Optimum CG Range

jcarlyle wrote:
There are summaries of Frank Irving’s 1981 OSTIV paper that say he
concluded the optimal CG for a standard class glider is 0.3 to 0.35 of
the Mean Aerodynamic Chord. DG’s web site has an entry that says the
2001 Akaflieg Braunschweig flight test results concluded best glider
results are obtained when the CG is 30-35% in front of the rear CG
limit.

I can calculate an optimal CG for my LS8-18 using the Akaflieg
Braunschweig results quite easily. However, I can't calculate an
optimal CG using Frank Irving’s results, because RS doesn't provide
the length or starting position of the MAC.

Are these two results in agreement? If not, is there some way for
getting them into the same frame of reference? (I'd prefer restating
Irving’s results in terms of CG rather than MAC, if possible.) Or have
these findings been superceded by something else in the last 8 years?

-John

CofG may be stated as a distance aft of a given reference station which
may be at (or in front of) the nose
OR
A percentage of MAC behind the wing LE
OR
A distance behind the Wing LE

The middle definition is probably the most fundamental, because there
is a known range of allowable CofG's in terms of %MAC which is similar
across a wide range of airframes

Brian W

OR
A percentage of MAC behind the wing LE
Brian W

Errrrr, not neccessarily so; The wing leading edge will be the zero
MAC point only if the leading edge of the wing is a straight line,
otherwise as in the LS-8, zero MAC will be located behind the leading
edge. I know a guy that made this incorrect assumption on the first
flight of an RS-15 and he flew the whole flight (rather short) with
the stick full back because his CG was forward of the forward limit.
He considered bailing out, but found he could keep the nose up if he
flew 80 knots. He landed OK touching down at 80.

I like to refer to the CG in a percentage of the allowable range. The
Genesis likes to be about 85% of the allowable range which is; 0 to
5.25" aft of root rib and 85% is 4.5"aft. After adjusting the CG, give
her a test drive. If you find you are trimming forward when entering a
thermal, your CG is too far aft.
Cheers,
JJ

JJ Sinclair wrote:
CofG may be stated as a distance aft of a given reference station
which may be at (or in front of) the nose
OR
A percentage of MAC behind the wing LE
OR
A distance behind the Wing LE

The middle definition is probably the most fundamental, because there
is a known range of allowable CofG's in terms of %MAC which is
similar across a wide range of airframes

Brian W


Errrrr, not neccessarily so; The wing leading edge will be the zero
MAC point only if the leading edge of the wing is a straight line
JJ

Quite so, leading to the slightly strained specification of definition #2:
A percentage of MAC behind the wing MEAN LE station

What the devil is a mean leading edge? :-)


Brian W

On Dec 13, 9:19*am, JJ Sinclair wrote:
OR
A percentage of MAC behind the wing LE
Brian W

Errrrr, not neccessarily so; The wing leading edge will be the zero
MAC point only if the leading edge of the wing is a straight line,
otherwise as in the LS-8, zero MAC will be located behind the leading
edge. I know a guy that made this incorrect assumption on the first
flight of an RS-15 and he flew the whole flight (rather short) with
the stick full back because his CG was forward of the forward limit.
He considered bailing out, but found he could keep the nose up if he
flew 80 knots. He landed OK touching down at 80.

I like to refer to the CG in a percentage of the allowable range. The
Genesis likes to be about 85% of the allowable range which is; 0 to
5.25" aft of root rib and 85% is 4.5"aft. After adjusting the CG, give
her a test drive. If you find you are trimming forward when entering a
thermal, your CG is too far aft.
Cheers,
JJ

Another useful approach is start at about 66% aft using manufacturer's
CG range. When making the tightest turn you normally do, if you run
out of elevator, you need to shift CG back a bit. You will probably
end up around 75%.There isn't a huge benefit in having the CG way
back, but there is a significant deterioration of handling which
requires better pilot skills to offset.The last little bit of glider
performance costs quite a bit in pilot workload until you are very
proficient. I usually take a couple pounds out of the tail in the
Spring and put it back in when my skills are back up to snuff.
FWIW
UH

