|Apr 23, 2004, 10:34 AM|
Punta Gorda, FL
Joined Apr 2002
I must apologise for my short answer. Although I know I have answered this question several times, I could not find them using the search function. So, here is a detailed answer:
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.
If the plane is a pure delta with a triangular wing planform and no canard surfaces or horizontal tail surfaces, then the neutral point is at the wing's aerodynamic center and the CG should be located about five to ten percent of the mean aerodynamic chord (MAC) ahead of the neutral point. The Aerodynamic Center (AC) of the wing is located at 25% of the MAC. The MAC is the chord through the center of area of half the plan form. In the case of a triangle, the center of area (centroid) is at 1/3 the semi-span from the center line. Therefore, the MAC is 2/3 of the root chord. For stability the CG should be 5 to 10% of the MAC ahead of the AC.
If you find this confusing, then give me the root chord measurement along the center line of the plane you are interested in and I will calculate the CG range and its location for you.
|Apr 30, 2004, 02:23 PM|
Ollie could have put it a bit more simply....
25% MAC is at 50% of the root chord on a full (not truncated) delta. Balace your delta No farther aft than 50% of the root chord, and preferably start about 40% and expect it to be just a little nose heavy. If the front end of the delta is truncated, simply project the leading edges out to where they intersect and use that as your root chord for balancing purposes. This general method will work alright unless the design has a significant truncation of the root leading edge or the wing tips.
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