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Optimum CG Range



 
 
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  #11  
Old December 14th 09, 09:53 PM posted to rec.aviation.soaring
Papa3
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Posts: 444
Default Optimum CG Range

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



  #12  
Old December 15th 09, 12:34 AM posted to rec.aviation.soaring
Herb
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Posts: 31
Default Optimum CG Range

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
  #13  
Old December 15th 09, 01:47 AM posted to rec.aviation.soaring
SoaringXCellence
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Posts: 385
Default Optimum CG Range

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.
  #14  
Old December 15th 09, 01:53 AM posted to rec.aviation.soaring
Eric Greenwell
external usenet poster
 
Posts: 1,096
Default Optimum CG Range

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
  #15  
Old December 15th 09, 01:55 AM posted to rec.aviation.soaring
Eric Greenwell
external usenet poster
 
Posts: 1,096
Default Optimum CG Range

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

  #16  
Old December 15th 09, 02:16 AM posted to rec.aviation.soaring
Brian Whatcott
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Posts: 915
Default Optimum CG Range

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
  #17  
Old December 15th 09, 08:51 AM posted to rec.aviation.soaring
Bruce
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Posts: 113
Default Optimum CG Range

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
  #18  
Old December 15th 09, 02:52 PM posted to rec.aviation.soaring
[email protected]
external usenet poster
 
Posts: 2,124
Default Optimum CG Range

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
  #19  
Old December 15th 09, 03:13 PM posted to rec.aviation.soaring
jcarlyle
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Posts: 522
Default Optimum CG Range

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.

  #20  
Old December 15th 09, 03:29 PM posted to rec.aviation.soaring
jcarlyle
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Posts: 522
Default Optimum CG Range

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
 




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