Generic FAQ on CG troubleshooting

[MGBGTRacer]

I hope I put this in the correct place...
I searched for info on troubleshooting/determining CG in a generic sense and couldn't find what I was looking for-

In general, what behavior of an aircraft indicates incorrect CG (in either direction)? Can one of you experienced folks help we newbies in troubleshooting CG problems? Thanks in advance for your help.

[Dax]

if your surfaces are all level (no trims) in flight and the plane points down, cg too far forward. points up, too far back.

if on handlaunch in sticks in the ground nose first, WAY too far forward. if it points nose up and falls over sideways, WAY to far back

[tim hooper]

MGB,

I'm moving this to Modelling Science where they know all about these things........

tim hooper:)

[steve lewin]

And if it is very unwilling to respond to the elevator the CG is too far forward. If it is very twitchy on elevator CG is too far back (hence the old saying plane with forward CG always flies poorly, plane with rearward CG only flies once ;)).

There's useful general trimming guide at http://www.willingtons.com/mymac/model_trimming.htm

For more specifics on CG including the famous "Dive Test" try http://www.polecataero.com/articles/cg_art.shtml

Steve

[Dax]

yep, the way I heard it was "nose heavy flies, tail heavy fries" but I like yours better.

[Ollie]

The CG location affects three things. They are control responsiveness, pitch stability and pitch trim.

Stability is the tendency to return to a trimmed flight atitude and airspeed after a disturbance such as a gust or abrupt return of elevator to neutral. The more forward the CG the faster the plane returns to the trimmed flight condition. As the CG is moved aft, it will come to a place where the time to return to the trimmed flight condition is infinite and in fact, there is no longer a trimmed flight condition and the plane goes where it is pointed after the controls are neutralized or the gust passes. This is the point of neutral (zero) stability also called the neutral point. If the CG is moved aft of the neutral point, the plane diverges from its flight path and becomes unstable. A slightly unstable plane can be controlled by a pilot with quick reflexes and intense concentration but as the CG is moved farther aft, the plane becomes uncontrollable.

Aerobatic aircraft are usually set up with the CG near neutral stability. Trainer aircraft are set up with lots of stability by putting the CG well ahead (5 to 10% of the wing mean aerodynamic chord) of the neutral point.

The location of the neutral point is influenced by many things. Long tail moment arms and large tail areas move the neutral point aft. Things ahead of the CG like propellers, fuselage nose area, landing gear, etc. move the neutral point forward a little.

The angular difference between the wing and horizontal tail (decalage) determines the pitch trim in glide. The angular relationship and location of the thrust line and the decalage determine the pitch trim under power. The CG location affects control sensitivity, stability and trim. Therefore the CG location adjustment should only be used to set the desired degree of pitch stability. The angular relationships between the thrust line wing chord and tail chord should be adjusted for the desired trimmed flight condition. The maximum control throw adjustment should be used to set the desired control sensitivity after the CG is properly adjusted.

[MGBGTRacer]

excellent information- thanks everyone!