Is the CG location shown on the plans for the Spectre 2 Meter correct?

The plans show the CG positioned at 4.25 inches behind the leading edge. This would roughly make it greater than 50% of the Mean Aerodynamic Cord. It just looks a little too far back to me when I balance it at that point.

From : Don Stackhouse

Yes, that's the correct C/G. There's a common misconception in the aeronautical community that believes that C/G is related to the aerodynamic center of the wing. The wing is only PART of the picture. The effects of the tail, the fuselage, any props (for powered aircraft, a prop acts like both a vertical and a horizontal flying surface, developing "lift" in response to any flow into the disk that isn't parallel to the axis of rotation), and any other surfaces hanging out in the breeze that can generate lift must also be considered. Of these other lift generators, the tail is by far the most important for most aircraft.

If the airplane is to be statically stable, the C/G must be ahead of the aerodynamic center, just like you've heard. However, it's not the aerodynamic center of the wing alone, but the aerodynamic center of the ENTIRE AIRCRAFT that matters. In the case of your Spectre 2-M, the fuselage is a very minor contributor, so (as with most airplanes) we can figure the aerodynamic center of the wing plus the tail, using a weighted average of their areas. That extra surface area way back there on the end of that long tail boom moves the aircraft's aerodynamic center back behind the specified C/G. This is quite common on sailplanes, where their relatively high aspect ratios and long tail moments combine to create this result. Don't worry, your Spectre will be stable and very well behaved at that C/G.

Don Stackhouse
DJ Aerotech