Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Longitudinal Center of Gravity Control
Table 15.1 also shows the longitudinal center of gravity (LCoG) computation. To do this, the LCoG values of all the SLS nylon and foam parts are calculated by the CAD Program, while those for the bought-in components are established by their locations in the design drawings, assuming that the individual centers of gravity lie at the center of each component. The analysis shows that the estimated LCoG is slightly forward of the centerline of the main spar (which lies at the quarter chord point). This is, of course, for an aircraft without gasoline in the main tank, which itself lies forward of the quarter chord point, so the aircraft will have positive trim stability even when all fuel is used.
Should the LCoG not be as required at this stage, consideration must be given to changes to the overall geometry of the airframe or the positioning of heavy internal components like the batteries. When designing with SLS nylon fuselage elements, we find it a relatively simple matter to adjust the LCoG by simply changing the lengths of one or more fuselage elements; then provided there are some relatively heavy elements in the nose of the aircraft, the LCoG can be adjusted as required. In the case of the aircraft tabulated, the batteries and main gasoline engine all lie well forward, so rather small adjustments in fuselage length gave good control of the LCoG without the need for major redesign, though extending the fuselage does, of course, increase the weight.
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