Flight Test Identification Methods for Loads Models and Applications 2011-01-2763
The Loads discipline contributes to the aircraft structural design by delivering shear, moment and torque (SMT, loads) all across the airframe resulting from application of aircraft airworthiness requirements as laid down in the CS 25/FAR 25 regulations and in some domestic ones. Loads computation considers the maneuver and gust conditions prescribed therein as well as other special design conditions. It is based on very detailed modeling, accounting for aerodynamics in all configurations, mass properties, flexibility of the airframe, flight control laws and retarded laws, hydraulic actuation, and specification of flight control system failure conditions. The resulting shear loads are processed and refined (e.g. nodal loads) and taken into account by the stress department for structural design.
Furthermore, the regulations require the validation of the (aerodynamic and structural) model assumptions used in calculating the design loads levels, which makes the Flight Test Identification (FT-ID) part of the Loads certification process. The result is a confirmation or possible update of Design loads levels for AC components. FT-ID comprises (a) the update of the aerodynamic model to match the flight mechanical behavior and (b) matching the measured loads on the components level. Two approaches are mainly used, each one suitable for the two tasks, which are based on (iterative, weighted) least square solutions applied to flight mechanical measurements and loads measurements. The first is like a shooting method (output error) that solves the equation of motion trying to iterate the matching of the flight test trajectory. The second one - presented and discussed in detail here - is an Equation Error Method (EEM). We provide the background for the EEM applied by the loads department to achieve the loads model validation target. The application of EEM method is explained in detail and examples are presented for adaptation of model outcomes (flight mechanics, loads) to FT data. Benefits and drawbacks of the method will be discussed.