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Non-Model-Based Approach for Drag Polar Determination
(1) Suboptimal smoothed states of an aircraft are obtained using the EKF-smoother [1,14]. Scale factors and bias errors in the sensors are estimated using the data compatibility checking procedure. (2) The aerodynamic lift and drag coefficients are computed using the corrected measurements (from step 1) of the forward and normal accelerations using the following relations:
(9.68)
The lift and drag coefficients are computed from Cx and Cz using
CL = —Cz cos a + CX sin a CD = — CX cos a — Cz sin a
CD versus CL is plotted to obtain the drag polar. The first step could be accomplished using the state and measurement models for kinematic consistency. Typical results of this case study are shown in Table 9.12.
The indicated maneuvers (Chapter 7) were performed on a supersonic fighter aircraft at several flight conditions. We can easily see from Table 9.12 that the nonmodel-based approach (EUDF-NMBA) gives better fit (lesser percentage fit errors) to the data and hence accurate determination of the drag polars from flight data. An NMBA could be preferred over a model-based approach as it requires less computation time and still gives accurate results for drag polars from flight data and is a potential method for real-time online determination of drag polars.
