



Go here http://www.mmaamodelairplanes.org/m...396f4db6c59109
About half way down the page is a link named "Mean Aerodynamic Chord Calculator" That dose what you want 





BTW I just noticed in your tailless pylon racer thread that you use Rhino 3D. I’m 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 pline 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 





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 

Last edited by rosco; Jan 04, 2008 at 01:52 AM.




Quote:







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 





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 





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 





Quote:







There is another way for those who are mathematically challenged.
Make an accurate thin card scale template of one wing, tip to centreline. 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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