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Old Jan 03, 2008, 09:22 PM
That motor isn't hot...
rosco's Avatar
Brisbane, Australia
Joined Aug 2005
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CG calc for wing with multiple sweep angles

Can someone direct me to a C.G. calc that will calculate te C.G. for a wing with multiple sweep angles?
I can only find ones for a wing with a single sweep.

Much appreciated
cheers
rosco
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Old Jan 03, 2008, 10:29 PM
internet gadfly
nmasters's Avatar
Colorado
Joined Aug 2006
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Go here http://www.mmaa-modelairplanes.org/m...396f4db6c59109


About half way down the page is a link named "Mean Aerodynamic Chord Calculator" That dose what you want
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Old Jan 03, 2008, 11:07 PM
internet gadfly
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BTW I just noticed in your tailless pylon racer thread that you use Rhino 3D. Iím not sure what the commands would be in Rhino but to find the centroid of an arbitrary polygon in Autocad you turn the lines or p-line into a region. Then go to <tools> <inquiry> <mass properties>. That will give you the approximate spanwise position of MAC but not the reference length. The length of MAC is simply area/span. So you take that reference length and put its mid point on the centroid.

--Norm
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Old Jan 04, 2008, 01:03 AM
That motor isn't hot...
rosco's Avatar
Brisbane, Australia
Joined Aug 2005
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Thanks nmasters.
I had to laugh when you used the word 'centroid'...i don't think I have heard that word before...just when I think I am getting O.k. at aerodynamics, I find somthing new that proves me otherwise

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rosco
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Old Jan 04, 2008, 01:16 AM
That motor isn't hot...
rosco's Avatar
Brisbane, Australia
Joined Aug 2005
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The software that I already have been using tells me that the MAC of my new shaped wing is 113mm. Is finding the C.G. a case of adding the static margin to the MAC?

cheers
rosco
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Last edited by rosco; Jan 04, 2008 at 01:25 AM.
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Old Jan 04, 2008, 01:45 AM
That motor isn't hot...
rosco's Avatar
Brisbane, Australia
Joined Aug 2005
425 Posts
Is this correct?



c= (S(A+2B)) / (3(A+B))
=(300(130+s*80)) / (3(130+80))
= 138.09


CG
= %MAC B.P.*(MAC) + C

CG= 25/100 * 113.71 + 138.09

= 167.51

The wing is shaped like this...


cheers
rosco
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Last edited by rosco; Jan 04, 2008 at 01:52 AM.
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Old Jan 04, 2008, 03:24 PM
internet gadfly
nmasters's Avatar
Colorado
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Quote:
Originally Posted by rosco
The software that I already have been using tells me that the MAC of my new shaped wing is 113mm. Is finding the C.G. a case of adding the static margin to the MAC?
No, MAC is the average chord length. The static margin is a percentage of that length measured from the aerodynamic center. For instance if you want a static margin of 3% you would multiply 113 by 0.03 to get 3.3 So you would put the CG 3.3mm in front of the aerodynamic center
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Old Jan 04, 2008, 04:33 PM
Red Merle ALES
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United States, Mt, Helena
Joined Apr 2002
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Rosco,

Do you have MS Excel? If so download Sailplane Calc from the link below and just put zeros in for the tail and your flying wing data will be accurate.

Curtis
Montana
http://h1.ripway.com/cloudyifr/files.htm
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Old Jan 04, 2008, 05:35 PM
That motor isn't hot...
rosco's Avatar
Brisbane, Australia
Joined Aug 2005
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CloudyIFR...
I thought christmas was 2 weeks ago...but I was wrong

Very nice work...

I can't quite work out which cell is the C.G. though...is it at D36?

Why does it say 'enter actual CG from leading edge' when I am trying to find out what the C.G. is?

Is that meant to be the C.G. given by the manufacturer?

Sorry for the silly question.
cheers
rosco
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Old Jan 04, 2008, 11:14 PM
Red Merle ALES
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United States, Mt, Helena
Joined Apr 2002
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Rosco,

Row 35 is the Neutral Point of the wing.
Row 36, 37 and 38 allow you to enter what you know, either the Static Margin, the Distance the model is balanced or the %MAC and it'll calculate the other two variables.

So if you have a model that has a balance location you can enter the inches aft and it'll give you the static margin and %MAC.

In your case you dont know the distance or the %MAC so you'll have to start with the static margin. This is the percentage of the MAC length forward that you're balancing the model. For a thermal duration model it'll be like 8% for a safe first flight and perhaps the same for a flying wing.

Oh, since it's a flying wing the Stab Efficiency would be 1, not totally accurate but close enough.

Hope this helps and thanks for the nice words.

Curtis
Montana
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Old Jan 05, 2008, 12:11 AM
That motor isn't hot...
rosco's Avatar
Brisbane, Australia
Joined Aug 2005
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Thanks again Cloudy IFR...
One last thing...It should be possible to put my measurments in millimeters, shouldn't it?
The only issue I can see would be the RE number calculated on thw 'wing' page.

What do you think?

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rosco
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Old Jan 05, 2008, 02:40 AM
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vintage1's Avatar
East Anglia, UK
Joined Sep 2002
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Quote:
Originally Posted by rosco
Can someone direct me to a C.G. calc that will calculate te C.G. for a wing with multiple sweep angles?
I can only find ones for a wing with a single sweep.

Much appreciated
cheers
rosco
Mine does multiple sweeps.
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Old Jan 06, 2008, 01:09 AM
That motor isn't hot...
rosco's Avatar
Brisbane, Australia
Joined Aug 2005
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Thankyou Vintage1.
I forgot about your one...couldn't find it though.
Have a great weekend.
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rosco
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Old Jan 09, 2008, 05:35 PM
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North London ENGLAND
Joined Dec 2006
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There is another way for those who are mathematically challenged.
Make an accurate thin card scale template of one wing, tip to centre-line.
Stick a pin in it at some arbitary point.Hang from that pin a piece of cotton with a weight on it , support by the pin.Mark the line where the cotton crosses the template. Do the same again at some other point, or even several times more. Where the lines cross is the centre of area. A line across that crossing point, parallel to the centre line, gives the MAC. If you wish a cg at, for instance,20% chord, divide the MAC into five, one fifth back from the LE gives the CG point when translated across to the centre line, (at 90 degrees of course.) Pro rata for other CG points.
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