U.S. Department of Defense


Date of this Version



Aerospace Science and Technology 39, 2014


U.S. Government work


A new approach for computing the unsteady and nonlinear aerodynamic loads acting on a maneuvering aircraft is presented based on linear and nonlinear indicial response methods. The novelty of this approach relies on the use of a grid motion technique for CFD calculation of response functions and the development of a time-dependentsurrogate model that fits the relationship between flight conditions (Mach number and angle of attack) and responses calculated from a limited number of simulations (samples). The reduced-order model, along with the surrogate model, providesa means for rapid calculation of response functions and predicting aerodynamic forces and moments during maneuvering flight. The maneuvers are generated using a time-optimal prediction code, each covering a different range of angle of attack and motion rates. The side-slip angle ranges from −5° to 5° for all maneuvers, and the model assumes that the lateral aerodynamics is linear with side-slip angle over this range. Results presented show that the aircraft studied in the current paper exhibits highly nonlinear roll moments even at low angles of attack which the linear model fails to predict. The results of the new model provide some evidence that, for a certain range of input parameters, in certain maneuvers considered, the predictions match quitewell with URANS CFD predictions. The models were at least better than traditional quasi-steady predictions. However, for aircraft maneuvering at high angles of attacks, discrepancies are found in lateral coefficients between the model and CFD. At these conditions, the lateral airloads become highly nonlinear with side-slip angle and the model fails to predict these effects. Also, the results show that the CFD calculation of response functions in the high angle of attack flight regime remains a challenging task.