Some recent posts (mine included) have given some attention to the Mean Aerodynamic Chord (MAC).  This is an important parameter used in full scale aerodynamics.  And it is useful, to some extent, in the design of our CLPA models.

However, and I am speaking in general terms, the taper ratio used on the wing planform on "most" or "typical" CLPA designs is "fairly" small compared to that found on full scale aircraft (most light planes excluded).  So the difference between the MAC and the average chord of our CLPA models is "fairly" small.  That means the search for a desirable starting point to locate the CG of the model, assuming say a 22% to 25% location of the CG on the MAC is very close to the 22% to 25% location of the CG on the average chord.  (This difference in those two CG calculations is even less when there is little or no "wing sweep"*).  After the model is flown and the CG is adjusted for best performance, that CG may change (probably a very slight change), but will likely still be generally very close to that initial CG calculation.

For me, it is more satisfying to determine the initial CG position based on the MAC, even though it probably is not more than 1/4" different than what the average chord calculation will give for my .40 to .60 powered models.  That means that I can generally position the range for the adjustable leadouts a bit more precisely based on the calculations of the leadout position relative to the CG, expected weight of the model, its speed, and expected length/diameter of the lines.

There is an equation in one of the recent threads that gives the location of the MAC relative to the root chord.  That works and is useful, even if there is only a slight amount of wing sweep.

(*Note:  Wing sweep is normally measured at the quarter chord line of the wing to the perpendicular to the airplane centerline.)

Notice to certain detractors:  I am not trying to start any controversy here.  I am only expressing my observations and experience in the design of our CLPA models.  It might be useful to those who do not want to bother with MAC calculations.

Keith

In full scale aircraft your dealing with multiple airfoil shapes from root to tip, add sweep, taper and planform and the difference between avg. chord and MAC is different. Not so much on a CL model where the airfoil is constant with a simple planform.

Here's a mathless, graphical way to calculate MAC.  I found this at boatdesign.net, but it is a standard method:

I think that the MAC in the comment above was being confused with lift distribution (dependent on aero and geom. twist, etc.). The MAC and its position are purely geometrical, as Howard's diagram indicates, but we have to keep in mind that in locating a.c.'s and thus cg's through MAC analysis, we are making the great simplification of assuming that the lift is the same at each infinitessimal point of the wing. This makes the computation of the aerodynamic center of the wing the same as finding the center of mass of a plate of constant thickness and density and the same size and shape as the wing, a significant over-simplification. There are aspects of the 3-D wing and its flow that alter this, but some apparently balance others. So we just have to get used to the variances and get on with the design job, being aware of trends. %-thickness of wings also marginally affects the approximately quarter-chord position of the a.c. The best reason to use MAC's, IMO, is to be able to predict the a.c. movement with varied sweep and taper. It seems to work pretty well, but I'd guess that the MAC (Edit 3: Oops, I meant a.c. or old center of lift) of each panel is further in toward the wing root than computed.

I don't think use of the diagram is always safe, because small inaccuracies in line positioning on paper can lead to large errors in where the lines cross. Also the a.c. of an elliptical wing of any sweep or aspect ratio is easier to compute than that of a straight tapered wing, but very difficult to approximate with diagrams. It's actually inside that of a straight-tapered wing of the same aspect ratio. This MAC's spanwise position is also listed wrong on several internet sites.

Edit: I did not address Howard's last computation, but finding the spanwise lift center is a good way of positioning the wing laterally in the fuselage, as long as the thrust line then passes through the center of mass.

Edit 2: ...or, better, of positioning the fuselage along the span, of course.

FWIW.

SK