Optimum CG Range

[jcarlyle]

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these findings been superceded by something else in the last 8 years?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

[Bob Kuykendall]

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/dj_questions/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/Schreder/Soaring_Aug_1976_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

[Brian Whatcott]

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

[JJ Sinclair]

> 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 Whelan[_3_]]

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/dj_questions/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?!?

[Brian Whatcott]

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

[jcarlyle]

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

[Bob Kuykendall]

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!

[Steve Leonard[_2_]]

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
:-)

[[email protected]]

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

[Papa3]

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

[Herb]

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

[SoaringXCellence]

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.

[Eric Greenwell]

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]

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

[Brian Whatcott]

Herb wrote:
> ...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

Reminds me of the old rule of thumb, if you could call it that:
CG too far forward, can't pull the nose up for takeoff.
CG too far aft, can't recover from a stall/spin.

This method can be expensive in airplanes though... :-)

Brian W

[Bruce]

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

[[email protected]]

On Dec 14, 7:55*pm, 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.
>
> --
> Eric Greenwell - Washington State, USA
> * Change "netto" to "net" to email me directly- Hide quoted text -
>
> - Show quoted text -

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

[jcarlyle]

Bob, thanks for figuring out the MAC of the LS8. I wasn’t clear on
how to “fudge” (DJ Aerotech’s term) the winglet and the wing root to
get the required area for use in their graphical calculation of MAC.

Your values of 736 mm for MAC, with a MAC LE of 41 mm aft of the LE of
the wing root, jibe quite well with my arithmetic chord of 700 mm and
my arithmetic chord LE of 50 mm aft of the LE of the wing root. I’m
not clear why you said it didn’t seem right.

But thanks to you I can now calculate the LS8 CG limits in terms of
MAC. The fwd CG limit is 280 mm aft of the LE of the wing root, and
the aft CG limit is 400 mm aft of the LE of the wing root (both values
from TCDS G14CE). So:

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

This result (a) makes the LS8 odd from the perspective of SE light
aircraft (typical CG range from 15% to 35% of MAC) and the HP-18 (was
25% to 40%, now 25% to 35%), and (b) means that the 1981 Frank Irving
optimum CG guideline of 30% to 35% of MAC isn’t useful. So that
answers my original questions.

-John

On Dec 13, 10:30 pm, Bob Kuykendall > wrote:
> 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.

[jcarlyle]

Thank you UH, P3 and J7 for your thoughts on optimal CG location. My
current CG on the LS8 lies between 45% to 55% of the available range,
depending on how well my diet is working. :-( I’ll start next
Spring without any rear weight until my proficiency returns, then move
the CG back to 65%, then 70%, etc. I’ll quit at 80%, which for me is 1
gal of water in the tail tank and thus serves as a very clear stopping
point.

-John

[Andy[_1_]]

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)

[JJ Sinclair]

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

[jcarlyle]

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

[jimboffin]

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

[Eric Greenwell]

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

[Martin Gregorie[_5_]]

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 |

[Eric Greenwell]

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

[Eric Greenwell]

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

[Andreas Maurer]

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

[Bruce]

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

[T8]

On Dec 15, 1:35*pm, Eric Greenwell > wrote:
> 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.

If you never fly faster than 25 second turns -- and a lot of us don't
-- then it really isn't an issue.

When I'm on my game, I can thermal with 14 second turns, full of
water. 12 seconds, dry. Good trick to have in the bag when presented
with an usually strong, tight core (are you reading, #44? -- good
fun!). And yeah, the cg has to be in the right place to do that.

"Ability to stall" is only an indicator that the ship is set up
correctly.

-Evan Ludeman / T8

[[email protected]]

On Dec 15, 10:42*am, 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)

My glider is set up for about 75% dry- about 315mm. Water goes in tail
tank to maintain this position when water goes in wings. My manual
doesn't have variable limit but does suggest a dry position of 300-310
mm.
My glider is also set up to retain relationship of tail to main water
during dumping.
Only exception is ridge flying when I use less tail weight to unload
ship in gusts.
FWIW
UH

[[email protected]]

On Dec 15, 1:35*pm, Eric Greenwell > wrote:
> 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

I want to be able to fly at 45 deg bank without running out of
elevator. I rarely thermal steeper than that because sink rate rises
and I'm mostly interested in the smallest circle.
Note that I am NOT advocating far aft CG position- actually quite the
opposite.
FWIW
UH

[Andy[_1_]]

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)

[[email protected]]

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

[JJ Sinclair]

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

[Andy[_1_]]

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

[[email protected]]

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

[Chip Bearden[_2_]]

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