Optimum CG Range

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these findings been superceded by something else in the last 8 years?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

On Dec 12, 7:36*pm, jcarlyle wrote:
...is there some way for getting them into the same frame of reference?

I'd recommend that you come to terms with MAC. As they say in Make
Magazine, if you can't open it, you don't really own it.

This article describes a graphical method for determining the MAC of a
double-taper wing such as that of your LS8:

http://www.djaerotech.com/dj_askjd/d...s/canard1.html

There are also calculators available on the Web for determining the
MAC of a multi-tapered wing numerically.

Once you determine the length of the MAC, locating it with reference
to the aircraft longitudinal axis is a matter of simple surveying that
you can do with a yard stick and some strings and plumb bobs.
Basically, what you do is find the location on the wing where the
actual chord equals the MAC, and then find the longitudinal location
of that chunk of wing, and that's where your MAC is. From there, all
you need to do is measure from the aircraft datum to the leading edge
of the MAC.

This Article on HP-18 weight and balance shows MAC location
graphically in relationship to the datum and side-of-body chord, and
the associated CG calculations:

http://www.soaridaho.com/Schreder/Sc...976_HP-18.html

Note that in the article, Schreder incorrectly equates mean chord with
average chord. However, in this instance the difference is very small
and makes the CG envelope more conservative, so I consider it a
forgivable simplification. But it is worth considering that you might
execute the same calculation in order to approximate the MAC length. I
think that the LS6 and LS8 in 15m trim have about 113 ft^2 like the
HP-18, so the average chord should be the same.

Thanks, Bob K.
www.hpaircraft.com

Bob Kuykendall wrote:
On Dec 12, 7:36 pm, jcarlyle wrote:
...is there some way for getting them into the same frame of reference?

I'd recommend that you come to terms with MAC. As they say in Make
Magazine, if you can't open it, you don't really own it.

This article describes a graphical method for determining the MAC of a
double-taper wing such as that of your LS8:

http://www.djaerotech.com/dj_askjd/d...s/canard1.html

There are also calculators available on the Web for determining the
MAC of a multi-tapered wing numerically.

Once you determine the length of the MAC, locating it with reference
to the aircraft longitudinal axis is a matter of simple surveying that
you can do with a yard stick and some strings and plumb bobs.

Sensible remainder snipped...

I'm not really intending to argue w. Bob K.'s position or reasoning,
just offering a slightly differing view...at least insofar as 'real
world' determination of CG is concerned.

For lots of sensible - if occasionally murky-at-first-glance - reasons,
the aerodynamic field has a love affair with mathematically elegant
approaches. While calculation of CG is arguably 'merely' a W&B
arithmetic exercise, the aerodynamic implications are obvious to anyone
who's ever flown models. That noted...

When it comes to *Joe Owner* verifying a ship's CG position, I've long
thought designers'/factories' use of MAC just a touch lazy. Since the
designer has obviously already done the math, IMHO Joe Designer should
take the next step and translate their (useful to those in the
aerodynamic field) MAC datum to some trivially-easy-to-locate fuselage
datum: less chance for user error, arguably reduced liability (sigh), etc.

Why have Joe Owner 're-design the wheel' every single time for every
single ship? If we assume CG-calc-accuracy is the goal, then failing to
make it straightforward to owners/others to perform, is (ruminatively):
thoughtless, lazy, obtuse, arrogantly didactic, etc. Personally, I don't
like RE-messing with plumb bobs when someone else already has...

Regards,
Bob - lazy, degreed AE sort - W.

P.S I blame my whine of a week+ of below 0 (F) temps prior to the onset
of winter. Where's Global Warming when you want it?!?

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.