On Dec 14, 8:56*am, wrote:
On Dec 13, 9:19*am, JJ Sinclair wrote:


Another useful approach is start at about 66% aft using manufacturer's
CG range. When making the tightest turn you normally do, if you run
out of elevator, you need to shift CG back a bit. You will probably
end up around 75%.There isn't a huge benefit in having the CG way
back, but there is a significant deterioration of handling which
requires better pilot skills to offset.The last little bit of glider
performance costs quite a bit in pilot workload until you are very
proficient. I usually take a couple pounds out of the tail in the
Spring and put it back in when my skills are back up to snuff.
FWIW
UH- Hide quoted text -

Also FWIW and purely as a first approximation, I've noted that the
following works reasonably well in most glass birds I've flown.

- At altitude, trim the glider so that it flys at "about" best L/D
airspeed. This is usually somewhere between 50-55kts.
- If that trim position results in significant up-elevator, you need
more weight in the tail. "Significant" in this context means that you
have more than a small percentage of the available up-elevator travel
dialed in to maintain best L/D airspeed.

Obviously, this is only a secondary check after you've already run the
numbers or done a proper W&B, but it seems to work pretty well. In
my LS8, the above works out to about 80% of manufacturer's aft limit
and results in very pleasant handling and obvious improvement in get-
home performance on weak days.

Anything wrong with this approach?

P3

On Dec 14, 7:56*am, wrote:
On Dec 13, 9:19*am, JJ Sinclair wrote:



OR
A percentage of MAC behind the wing LE
Brian W

Errrrr, not neccessarily so; The wing leading edge will be the zero
MAC point only if the leading edge of the wing is a straight line,
otherwise as in the LS-8, zero MAC will be located behind the leading
edge. I know a guy that made this incorrect assumption on the first
flight of an RS-15 and he flew the whole flight (rather short) with
the stick full back because his CG was forward of the forward limit.
He considered bailing out, but found he could keep the nose up if he
flew 80 knots. He landed OK touching down at 80.

I like to refer to the CG in a percentage of the allowable range. The
Genesis likes to be about 85% of the allowable range which is; 0 to
5.25" aft of root rib and 85% is 4.5"aft. After adjusting the CG, give
her a test drive. If you find you are trimming forward when entering a
thermal, your CG is too far aft.
Cheers,
JJ

Another useful approach is start at about 66% aft using manufacturer's
CG range. When making the tightest turn you normally do, if you run
out of elevator, you need to shift CG back a bit. You will probably
end up around 75%.There isn't a huge benefit in having the CG way
back, but there is a significant deterioration of handling which
requires better pilot skills to offset.The last little bit of glider
performance costs quite a bit in pilot workload until you are very
proficient. I usually take a couple pounds out of the tail in the
Spring and put it back in when my skills are back up to snuff.
FWIW
UH

Here's another gem piece of advice: With my 3D model airplanes I roll
inverted and check if I need down elevator to stay level. If so, the
cg needs to be moved further back. A well set up model will happily
fly inverted without elevator movement! Haven't tried that in my LS8,
though.
Seriously, Hanks and Eric's methods will both work well. As long as
inside the book range, find the cg that gives you good handling and
enough up elevator to stall the plane in a steep turn. It'll be at
75%-90% aft.

Herb, J7

On Dec 14, 3:34*pm, Herb wrote:
On Dec 14, 7:56*am, wrote:





On Dec 13, 9:19*am, JJ Sinclair wrote:

OR
A percentage of MAC behind the wing LE
Brian W

Errrrr, not neccessarily so; The wing leading edge will be the zero
MAC point only if the leading edge of the wing is a straight line,
otherwise as in the LS-8, zero MAC will be located behind the leading
edge. I know a guy that made this incorrect assumption on the first
flight of an RS-15 and he flew the whole flight (rather short) with
the stick full back because his CG was forward of the forward limit.
He considered bailing out, but found he could keep the nose up if he
flew 80 knots. He landed OK touching down at 80.

