(
Recommended CG: 5.25" to 5.5" from the LE at the root of the wing)
Find the Center of Gravity
Mean Aerodynamic Chord (MAC)
One theory is that all aircraft should balance at a point defined as 25% of the Mean
Aerodynamic Chord, with no consideration given to tail area or moment arm. For the scale
model RC airplane, I prefer to take these facts into consideration.
When designing a scale model we do not have the ability to change dimensions or ratios-we
are restricted by our wish to build an accurate reproduction of the full size.
The formula I have used, came from a book written a few years ago by Gordon Whitehead
titled "Radio Control Scale Aircraft Models for Everyday Flying" ( A great book that every scale
modeler should own)
Here is the formula- CG POSITION = MAC/6+(3 X TAIL AREA X TAIL MOMENT ARM ) / 8 X
WING AREA. Note that the Moment Arm is defined as the distance from the 25% MAC of the
wing to the 25% MAC of the tailplane.
For a constant chord wing the MAC is obvious but for tapered or swept wings it is not
so easy. However a simple graphical method is shown here-
•
Draw or trace the wing shape as shown. Then extend the Leading Edge ( L.E.)
and Trailing edges to the center line of the plane. Also square off the tips. Now
extend the opposite chord lengths as indicated on the drawing; i.e. lengthen tip
chord B by root chord A and A by length B. The four extreme points are then
connected by diagonals. The point where these lines intersect will indicate the
Mean Aerodynamic Chord (MAC). To find the 25% of MAC position we must
measure 25% of the MAC back from the L.E. and extend this to the aircraft
center line.
