I see that many people are running CG's a long way back in HLG and they seem to perform well...

Is this due to the fact that the often highly asymmetric airfoils have a center of pressure that moves fwd with the higher angles of attack that exist with low speed HLG craft?

From : Don Stackhouse

No, that has nothing to do with it. Many HLG's with well-aft C/G's have airfoils with relatively little camber. The airfoil characteristics have very little (but not quite zero) influence on C/G. BTW, center of pressure has been pretty much discarded in modern aerodynamics. We now use aerodynamic center, which is not quite the same thing.

To understand C/G you first have to understand the concepts of neutral point and static margin. It's quite simple really. The neutral point nothing more than the C/G location where the static stability is exactly neutral. BTW, in most discussions we refer to the neutral point for pitch stability, but there is a neutral point for yaw stability as well, and the two are not necessarily the same.

Static margin is the distance between the C/G and the neutral point. If the C/G is ahead of the neutral point, then the static margin is positive, and the static stability is positive by an amount that is related to the static margin. If the C/G is behind the neutral point, then the static margin and static stability are negative (i.e.: the model is statically divergent, if you pull the nose up it wants to go up even more, shove the nose down and it wants to tuck).

Now the catch: the location of the neutral point depends on the design of the ENTIRE AIRCRAFT, not just the wing. This rule of thumb of specifying C/G as a percentage of wing chord without considering anything else is actually quite bogus. We get away with it because for the most part conventional aircraft layouts are reasonably similar, and because the static margins that usually result from these rules of thumb are typically quite conservative. It all seems to work fairly well until we notice that some airplanes can handle C/G's far aft of what the old rules of thumb say should cause instability.

When we consider the aerodynamic center of the entire aircraft we can see why. The tail is well aft of the wing. When we find the aerodynamic center of the entire aircraft (which should include the effects of the fuselage too, although with the slim fuselages typical of our models we can usually ignore it), we find that the total aerodynamic center is typically in between the aerodynamic centers of the wing and tail, proportional to the ratio of their areas. For example, if the wing had an area of 2 square feet and the tail had an area of 1 square foot (area of the total is therefore 3 square feet), then the aerodynamic center of the aircraft (neglecting the fuselage) is located 2/3 of the way from the tail's aerodynamic center to the wing's aerodynamic center. BTW, the neutral point will usually be pretty close to the aircraft's aerodynamic center.

A large tail and/or a long tail moment can obviously shift the aircraft's aerodynamic center ("AC") and the corresponding neutral point well aft on the wing, even aft of the trailing edge.

Go back to the design you're studying, find the aerodynamic center of the entire aircraft, then determine the static margin based on that overall AC. I think you'll find that it makes a lot more sense this way.

BTW, this same method works for canards and other unusual layouts.

Don Stackhouse
DJ Aerotech