I like to refer to the CG in a percentage of the allowable range. The
Genesis likes to be about 85% of the allowable range which is; 0 to
5.25" aft of root rib and 85% is 4.5"aft. After adjusting the CG, give
her a test drive. If you find you are trimming forward when entering a
thermal, your CG is too far aft.
Cheers,
JJ

Another useful approach is start at about 66% aft using manufacturer's
CG range. When making the tightest turn you normally do, if you run
out of elevator, you need to shift CG back a bit. You will probably
end up around 75%.There isn't a huge benefit in having the CG way
back, but there is a significant deterioration of handling which
requires better pilot skills to offset.The last little bit of glider
performance costs quite a bit in pilot workload until you are very
proficient. I usually take a couple pounds out of the tail in the
Spring and put it back in when my skills are back up to snuff.
FWIW
UH

Here's another gem piece of advice: With my 3D model airplanes I roll
inverted and check if I need down elevator to stay level. *If so, the
cg needs to be moved further back. *A well set up model will happily
fly inverted without elevator movement! *Haven't tried that in my LS8,
though.
Seriously, *Hanks and Eric's methods will both work well. *As long as
inside the book range, find the cg that gives you good handling and
enough up elevator to stall the plane in a steep turn. *It'll be at
75%-90% aft.

Herb, J7- Hide quoted text -

- Show quoted text -

Herb means 75-90% aft of the CG RANGE not the MAC.

Herb wrote:
Another useful approach is start at about 66% aft using manufacturer's
CG range. When making the tightest turn you normally do, if you run
out of elevator, you need to shift CG back a bit. You will probably
end up around 75%.There isn't a huge benefit in having the CG way
back, but there is a significant deterioration of handling which
requires better pilot skills to offset.The last little bit of glider
performance costs quite a bit in pilot workload until you are very
proficient. I usually take a couple pounds out of the tail in the
Spring and put it back in when my skills are back up to snuff.
FWIW
UH


Here's another gem piece of advice: With my 3D model airplanes I roll
inverted and check if I need down elevator to stay level. If so, the
cg needs to be moved further back. A well set up model will happily
fly inverted without elevator movement! Haven't tried that in my LS8,
though.
Seriously, Hanks and Eric's methods will both work well. As long as
inside the book range, find the cg that gives you good handling and
enough up elevator to stall the plane in a steep turn. It'll be at
75%-90% aft.
Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.

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

Herb wrote:
Another useful approach is start at about 66% aft using manufacturer's
CG range. When making the tightest turn you normally do, if you run
out of elevator, you need to shift CG back a bit. You will probably
end up around 75%.There isn't a huge benefit in having the CG way
back, but there is a significant deterioration of handling which
requires better pilot skills to offset.The last little bit of glider
performance costs quite a bit in pilot workload until you are very
proficient. I usually take a couple pounds out of the tail in the
Spring and put it back in when my skills are back up to snuff.
FWIW
UH


Here's another gem piece of advice: With my 3D model airplanes I roll
inverted and check if I need down elevator to stay level. If so, the
cg needs to be moved further back. A well set up model will happily
fly inverted without elevator movement! Haven't tried that in my LS8,
though.
Seriously, Hanks and Eric's methods will both work well. As long as
inside the book range, find the cg that gives you good handling and
enough up elevator to stall the plane in a steep turn. It'll be at
75%-90% aft.

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

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

Eric Greenwell wrote:
Herb wrote:
Another useful approach is start at about 66% aft using manufacturer's
CG range. When making the tightest turn you normally do, if you run
out of elevator, you need to shift CG back a bit. You will probably
end up around 75%.There isn't a huge benefit in having the CG way
back, but there is a significant deterioration of handling which
requires better pilot skills to offset.The last little bit of glider
performance costs quite a bit in pilot workload until you are very
proficient. I usually take a couple pounds out of the tail in the
Spring and put it back in when my skills are back up to snuff.
FWIW
UH


Here's another gem piece of advice: With my 3D model airplanes I roll
inverted and check if I need down elevator to stay level. If so, the
cg needs to be moved further back. A well set up model will happily
fly inverted without elevator movement! Haven't tried that in my LS8,
though.
Seriously, Hanks and Eric's methods will both work well. As long as
inside the book range, find the cg that gives you good handling and
enough up elevator to stall the plane in a steep turn. It'll be at
75%-90% aft.

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...

Bruce