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Old Mar 18, 2012, 02:57 PM
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finding cg for double delta

any source for calculating cg on a double delta? (sort of diamond shape).
thanks
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Old Mar 18, 2012, 09:53 PM
"...certainty is absurd."
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Phil,

This calculator should give you a reasonable answer regardless of the planform shape:

http://www.rcgroups.com/forums/showt...=cg+calculator

Kevin
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Old Mar 19, 2012, 05:02 AM
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double delta shape

thank you, Kevin, seems great for many uses.
my problem is my limited knowledge of computers. and the calculator specifies something about security settings that are chinese to me. i will ask my son, who knows more than me on the subject.
but if you or some1 could provide an easier way to calculate just a doble delta would be great.
thanks again.
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Old Mar 19, 2012, 08:12 AM
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Phil,

Doble delta? (sort of diamond shape).

See my pictrure.

Is the plan form of your delta different? Show us a scaled top view!


To know the location of the CG of my example we first have to know the position of the mean aerodynamic chord (MAC). We don't need a computer for that, only paper and pencil will do the job.
Normally the green dots are the tip dimensions which probably are 0 inch when I do understand your diamant shape plan form.



General rule
CG position of a delta to start with is at 20 a 25 % of MAC (IMO!!!) and so in more forward position compared with a "normal" plane/wing.

Cees
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Old Mar 19, 2012, 09:54 AM
"...certainty is absurd."
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Phil,

Excel always asks if you want to enable macros, which you must do for that CG calculator to work. There are simpler on-line versions that ignore a lot of effects, and I'm not sure if they will handle your planform?

http://fwcg.3dzone.dk/

MAC calculator:

http://www.geistware.com/rcmodeling/cg_calc.htm

I'm not sure what a "double diamond" shape looks like. There are graphical methods of finding the mean aerodynamic chord (MAC), as TF showed above, and graphical techniques for planforms with multiple tapers also.

http://www.nsrca.org/technical/tip_tricks/mac_cg.htm

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

http://soartech-aero.com/ivens.htm

If this is a flying wing, you probably want to start with the CG at about 18% of MAC (7% static margin for a flying wing). It may be nose heavy there, but it is safer to test fly.

Kevin
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Old Mar 19, 2012, 10:19 AM
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I think that graphic shows the mean geometric chord. The calculation of the mean aerodynamic chord includes the section additional lift coefficient (meaning the C_l for the untwisted wing), and we cannot trivialize this for such a low aspect ratio wing (if that really is what a "double delta" is).
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Old Mar 19, 2012, 11:03 AM
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shape

the idea is to balance at 33% of the mean geometric chord.
from my early memories, i recall doing this with geometry by calculating centroids.
but i forgot that.
if you remind me of that, the rest would be easy.

let's start with a delta: straight trailing edge.
then add another delta (reversed) at the rear. that is, if span and lenght are same, we have a square, tilted 45 degrees.
with that, the thing will balance...where? at 33% of the area?

now, if we change the lenght of the rear delta to look like the drawing on #4 post, that is, the lenght of the rear delta smaller than the front, how we calculate the cg?
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Old Mar 19, 2012, 11:21 AM
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same planform

Quote:
Originally Posted by Taurus Flyer View Post
Phil,

Doble delta? (sort of diamond shape).

See my pictrure.

Is the plan form of your delta different? Show us a scaled top view!



To know the location of the CG of my example we first have to know the position of the mean aerodynamic chord (MAC). We don't need a computer for that, only paper and pencil will do the job.
Normally the green dots are the tip dimensions which probably are 0 inch when I do understand your diamant shape plan form.



General rule
CG position of a delta to start with is at 20 a 25 % of MAC (IMO!!!) and so in more forward position compared with a "normal" plane/wing.

Cees
the planform is very much like the 1 you have on the drawing
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Old Mar 19, 2012, 12:04 PM
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Phil,


Taurus flyers do know what a diamond shape is, that's why I did post.
What for you is the shape of your whole plane is that stabilizer (cross sectional) for us!

