<aerodynamics> <axis name="LIFT"> <coefficient name="CL"> <function> <table> <independentVar lookup="row">aero/alpha-rad</independentVar> <independentVar lookup="column">fcs/camber-command</independentVar> <!-- data from wind tunnel: rows alpha (-0.2 to 0.4 rad), cols camber (0 to 0.05) --> <tableData> -0.2 -0.4 -0.35 ... 0.0 0.2 0.25 ... 0.4 1.2 1.3 ... </tableData> </table> </function> </coefficient> </axis> </aerodynamics> He does the same for drag and pitch moment. For sideforce, yaw, roll, he uses simpler stability derivatives.
<flight_control name="FCS"> <channel name="pitch"> <pid name="elevator_pid"> <kp> 0.8 </kp> <ki> 0.05 </ki> <kd> 0.2 </kd> <input> aero/qbar-psf </input> <!-- dynamic pressure --> <output> fcs/elevator-cmd-norm </output> </pid> </channel> </flight_control> He runs a quick test using JSBSim’s command‑line tool: jsbsim tutorial
The X‑1 lifts off at 45 knots, climbs, then slowly rolls left due to an asymmetric thrust he forgot to model. Alex corrects by adding a trim tab definition under <flight_control><channel name="roll"> . Alex corrects by adding a trim tab definition
<metrics unit="KG" unit_area="M2" unit_length="M"> <wingarea> 12.0 </wingarea> <wingspan> 10.0 </wingspan> <chord> 1.2 </chord> </metrics> All units are SI internally, but you can specify units per value. JSBSim converts. Part 3: The Aerodynamics Puzzle – Coefficient Tables Now the hardest part: the X‑1 has a variable‑camber wing (no flaps, but morphing trailing edge). No existing table works. You visualize separately.”
Alex’s first morning. The lead engineer, Dr. Maya, slides a USB drive across the table. “The X-1’s aerodynamics are unconventional. We’re using JSBSim. Not Simulink. Not X‑Plane. JSBSim. Go learn it by noon.”
Maya smiles. “You don’t fly it. You build the laws of physics for it. JSBSim is a library—a simulation engine. It takes an XML model and outputs time‑step states: position, orientation, velocities. You visualize separately.”