Optimum CG Range

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)

jcarlyle wrote:
Fwd CG limit = 100 * (280 - 41) / 736 = 32% MAC
Aft CG limit = 100 * (400 - 41) / 736 = 49% MAC

John,
My experience and judgement tell me your figures are way too far aft.
I suspect the distance aft of the leading edge to the zero MAC is in
error. One sure way to check this is to mark the MAC on both wings
then assemble the wing without the fuselage on saw-horses. Then snap a
string from both zero MAC's and measure the distance from the string
to the leading edge at root rib.
Cheers,
JJ

JJ, I place great weight on your experience and judgement! I agree,
what I calculated is way far back. But I've looked over my figures
very carefully a number of times, and if there's an error I sure can't
find it.

As for the LE to zero MAC dimension causing the problem, we can get a
figure of merit by calculating the CG limits for the root chord:

Fwd CG limit = 100 * 280 / 900 = 31% root chord
Aft CG limit = 100 * 400 / 900 = 44% root chord

Of course it's different from the 32% to 49% MAC, or the 33% to 50%
for the arithmetic chord, but all three result are in the same ball
park. Measuring the actual wings as per your write-up is a good
suggestion, but due to available work area and the weather I won't be
able to try that for about 5 months at the earliest.

-John

On Dec 15, 10:49 am, JJ Sinclair wrote:
jcarlyle wrote:

Fwd CG limit = 100 * (280 - 41) / 736 = 32% MAC

Aft CG limit = 100 * (400 - 41) / 736 = 49% MAC

John,
My experience and judgement tell me your figures are way too far aft.
I suspect the distance aft of the leading edge to the zero MAC is in
error. One sure way to check this is to mark the MAC on both wings
then assemble the wing without the fuselage on saw-horses. Then snap a
string from both zero MAC's and measure the distance from the string
to the leading edge at root rib.
Cheers,
JJ

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

Bruce wrote:
Eric Greenwell wrote:

Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.

I expect it has to do with efficiency.

If your CG is such that your control inputs are minimised - you reduce
drag. In the case of steep thermalling, it reduces safety - because
you can now stall and theoretically spin.

There can be few things as frustrating as my experience with my (new
to me) Kestrel 19. First flight I wanted to be cautious so set the CG
at 35%. Then the day was booming - but with tight strong thermals, and
I was continually running out of elevator. Stick against the back stop
and the thermal is still tighter.

My Cirrus with it's all flying tail never had that problem. Of course
you could depart controlled flight if you got too enthusiastic...

At 35%, you are a long way from stalling in a steep turn. With a more
rearward CG, you would turn tightly enough, but still without enough
elevator to stall in a "tight" turn.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

On Tue, 15 Dec 2009 07:42:51 -0800, Andy wrote:

For the ASW-28, and probably other modern gliders, the "manufacturer's
approved CG range" is dependent on the glider mass.

That's not just for modern gliders and not necessarily connected with the
amount of ballast on board or the pilot's weight.

The Libelle 201B handbook contains a W&B diagram on page 6 (of both
German and English sections). This shows that the GC limits for the
aircraft move forward as its empty weight increases.

I don't understand why this would be the case - just that its documented
in the handbook.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

wrote:

Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.
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

How steep is the turn for which you still desire the ability to stall?
And how close to the ground or hillside do you expect to need this ability?

It's not clear to me this would be a useful ability for a pilot that is
trying to thermal away from a hillside, mountain, or during a low save.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

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?
The flight manual for my ASH 26 E also shows a reduction in the aft cg
limit above a certain mass, and a far greater change in the forward cg
limit over the entire mass range. It also shows the "Favorable CG range
for optimum straight flight performance" that is quite broad, about 80%
of the permissible range at full gross, and about 40% at the lowest
weight (100 pound pilot!). There is no chart for "optimum"
maneuverability in thermals, nor is there a tail tank.

It's a flapped ship, so I suspect it has a larger range than comparable
unflapped gliders, regardless of the criteria chosen.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

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

Eric Greenwell wrote:
Bruce wrote:
Eric Greenwell wrote:

Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.

I expect it has to do with efficiency.

If your CG is such that your control inputs are minimised - you reduce
drag. In the case of steep thermalling, it reduces safety - because
you can now stall and theoretically spin.

There can be few things as frustrating as my experience with my (new
to me) Kestrel 19. First flight I wanted to be cautious so set the CG
at 35%. Then the day was booming - but with tight strong thermals, and
I was continually running out of elevator. Stick against the back stop
and the thermal is still tighter.

My Cirrus with it's all flying tail never had that problem. Of course
you could depart controlled flight if you got too enthusiastic...

At 35%, you are a long way from stalling in a steep turn. With a more
rearward CG, you would turn tightly enough, but still without enough
elevator to stall in a "tight" turn.

Hi Eric

That is the point I was trying to make.

With the CG so far forward the behaviour is really benign, but I can't
get enough elevator to stall her. Even straight and level the nose just
wallows around at the back stop with ~38kt indicated.

I will be moving the CG back until I can stall it, or the handling
deteriorates, then move it a little forward. There is no virtue in being
able to stall in a tight turn, just efficiency in not holding undue
control deflection.

Bruce