Tech Briefs

AFRL develops a new aerodynamic analysis program.

In addition to collecting flight data through radar-based and onboard sources, analysts using the new testing procedures measure atmospheric data just prior to the experimental firing. The atmospheric data they collect consists of pressure, temperature, and wind speed and direction as a function of altitude. Based on the projectile's position, the analysts can interpolate the atmospheric data to calculate local conditions of Mach number, air density, and wind speed and direction. Analysts also record the projectile's physical properties (mass, moments of inertia, and diameter) immediately before initiating the experiment; they then integrate this information with the acquired flight data for all points along the trajectory.

Since past research has revealed that aeroballistic parameters vary nonlinearly with angle of attack and Mach number, the new process parses flight data by Mach number, at trajectory locations where aeroballistic parameters are assumed to be nearly constant in Mach number. For each grouping, the analysis accommodates nonlinearity with angle of attack and small variations in Mach number. Test engineers then analyze each Mach number group individually. Using the equations of motion, they determine the aerodynamic forces and moments for each instantaneous state of motion within each parsed group. Employing a multidimensional, leastsquares data-fitting methodology called Chi square, the analysts next fit the force and moment coefficients to the aerodynamic parameters to obtain the aerodynamic coefficients and stability derivatives. They repeat this sequence of activities for each of three distinct projectile motions: drag/swerve, roll, and pitch/yaw.