Feb 22, 2012, 10:54 PM DONT PANIC! :) United States, FL, Lehigh Acres Joined Jan 2012 1,548 Posts 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)
 Feb 22, 2012, 11:19 PM Registered User USA, IL, Wheaton Joined Oct 2010 422 Posts 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 Last edited by DuPageJoe; Feb 22, 2012 at 11:24 PM. Reason: misspelling
 Feb 22, 2012, 11:28 PM DONT PANIC! :) United States, FL, Lehigh Acres Joined Jan 2012 1,548 Posts 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.
 Feb 23, 2012, 12:34 AM DONT PANIC! :) United States, FL, Lehigh Acres Joined Jan 2012 1,548 Posts my browser is buggin. sorry for the double post.
Feb 23, 2012, 12:36 AM
DONT PANIC! :)
United States, FL, Lehigh Acres
Joined Jan 2012
1,548 Posts
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.

# Images

 Feb 23, 2012, 08:40 AM Registered User USA, IL, Wheaton Joined Oct 2010 422 Posts 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.
Feb 23, 2012, 10:38 AM
DONT PANIC! :)
United States, FL, Lehigh Acres
Joined Jan 2012
1,548 Posts
Quote:
 Originally Posted by DuPageJoe 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!
 Feb 23, 2012, 10:44 AM An itch?. Scratch build. South Wales U.K. Joined Mar 2003 11,676 Posts 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.