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Old Feb 22, 2012, 09:54 PM
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United States, FL, Lehigh Acres
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Question
what is 27% MAC?

yes, it seems to be another dumb question. my manual states "for CG proper position should be at 27%MAC.

(is this telling me the distance from the leading edge?)

how many inches/cm/mm is 27%MAC? (im confused lol)
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Old Feb 22, 2012, 10:19 PM
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USA, IL, Wheaton
Joined Oct 2010
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MAC is Mean Aerodynamic Chord

Hi
Once you get past the acronym, this isn't hard to figure. Mean Aerodynamic
Chord (MAC) is the width of the wing taken as an average over its span. If the wing is a simple rectangle in shape, you simply measure from the leading edge to the trailing edge to get the chord, which is also the MAC. Multiply that by 0.27 to find where the CG should be. Then mark it on the wing, measuring from the leading edge. For simple tapered wings, you add up the chord at the tip and the root and divide by 2. If you have a wing with straight and tapered sections, you need to factor in the areas of the different pieces. The average chord of the straight part times the fraction of the wing area it represents is added to the average chord of the tapered parts times their area fraction. It gets a bit more complicated if the airfoil changes from point to point.

Regular looking model wings use the simpler averaging. Don't ask me to do a Spitfire wing in my head. You can get close by using the chord half way out on the wing. Deltas are the trickiest for this. If you're modeling an B-70, you may just need to build a cardboard model and add paperclips to the nose till you get it right.

When you express things this way, it allows you to change the wing top view and still get a reasonable way to find the CG.

Hope this helps

Joe
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Last edited by DuPageJoe; Feb 22, 2012 at 10:24 PM. Reason: misspelling
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Old Feb 22, 2012, 10:28 PM
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thanks joe. ill have to read this a couple more times lol. i have an aerobatic 3d plane.

i was told a good point to start is at the thickest part of the wing or the wing tube.
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Old Feb 22, 2012, 11:34 PM
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my browser is buggin. sorry for the double post.
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Old Feb 22, 2012, 11:36 PM
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actually by looking at this illustration i found actually helps and should or will eventually help someone else. (although this pic only represents rectangular wings. mines is an sbach wing i guess)

so what im looking for is 27% of my MAC from the leading edge? feel free to correct me.
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Old Feb 23, 2012, 07:40 AM
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I found the calculator you need from a quick Yahoo search. The National Association of Scale Aeromodellers has an on line calculator and a very good diagram here to go from wing planform and % MAC to CG position at the wing root.

http://www.nasascale.org/howtos/cg-calculator.htm

You've got the idea for the rectangular wing. If you run that calculator, you should get the tapered wing too.
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Old Feb 23, 2012, 09:38 AM
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Quote:
Originally Posted by DuPageJoe View Post
Out of stock
I found the calculator you need from a quick Yahoo search. The National Association of Scale Aeromodellers has an on line calculator and a very good diagram here to go from wing planform and % MAC to CG position at the wing root.

http://www.nasascale.org/howtos/cg-calculator.htm

You've got the idea for the rectangular wing. If you run that calculator, you should get the tapered wing too.
thank you so much! so my wing is called a tapered wing? i dint know. thanks ever so much!
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Old Feb 23, 2012, 09:44 AM
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Here is a very good CG calculator that also take the tail area and distance into account.
There is also the facility to change the 'Static Margin' for sensitivity.
http://www.geistware.com/rcmodeling/cg_super_calc.htm

There are also calculator around on the forums for canards, and flying wings.
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