When the system equations are nonlinear or the input is nonstationary the foregoing methods of analysis all fail. In such situations one approach is to construct an appropriate mathematical model of the system—analog or digital—and feed in random inputs representing the turbulence. The statisti­cal properties of the output can then be determined by analogue or digital analysis techniques.

In this connection mention should be made of a possible experimental technique that does not appear to have been applied yet. It would consist of exposing a rigid model of the vehicle to a wind-tunnel flow simulating the real turbulence. Force transducers could then produce time records of the actual input forces, thus bypassing the whole problem represented by the second box of Fig. 13.1. The measured forces and moments provide directly the required inputs to the mathematical model, which could be connected on line. In transient situations an ensemble of records could supply the appropriate statistics.