With what you show we only can make an estimation, and that I did show in my post 4.
CG never can be located behind 25 % and expect it's around 15 20 % of that MAC.

Cees
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Old Mar 19, 2012, 02:29 PM
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I remember that method to find the MAC, but something seems wrong to me, though I can't put my finger on it. The 25% MAC calculated that way seems a bit too far forward. I'll have to dig up an old book, I guess.
[edit] No, the method is quite right. I think there is some small inaccuracies in the drawing itself that make the MAC chord appear more inboard than it actually is, and on a strongly tapered wing this causes a rather large error. I think the line going from the lower left to the upper right is missing the wing root chord and is a little shifted to the right.
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Old Mar 19, 2012, 02:44 PM
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Quote:
Originally Posted by Brandano View Post
I can't put my finger on it
The back of Etkin and Reid says the MAC is defined by:

xbar = 2/(C_L*S) * int(C_l_a*c*x*dy, from 0 to b/2)

I those characters are annoying to read....
C_l_a is the section additional lift coeffient (from the spanwise lift distribution of the untwisted wing)
x is the chordwise coordinate of the section aerodynamic center
c is, of course, the chord
y is the span variable
Integrate the location of the section AC weighted by the chord as well as how important that section is to the lift distribution. Do this integration along the span from y = 0 to y = b/2.


I didn't work it out, but I'm pretty sure using a C_l_a of unity gives you the mean geometric chord.
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Old Mar 19, 2012, 02:58 PM
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The CG should be forward of the 25% area line. 25% of the area of a Delta is at 50% of the chord.

Just build a 1/16th sheet model of the planform and balance it with clay. Easier!

Tom
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Old Mar 19, 2012, 11:54 PM
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NACA Report 824 says....
"The chordwise position of the a.c. is behind the quarter chord point for the NACA 6-series airfoil.... " which are useless for models.
"For the NACA 00 and14-series airfoils less than 12% thick, the chordwise position is at the quarter-chord and does not vary with airfoil thickness"...
"For the NACA 24-, 44- and 230- series, thicknesses between 12 and 24 percent the a.c. is ahead of the quarter point and moves forward with increase in thickness.."
The a.c. for models is reasonably placed therefore, at 25%.
It's a waste of time to play with small numbers for small results.
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Old Mar 20, 2012, 05:22 AM
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Carl Gustav Hempel, paradox of the ravens!

Sparky Paul.


Quote:
Originally Posted by Sparky Paul View Post
NACA Report 824 says....
"The chordwise position of the a.c. is behind the quarter chord point for the NACA 6-series airfoil.... " which are useless for models.
"For the NACA 00 and14-series airfoils less than 12% thick, the chordwise position is at the quarter-chord and does not vary with airfoil thickness"...
"For the NACA 24-, 44- and 230- series, thicknesses between 12 and 24 percent the a.c. is ahead of the quarter point and moves forward with increase in thickness.."
The a.c. for models is reasonably placed therefore, at 25%.
It's a waste of time to play with small numbers for small results.

You write about black ravens:

"For the NACA 00 and14-series airfoils less than 12% thick, the chordwise position is at the quarter-chord and does not vary with airfoil thickness"...
"For the NACA 24-, 44- and 230- series, thicknesses between 12 and 24 percent the a.c. is ahead of the quarter point and moves forward with increase in thickness.."


But what has that to do with the thread? Do you know what airfoil is used for the delta??


The white raven


Because I am a modeler for a while I know there is also a NACA 231xx serie, often used for tailless airplanes and delta's, of which I show an example in picture 1.

When we take an example of a delta with diamant shape, see picture 2, we see a location of CG on 16.4 % of the MAC calculated with ratio of line lengths (in pixels) on the graphic constructed line for MAC.

Cees



MODELING SCIENCE?
Google for: Carl Gustav Hempel, paradox of the ravens!
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Old Mar 20, 2012, 09:32 AM
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double deltas

back into the issue.
ok, guys. i have had the deltas and flying wings well worked out.
what i need is about double deltas: the kind that looks like a square tilted 45 degrees.
can you help me with it?
thanks
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