Mass, Inertia, and Center of Gravity Characteristics

Beside other requirements, an adequate knowledge about the mass and inertia characteristics is also important for accurate flight simulation and identification of aircraft stability and control derivatives. Nondimensional moment derivatives are directly influenced by the inertia calculations, while force derivatives will be straightway affected by the errors in aircraft mass calculations [7]. The moment of inertia information is difficult to obtain as it not only depends upon the placement of the test equipment but also on center of gravity (CG) variation during flight. The way the fuel is consumed during the flight, e. g., the forward tank, followed by the wing tanks and so on, can affect the moments of inertia. Determination of inertia from special oscillation test rigs is not practical [8]. A manufacturer’s data are mostly for

moment of inertia calculations. For a 4 t class of trainer aircraft, typical values of the moment of inertia Ix, Iy, Iz, and Izx will be like 4,500, 18,000, 20,000, and 1,500 kg m2. The cross-product of inertias Ixy and Iyz is small and hence usually neglected. Information on fuel consumption is useful to compute CG travel and the actual mass of the aircraft at any time during the flight. To correct the flight data for the offset in sensor locations from CG, we need to compute the actual distance of CG from the reference vertical datum rather than express it in terms of percentage of the mean aerodynamic chord (MAC). The actual distance of CG in meters from the vertical datum can be computed as follows:

where c is the mean chord length and xLE is the extreme leading edge location of the MAC.