PIK, Nimbus II, et
al.).

I followed UH's (and Herr Waibel's) advice when I got my ASW 24 and
set the CG about 2/3 to 3/4 back in the range. Flies and performs
great. Which is good because I don't have nearly as much time to
fiddle with gliders as I did in earlier times.

Chip Bearden
ASW 24 "JB"
USA

On Dec 16, 6:48*am, wrote:

OK oops- I have to admit not paying large amount of attention to
variable CG since I don't remember the last time I flew above about
480kg.

Glad to know we have the same limits. Even out West I have never
filled the glider. My typical ballast load puts me at 497kg. (9.7 psf
loading)

So back to the point that got me interested in this discussion:


Gives adequate pitch authority to pull to max lift coefficient, thus
tightest turn. From my experience, this is usually about 75-80% aft in
manufacturer's approved range.
UH

It's not quite that simple though is it?

The statement about optimum cg should perhaps be:

"Optimum cg is usually about 75-80% of the manufacturer's approved
range at minimum flight mass, but no further aft than the aft limit at
the actual flight mass."

BTW I manage my cg to be about 320mm dry and 305mm ballasted. I want
the cg to go aft for better climb performance if I dump because of
weak conditions. I have to change that at Parowan (strong conditions
and no ballast) so I add a nose trim disc to bring the dry cg to about
310 so I don't run out of trim in the fast glides. (For those that
want that in percent of limits the minimum flight mass range is
227-345mm).

Andy