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these findings been superceded by something else in the last 8 years?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

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

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?!?

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

Thanks, guys - I've read all your responses, thought about them and
have been Googling to learn more. It looks to me like Bob W's "rant"
was correct, and that JJ's method is the best way to go.

Brian said "there is a known range of allowable CofG's in terms of
%MAC which is similar across a wide range of airframes", so I decided
to see what that might be. The best I could tell a reasonable range
was 15% to 35% of MAC. However, it's really a loosey, goosey "range" -
the HP-18 Bob K referenced has a 25% to 40% of MAC actual range, while
a DC8 has a 8% to 18% of MAC actual range! I did discover that by
using the 15% to 35% of MAC range Frank Irving's optimal CG is 75% to
100% of the allowed CG range (where 0% is the fwd limit and 100% is
the aft limit). It isn't very precise, though, and it doesn't agree
with the Akaflieg Braunschweig findings.

I then calculated the arithmetic mean chord of the LS8 wing by
dividing span by aspect ratio, and got 700mm. This looks about right,
since the root chord is 900mm, and yes, I know it's not the MAC. Then,
I found a scale drawing of the LS8 at: http://www.dg-flugzeugbau.de/Data/3s-ls8-s.pdf
If I'm reading it right, the 25% MAC will be located 225mm behind the
wing LE. Using the arithmetic mean chord of 700mm and the allowed CG
range of 280mm to 400mm behind the wing LE, I calculated that the LS8
has a CG range on the order of 33% to 50% of MAC. That seems wrong
enough that it isn't worth the bother of actually going through the
geometric excercise of calculating the MAC on the triple tapered LS8
wing. I cheerfully admit there's a possibility that I don't know what
I'm doing, but at this point I'm going to drop Irving's approach.

This gets me to JJs advice. I'll follow the 2001 Akaflieg Braunschweig
method, and use the tail tank to set the CG to 65% of the allowable
range. Once I'm familiar with how she flies there, I'll move it back 5
to 10% at a time until I either reach 90% or get to JJ's criteria
point (I have to trim forward when entering a thermal), whereupon I'll
bring it forward 5%.

I'd be grateful for any further comments or suggestions.

-John

On Dec 13, 1:34 pm, jcarlyle wrote:

...I then calculated the arithmetic mean chord of the LS8 wing by
dividing span by aspect ratio, and got 700mm. This looks about right,
since the root chord is 900mm, and yes, I know it's not the MAC. Then,
I found a scale drawing of the LS8 at: http://www.dg-flugzeugbau.de/Data/3s-ls8-s.pdf
If I'm reading it right, the 25% MAC will be located 225mm behind the
wing LE...

Hmm... That doesn't seem right. Using the DJ Aerotech graphical MAC
method, the same LS8 drawing, and information on the LS8 from Thomas'
Fundamentals of Sailplane Design (thanks again, Judah!) that places
the planform break at 0.6 semispan, I got:

* MAC length of ~736mm
* MAC LE location of ~41mm aft of the wing LE at side of body _or_
* MAC LE location of ~45mm aft of the intersection of the projected
leading edge and the plane of symmetry (yeah, who uses that?)

Given that the LS8's double-trapezoid planform gives it more MAC per
unit area than the HP-18's eminently buildable single trapezoid
planform, and that the LS8's wing is unswept along the 25% chord line
as opposed to the HP-18 being unswept along the 41.25% chord, the MAC
and MAC LE numbers I got sound about right to me. But, hey, I'm a
college dropout with no engineering training, what do I know?

As regards the suggested CG location for the HP-series, Dick Schreder
typically suggested 25% to 40% MAC as the allowable range. Based on an
analysis of the margin of static stability of the HP-18 done by Steve
Smith (that's Dr. Smith to you Mythbusters fans), and based on my own
experience flying an HP-18 with CG back around 40%, I currently
recommend that HP operators limit their operation to 25% to 35% MAC.

For an extra 245 Europes, I will be glad to translate that into a
dimensional range aft of the wing leading edge at side-of-body. For
245 Australias, I will do all the above and throw in a wisecracking
reality-show cameraman. We are not accepting any other continents at
this time.

Thanks, and best regards to all

Bob K.
www.hpaircraft.com ---- now with 245% less ondulation!

On Dec 13, 9:30*pm, Bob Kuykendall wrote:

For an extra 245 Europes, I will be glad to translate that into a
dimensional range aft of the wing leading edge at side-of-body. For
245 Australias, I will do all the above and throw in a wisecracking
reality-show cameraman. We are not accepting any other continents at
this time.

Thanks, and best regards to all

Bob K.www.hpaircraft.com---- now with 245% less ondulation!

So, Bob, does this mean you are only accepting two out of continent
offers, and no in-continent offers at this time? Will this now
qualify me as the wisecracking reality show cameraman?

I have various manuals on various planes that say anything and
everything from X% to Y% of the root chord, to X% to y% of the chord .
5 meters outboard of the side of the fuselage. Best reference that
can be given for the average pilot is a set of dimensions from an easy
to identify point. Forward face of the forward drag spar in the
fuselage on an HP-18 is an excellent datum.

Steve Leonard
:-)

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